Epiphone Valve Standard to 18W TMB Conversion

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

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bigjoe
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Epiphone Valve Standard to 18W TMB Conversion

Post by bigjoe »

I am modding a Valve Standard. I did the following mods as per this thread as best as I could figure out. It’s a little long and disjointed. I have only modded the pre-amp so far.

I’ve not done the power amp stuff as I wanted to see what the pre-amp stuff would get me. I’m not sure about the power amp it looks like it has a 12AX7 (I assume this is a part of the power amp) and 2 EL84s. This is my first venture into an amp mod.

The reason I’m modding is it’s a little farty ( I think that’s the term) in the low end. I like the sound, but open position rhythm/solos was to muddy not very well defined tonal. I could have lived with it, but seeing these posts made me think I could help it out.

I did the mods below and now it really distorts, more so than over drive, which I was not crazy about and it almost sounds muffled (muffled is not the best word, but I’m at a loss for one, it reminds me of the marshall tone but– its like “Purple Haze” and I want more of a “Castles Made of Sand” sound).I think its louder than prior to the mod. The thing I did like before the mod was the nice fat, well defined, mid and high tones with a chimey bite. The low end stuff was really getting to me though. With mods below the low end is not much better and the mid/high bite it had is gone. I play a 1982 60’s reissue strat mostly blues if it matters.

One thing I noticed when trying to dial in the sound (post-mod) was its considerably louder with the treble turned all the way DOWN. Also the middle and bass have virtually no effect on the tone.

Mods:
bypassed Solid state
C1 and C9 - 1 uF electrolytic 50V ( I cannibalized these from the SS circuit)
R12and R13 changed to 100K
VR6 put 2 100pf caps in parallel across it (didn’t have one 180 pf cap)
removed DSP circuit board
I might try the R12 with a 1M audio pot

When bypassing the solid state the schematic for the valve special and valve standard was not the same as my board (someone already mentioned this). The master volume had a resistor across it (50 K I think someone posted a pic mine is the same way). I left it and ran the wire from R17 to the outer most lug of the pot. Not sure what that resistor does nor am I sure of the board component number.

I also noticed C6 on my board was not connected to Master volume wiper as the schematic suggests but directly to the solid state op amps on the main board.

Did I miss something? Will power amp mods help? Can I do the mods to the power amp that are listed here for the special, and if so any special considerations?

Over all it went good and only took an hour or so of work, so if I have to change it back it won't be rough.

Joe
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bigjoe wrote:The thing I did like before the mod was the nice fat, well defined, mid and high tones with a chimey bite. The low end stuff was really getting to me though. With mods below the low end is not much better and the mid/high bite it had is gone. I play a 1982 60’s reissue strat mostly blues if it matters. ....
This doesn't make much sense, since these mods will both tighten up low end, and improve clarity and definition at the high end.
bigjoe wrote:C1 and C9 - 1 uF electrolytic 50V ( I cannibalized these from the SS circuit)
This is a well-proven mod which will tighten up the low end. Ideally you should also reduce R8 to a JTM45ish 820 ohms (or less) to help tighten the low end some more. You could also reduce R9 to 820 ohms and disconnect the bypass cap C9, which is also like the JTM45.
bigjoe wrote:R12and R13 changed to 100K
That mod improves sparkle and definition. Also do the same with R17, and make sure it's discoonnected from C7/R18.
bigjoe wrote:VR6 put 2 100pf caps in parallel across it (didn’t have one 180 pf cap)
That also improves definition and sparkle.
bigjoe wrote:I might try the R12 with a 1M audio pot
Which one's R12? I couldn't find it on the Valve Special schematic... :?
bigjoe wrote:When bypassing the solid state the schematic for the valve special and valve standard was not the same as my board (someone already mentioned this). The master volume had a resistor across it (50 K I think someone posted a pic mine is the same way). I left it and ran the wire from R17 to the outer most lug of the pot. Not sure what that resistor does nor am I sure of the board component number.
A 50k resistor across the MV will kill a lot of signal to the power amp.
bigjoe wrote:I also noticed C6 on my board was not connected to Master volume wiper as the schematic suggests but directly to the solid state op amps on the main board.
So what does your MV connect to? Or is it that C6 sits between the op amp output and one of the end lugs of the MV pot? That would also work, and keep the tube and SS voltages separated from each other.
bigjoe wrote:One thing I noticed when trying to dial in the sound (post-mod) was its considerably louder with the treble turned all the way DOWN. Also the middle and bass have virtually no effect on the tone.
That doesn't make sense. The tone stack in this amp is just a regular Marshall tone stack - which BTW is known to have a somewhat ineffective bass control, which is easily fixed. The behaviour you describe with the treble control suggests something's broken, or maybe there's a bad solder joint somewhere in there.
bigjoe wrote:Did I miss something? Will power amp mods help? Can I do the mods to the power amp that are listed here for the special, and if so any special considerations?
Can you post a schematic for your amp? It's hard to really know otherwise.

