Epiphone Valve Standard to 18W TMB Conversion

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

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Post by zaphod_phil »

No, it would just give clearer high frequencies from your guitar. Basically more definition and sparkle. I usually use a 10k value for the input grid resistor, instead of the old Fender 68k, but you can short it out entirely if you want. 10k still gives some protection against radio stations breaking in.
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Post by bigjoe »

Phil, are you out there?

I think you'll be happy to hear that I ripped out the PCB in the Epi and put an 18 watt TMB circuit in its place with point to point wiring. I also installed the GDS 18 watt OT.

I love the OT.

I've got it playing pretty good I think. Its only one channel, so I paralleled V1 with itself. I used a schematic form 18 watt. Its mostly stock except for a 1000uf cap and 180 R on the power amp cathode.

Sounds good with one exception below.

I'm getting a strange static noise when I pick a note and hold it. As the note rings there is a static like noise that tails off with the note. I've gone over the solder and double checked component location/values. It is worse with the MV cranked, but I have with the volume cranked also, just not as bad. Should I be looking hard ariund the PI?
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Post by bigjoe »

Got an update.

I connected the PI input and cathode resistors directly to the sockets (470K ran separately to 2,7 and 820 on Pin 3 jumpered to 8 ). This fixed the "static" problem. I got the idea from reading posts about pin 2 on V2, and saw some posts mention they put the resistors right on the pins on the PI also. Maybe it was resoldering the wire or slightly repositioning it, but its working. No more static.

Its sounding good. I get a squeal with both MV and V dimed, but I can't see myself ever needing to have this configuration.

With MV at about 6 o'clock and Volume dimed, the lows get a little mushy and undefined with the neck pup. This is normally my favorite position to play in. I love the tone of this pup.

I said earlier the only change to the stock circuit I made was the cathode bypass cap on V4/V5. Actaully I increased the cathode resistor on V1 to 2.7K, and tone stack slope resistor is 33K. I'm thinking of putting V1 cathode R back to 820. Could this be make the low end mushy? I have it this way on both V1 cathodes, could that be overdoing it a bit? I'll try different value tomorrow, to late now.

The neck pup tone sounds a little "womanish", but I suspect taking care of the undefined low end might help this.

I do like the warm clean tones I get by rolling the git volume back a little bit to take the bite out. Or with MV and V both on 6 it sounds great.
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Post by zaphod_phil »

So you've basically completely replaced the guts of the Valve Standard you had?
bigjoe wrote: Actaully I increased the cathode resistor on V1 to 2.7K, and tone stack slope resistor is 33K. I'm thinking of putting V1 cathode R back to 820.
Yes, reducing the cathode resistor back down to 820 ohms will definately bring the low end under control a bit better. Also I reckon you'll prefer the amp's tone with a 56k resistor in the tone stack. IMHO the mid emphasis can be excessive with a 33k in there, unless playing Strats most of the time. Your treble cap in the tone stack should also be 470pF, not 250 or 270pf, like in the JTM45 and some of the 18W TMB schematics.
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Post by bigjoe »

So you've basically completely replaced the guts of the Valve Standard you had?
Yes, I did completely replace the guts. I thought that might actually make you happy, but I think about how you helped me trying to get that PCB to sound right. I apologize for that, but this valve standard will not be the beta version of an 18 watt conversion unfortuantely. I do appreciate all your time and expertise on this project and hope your not insulted by the fact I pulled the PCB. There was another member who I have kept in touch with who may still be modding.

Around the time I got the GDS OT the 18watt went on vaction. The amp sounded bad with the new OT. I got quite frustrated trying to work around the PCB. I was getting a lot of oscialltion, due to how I had to lay things out on the PCB I think. I had most of the components on had and I decided to trash the PCB. This was before I knew the board was coming back on line, and I was a little desparate. Best decision I’ve made as far as this amp is concerned, that and the OT. The only stock items from the Epi are the chassis, jacks, the PS, and speaker. I like the speaker BTW.

I only play strats, I have it modified with a push-pull pot switch to change pup configuration. That’s about as close to a HB as I get. I have a 1959 Silvertone hollowbody w/ the original DeArmond soapbar pups that’s gets some time also. Played very little the last 30+ years.

I remember putting that 33K in there, and trying a 56K at one point, and went back to the 33K. I think that was when the PCB was still in the amp though. I’ll throw a 56K in there and see what happens just for the sake of doing it. The schematic I used did have a 250pf cap in the tone stack, but I think I used a 470 pf based on previous posts with you. I’ll have to check that.

