maximizing output tube overdrive, minimizing pre-amp fuzz.

[tonequester]

Greetings to ALL.

I bought a VHT Special 6 Ultra(made for modding). My intent is to max the power tube overdrive, and minimize the pre-amp overdrive.....IF this is possible. My tube vendors tech advised my to replace the first position pre-amp tube(12ax7) with a 12ay7. Since the stock tubes did not sound good at all, I took his advice. Now I have an Electro-Harmonix Gold 12ay7 as the first pre-amp tube. This is followed by a Tung-Sol 12ax7 Gold. For the power tube I have a Tung-Sol 6V6GT. I also bought their adaptor to El84, and have tried out a Mullard re-issue El84(breaks up early but sounds a little harsh).
O.K., I would like to know if using the 12ay7, which has a gain of 47 instead of the 12ax7's 100, is a good approach to use in obtaining my purpose. In other words, will cutting the gain of the pre-amp by about 25% "tame" the pre-amp, yet still create enough signal strength to overdrive the 6V6. ANY opinions are WELCOME.

I tried to ask this question on another forum, but nobody gave any opinions to my query. I got a couple of replies that merely suggested different brands of 12ax7 tubes. I know that the 12ay7 is at times suggested for giving an amp more headroom as I've seen this suggested several times before. However, I don't know if headroom applies here or not. If headroom only applies to what occurs in the output section, it seems that while making the pre-amp distort less, I may also have a harder time getting output overdrive, giving me a Catch 22 situation !? Again, any opinions are welcome concerning my questions. Thanks in advance,.......tonequester.

[Firestorm]

Use the clean input, push in the boost knob to turn the boost off and set the volume about half. Then turn down the Watts control until amp starts to break up. That will be 100% output tube distortion. If you like how that sounds, you can experiment with the 12AY7, but generally unless you have an exceptionally hot pickup, that first gain stage won't distort and the second one is driven by the volume pot so can be controlled from that.

[Stevem]

until you test any tube there is nothing cut in stone that for example a 12ax7 who`s spec is 100 gm will have that expected level of gain.
A new but weak 12ax7 may not have the voltage gain or current drive of the 12ay7/6021 tube.
If you have a way to drive a test tone thru the amp do that and meaure the signal level on the out bound side of each tubes plate coupling caps as you swap in different tubes.
If it where me I would start off with swapping a 12at7 first and drop down in gm if you need less preamp kick by then doing the 12ay7 thing.
A lot of compression and sustane comes from over driving the preamp section, a unliked fuzzy distortion can be sweetend up by placing low value caps across one or more preamp plate load resistors and still leave the touch and feel of a higher gain tube intact.

[tonequester]

Greetings Firestorm.

Thanks for your helpful info on the set up of my amps control. This amp may have too many controls for me. It would be nice if VHT would thoroughly
explain how each of the controls work ! Here I'm ready to sink another 2-300 in it to get what I want, and there is a simple setting which should get me close. Thanks for potentially saving me a lot of money. I don't always get my questions answered, although I always get plenty of advice. Your approach is perfect for helping beginners like me. Now, I at least have a starting point.

Thanks so much for your concise reply. I hope that YOUR projects meet with complete success !.........tonequester.

[tonequester]

Greetings Stevenm.

Thanks for the helpful advice on the tube swapping. I am very interested in what you are trying to get across about the caps in parrallel with the plate load resistors. I looked at my schematic, and if I am identifying the plate load resistors, as those that are in a direct line away from the plates, this is what I have.(The resistors terminate on the non-tube end at B+4 and B+3) On V1A R4 is a 110K Ohm R. It has a cap marked 102(.001uf) bypassing that resistor. V1B has R10 a 100K Ohm bypassed by NO cap ! V2A has no resistor, or Cap ! V2B has R25 a 220 K Ohm bypassed by a cap marked 472(.047uf) Can you suggest what I should do here. I can see changing the value of the caps already used, but what about the V1B tube which has no resistor, and the V1B with no cap at all. If I should do anything with those missing components, what values should I experiment with for starters. I really want to max out power tube overdrive while maintaining the lowest possible pre-amp fuzz/distortion

Noise is also a problem with this amp, especially from the pre-amp(mostly at high gain settings). I am going to add a choke(Hammond 157M,8 Henry) and use shielded cable on the input jacks of both channels. I realize that I may be barking up the wrong tree here, but I bought it to mod, and noise lowering mods are worth the experience. If you have time, give me your thinking about the caps, now that you know what I have and don't have. I truly appreciate your advice and patience with my ignorance.........tonequester.

[Stevem]

Yor mean one section of V2 is missing the resistor and cap, that is due to that section of that tube, or whatever tube not being used.
These amps and a lot of new amps are designed to use todays crummy low gain tubes and sometimes when you get one or two really good tubes in such a amp the gain hiss level can be quite high.
That's why its so important to know how well your tubes test out in regard to their rated spec.
Shielded wire and a choke will not help one bit with a hiss issue, but anytime you replace a power supply dropping resistor with a choke, the amp will sound and play better overall!

[Firestorm]

V2A is the effects loop send buffer. The signal is taken off the cathode so there is no plate resistor. This is a series loop so V2B is the effects return amp (when the loop is used) or simply the final preamp stage when nothing is in the loop. The .047 is the coupling cap, not a bypass.

Where are you planning to put the choke? The 157M should be able to handle the full amp's current if you wanted to do it like a 5E1 Champ.

[tonequester]

Greetings firestorm.

Sorry to be late in reply about the choke. According to the instructions for the mod, from Rod Robinette's website, the chokes leads replace R43. If you have what you think is a better placement, please feel free to let me know. I try to keep an open mind on every mod I attempt. This is the only way I can have time to both mod, and play. I just try to learn along the way. Thanks again for your interest. You can bet I'll have more issues to post as I try to make this amp sound like I want it to.

tonequester.