potentially maxing output distortion, minimizing pre amp hum

[tonequester]

Greetings to ALL.

I am not sure that the title matches my queries perfectly, but here goes. I have VHT Special 6 Ultra, "made for modding" amp. The only mod I've done to it so far was to add a choke, which did lower the noise level considerably. My goal is to be able to play clean(on the clean channel), ideally just getting into output tube saturation with the volume at about 9 1/2. I am not a big fan of the amps Wattage control/tube starvation tone. In order to go farther into saturation, I have to switch to the Ultra channel, which has a drive control. This drive/gain control is a push/pull pot. Pull it out for boost. The amp also has a foot switch(tone control bypass) which when engaged, gives one another significant boost(maybe too much).
Without ANY boost applied, there is too much "gain hum" with the drive control at only 4. I have tried a variety of tubes, and found out that a very quiet tube in position V1(V1a for clean, and V1b for Ultra) made a big difference. However, there is still too much hum for my taste.
Would substituting a lower gain tube(12at7,...12au7...) lower the noise level as it lowers the overall gain ? Would lowering the gain thwart my desire to overdrive my output tube(6V6GT) ? I have read in several places that using a lower gain tube in the pre amp, would make the output tube "work harder" to put out the same current. Does working the output tube cause the type of over driven tone I desire ? It seems to me that lowering the gain too much might preclude output tube distortion, as too much signal Voltage causes the output tube to clip. Finally, does anybody have an opinion as to whether this amps circuit(without whistles and bells) resembles any vintage amp in design ?
I apologize for the length of my post, and I don't know how to make a quick link to my schematic(although I have posted in Garage Talk, asking how to do this). Anyone can view the schematic, searching "VHT Special Ultra-Thomann", and scrolling to page 15 of the manual. You will need to zoom it to read values. Once again I appreciate anyone's replies on all or part of my post, and again I apologize for my forum ignorance. I am trying to learn. Thanks "big time",.......tonequester

[Colossal]

Since you keep mentioning hum in your posts about this amp, you need to consider the possible sources and rule each one out before changing anything in circuit. Hum can be caused by your guitar's pickups, a bad ground, a bad guitar jack, a bad input jack, radio source near your amp, a dimmer module on the same circuit as your amp, dirty AC power, a failing power capacitor, or a bad tube...to name a few. Have you carefully tested each variable and ruled all of these things out? Your amp should not hum. It should have some added hiss in the higher gain mode but the added choke should only make the amp quieter and reduce the background noise even further. If the hum is loud, then you have something else going on.

Oh and yes, as I mentioned in your other thread, this amp resembles a Fender Champ with an added effects loop.

[tonequester]

Greetings Colossal.

Believe it or not, I have checked all of the usual outside offenders. The amp makes too much noise when no guitar is even plugged in. The choke that I installed lowered the noise by maybe a third. I made it almost tolerable by installing a TAD 12ax7 in V1. The clean channel is now noise free. The Ultra channel is still too noisy, and it's worse yet when you pull out the Ultra pot. It's worst of all when using the foot switch tone control bypass. I emailed VHT about this, and they gave me a list of about 15 resistors I can "play with", but they gave no info as to what ELSE the resistor swaps would do. I've already sent one amp back to them, and it DID NOT hum ! Every pot on the amp was scratchy to the extreme however.
I only keep mentioning hum, because the amp still hums. I am told that this amp is NOT a high gain amp. I know that the more gain you have, the more noise you will have to a degree. I have some more tubes ordered, and if they don't work I guess I'll start trying resistor replacements. I only posted this again(just the first part of my post) because I have followed a couple of VHT clubs, and nobody seems to have as much trouble as I am. Thanks for your input. I hope that someone might answer the other part of my post, besides hum and what vintage circuit the VHT might resemble. I apologize because I forgot that you had pointed to the champ before. Best Wishes................tonequester.

[Colossal]

TQ,

Ok, that's good that you've ruled out external/environmental sources. So you said that the amp does not hum in clean mode (V1B) but hums when the second stage is added (ultra mode). If your other amp was quiet in both modes, but this one hums, the issue lies in V1A (see schematic). What is telling here is that it hums with no guitar plugged in; that ain't right. Do you have an scope?

Also, did you replace the MOSFET in your power circuit?

[tonequester]

Greetings Colossal.

I installed a new mosfet without incident. I'm sure that you are right about V1. I've tried 3 12ax7's there, and each time the noise level changed. I have a TAD installed right now, and the noise level isn't half what the amp had new. The stock tubes were black. You couldn't see the elements inside. They just said "made in China" on the outside. The amp was useless from all of the noise. The second tube tried was a new Electro-Harmonix gold pin, which was a little better, maybe 25%. The TAD helped the most. The amp now makes about a third of the original noise. I could play at "gig" level now, but anything with less volume has just too much noise.
Man, a scope is on my "wish list", but for now I don't have access. I have used the chop stick method of checking lead dress. I've tapped tubes, and I've checked the grounding of the amp. All seems fine. Here's something that might have meaning to you, that I thought strange. The amp hums the second it warms up enough to amplify, without the guitar plugged in. Plugging the guitar in does nothing to the noise level. HOWEVER, when you touch the strings or bridge the noise decreases considerably. If I played with the heel of my hand on the bridge, I think that the noise would be tolerable. Am I looking at star grounding this thing ???
Any opinions/tips are most welcome. Thanks for your continued help, I really appreciate it. I've got a reverb mod to install, but I figured one thing at a time(that's about all I can handle !). tonequester

[ampdoc1]

I'd suggest going thru the amp (carefully) and touch a grounded probe to the output side of the signal capacitor coming off each preamp tube section. If you find a node where the probe silences the hum, you know the problem is between that node and the previous node. That should pare down the number of components you need to check.

Again, be careful and DO NOT handle the probe directly,..

a'doc