Epiphone Valve Standard to 18W TMB Conversion

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

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

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.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

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. :)
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

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.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

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.
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

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.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

So V5 is still red-plating - as you say you still have the problem?? Yet, the amp is sounding much better now...?? :?

Things to look at in voicing the amp:

IMO you should put C1 back, which is important to limit mushy overdrove, and fartiness from too much bass.

If mids are too strong, replacing the 33k slope resistor in the tone stack with a 56k, which will provide more scoop. 56k is also a common Marshall value. To make the controls a little more effective, you can also change the 0.022uF mid cap, with a 0.01 uF, and solder a 470k resistor across the two end lugs of the bass pot.

In the PI, you have a 1.2k cathode resistor, which will tend to make for some fizziness when the PI gets overdriven. I recommend changing it to 820 ohms, as in found in Marshall 18Ws.

The voltage issue is a tough one. IMO you need to knock a load of volts down. Five 9V zeners hooked in series should get you onto about the right ball park. Or if you want use fours zeners and a 180 ohms 5W resistor, if you waould like to introduce some sag. You can connect them in series zig-zag fashion on a piece of tagboard and then connect that between the rectifiers and the first filter cap. This is also a good place to insert a standby switch. Then change the cathode resistor to 180 ohms, and the two screen grid resistors to 1k. Then you should be OK to try your JJ tubes. That's assuming your V5 red-plating issue has been fixed, of course.

Your amp is now starting to look somewhat like a single-channel version of an 18W TMB, except that instead of a Normal channel, the first triode in the TMB preamp has been paralleled, providing a richer, fatter tone. Currently with the 220k grid leak resistors (R22 & R23) on the EL84s, the power amp will stay chimey and clean until driven pretty hard, like a Vox amp. You can experiment with larger values, such as 330k or 470k to get more 18W style power tube overdrive.

The icing on the cake for this amp will be to change the output transformer for a real 18W clone OT, such as from GDS/Heyboer, Trinity, MM or Shinrock. Then you'll really be getting into some real tone!

Anyway, there's a few things for you to be looking at and trying out....
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

You posted as I was writing this one. I guess I did not make it clear that fixing the wire and changing C27 fixed it. See below for details on tone etc... Much better now. Still some tweaking that can be done.

No red plating, sorry I gave that impression.

Put C1 back? Its not removed. You suggested lowering to 1uF.

PI cathode resistor already 820.

Can you direct me to a schematic or something with your suggestion to lower the voltage with zener diodes? Not sure about the whole thing and where/how to insert into circuit.

When loud it has a high pitch, sounds like feed back but disappears when playing a note, immediately returning after note is silenced (I really don't think its feedback). It also motorboats. It's quite loud when all this takes place. I would have to guess it is ever bit as loud as pre-mod.

Volume at about 12 o'clock and gain just past 1 o'clock. I might be employing the "what do you mean I have no money, I still have checks" theory to the volume. Its at 12 o'clock and figure I still got a lot of knob left so I should able to use most of it.

Over all at volume tone is not bad. Bass could be more defined, but better than pre-mod. Its not so sloppy. The mids and highs are a little lacking. Can't get those chimy mids and highs associated with a Strat, Over lack of depth to tone.

The fizziness has disappeared, with good headroom.

Overall I'm pleased. After the week from hell.

Mod list to date:
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.
* Removed 47K esistor off VR6
* PI cathode bias resistor changed to 820 from 1.2K
* Power amp coupling caps C5 and C6 changed to .01 uF from .002 Uf
NOTE: The schematic says VR6 is 50K that is wrong or the wrong part was installed, its 1 MEG.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

