Correction bigjoe now has the GDS 18 watt OT. Very nice. I'm kind of slow I suppose, but I never realized that GDS was Graydon from 18 watt.

I'm guessing that mods to the PCB can be done in such a way that the VS can be upgraded into an 18 watt like clone, however I really had some problems with oscillation when doing it. I don't think the PCB layout lends itself to preventing these problems. I started to mod the PCB so heavily it was becoming a hybrid of point to point and PCB. So I completely removed the PCB and went all ptp. Its much easier to work on and debug.

One thing I have noticed when working point to point that has made a difference is putting V2 grid resistor right on the tube socket itself, which I had not done on the PCB if I recall correctly. I also went a little further and put the PI Grid, and cathode resitors right on the tube sockets (those resitors have a name and it slips my mind right now). I'm not sure if this is standard practice, but it helped me get rid of an annoying static noise as the note rang out.

I'm basically finished with it. The hard work anyway. I am going to make a few final tweaks this evening to tone stack and V1 cathode resistor and put it back in the cabinet.

Joe <*}(({{>>{

Thanks for the heads up on the tubes you are using, I'm picking up my VStandard this week and was just thinking about tubes, Do you or anybody out there know what are the tubes that will work with this amp pros and cons, and do I have to bias this amp?
Thanks Keith.

Okay, I just snagged a Valve Standard for a decent discount, and now that it's been gutted and I'm waiting for the paint (on the faceplate) to dry, I was wondering if anyone would care to comment further on the various 18watt transformers available.

I'm considering a new PT so I can use 5 preamp tubes for the total TMB, trem, and reverb package. I'm also very curious about OT options as well, especially regarding Hammond versus the various RadioSpares clones, and would appreciate any opinions you might want to share. i.e. Which ones sound better and why?

Gil...

i had another thought:

I was looking at the schematic and did not see where the bypass switch for the dps circuit is? Does it go through the TL072? Would the way to totally bypass the dos circuit be to run a jumper from the wiper leg of the treble pot to the non grounded / non-wiper leg of the volume pot? or would there need to be some sort of coupling circuit or step down resistor or something inbetween?

if SW3 is the dps bypass switch, then it looks like your signal goes through 3 stages of TL072 (the final one being a summing amp?), even if you are in dps bypass.

Matt,

thank you for the great advice!

I'll do the mod and post my results...

any thoughts on a few things to do to make this a bit more ac-15 sounding?

thanks!

-steven

Double post....

R3=220k
C12=1200pF (ie 1.2nF or 0.0012uF)
Short out R12 and R13
C27 = 0.01uF

That will get you pretty close, although the tone stack is a Marshall one and not a Vox. However, that's not necessarily a bad thing as it should allow you to get thicker mids, more like Brian May's sound. I might just be tempted to increase the tone stack's scoop a little by changing R15 to 56k, from the original 33k.

A Hammond 125E would make for a fine OT, for an AC15ish amp, or if you want a bit more of a beefy sound, you could go to an 18W clone OT, from GDS, Brownote or Trinity or Ceriatone.

OK, here are some clarifications:

1- The PI is the phase inverter, consisting of V3. We need to get the signal from VR2 via R17 (which now needs to be changed for 220k for the Voxy mods) to the top lug of VR6. The SS DSP circuitry has to be completely disconnected from the signal path at each end. One way would be to cut the PCB track after R17 and between VR6 and U2B/C23/R41, then solder a wire from R17 to the top end of VR6. Then you can forget about removing R18 and C7, as they won't be connected anywhere.

In bigjoe's amp VR6 was 1M with a resistor (IIRC either 47k or 56k) soldered across the two outer lugs. If your amp is the same, then all you need to do is to remove that extra resistor. Otherwise, you will need to change the pot, but hopefully that won't be the case.

3 - Both EL84s have a Pin 7, which is the anode (aka "plate") connection, or else the amp wouldn't work. So bigjoe is saying he measured about 343V on each of the EL84s' anodes after the mod. Your voltages may be slightly different, but within the same ball park.

For convenience bigjoe inserted the zener string in the vicinity of FS3. The zeners should all be connected in series with each other, with their cathodes (striped ends) pointing back towards the rectifiers and power transformer, since zeners operate in reverse to regular diodes.

For the OT you may also like to try an AC15 clone OT, as an alternative to the ones I suggested. I'm told that Vox's own AC15 replacement OTs are c**p though, so don't try one of those. If you've got the money to spend, Mercury Magnetics, probably do some of the best. But if not, an 18W OT or Hammond 125E will do fine.

FYI the blow-by-blow account of what bigjoe and others did is in the thread on 18watt.com, as well as this thread here - http://www.ppwatt.com/node/11922

Hi,

I am trying to accomplish this mod on a Valve Special and markings do not match.

I am attaching 2 small pics to this message. Can you please confirm the connections I need to make on my PCB ?

Thanks !

Michel

I think there is a new revision of the Valve Special. I asked Gibson for the schematic to my new VS (serial #07051409) and got a quick reply. They sent a low-res JPG of the schematic, which I'll attach. This version puts VR6 just after VR2, so the master volume control is before the solid-state section. I got half-way through doing the old-style SS bypass (connecting VR2 -> VR6) before I realised that it was already done and the bypass needs to be done at a different point

Did you end up doing the bypass on yours Michel?

