Epiphone Valve Standard to 18W TMB Conversion

[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....

[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.

[zaphod_phil]

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.

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.

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.

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.

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.

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.

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.

[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.

[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....?

[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.

[zaphod_phil]

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.

[bigjoe]

I think I'm 99% of the way there. Very pleased with the response, but bass is still lacking . 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.

[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?

[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! 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....

[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.

[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

ing 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.

[zaphod_phil]

* 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.

[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

[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

[bigjoe]

I think you have most of them. I edited your photoshop schematic with what I changed.

zaphod_phil deserves all the credit for the mod ideas.

[bigjoe]

I had a mod go bad.

The 5 zener diodes in series in the power supply to adjust cathode bias on V 4/5 went bad. Either the guy gave me the wrong values, or I did not specify the wattage correctly when buying them. Most likely the latter. Looking at the package I still had from one of them they are only 1 watt. Zaphod_phil suggested 5 watts.

The amp had been sounding differently lately. On a few occasions the volume would mysteriously fluctuate up and down, and then be fine. The master volume (VR6) went bad and I thought viola, it’s a bad pot. After changing the master volume I thought the amp sounded different. Kind of fizzy. I suppose it was the diodes slowly going bad. When I originally put the diodes in it definitely changed the voicing of the amp.

I decided to try a few mods phil suggested to get rid of fizz, and the bass could still be better.

Playing tonight and monkeying around with amp and guitar mods (added push/pull tone to my strat, and I highly recommend this) I smelled something electrical and the amp was cutting in and out. The tape I had around the diodes tag board was melting. One of the diodes was completely burnt open.

The mods I added before the diodeswent bad: changed R21 to 470 from 820. Phil said that should help the fizzness
C12 to 1000 uf 35v to help tighten up the bass
R17 to 220K and added 1 meg resistor across VR6 (master volume). I did the R17 mod previously, but I think I forgot to add the 1 meg resistor across VR6.

Gitella I think I told you in an e-mail I would check the jumpers I pulled to isolate the solid state circuit. JP76 was one of the jumpers I pulled close to the master volume. C7, which was marked JP73, was the other component pulled to isolate the solid state circuit.

Mods to date:
* R8 and R9 820 from 1.5 K
* C1 uF from 22uf
* C9 removed.
* R12 and R13 100K from 1 meg
* VR6 shielded wire to R17. (one end of shield grounded to PCB)
* Isolated SS via jumpers next to R17 (JP73 is actually C7) and VR6 (JP76).
* Removed 47K resistor off VR6
* Power amp coupling caps C5 and C6 changed to .01 uF from .022 uF
* R25 and R26 1K 5watt from 100 3 watt
* R24 180 5 watt from 120 5 watt.
* 5 9 V zeners (5 WATT) 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 from .022 uf
* R21 PI cathode bias resistor changed to 470 (previously 820) from 1.2K
* C12 1000 uf 35 volt from 100 uf 35 volt
* R17 220K (from 470 K) / 1 Meg resistor across VR6 Master Volume

[zaphod_phil]

Yes, you must use 5W zeners. And don't wrap tape or anything around them, as they need free air to dissipate the heat. I once made the mistake of putting heat-shrink tubing on a string of zeners. So eventually they started burning out. Now I prefer to solder them on a small tag board, and mount that somewhere convenient near the power supply end of the chassis.

[gittela]

That sounds like fun!


I suppose all resistors are standard type 1/4 watt unless otherwise stated?

Small question to Phil:
It seems that 9(actually 9.1)volt zeners are out of stock at my local supplier due to lack of RoHS compliance. closest options are either 6.8 or 10 volts.
Any suggestions?

Howard

[bigjoe]

git,

I have been replacing resistors with same wattage rating as the old. Unless otherwise specified. I think most are 1/2 watt.

Lesson leearned on the diodes . I have on tag board, but was looking for a good place for them so I wrapped with electrical tape until I could figure that out. I guess I have to figure it out now.