We could easily make your preamp quite similar to a JTM45's, but I think we should first try to fix the strange way its behaving at the moment.
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Post by bigjoe »

Sorry I menat to attach the schematic last night when I posted. It was late and I forgot. Gibson Support sent it to me.

I also got my JJ tubes yesterday. I want to hold off until I get things straightened out.

I’ll reduce R8 and R9 to 820 (across C1 and C9 respectivley), and disconnect C9.

You said to change R17 to 100k? I thought it was supposed to be be removed. I pulled up one side of it and ran a wire from where the resistor lead was over to VR6 lug. Having disconnected R17 do I need to disconnect C7/R18. Or do I need to change R17 to 100K and run wire from it to the VR6 and then disconnect C7/R18?

The 2 100pf caps across VR6 did brighten it up some. I added them after I made other mods and thought it was way to dark.

R12 is input for VR1 on both the special and standard. Someone said the made it a VR to helping rid the dark sound. quote page 1 “The large resistors in series with the signal chain are killing the highs - R12, R13 and R17, as is the gain pot VR1 to some extent.”

So remove the resistor on VR6, or reduce its value? If so what do I change it to?

Not sure what I was saying about C6. Looking at the diagram it doesn’t connec to the VR6 pot at all. I must have meant C27. I was having a bit of a hard time tracing back to components on the PCB. DO I want to jumper C6?
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Post by zaphod_phil »

bigjoe wrote:You said to change R17 to 100k? I thought it was supposed to be be removed.
Doh, I forgot that one! :oops:
I pulled up one side of it and ran a wire from where the resistor lead was over to VR6 lug. Having disconnected R17 do I need to disconnect C7/R18.
bigjoe wrote:Or do I need to change R17 to 100K and run wire from it to the VR6 and then disconnect C7/R18?
There is the real possibility of overdoing the distortion on your amp, and the overdriven tone getting blatty. So I'm tending to think it may be preferable to have some attenuation between stages. Now the MV, VR6, is shown in your schematic as 50k, and it looks from your previous post like they've probably just stuck a 50k resistor across a 1M pot to approximate to that. In which case I would remove the 50k (or whatever it is) resistor and then use the 100k value for R17. It can then be tweaked a little more later - maybe even place a 470k resistor across VR6.
bigjoe wrote:R12 is input for VR1 on both the special and standard. Someone said the made it a VR to helping rid the dark sound. quote page 1 “The large resistors in series with the signal chain are killing the highs - R12, R13 and R17, as is the gain pot VR1 to some extent.”
OK, yes, which is why I suggested replacing it and R13, both with a 100k value. FYI, the JTM45 uses a 270k in the R13 position, with no R12 (ie shorted). So that might also be interesting to try out. You might also like to try a bright cap across VR1, like you did with VR6 to get you a bit more high end sparkle.
bigjoe wrote:Not sure what I was saying about C6. Looking at the diagram it doesn’t connec to the VR6 pot at all. I must have meant C27. I was having a bit of a hard time tracing back to components on the PCB. DO I want to jumper C6?
Don't jumper C6 or your amp won't work! In the interests of tightening and brightening the tone you could replace both C5 and C6 with 0.01uF caps.

I'm still puzzled about what's gone wrong with your tone stack. So I think you will need to do some more checks around it, before proceding further.