I could sure use some help on the conversion of an Epiphone Valve Standard to pseudo-18watt thing with elements of the 45watt putting in some appearances too. I have read through the several subject posts I've found here and a couple other places and put together a list of mods to make to a broken Valve Standard I picked up on the cheap. I have no idea what it used to sound like so part of my problem is I have no benchmark. But judging from the clips I've heard of real 18Watt and 45Watt amps, I shouldn't have these issues. Basically the amp sounds really good clean but stops being clean pretty quick. That's not a big issue 'cept the overdrive does not sound very good. It is very harsh, almost brittle sounding and the bass is flubbery - or maybe blatty is a better description with the harsh vibe on top of the flubberation (working on a new vocabulary - it too still has some rough edges ). Follows is a description of the mods and enclosed is a schematic that I made up of the amp as it is currently modified (I was going blind looking at that tiny little schematic Epiphone gives out).

Ok, first off, I fully realize that I went way overboard on this putting way more work into it than it would have been to gut it and build a "real" 18watt. But it was also a learning experience for me and in that at least, it has been quite successful. I just wish the amp was a little bit more successful! Mods to the amp include the following (you'll recognize a lot of them from Zaphod's and BigJoe's lists):

• Disable and remove the solid state kruft. I don't mean just disconnect .. Perhaps I was bored or perhaps the SS stuff offended me in some fashion by sharing space in my tube amp so every solid state component, every jumper, every wire, etc that was not part of the ALL TUBE amp I wanted to be left was removed. In places where long traces went from tube stuff to solid state stuff that was no longer in use, I cut the traces near where the tube stuff was to eliminate any little "antennas" that could pick up noise or generate capacitance or whatever.
• The heaters were disconnected from the PCB board traces and soldered to the backs of the sockets (again cutting the old traces near the sockets).. Absolutely WORLDS of difference in the hum/buzz noise level. Amp is now quiet as a mouse 'cept right near the top of the MV dial.
• Grid stop resistors were often several inches and/or many snaking loops and turns of PCB traces away from the tube grid they were connecting to. These were replaced with shielded cable and the resistor soldered right to the back of the socket. This cured some oscillation issues I had been having.
• After reading the forums, a common theme that emerged was the shoddy quality of the components used, especially the polys. Since I had them around, I replaced ALL the caps .. both electrolytics and polys. Polys were replaces with a mixed bag of Mallorys and Wimas. Ceramics replaced with Silver Micas.
• My amp had two chassis ground points, one on the left side of the board, one on the right. Consolidating these got rid of yet another ground loop hum problem (Amp started out with more hum than guitar volume).
• C3 (tone stack) changed to .01uF from .022uF.
• R8 and R9 (cathode resistors) changed to 820R from 1K5 ohms.
• C1 (cathode bypass) changed to 1uF from 22uF.
• C9 (cathode bypass) was removed.
• R12 and R13 were changed to 220K from 1M. I did have R12 jumpered but put it back to 220K to try to calm the flubby bass issue. It dampened it along with the rest of the amp so I don't believe it is the problem and after the flubby bass issue is solved, plan to put it back to a jumper.
• C5 and C6 (power amp coupling caps) to .01uF from .022uF.
• R25 and R26 (screens) to 1K/3W from 100R/3W.
• Inserted two 9V/5W zeners between rectifier and fuse to bring voltage down a bit so EL84 plates do not exceed 350V (my voltages were not as high as BigJoe's).
• R21 (PI cathode resistor) changed to 470R from 1K2 ohms.
• C12 (power tube cathode bypass) changed to 1000uF/35V from 100uF.
• R17 changed to 220K from 470K (thinking I might change this back in case it is the PI generating the harsh overtones).
• Originally listed in the mods was changing R22/R23 to 330K from 220K but I had oscillation and *really* ugly tone issues doing this so this was undone and the resistors are back to 220K now. These *may* have been solved by some of the other oscillation changes but since this change added gain, I wasn't going to retry until the harsh blatt was controlled..
• R15 (tone stack) changed to 56K from 33K.
• C5 (tone stack) changed to 250pF from 500pF. Did this (and the 56K change) to brighten the tone a bit which was unaccountably dark before.
• I had changed the master volume pot to 500KA instead of 1MA but I'm now getting pretty much full volume within the first 1/8th to 1/4 turn. I think it was better with the 1MA pot but I haven't switched it back just yet.
• A 1M ohm resistor was put across VR4 (the 1MA bass pot) to make it effectively a 500KA pot.
• Output transformer changed out for a "real" 18Watt OT from GDS.
• Tubes changed to 3 Chinese 12AX7s and 2 JJ EL84s all from GDS.

