Epiphone Valve Standard to 18W TMB Conversion

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

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bigjoe
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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.
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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.
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zaphod_phil
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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.
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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
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gittela
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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
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bigjoe
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Post by 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.
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Post by 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
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Post by 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.
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gittela
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That burning sensation....

Post by gittela »

That sounds like fun!
:D

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
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bigjoe
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Post by 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 :oops: . 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.
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Re: That burning sensation....

Post by zaphod_phil »

gittela wrote: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.
Find some other combination of zener values that gets you to approximately 45V - they don't all have to be the same voltage. Just be careful with higher voltage zeners, as they will also have to dissipate more power as heat (volts X amps). Use zeners that are rated for twice the power they will normally have to dissipate, and allow for free air ciculation around them.
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bigjoe
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Post by bigjoe »

Had the same problem gitella. Had to mail order. I'm expecting them Wednesday.
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Post by bigjoe »

Replaced diodes today.

Amp sounds good, bright and chimey, with good low end response. So the most recent mods I did really helped the bass.

Most recent mods:
* 5 9 V zeners (5 WATT) in series placed in circuit at FS3. (Pin 7 346volts on EL84's)
* 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


Any suggestions for more headroom. Its gets dirty way too quick. I can live with it, but more headroom would be great.

gitella, hows are the mods going for you?
Last edited by bigjoe on Sat 05/05/07 9:37 pm, edited 1 time in total.
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Post by zaphod_phil »

Congratulations on your 18W TMB!! :D
bigjoe wrote:Any suggestions for more headroom. Its gets dirty way too quick. I can live with it, but more headroom would be great.
Can you explain a bit further. It get's dirty too quick as you increase the preamp gain control (VR1), or as you increase the MV (VR6)? What happens, for example if you reduce VR1 until the sound is clean and then bring up the volume level on VR6?
bigjoe wrote: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.
I don't know anything about them. But a 7k impedance will make the power amp break up slightly sooner.
bigjoe wrote:If you scroll down it's T025
http://www.allenamps.com/parts.html#transformers
I got a Page Not Found error. :(
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a slow day in mod country

Post by gittela »

bigjoe wrote:Replaced diodes today.

gitella, hows are the mods going for you?
Mods are going a bit slow. Haven´t had time to buy the stuff needed, but I figure I´ll get some of it done this week or weekend.

But what I have done is doing a bit of reading on an old EAM86 tube i have, an indicator tube. This is usually called a "magic eye" in english, in Norway we call it a "Troll´s eye".
Wondering about stuffing one of these in the am just for show. No sonic value, just cool to watch.

And I´ve been doing some more work on the schematic, of course. Will post it as soon as I get my mods done, and everything is up to date.

:-)
Howard
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Post by bigjoe »

Go to: http://www.allenamps.com/
Click transformers on the left. scroll down. Not sure why link didn't work as thats what comes up when I navigate to it.

“What happens, for example if you reduce VR1 until the sound is clean and then bring up the volume level on VR6?”
reducing VR1 until clean and bring up volume it stays clean. Theres not a lot of knob bewteen Clean and dirty though. With master up its too dirty for me at around 10 o’clock on the the gain knob.

Maybe I’m being picky. As I said I can live with it, just though I would ask.
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Post by zaphod_phil »

bigjoe wrote:Go to: http://www.allenamps.com/
Click transformers on the left. scroll down. Not sure why link didn't work as thats what comes up when I navigate to it.
OK, I had a look at this transfomermer, and although it would work, it's clearly a Fender replacement OT and not a Marshall 18W transformer. So while it may sound fine, you won't get *that* legendary 18W tone out of it. Also, with the 7K primary impedance it will make the power amp break up somewhat sooner. IMHO, I would save your money up for a real-deal 18W OT from GDS(Heyboer), Brownnote or Trinity, say. Those also have three speaker impedance taps, for 4, 8 and 16 ohms, rather than just for 4 and 8 ohms.
bigjoe wrote: Theres not a lot of knob bewteen Clean and dirty though. With master up its too dirty for me at around 10 o’clock on the the gain knob.
OK, so it's mainly the range on the MV you're concerned about? Did you change the two EL84 grid leak resistors, R22 and R23 to 470k? I don't remember if I recommended you to try that or not. If you did, you may need to reduce them again...
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bigjoe
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Post by bigjoe »

Phil Pg 9 2nd post down Quote:
"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. "
"OK, so it's mainly the range on the MV you're concerned about? Did you change the two EL84 grid leak resistors, R22 and R23 to 470k? I don't remember if I recommended you to try that or not. If you did, you may need to reduce them again... "
See first quote above in this post. When I read the part about chimey and clean I decided not to change R22 & R23 from stock values.

Its the gain I think I'm worried about. I'd like loud and clean, or loud and slightly overdriven. It just seems that theres not much in between clean and heavy overdrive. I usually play with the MV up and the gain turn fairly low.

THanks for you thoughts on the transformer also.
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Post by bigjoe »

Got an update.

My gain pot was bad. I pulled it and it was very erratic when checked with DMV. Resistance would bounce around quite a bit. I know its supossed to change as you turn the shaft, but this would bounce by thousands and never really settle on a value. I changed it, and I think it fixed my overdrive problem.

While changing pot I decided to change my MV wire to R17 and wire from MV to C27. Just wasn't happy with it, wire was very stiff and awkward. Used 2 conductor wire this time as opposed to braided shield. I grounded one of the 2 conductors (one end only) to the PCB. I would imagine this is OK. The factory wiring on this amp used a similar setup as opposed to shield braided wire.

I have more headroom before it gets too dirty. Over all, the overdrive is much smoother even when pushed.

While I had it apart I decided to try a few of Phil's suggestions I had not yet implemented. I change R22/23 to 330K. Also changed R15 in the tone stack to 56 K.

I happy with the tone, nice chimey bite. Well defined highs and mids and the bass is solid.

Volume not as loud when cranked. Once I get past 12 o'clock on the MV I have no noticable increase in volume. I mentioned this previously, thinking I was wrong to be able to use it all. I think Phil concurred I should probably have more volume.

Still fizzy as ever.

Mods to date:
* C3 .01 uf (forgot to mention this previously)
* 1 meg across outer lugs of VR4 (forgot mention this previously)
* 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 7 343 volts on EL84's)
* 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 470K)
* 1 Meg resistor across VR6 Master Volume
* R22/23 330K from 220K
* R15 56K from 33K

Probably buying new OT within 2 weeks.
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Trying to do some of these mods I've

Post by dthill »

screwed up the PCB. It's got a pretty good crack in it now. I asked my tech about it (in general terms) one day and he said to trash it. Is that what I should do Are they not repairable? Are there replacements? Use the cab for another amp or what?
Not trying to hijack this thread, actually I want a mireacle of some type!

dthill :cry:
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