This particular amp has a lot of potential overall. It has a great preamp topology that could be voiced either to be Marshall or Vox/Matchless like. The tone stack is totally Marshall, while the power amp is also similar to a Vox/Matchless or Marshall 18W. If the mods are done right, there's no reason it should be the least bit dark or muddy. It's actually more likely to be too bright. Keep on persevering. The end result will be worth it!
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Post by liquidsuspension »

This particular amp has a lot of potential overall. It has a great preamp topology that could be voiced either to be Marshall or Vox/Matchless like. The tone stack is totally Marshall, while the power amp is also similar to a Vox/Matchless or Marshall 18W. If the mods are done right, there's no reason it should be the least bit dark or muddy. It's actually more likely to be too bright. Keep on persevering. The end result will be worth it!
After tweaking on my valve special, it has become almost too bright now (with the treble pot turned up all the way). not a bit dark at all, which was my primary gripe. bass sounds tight, especially through an external cab. the mods made quite a difference to this cheap amp. still haven't replaced the OT. i've ordered up an 18w tmb kit instead :wink:
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Post by bigjoe »

I don’t want you all to get the impression I’m down on this amp or think that it’s a POS. Quite the opposite. I appreciate everyone's time and advice.

I need to make sure I have C7/R18 removed/isolated. I’ not so sure that they are. I’ll put R17 back in the circuit. Get rid of he VR6 pot resistor and run the wire from R17 to the VR6 lug (outer most lug) . The overdrive is quite intense (really distorted). I get the impression this should mellow that back out.

Also just clarify a few things about my master volume. The jumper wire (from pre-amp/R17 is on the out most lug (outer = closest to the outside edge of the PCB). The 50 K resistor was on the other outside lug.

I placed a meter across VR6. I don’t think its 50K. I measured across it (it was not isolated though) and side to side 1M, one lug to center would vary with the pot. The pot also says 1M on it as the value. So I don’t think that the pot is 50K, but you said having the resistor across would approximate this?

A bright cap across VR1…. being another 180 pf to get the brighter tone?

I’ll try this stuff and post. The things that really got me was how loud the amp was with the treble turned down completely and would get softer as you turned it up. Mid and Bass were basically ineffective.

This is my first amp mod. I was going to build one and this standard dropped in my lap for $80. I’ve got a few dollars in parts and a new set of tubes added to that so far (not much more than a VJ would have cost). It had a problem cutting out that I knew about. After 5 minutes of poking around I found 2 bad caps, I think C5 and C6, the component slots they occupied both said they were jumpers.

I tapped on the caps and it re-produced the cut out problem. I had one other problem with DSP (now disconnected, but wanted to make sure I have a fully opertaion amp before modding) self induced with sloppy soldering. But it’s a great amp that I like more than I had ever dreamed I would. Even if the mods don’t work out and I put it back stock I can live with it. Probably the best $80 I have ever spent honestly.
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Post by zaphod_phil »

bigjoe wrote: I need to make sure I have C7/R18 removed/isolated. I’ not so sure that they are. I’ll put R17 back in the circuit. Get rid of he VR6 pot resistor and run the wire from R17 to the VR6 lug (outer most lug).
Sounds like a good plan.
bigjoe wrote:The overdrive is quite intense (really distorted). I get the impression this should mellow that back out.
Replacing R17 should help with that, as well removing C9.
bigjoe wrote:So I don’t think that the pot is 50K, but you said having the resistor across would approximate this?
Yes, exactly that.
bigjoe wrote:A bright cap across VR1…. being another 180 pf to get the brighter tone?
Sure try 180pF or 220pF and see how it sounds.
bigjoe wrote:The things that really got me was how loud the amp was with the treble turned down completely and would get softer as you turned it up. Mid and Bass were basically ineffective.
There's something badly screwed up there. Maybe a bad joint, a resistor gone open circuit or a leaky cap....

Once that's fixed I'll tell you what values to use to make the tonestack more effective, as well as a small tweak to the PI to make it sound less fizzy when overdriven.
bigjoe wrote:This is my first amp mod. I was going to build one and this standard dropped in my lap for $80. I’ve got a few dollars in parts and a new set of tubes added to that so far (not much more than a VJ would have cost).... Probably the best $80 I have ever spent honestly.
You really got a bargain then. The mods I'm giving you are gradually taking it towards becoming an 18W TMB type of amp - since you talked about Hendrix tones earlier. Alternatively, you could even convert it into a Matchless Lightning. If you were to splash out a little money on a real 18W OT, plus those new JJ tubes you just bought, the tone of this amp could really become a killer!
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Post by bigjoe »

I have C7/R18 isolated. I found a very convenient jumper R17 ran to, I pulled up jumper and isolated the SS stuff that way.