Phil I apologize once more, but I am much happier with the amp though. Not to mention it’s a dream to work on. Much easier to debug also.
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Post by zaphod_phil »

Hey, no need to apologize for anything. You've got your amp sounding the way you want with real 18W tone, and you've also learnt a heck of a lot along the way. That's what counts. :D
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Post by bigjoe »

Thanks for undetstanding Phil. I think I have learned a lot and most of it since going ptp.

After reading the first line of your post I thought,wow this guy phil helped me mod the heck out of that PCB. THen I go and throw it in the trash and start over.

All is well though. Love the amp so far. I hope to play at some local blues jams next week with it. I think I can hang without being mic'd. They are fairly small clubs. I was playing a 35 watt ss Fender amp at them.
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Post by bigjoe »

I changed the components mentioned in zp 's previous post. I like it. 56 K slope, 500pf cap in tonestack as opposed to 250 pf, and the cathode resistors back to 820 on V1.

The low is great now at high volume. I love the sustain on this amp. Killer tone, very responsive. Great with my Strat.

Still have the squeal with Volume dimed and MV at about 3 o'clock. I didn't think I would ever use that range, but I'm liking the sound I'm getting is the component changes above and could see using it if I needed the extra volume.

Moving the OT wires has a definate effect on it. I put long jumper wires on the OT leads and was able to dime both volume controls. I ran them behind the amp and straight to the pins from the back side of the amp. Is this fix that simple? Make the wires longer and run them to the pins and I'm done. This might be a stupid question, but would it help if I some how shielded the OT leads. Not sure how, but I have a good imagination. I moved other wires it seemed the only one that fixed it was the OT leads.
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Post by zaphod_phil »

Also try twisting those long OT wires together as a pair. That often fixes the problem.
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Post by bigjoe »

So your saying lengthening them is a viable fix?

They are already twisted, not very tight though. over the foot or so of wire there is maybe 5 twists.

Should I only twist the plate wires (blue and brown), or include the red B+ wire also.

Just to clarify I am talking about the primary OT wires.
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Post by zaphod_phil »

Yes the OT primary wires. If you twist them pretty tight it will cancel out signal radiation from them. I would have thought your B+ wire would be traveling a different path though, so it wouldn't make much sense to twist it with the primary wires. If you've got long secondary wires, you can twist those as well.
Sometimes just swapping around the OT primary connections also helps kill off squeal.
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Post by bigjoe »

I got it taken care of. I rerouted the wires and twisted. Added a little bit of length to them . No more squeal.

I had a scare last night. I put the amp back in the cab (finally). I played for at least an hour without any problems. I stopped for a while and decided to put the JJ tubes in to see how they sounded. Turned amp back on and no sound. Looked in the back and no tubes are glowing. I smelled electrical so I powered off.

Pulled the chassis. I found a heater wire that had shorted to something. What I don't know. THere was nothing really in the vicinity for it to short to. I check the mount bolts, the wires weren't high enough to touch the top of the cab to short against the metal shileding tape either.

I checked voltages on the PS transformer wires with nothing connected and they were all good. I re-ran the heater wires for good measure. The weird thing is I found a open resistor on the 32uf caps (metal film, 3 watt), which I was not even looking for really. So I don't know what happened, but I'm going over it with a fine tooth comb before it put back in the cab. I powered up again last night. Its working just fine again at low volume. It was too late to crank. This really bummed me out.

The day I put it back in the cab after working and tweaking it for so long, finally have it sounding the way I want it, and bam, open resistor and a short in the heater wires. I guess the nice thing is I’m done tweaking, and it should be easy enough to fix.
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18 watt TMB -- first jam

Post by bigjoe »

I finally got to take my 18 watt out to a local blues jam. A fairly small venue. Its a single channel 18 watt TMB. As you can see from all the previous posts I was modding and Epiphone valve standard and decided to pull the PCB and go with ptp wiring. I put a GDS OT in it also. The PT cab and speaker are bascially the only Epi components.

My only concern was that it might not be loud enough. That concern was a waste of energy. The band I jammed with plays at un-typical low volume in comparison to most that play there. I think with one of the other bands that play at the jam I will be able to have this amp reach its fullest potential.

I really couldn't crank it quite as much as I would have liked to get the best tone/overdrive.

I had the master dimed and the volume at about 10 o'clock. Regardless it still performed great. i Played for 2 hours straight. Great response and overall tone.