bigjoe wrote:Can you direct me to a schematic or something with your suggestion to lower the voltage with zener diodes? Not sure about the whole thing and where/how to insert into circuit.
It's not complicated enough to have schematic. You just connect five 9V 5W zener diodes in series, and insert them anywhere between the rectifiers and the first filter cap. Somewhere in the vicinity of FS3 would work fine - I think that's a fuse. Just remember zener diodes operate in reverse, so you would need to connect them with their stripe ends pointing back towards the rectifiers and PT. I recommend doing this important mod before working on the voicing of the amp, as it will also probably affect the tone in some way.
bigjoe wrote:When loud it has a high pitch, sounds like feed back but disappears when playing a note, immediately returning after note is silenced (I really don't think its feedback).
A high pitch what? A squeal, or a whistle? Is this the same you had before, which would go away when you pulled the PI tube? Check that the OT secondary is grounded on one side. Also try swapping the OT primary connections. That often helps. Also it may go away after you lower the B+ voltage with the zeners.
bigjoe wrote:It also motorboats.
Usually caused by faulty filter caps. To find out where the motorboating is happening, see what happens with V1 removed. Do you still get the motorboating? Then repeat the experiment with V2 removed, and again with V3 removed. The stage at which the motorboating disappears probably needs to have its nearest filter cap replaced and/or beefed up.
bigjoe wrote:Its at 12 o'clock and figure I still got a lot of knob left so I should able to use most of it.
Yes, I agree.
bigjoe wrote:Over all at volume tone is not bad. Bass could be more defined, but better than pre-mod. Its not so sloppy. The mids and highs are a little lacking. Can't get those chimy mids and highs associated with a Strat, Over lack of depth to tone.
I'm pretty sure you will need to upgrade the OT and then the tubes to get really good tones out of this amp. The OT will have the most effect. With R17=420k, you're killing some of the higher frequencies, so you probably need a bright cap across. You may like to try the Marshall standard 470pF.
bigjoe wrote:VR6 shielded wire to R17. (Shield grounded to PCB is that bad?)
Connecting the shield to the PCB ground is correct. Make sure only one end of the wire is grounded.
bigjoe wrote:Removed 47K resistor off VR6
The schematic says VR6 is 50K that is wrong or the wrong part was installed, its 1 MEG.
The 47k across the 1M was intended to approximate to 50k-ish.
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

“A high pitch what? A squeal, or a whistle?”
The high pitch, sounds like feedback. It occurs with guitar plugged in or not. Turn the volume past 12 o’clock and it sounds like a guitar feeding back, even though one is not connected. This is when it sounds like a motor boat also. Does not motor boat constantly.

I’ll try the zeners. It does sound straight forward, striped ends on PT side.

BTW -- FS3 is a fuse, and amp already has standby switch.

Should I just change R17 to 100k? Reviewing previous posts I changed it back to 470K when I was having overdrive problems. I never changed it back to 100K, you also recommended at some point making it 220K. If I change it do I want to bridge with a bright cap ? I suppose I do it both ways and evaluate.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

OK if the amp already has a standby switch, that's probably also inline with where the fuse is, so that would be a perfect place to put the zeners in line with. Like I said previously, it may be easier to mount them on a piece of tag board and bolt that to the chassis somewhere convenient.. Your amp may also start behaving better with the reduced voltage.

If you reduce R17, also solder a 1M resistor across VR6, to turn it into the equivalent of a 500k pot, to keep the signal level hitting the PI from getting too big. That will also make the MV pot less of a tone sucker, when it's turned down. Yes, also try R17 with and without a bright cap.

Try swapping the OT primaries, like I recommended, for the feedback problem. From what you're saying, you're getting both low frequency and high frequency feedback inside the amp....?
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

From what you're saying, you're getting both low frequency and high frequency feedback inside the amp....?
I don't understand the question. The sound I get sounds very similar to, if not just like guitar feedback (like when standing in front of the amp with volume to high). It will do this without a guitar plugged into the amp. I guess this is what you are calling "feedback inside the amp". If so then yes I am getting feedback inside the amp, but as far as frequency is concerned not sure. Its a high pitch sound, so I would say its high frequency.

I'll swap the OT wires per recommendation, but I wanted to clarify the feedback thing.

BTW pin 9 has the screen resistors you posted about earlier correct. Changing them from 100 3 watt to 1K 3watt.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

bigjoe wrote: I am getting feedback inside the amp, but as far as frequency is concerned not sure. Its a high pitch sound, so I would say its high frequency.
In that case, it wouldn't be refered to as motorboating, which is a low-frequency oscillation, which sounds like a motorboat or motorcycle. The difference is importnat as they tend to have different causes.
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

I think I'm 99% of the way there. Very pleased with the response, :D but bass is still lacking :huh: . The mids and high are nice and chimey with a little bit. The lowsjust have no depth.

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.
* Removed 47K resistor off VR6
* R21 PI cathode bias resistor changed to 820 from 1.2K
* Power amp coupling caps C5 and C6 changed to .01 uF from .022 Uf
* R25 and R26 1K 5watt
* R24 180 5 watt.
* 5 9 V zeners in series placed in circuit at FS3. (This very went well Pin 7 voltage 346volts on EL84's)

The high pitch squeal (feedback like sound) is gone. Maybe a combo of R24,R25,R26 and adding the 5 zeners? BTW Pin 7 on the EL84's is at 346. You hit that one right on the head.

I'm pleased with high/mid response. I don't want to monkey with it too much any more I don't think. The zener mod did effect the voicing in a good way. I think I might leave R17 at 470K unless you think it absolutely needs changed.

Bass is still lacking defintion. Kind of sloppy. I am going to try the tone stack stuff you mentioned in previous posts. I think you suggested C3 (mid tone stack cap) change to .01uf and a 470K resistor across VR4 bass pot. Lowering R17 would not have any effect on the bass would it? I would think not.