Changed R17 to 100K as suggested ran wire from R17 to outer lug of VR6.

Removed 28 K pot resistor off VR6 (previous said it was 50K, but was not sure).

Changed R8 and R9 to 820.
** Edited by bigjoe should I still bridge these with a bright cap of 100pf, 220pf or 470 pf as suggested? I was reviewing thr previous posts and saw I never did this). ***

C1 is 1uF and C9 disconnected.

Added bight caps to VR1 (I put the 180 pF on VR6, so I put 100 pF in parallel on VR1).

The overdrive is cleaned up and sounds good again. Overall the tone is as ok, but… there’s always a butt. The treble is the only EQ working Mid/Bass have no noticeable effect. Treble turned all the way down the amp is probably louder than previous, but its not an acceptable tone. The low is still lacking and muddy.

I can get a good tone, but is ridiculous how low the volume is. Its not a whisper, but its no where near what it was before. Before the mods I turned treble & mid all the way up and bass at around 2/3. It had a great mid & high tone and bass was lacking. Its almost the opposite now. The treble all the down and terrible tone, but the volume is there.

I’m just thinking out loud now, but is is possible the SS stuff did something to signal that’s not being accounted for now? Do I need to pull the outer leg of VR6 off the board so it is not attached to the SS circuit, and only attached to the wire from R17? The SS is isolated as mentioned above, but could the signal be tracing back through the SS circuit via the VR6 leg? The reason I asked is because the wire is basically connected directly to the Treble pot (realizing it has R17 between). if I pull the leg off the board and solder wire directly to it it eliminates the SS completely.
** Edited by big joe ** I found a jumper quite close to the leg of VR6 that went to the SS circuit and pulled it up. No better really. The treble is really muffled when turned down, but loud. I know there is more potential for more volume. The more I look and work on this I get the feeling as someone else said the DSP was going to be on board and not another card, also I think they set the board up to make a model without the DSP****

I might try and pull the lug off the board and connect wire directly to it. I don’t think it can hurt anything. (see edit above didn't matter).
I’m going to start looking for components gone bad I guess. Any ideas where to start? Should I even bother putting in the JJ’s at this point?
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Post by zaphod_phil »

bigjoe wrote:The treble is the only EQ working Mid/Bass have no noticeable effect. Treble turned all the way down the amp is probably louder than previous, but its not an acceptable tone. The low is still lacking and muddy. I can get a good tone, but is ridiculous how low the volume is. Its not a whisper, but its no where near what it was before. Before the mods I turned treble & mid all the way up and bass at around 2/3. It had a great mid & high tone and bass was lacking. Its almost the opposite now. The treble all the down and terrible tone, but the volume is there.
OK, I'll spell it out again. Your tone stack is broken. I don't know what it is, but it's nothing directly caused by the mods. Maybe a bad pot, resistor or cap, that's messing up the whole way the stack works. Maybe even a damaged track, a bad joint or a solder whisker somewhere. You need to fine tooth comb that whole part of the circuit. Also make sure there's nothing still connected to the tone stack or R17 that's somehow pulling the signal level down. Also check that the cathode voltage of V2B is close to the preamp rail voltage, which would indicate that the cathode follower is working correctly.
bigjoe wrote:...is it possible the SS stuff did something to signal that’s not being accounted for now?
No - because without the SS stuff, you end up with a perfectly good tube amp design - which could become a real good tube amp after a few tweaks. The silly thing is that with the SS circuitry in place, the signal first has to be attenuated by those big resistors so the SS stuff can handle it, and then it has to be boosted again afterwards to feed the power amp.
bigjoe wrote:Do I need to pull the outer leg of VR6 off the board so it is not attached to the SS circuit, and only attached to the wire from R17? The SS is isolated as mentioned above, but could the signal be tracing back through the SS circuit via the VR6 leg? The reason I asked is because the wire is basically connected directly to the Treble pot (realizing it has R17 between). if I pull the leg off the board and solder wire directly to it it eliminates the SS completely.
Definately do that, in case that's what's screwing up your tone stack. Also, just for fun try disabling your tone stack by lifting the ground end of the Mid pot, VR3.