Thanks again zaphod_phil. I've learned a lot building this amp. This amp is killer and to quote phil diabolically loud.
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Re: 18 watt TMB -- first jam

Post by michel »

Hello !

I have been reading this thread a couple of times already, I am NOT into hardware electronics, rather software, I am a programmer. :?

I own a Valve Special too and I would like to make some modifications, but one-by-one... :oops:

First get rid of the DSP. Did you finally manage to find a simple way to disconnect the DSP ? I don't understand your messages about this part because I can trace the circuit from R17 to VR6 outer post already on the PCB !?! Why would I want to add a wire between those two connections ?

Are there different versions of the Valve Special ? Mine says HB01573 - VER050305...

Thanks,

Michel
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Post by bigjoe »

THe easiest way to get rid of the DSP is to pull the PCB and throw it in the trash. I know thats not what you asked though. I did like the sound of the amp before mods, a little muddy though. I LOVE the sound of of my 18 watt. It will get to be a chore modding the board on thing at a time. It s pain pulling it, so I would come up with a good way of leaving it out to try out your mods.

The schematic as it relates to the circuit is more of a guideline. They do not match excatly.

Its been a while since I've modded the Epi PCB, so everything I tell you is based on memory and looking at my old posts.

I'm not doubting your board tracing skills, but I would get a fine tipped marker and trace the circuit from R17 to VR6 again. Pay attention to the skinny wires and jumpers on the board. I assure you that R17 does not directly connect to VR6 unless they have drastically changed the circuit.

I'm looking at my Epi PCB right now (I lied in previous post about throwin git in the trash, mostly for effect). It has a lot of stuff pulled off of it and is no where near stock configuration anymore. My board numbers are:
Mine says HB01574 - VER050305...

There is a jumper that sits right behind VR3 (Mid pot). Pull either side of it up off the board. Pull up one side of JP76 sits right behind VR5 (DSP pot). This should isolate the DSP. Then run a shielded wire from R17 to VR6 (leg closest to outside of board I think). Make sure to ground one side of the jumper, not both.
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Post by michel »

Hi,

Like I said I am a newbie. I posted some pics on this thread:

http://www.ppwatt.com/node/15351

I am pretty sure though I can follow the trace from the R17 to the VR6 outer leg...

I took some pics showing the different numbers on the PCB...

http://www.ppwatt.com/files/ppwatt/vr6.jpg
http://www.ppwatt.com/files/ppwatt/r17.jpg

I have a pic of the trace also in attachment...

Since I don't konw what I am doing, I must rely on a schematic, but PCB does not seem to know it... ;-)

Thanks,

Michel
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Post by cGil »

bigjoe wrote:THe easiest way to get rid of the DSP is to pull the PCB and throw it in the trash.
My sentiments, exactly. I tried to warn 'em.
bigjoe wrote: Mine says HB01574 - VER050305...
Mine, too! My board was hiding under the bench, but I dug it out for this occasion. This one was a modded mess when I got it. Lots of post-factory resistor and cap additions and jumper changes not consistent with the schemo at all, but that seems to be par for the course.

After reading this thread and then finally seeing it up close for myself, I understood immediately why there's not much hope for this thing. Too messy, and far too flexible a board to last very long no matter what you try to do. I stripped mine of anything really useful within hours of getting it home, but then, I bought this amp just to rebuild it from scratch anyway.

Gil...
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Post by michel »

cGil wrote: Mine, too! My board was hiding under the bench, but I dug it out for this occasion. This one was a modded mess when I got it. Lots of post-factory resistor and cap additions and jumper changes not consistent with the schemo at all, but that seems to be par for the course.
So, you never worked with an original Valve Special. That must explain the difference between your description and my board.

Will keep looking...
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Post by cGil »

What? Stripping my Valve Standard wasn't work? I beg to differ! :)

And seeing pictures of your Valve Special board and comparing it to the same exact traces on mine tells me you really should quit whining about insignificant minor molehills and simply follow the footsteps you've found in here. The fact that you're here says you've found all the information you need. What more do you want?

Gil...
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Post by bigjoe »

look at the other side of the board on your trace pic. I'm certain that on mine R17 didn't have a direct route to VR6. It took me a while to figure it out. I was convinced at one point they did have a direct route. The reults I got said different.

My board is stripped clean of any factory wiring so I'm at a disadvantage. I might suggest you pull the components legs that I suggested and run the wire from R17 to VR6. Mine seemed to have a lot of post automated production mods also.
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