This might be a stupid question, but would I really want to change C3 (mid tone stack) if I'm happy with the mids, maybe just add the 470K resistor across the Bass pot instead. Is my logic correct or does the midmod have kind of effect on the bass so

Its late now I'll have to get to the tone stack stuff tomorrow. Going totake the JJ's out of the box tomorrow.
0 x

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

Yes the bass is too woofy I suppose. flabby or sloppy are words that come to mind also.

I’ll give the C3 .01uf / 1M across Bass pot and change C12 to 1000uf I happen to have all those parts on hand.

I’ll have to look at OT’s I know you mentioned a few previously.

I’ll let you know how it goes and how the JJ’s sound.

BTW the R25 and R26 5 watt resistors. All the had were 1 watt and 5 they were out of 3watt otherwise I would have gotten them. I don’t that will matter really do you?
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

I just scratched my head and got a splinter in my finger wondering how you managed to fit two 5W resistors onto the PCB! 8O 2W resistors would also be fine.

Any of the usual 18W clone OTs would work real fine in your amp. GDS/Heyboer, Trinity, Brownnote/Ceriatone, Mercury Magentics, Shinrock, etc....
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

R25 and R26 were large 3 watt already with quite a lot of room around them. The 5 watt is only slightly wider. Piece of cake.

Phil you have the patience of a Saint. Thanks you for all the help.
0 x

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

I figured I would post all the mods I did since everything seems quite stable at this point. I did the tone stack mods and it really helped my bass response. It’s no where near as sloppy. I supposed the next step will be changing the OT, but I'll need to lay low on that one for a bit. My $80 amp is getting expensive, but it’s still cheaper than retail at this point. IMO the tone on this amp is exceptional at this point.

Mods:
* R8 and R9 820
* C1 uF
* C9 removed.
* R12 and R13 100K
* R17 420K
* VR6 shielded wire to R17. (one end of shield grounded to PCB)
* Isolated SS via jumpers next to R17 and VR6.
* Removed 47K resistor off VR6
* R21 PI cathode bias resistor changed to 820 from 1.2K
* Power amp coupling caps C5 and C6 changed to .01 uF from .022 uF
* R25 and R26 1K 5watt
* R24 180 5 watt.
* 5 9 V zeners in series placed in circuit at FS3. (Pin 7 346volts on EL84's)
* Added 1 M fixed resistor across the outer lugs of VR4 (bass pot)
* C3 .01uF

Reading back through posts I never changed R10, R11 to 8.2K or 10K. You said leaving them at the current value would help keep it chimey and clean unless driven hard. I like the way it sounds and breaks up nice and smooth.

Things I tried, but changed back.
* Bright cap across R17. Took it off a little did not seem to do much.
* Changed R17 to 220K. Got too muddy. Changed back to 470K.
* C12 1000uF. This seemed to make it break up early and cutout when driven hard. Not sure why. Maybe a bad cap? Another thing I noticed was after playing for a few minutes I would lose almost all the volume then slowly it would come back. It only did this a few times when I first turned started to play through it. It never seemed like it ever came back to the volume it was previously.
* The mini cat mod gone bad.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

bigjoe wrote:* C12 1000uF. This seemed to make it break up early and cutout when driven hard. Not sure why. Maybe a bad cap? Another thing I noticed was after playing for a few minutes I would lose almost all the volume then slowly it would come back. It only did this a few times when I first turned started to play through it. It never seemed like it ever came back to the volume it was previously.
Almost certainly a bad cap. The 1000uF value stiffens the power stage, so it behaves a bit more like a fixed-bias amp and overdrives more smoothly, especially on low notes.
0 x
Nature abhors a clean tube amp

bigjoe
Newbie
Newbie
Posts: 50
Joined: Sat 03/31/07 2:00 am
Location: Unknown

Post by bigjoe »

Phil do you have any experience with allen amps? They have transformers that are "Custom wound exclusively for Allen Amplification and made in the USA by Heyboer". Heyboer is one of your suggetsion to me for an OT, but this one at Allen Amps is considerably cheaper. They say its an "Excellent choice for 18 watt Marshalls and similar EL84 amps".

I think my primary is 8K and it says the primary is 7K on the one I'm looking at.

If you scroll down it's T025
http://www.allenamps.com/parts.html#transformers
0 x

User avatar
gittela
Unrated
Posts: 5
Joined: Wed 04/25/07 2:00 am
Location: Norway

Modding of Valve Standard

Post by gittela »

Hey, bigjoe!
I´ve emailed you a couple of questions, but after reading closely I think I´ve figured out most of it.
Made a half-a**ed schematic out of it, reflecting most of the changes. If you guys could have a look at it and give me some feedback on the correctness, that´d be great!

:-)
Howard
0 x

Post Reply