JJ tubes tend to be a little dark, so I suggest you don't bother using them until these issues have been fixed. Once the tone stack problem has been fixed you'll be pretty close to having a killer amp!
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Post by bigjoe »

Phil,

Thanks for the help.

Tone Stack Problem solvedn nice and bright now (glad I pulled the bright caps). The treble/mid/bass all work.

I checked solder conn., for bad resistors, for proper value resistors (R8 and R9 was 820 changed to 1K, originl value 1.5 see below). Re-ran wire from R17 (changed back to original 470K see below) to VR6. Everything looked fine and R values were correct.

The SS is absolutely isolated.

For good measure I took the caps off of VR6 and VR1 (180 PF). I thought I'd simplify it as much as possible.

While I was at it I changed C3/C4. They are in the tone stack. I think caps C3 and C4 were the culprit. The same type/value caps (C7) were bad when I bought the amp ( C7 was easy to troubleshoot tapping on it duplicated problem, glad I bought extras).

After all of the above, NO HEADROOM. It got dirty really fast. Sounds OK, just not my thing really. So with what little knowledge I have of tone circuits from here and internet searches I made some tweaks. I left C1 and C9 1uF (originally 22uF), but I changed R8 and R9 to 1k. From what I have read this will reduce gain (higher the R value less gain?) the smaller Caps @ 1 uF are for more bass correct? Any ideas for tweeking these. I'm thinking about putting the stock parts back or going higher to see what happens if I'm still lacking when it comes to the lows. Originlly the bass was very lacking kind of muddy without any tonal definition.


Also changed R17 back to original 470 K. from what I have red this will also help decrease the heavy gain I was experiencing. After the R8, R9, R17 changes much cleaner and does not break up as soon. Bass sounds good at bedroom volume. Its late and I will try to see how bass reacts in the AM at high volume…

Tried out the amp this morning with it cranked.
The bass is better when loud, but not great.

With master turned up and gain very low it breaks up and cuts out (completely almost) gets a fuzzy at vairous volumes along the way. ( my gut tells me this is due to gain and not the tone stack). I thought it was the speaker, but I plugged it into a micro stack with 2 10” celestions same result. I have 1uF cathode bypass caps on V1 and V2.

Any ideas. I have to say its really come around and is getting better. I don’t think I have as much volume as pre-mod. At least not with it cutting out like it is. Thinking about going back to stock R8 and R9 values . This would help it cutting out at higher volume correct? I thougth maybe going back to stock parts and seeing how it behaves and then go from there. I did not have a problem with the mids/highs with it stock only the crappy lows.
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Post by zaphod_phil »

bigjoe wrote:Tone Stack Problem solved nice and bright now ...
What did I tell ya :D I have a couple of tone tweaks for that tone stack, when you're ready to try them, but let's get the more important things fixed first.
bigjoe wrote:So with what little knowledge I have of tone circuits from here and internet searches I made some tweaks. I left C1 and C9 1uF (originally 22uF), but I changed R8 and R9 to 1k. From what I have read this will reduce gain (higher the R value less gain?) the smaller Caps @ 1 uF are for more bass correct?
Let me save your time on the internet :) Increasing those two cathode resistors has *no* effect on stage gain, since they are bypassed. IMHO you should have left them at the Marshall values I originally recommended. From your refernces to Hendrix I'm assume you like a Marshally vibe. Removing C9 however, will reduce the amount of gain and distortion in the preamp, and make the 2nd stage like a JTM45's. Reducing C1 from 22uF and R8 to 820 ohms also helps reduce and tighten up the bass. I recommend you leave them like that.
bigjoe wrote:Also changed R17 back to original 470 K. from what I have red this will also help decrease the heavy gain I was experiencing. After the R8, R9, R17 changes much cleaner and does not break up as soon.
R8 and R9 had nothing to do with it. Just the increasing of R17 to 470k did. However, as you pointed out your amp doesn't seem to get quite as loud though. So you may want to play around with that 470k. Maybe also try a 220k as an in-between value - but only after you've got the power amp problem fixed.
bigjoe wrote:Bass sounds good at bedroom volume.
Mainly due to the reduced C1 and C9 values.
bigjoe wrote:With master turned up and gain very low it breaks up and cuts out (completely almost) gets a fuzzy at vairous volumes along the way. ( my gut tells me this is due to gain and not the tone stack). I have 1uF cathode bypass caps on V1 and V2.
It appears that you're getting blocking distortion within the power stage, as well as probably some fizz from the PI being biased cold. Since the preamp gain is low and the MV turned high, this indicates that the problem isn't directly related to preamp gain. The classic fix for blocking distortion in the power amp is to reduce the coupling caps to the power tubes, C5 and C6, to 0.01uF. However, if this effect is only happening at certain volume settings, it suggests there may be some parasitic oscillation contributing to the problem, which usually we would try to fix by means of screened wire. So I'm thinking that your wire connecting R17 to VR6 might need to be shielded. Also reducing the PI's cathode resistor from 1.2k to 820 ohms like in a Marshall 18W or 470 ohms like in a JTM45/Bassman or Plexi will stop fizziness from the PI.
bigjoe wrote:Thinking about going back to stock R8 and R9 values . This would help it cutting out at higher volume correct?
No, it will just increase bass and make the preamp overdrive sound more fizzy.
bigjoe wrote:I thougth maybe going back to stock parts and seeing how it behaves and then go from there. I did not have a problem with the mids/highs with it stock only the crappy lows.
There are several problems with the component values used in the original design, in particular C1/R8, C9/R9, R12 and R13. These will all lead to muddiness, farty low end and increased hiss.