For voltages, I have the following:

V1: pin1=121V pin2=0.6mV pin3=0.1V pin6=120V pin7=.5mV pin8=1V
V2: pin1=148V pin2=-2.3mV pin3=0.88V pin6=255V pin7=148V pin8=150V
V3: pin1=202V pin2=51V pin3=82.3V pin6=207V pin7=52V pin8=81.7V

V4: pin2=14mV pin3=13.2V pin7=349V pin8=343V
V5: pin2=8mV pin3=13.2V pin7=349V pin8=343V

Basically looking for ideas to try to reign this thing in and smooth it out. Like I said, I like overdrive. I like it a lot. But I have several high gain amps and don't want this to be one of them. Some decent smooth breakup would be good and then should I want more, I can push it with a pedal or something. It has a unique tone that I really like and I want to keep that.

Enclosed is a schematic that I made of the amp as it exists. It is basically the Epiphone schematic sans all the SS crap and drawn to a scale that can be read when printed and it of course includes the mods above. I think I got everything right but if errors are spotted, please let me know. Done with TinyCAD hence the .png export. Note that the component numbers are unchanged from the Epiphone doc.

Thanks for any and all suggestions!

Interestingly enough, I too have the Hammond filament xfmr in mine. One of the several things borked in the broken amp I got was fried filament windings in the PT. Didn't know the rating was that low on the stock heaters. After looking at the "quality" of the trace routing on the board (let's wind those filaments around every possible component to generate some bitchin noise!) I no longer wonder what they were thinking .. This thing was done on the cheap cheap cheap cheap with zero quality control at all. The price paid is good for the cab, chassis, a few odd components and *maybe* the PT. I've heard the speaker is no great shakes but I've never actually heard it. And yes, the GDS OT was a *major* upgrade ..

Wish there was a way to get this one to work as-is though.. I have an organ chassis I was going to do a full "normal" 18watt build in. Amp has plenty of bass in it. Just when I turn the MV pas about 10oclock, starts to pick up a real harsh top end like the PI is distorting badly or something. Same thing happened with the earlier sovtek tubes so I don't think it is tube related .. tiz something in the amp itself.

The Valve Standard is closer IMO to an 18W TMB amp, such as Richies' TMB on 18watt.com than an 18W Lite IIb. Basically you get an 18W TMB with a parallel triode first stage and no Normal channel. In any case, the mods you suggest are good, as they bring the Valve Standard closer to an 18W. However, I wouldn't necessarily jump to using a 100uF cathode cap on the EL84s with the 150 ohm resistor, as the 1000uF/180 ohms will deliver smoother distortion and tighter bass. I think it might be good to experiment with those different values, since tastes vary a lot. Maybe also try increasing the drive on the el84s, 18W style, by increasing R22 and R23 to 330k or 470k, depending on how much you want to push the power tubes.

To control the low end C2 can be reduced to 0.01uF. Some people also like to use a 2.7k value for R9, with a 0.68uF across it when more boost is required, which can also really help reduce any woofiness. For R1 I would actually recommend using 10k rather than 68k, as this will be less likely to pick up buzz, while allowing more sparkle from the guitar to come through.

Just a few ideas....

I had someone bring me their standard to bypass the DSP. To do it right is a good couple of hours(PCB removal,job and re-assemble) and a new MVpot as noted above. It would be silly to just do that mod once the chasis is already out. I decided not to take the job. If I owned the amp and wasn't billing myself I would do all sorts of stuff. I told the guy it's not worth it unless he did the work himself. How much time do feel you've invested bigjoe?

Remember to keep one hand in your pocket!

I have made progress. Latest schematic is attached. I had a misunderstanding on what the voltages should be for the EL84. I thought they were supposed to be under 350V but looking at a JJ tube data sheet last nite said under 300V so my 348V was way too high. I added in the other 3 zeners I hadn't added (total now 5 as was originally recommended) and that brought the plate voltages down to the 320V range which is inline with the voltages bigjoe posted near the top of this topic.

Still had over-fuzz condition on preamp (and no headroom). My Schecter C1 Classic (JB/Jazz Seymour pups) got dirty on the bridge pickup with mv and pre just cracked enough to make noise. So I cleaned up the preamp some more, which got me a decent clean yet still got dirty on the high end. Has sufficient treble too which is cool. I also had to give the PI some more headroom going back to many of the original epi values here. It still gets really loose and loses all definition and coherency when you dime it. Read as zero articulation. Power chords just sound like loud blatts (could be blocking distortion but from where??!?!). But it sounds quite nice so long as gain and mv don't go past about 3/4s .. is this typical of 18 watts? I've never played one so have no basis for reference. It also isn't all that loud anymore. I would have liked more volume out of it but it seems hitting the power tubes with that much boost drives 'em nuts so to speak..

Biggest annoyance right now is with all those zeners in there I've picked up a really annoying constant buzz. Very harsh. I've heard zeners are excellent buzzsaw noise radiators so I'm assuming that is what I have. They are located on the board but up in the upper right hand corner where a bunch of solid state stuff used to be. They aren't really near any thing except they are about 1.5 inches away from the el84s (which are on the other side of the board. Not sure if they are able to radiate that far or if the el84s could somehow be better shielded or if I just need to get a real power supply and quit messing around with this epiphone crap (which would also get rid of the heater supply xfrmr I'm using right now too). The PT275 from musicalpowersupplies.com looks like it could be up to the job. And the price is certainly right!