Epiphone Valve Standard to 18W TMB Conversion

[zaphod_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.

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.

[bigjoe]

Had the same problem gitella. Had to mail order. I'm expecting them Wednesday.

[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?

[zaphod_phil]

Congratulations on your 18W TMB!!

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?

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.

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

I got a Page Not Found error.

[gittela]

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

[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.

[zaphod_phil]

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.

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...

[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.

[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.

[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

[zaphod_phil]

I have more headroom before it gets too dirty.
* R17 220K (from 470K)

If you find it's breaking up to early on the MV, increase R17, either back to 470k, or the in-between value of 330k. If you lose some high end with that, you can always place a bright cap across it, say 250pF.

Volume not as loud when cranked. Once I get past 12 o'clock on the MV I have no noticable increase in volume. .. I think Phil concurred I should probably have more volume.

A cranked 18W amp should be diabolically loud, assuming reasonably efficient speakers. However Marshall 18W amps are also well known for getting loud and breaking up quite early on the dial, then not getting much louder - just dirtier. You can adjust the breakup point by increasing R17.

Still fizzy as ever.

It shouldn't be fizzy. Can you explain more about when you get the fizz? Do you get it with the MV turned down? In any case an upgraded OT will probably fix it.

Probably buying new OT within 2 weeks.

Perfect! If it's a genuine 18W OT it should improve the tone of your amp a whole lot.

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!

The solution is pretty simple. Swap the board. Gibson should be able to sell you a replacement PCB. Do the mods on it first, before you fit it into the amp. Alternatively, just buy yourself an 18W TMB board from any of the outfits selling TMB boards - eg Trinity, Ceriatone, s2, etc. Just tell them that you don't have space for a Normal channel in your amp, so you want the first triode on the preamp to be wired in parallel. Also tell them that you're using solid-state rectifiers, so you need a sag resistor included. That will give you a full-blown TMB 18W amp, especially if you also upgrade your PT.

[hartwell]

Alternatively, just buy yourself an 18W TMB board from any of the outfits selling TMB boards - eg Trinity, Ceriatone, s2, etc. Just tell them that you don't have space for a Normal channel in your amp, so you want the first triode on the preamp to be wired in parallel. Also tell them that you're using solid-state rectifiers, so you need a sag resistor included. That will give you a full-blown TMB 18W amp, especially if you also upgrade your PT.

Sounds like a great idea. I guess the OT as well ...?

[zaphod_phil]

Sounds like a great idea. I guess the OT as well ...?

The amp will work with the stock OT. It will sound killer with a real deal 18W OT!

[bigjoe]

If you find it's breaking up to early on the MV, increase R17, either back to 470k, or the in-between value of 330k.

I'll try the tweener 330k. I'll liked the tone with the 470K value actually. It's not bad its definatley brighter with the 220K value.

A cranked 18W amp should be diabolically loud, assuming reasonably efficient speakers. However Marshall 18W amps are also well known for getting loud and breaking up quite early on the dial, then not getting much louder - just dirtier. You can adjust the breakup point by increasing R17.

Diabolically loud?? This is defiantely not that, its loud though. Thats one of the reasons I dropped R17 to the 220K value. Hoping to get more volume.

I'm using the stock speaker. I also borrowed a cab from a friend that had 2X10" celestions in it, can't remember model. I tried with 2X10 celestions I had from one of those micro stacks, but my 1X12 stock speaker was louder in either case.

The fizz seems to be present pretty much at any time. Low, High volume does not matter. The MV and Gain configuration do not matter either. Its just there all the time. Not nearly as noticable at lower volume, which I guess is some what expected.

I've been traveling the last week and a half and before I left last weekend I had water in my basement. If you live in states and have seen the news in the last week you probably know that close to Kansas City is flooded. I had my amp taken part before I left and I have not had a chance to tinker at with yet. I'll try the tweener R17 @ 330K and probably order a Hyober OT once my I get the proceeds from my travel voucher back.

Just my 2 cent on the PCB. Is it a huge crack, you know like hal way across the PCB? If there is any board around the crack that does not have traces across you could try and stop drill the crack. Its good enough for aircarft wings why not an amp PCB. I trashed a few traces on my board and I got creative with some fairly large solid copper wire, like for house wiring, to fix the larger traces. The smaller traces can be repaired with the scrap pieces of the components conductor that you cut off after installing them. I did PCB repair for about 2 years on car computers right out of high school. If you have a good mechanical connection there is no reason it should not work.

If your board is cracked nearly all the way across or something really bad like that I would just trash it as someone suggested (probably not worth all the effort). I had plans of using my Valve Standard cab and chassis for an amp I was going to build from scratch (I got it on the cheap), but I liked the sound of it, it just needed some help.

[bigjoe]

Changed R17 to 330K. I like this value there. It doesn't break up nearly as early and is a lot smoother when it does. Just need the OT at this point.

Mods to date:
* C3 .01 uf
* 1 meg across outer lugs of VR4
* 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 330K w/ 220 pf cap (from 220K [200k broke up too early] from original 470K)
* 1 Meg resistor across VR6 Master Volume
* R22/23 330K from 220K
* R15 56K from 33K

[zaphod_phil]

Looks good! Like I said, this amp should be very loud, just like any other 18W. Certainly it should be plenty loud when you get your new OT in - unless your speakers are inefficient....

[bigjoe]

phil,

One thing I noticed is the fizzy sound gets worse with my strat Bridge/Neck pup in series. I modded my strat with this pup combo. Just the neck pup is fizzy, but much more noticable with the presumably higher output of the series Neck/Bridge pup combo. I have a cheap epi SG knock off I bought for the kids with cheap HB pups and its even worse than my strat.

Thought it might be worth mentioning, maybe not.

[zaphod_phil]

First upgrade your OT to a real 18W clone OT. Then let's see what fizz is left, if any.

[bigjoe]

Probably going with this one. I really want to play out with this amp.

http://www.gdsamps.com/18wattbuyparts.html

scroll to #13a.

Any opinions? $75 includes shipping. Its at the top end of my affordable.

[bigjoe]

I'm curious about something. What would happen if I reduce R1 to a lower value. Would it result in more volume? I don't plan on doing this I'm just curious. I'm going to be mod-free until I get the OT.

Well hopefully mod free after that also.