So here's the next thing. Once you get this amp's basic bugs fixed, it has the basis of a real killer amp. Why settle for a mediocre amp, when you could have a fantastic one? You could quite easily tweak it to be an 18W TMB or a Matchless Lightning - both using tried and proven component values. It's just your choice whether you want a the Marshall vi be or a Matchless one. Personally, I would prefer the 18W TMB, but that's just me. It's your amp and your choice.
dram56 wrote:I was wondering if anyone has put together a list of the common mods for the Valve special. Something alone the lines of the Gold Mods for the valve junior. I have a valve junior that I am modding now, and I also have a valve special that badly needs help next. The valve junior repository would be a great place for the valve special mods (summary) to reside. Might also be a great project for Ken at turretboards to tackle next, a "drop-in" board for the special.
I don't think there have been enough Valve Special modders to arrive at a consensus of which mods are good to do. You could of course convert the preamp to a Marshall 18W TMB or JTM45ish preamp, or a Matchless Lightning preamp. Then do the VJr mods on the power amp section. If you wanted to try a new board, this amp already has the tube complement to be an AX84 High Octane, so I would just do that.
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Post by bigjoe »

Dr Phil call an ambulance.

It WAS better. Still no headroom, breaking up too early for my taste. The clipping was gone and it was VERY LOUD.

Bass was muddy. Not much tone or definition. The highs were too high.

What I did.
VR6 shielded wire ran to VR1.
I also did the following from page 2 since my highs were “like an ice-pick”.
“bad cat minicat mod” for the power tubes on page 2 to help with ice-pick highs. I connected a 100K resistor in series with a .002 uF cap to ground.

Turned on the amp and immediately had a high pitch squeal with gain and volume turned down completely. So I turned it off and took resistor and cap back off. Still had the high pitch squeal. Now something is hosed. Checked solder connections to make sure I did not bridge something. Everything looked good.

Did I screw up a tube? They all were glowing orange but V1 was glowing a little brighter than the other 2 pre-amp tubes, which I had never noticed before. Is there something obvious that I might have trashed with the “bad cat minicat” mod?

Still have the JJ’s in their boxes.
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Post by bigjoe »

Dr Phil call an ambulance.

It WAS better. Still no headroom, breaking up too early for my taste. The clipping was gone and it was VERY LOUD.

Bass was muddy. Not much tone or definition. The highs were too high.

What I did.
VR6 shielded wire ran to VR1.
I also did the following from page 2 since my highs were “like an ice-pick”.
“bad cat minicat mod” for the power tubes on page 2 to help with ice-pick highs. I connected a 100K resistor in series with a .002 uF cap to ground.

Turned on the amp and immediately had a high pitch squeal with gain and volume turned down completely.Not feedback but a very high ear peircing squel. So I turned it off and took resistor and cap back off. Still had the high pitch squeal. Now something is hosed. Checked solder connections to make sure I did not bridge something. Everything looked good.

Did I screw up a tube? They all were glowing orange but V1 was glowing a little brighter than the other 2 pre-amp tubes, which I had never noticed before. Is there something obvious that I might have trashed with the “bad cat minicat” mod?

Still have the JJ’s in their boxes.
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Post by bigjoe »

Quick update. I swapped power tubes positions (V4 to V5 vise versa).

V4 is glowing much brighter than V5 now.

Also when I tapped on V4 it came through the speaker. Only did thisthe first time I tapped on it.

High pitch squeal with not no other output still there.

I guess I'm going to check components around the circuit there. If that all checks good I suppose I could put in the spare tubes I have, but I'm not crazy about doing this.
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Post by zaphod_phil »

OK, I thought you said a preamp tube was glowing brighter. :? So when you swap power tubes between sockets, it's the same one that stays glowing brighter than the other? If so, then yes, you've probably got a bad tube. Time to try the JJs. JJs are usually considered to be pretty tough, so you should be OK to try them. Just to play safe, it may be a good idea to use a larger value for the cathode bias resistor R24, say 180 ohm 5W, for now. I suspect 120 ohms may be on the low side, and if you can measure the voltage on pin 7 of each EL84 that will give us a better idea.

If the amp still squeals with two good new EL84s, then it's probably time to try a couple of other things. If you pull the PI tube (V3) out, does the amp still squeal? What happens if you reverse the OT primary's connections?

The "Mini Cat Mod" is for a single-ended amp, which this isn't. One of the reasons for this amp's very bright top end is the EL84 tubes, which only have 1.5k resistors (R10 & R11) on their grids. In an 18W these resistors are 8.2k, and you could even use 10k to help roll off some of the sharpness. In Vox amps, they also use a "Cut" control, consisting of a 250k pot in series with a 0.005uF cap between the power tube grids - see http://www.schematicheaven.com/voxamps/ac10_1.pdf With the SS stuff removed, you may have a place now on your front panel for one of these.

With regard to the muddiness and gain being higher than you want, did you put the preamp back to the way I recommended? I mean R8=R9=820ohms C1 = 1uF & C9 removed? Please tell me exactly every step you've done. The voicing of the amp needs to tackled scientifically and methodically, just one step at a time.

But first you need to fix your power tube issue.....
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Post by bigjoe »

Sorry Phil I was up late last night when I made the update. Pre-amp is configured per your suggestions yes. Don't have a larger value for the cathode bias resistor R24. I'll try and get one tomorrow.

I can use a 3 watt to replace a 1 watt of same resistance, but I would not want to replace a 3 watt with a one watt correct?

I did not put the JJ's in yet BTW. Too many weird things. The squeal is probably about as high of a frquency that is audible, its not loud but ear peircing. No sound from guitar comes through when squealing.

Mods to date:
* R8 and R9 820
* C1 uF and C9 removed.
* R12 and R13 100K
* R17 420K
* VR6 shielded wire to R17. (Shield grounded to PCB is that bad?)
* Isolated SS via jumpers next to R17 and VR6.
* Tried the bad cat mod (it was not good) took everything back off. This is when the squeal started.
* Removed 47K esistor off VR6
* PI cathode bias resitor to 820 from 1.2K

NOTE: The schematic says VR6 is 50K that is wrong or the wrong part was installed, its 1 MEG.

After reviewing notes and posts I think that is all the mods.

Squeal stopped with PI tube removed (V3 correct?). Does this point to the power amp as the source of the squeal, or I guess I should say something after the PI.

I checked V4 and V5 voltage with PI removed. 386V on V4 and 387V on V5 Pin 7(this is DC voltage correct? AC volts across pins 4 and 5 of the EL84?). Not sure that means much with PI tube out, but as soon as the amp warms up I can't read anything across pin 7 with PI installed. It starts at about 420 and drops as the squeal gets louder until eventually I read nothing. V5 glows very bright regardless which tube EL84 tube is in that position.

Another notable item. I was curious about the voltage on V4 grid resistor (don't know why, I guess because V5 was glowing brighter, though it was actually V4. on the board the tubes go 1,2,3,5,4). When I placed the probe on R10 to measure V it kind popped thru the speaker. I'm embarrassed I don't recall the voltage, but when it popped the squeal went away. So I quickly plugged in and heard my guitar very softly through the speaker, but the volume and gain were cranked and it was just a whisper. V4 and V5 now glowed at the same brightness.

I chopsticked the caps and R10 the squeal came back V5 started glowing bright again. I have a few new .022 uF caps on hand. I'm probably going to change the caps just because I've had problems with them and check R10, R11, R22, and R23 resistance for good measure. These .022 caps are dogging me. ** Update changed caps, but don't have the resistors they check good with a meter though ***

I need to swap OT wires. Maybe moot at this point with the items noted above though, but I'm new to all this.

I'm going to take your advice and slow down a bit and take one thing at a time. I know I got a good amp here I just need to exercise some patience and it'll get there. quote phil "The voicing of the amp needs to tackled scientifically and methodically, just one step at a time. "

V5 power tube was starting to glow purple with V3 installed. I aborted everything after I saw this.

Phil I really can't thank you enough.
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Post by zaphod_phil »

So the red-plating is only happening in the V5 socket position. Was V5, by any chance, the side that you did the Mini Cat mod to? In any case I'm guessing that V5 isn't biasing correctly due to a problem with R11, R23 and/or C5. There needs to be a solid path to ground through those two resistors, and C5 needs to isolate the grid from the high voltage at V3B's anode. If either of those resistors are faulty, or you've got a bad solder joint or cracked track there, V5 will red-plate. Same if C5 is leaky. The grids of both tubes should be at 0V DC.

Once that issue's fixed, then we can see if the amp still squeals. If it does, there's a possiblity that something's wrong with C10 on the PI, which cancels out very high frequencies.

BTW the voltages on those tubes are ridiculously high, at 386V and 387V, and with a 120 ohm cathode resistor, they're running on the edge of burn-out. Also EL84s generally bring out their best tone in the 320V to 350V range. So *after* we've got the V5 problem and the squeal problem fixed, we will need to address the voltage and biasing thing, before putting in your JJs.

Just one step at a time. Remember, curiosity killed the cat. :)
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Post by bigjoe »

I did the mini cat mod to V4 and V5.

I changed c5 and c6. I'll try to get resistors today to replace r11and r 23 along with r10 and r22.

Just a remider the voltage readings on pin 7 were without V3 in the circuit.
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Post by zaphod_phil »

I think maybe you're being too hasty again. Just check your solder joints and tracks in the vicinity V5's grid, and R11/R23/C5. Check that the voltage on V5's grid really is 0V. I suspect you may have just got a bad solder joint or damaged some track doing the "Mini Cat Mod".

If you were going to change C5 and C6, you would have done better swapping them to 0.01uF ones (400V or above), to get get the bass better under control. Next time you're buying bits also buy a couple of 470k and a couple of 330k resistors (1/2W is fine), for future experiments with power amp overdrive.

BTW those tube voltage readings are fine, as long as both V4 and V5 were both plugged in and working at the time.
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Post by bigjoe »

Got it figured out. :D
I suspect you may have just got a bad solder joint or damaged some track doing the "Mini Cat Mod".
It was not one of my solder joints from the mod gone bad, but there was one probably contributing to the problem.

Posted a message with voltages previous to this one, but deleted it as it was long and moot at this point. The grid voltage on V4/V5 is zero.

Went over everything, not just the mods.

Found a shielded wire coming from VR6 (Master) installed at the factory to C27. Checked it with meter and was flaky when the wire was wiggled. Repaired it and hooked everything back up and still had the problem.

C27 is one of those .022 Uf Caps that has been dogging me. Had a new one on hand and replaced it.

Played at soft volume and it sounds good. The bass is more defined, and the highs are no longer ice picky. I'll need to play it cranked tomorrow. It's not breaking up as early and the over all sound is clean as ever. The tone seemed a little flat, but couldn't tweak much with EQ.

I'll play it tomorrow and play with the EQ to try and dial in the EQ. The only bad thing is my wife works over night at a hospital and gets home at 8 AM and goes to bed immediately. I'll have to get to it right away in the AM.

Looking forward to moving on and discussing the power amp and tone stack stuff. You mentioned a 180 ohm high wattage R24 cathode bias for V4/V5. Pin 7 is still 387/388 volts. Voltage across pin 3 on V4/V5 is 12 volts.
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