getting overdrive without distorting the pre-amp

[tonequester]

Greetings to ALL !

I am interested in how one would keep the pre-amp from distorting, and yet have enough gain to overdrive the output section of the amp. I am modifying a VHT Special 6 Ultra. The clean channel will NOT distort without a pedal. It has only 2 gain stages(1/2 of each 12ax7's). The Ultra channel has 3 gain stages, again, each is 1/2 of a 12ax7. It will distort, but the tone is too "fuzzy". If it is distorting the output section of the amp (a single 6V6), it is being overwhelmed by the pre-amp, and the resulting tone is dissonant.
I have tried a 12au7 in the first position. This did not help, and I really noticed little difference in the tone. I tried a clean booster pedal in the effects loop, and this gave me the best results, but is still too dissonant. Unfortunately, 1 X 12ax7 follows the effects loop. The amp has a Wattage control, which attenuates the plate Voltage, but this type of distortion is still not "sweet".
I do not know how to figure out the limit of a gain stage. An answer to this problem may help me understand the phenomena. Also, is there a limit to a 12ax7's input Voltage(A.C.) ? Is this a choice of tube problem ? I have found that there is definately a difference in tube brands. I bought VHT's EL84 adapter, and tried it with a Mullard RE-issue EL84. It sounded much too fuzzy, and it was next to impossible to play clean with it.
If my problem IS tube related, what parameters are we talking about. Do I need to minimize gain to keep the 12ax7's from distorting. I would think that when I tried the 12au7 this would have happened. Is there such a thing as headroom for a pre-amp tube ?
This is an experiment that I am doing. I'm not saying that output tube only distortion is best. However, I recently spoke with DeLisle Amplifiers about buying one of their custom amps. They told me that their amps are low gain, and made to distort FIRST at the output section, then the phase inverter, and lastly at the pre-amp, so there is precedent for emphasizing output distortion.
All comments and opinions are most welcome, and answers to my questions are greatly desired ! tonequester.

P.S. I emphasize............I'm not against pre-amp distortion, just don't like fuzz !

[brewdude]

Do you have schematic?

Edit: is this it? I found if at http://robrobinette.com/Special6UltraMod.htm This is a pretty busy schematic, lots of local NFB and snubbers… Looks like a fixed tone stack… and do you need 11 different cathode bypass caps?

[tonequester]

Greetings brewdude.

Thanks for replying to my post. I DO have the exact same schematic, but I need to get it blown up so I can read it(poor vision). brewdude, I'm not as advanced in tube amp theory as you are. I don't know what "local NFB" is. Nor do I know what "snubbers" are. As far as 11 different cathode bypass caps, that sounds like a lot even to ME. If you can clarify, I would be humbly thankful. I'm here to learn, more than anything. I'm getting ready to email VHT over some of these issues, but I always like to get more than one opinion, especially when they aren't the manufacturer. When I emailed them about excess hum on the Ultra channel, they wanted me to change the value of about a dozen resistors. I suspect the resistor changes would have lowered the gain, and made it quieter. However, i did not want to lower the gain unless necessary. I did Rob Robinette's "add a choke" mod, and replaced the first position 12ax7(Chinese) with a less noisy tube(TAD) and now the amp is real quiet.
I don't know if the issues you brought up are related to my "fuzzy distortion" problem or not. This is the first amp that I have bought planning to do major mods to, but I haven't found a mod that would apply to the fuzzy sounding distortion. I have a reverb mod, a channel switching mod, and a tube type rectifier sag mod to do, but I don't want to go there until I have gotten good tone out of this "mod friendly" amp.
If you have the time to clarify that which I don't understand, I am in your debt. Sincerely,.........tonequester.

[brewdude]

Honestly, I'm no expert. But, I seem to prefer much simpler designs. I'm not sure I know where to start.

How do you tend to set the controls? Depth switch? Triode/pentode? Power regulator? Do you use the loop? What kind of sound are you hoping for?

Personally, I would start by trimming some of the bells and whistles. Too many choices, I can't believe that they could all sound good--probably difficult to dial in a single great tone… but I have never seen or heard one of these amps, let alone played one.

[matt h]

http://ampgarage.com/forum/viewtopic.php?t=25226
http://ampgarage.com/forum/viewtopic.php?t=25169
http://ampgarage.com/forum/viewtopic.php?t=25201
http://ampgarage.com/forum/viewtopic.php?t=24872
http://ampgarage.com/forum/viewtopic.php?t=24939
http://ampgarage.com/forum/viewtopic.php?t=24929
http://ampgarage.com/forum/viewtopic.php?t=24797


http://sd.keepcalm-o-matic.co.uk/i/keep ... oll-22.png

[brewdude]

It looks like dejavu all over again.

[tonequester]

Greetings matt h.

I guess that you take issue with my posting about the same related topics. I apologize if I somehow upset you. I hope that no one is forcing you to read my posts. However, I tend to be redundant in pursuit of knowledge. This is especially so when the replies that I get do not have anything to do with my questions.
This "troll" thing is confusing. I guess that I could be called a "troll" because I submit more than one post concerning one issue. However, I have heard the term used to describe those individuals who have nothing better to do but criticize, whether it be from their own conception of over-posting, grammar, paragraphs and spacing, etc., etc., etc. I only know that I am sincere in my search for knowledge, and feel that if ANYONE does not like the way I post, they can read and reply to someone else's posts.
As of yet, I have not found the answers to MOST of my questions posted. I intend to keep trying on this, and other forums. As I do this, I will not criticize or bully ANYONE. I am quite capable of NOT letting my testosterone levels override my neurotransmitter levels. I also look at your attitude like this : I must forgive in order to be forgiven.
brewdude, I am beginning to wonder myself if I made a big mistake in buying this amp because of the multitude of "bells and whistles". Originally, I was going to buy the plain Jane Special 6. Then I found out that the reverb mod I had found would not work on it. That's why I popped for the Ultra. I truly appreciate your attempt to help me out, and for NOT being turned off because I have posted related matters before.

Sincerely, Larry Boydston.

[ToneMerc]

Greetings matt h.

I guess that you take issue with my posting about the same related topics. I apologize if I somehow upset you. I hope that no one is forcing you to read my posts. However, I tend to be redundant in pursuit of knowledge. This is especially so when the replies that I get do not have anything to do with my questions.
This "troll" thing is confusing. I guess that I could be called a "troll" because I submit more than one post concerning one issue. However, I have heard the term used to describe those individuals who have nothing better to do but criticize, whether it be from their own conception of over-posting, grammar, paragraphs and spacing, etc., etc., etc. I only know that I am sincere in my search for knowledge, and feel that if ANYONE does not like the way I post, they can read and reply to someone else's posts.
As of yet, I have not found the answers to MOST of my questions posted. I intend to keep trying on this, and other forums. As I do this, I will not criticize or bully ANYONE. I am quite capable of NOT letting my testosterone levels override my neurotransmitter levels. I also look at your attitude like this : I must forgive in order to be forgiven.
brewdude, I am beginning to wonder myself if I made a big mistake in buying this amp because of the multitude of "bells and whistles". Originally, I was going to buy the plain Jane Special 6. Then I found out that the reverb mod I had found would not work on it. That's why I popped for the Ultra. I truly appreciate your attempt to help me out, and for NOT being turned off because I have posted related matters before.

Sincerely, Larry Boydston.

I think what he is simply trying to say is that we heard you the first time.

Perhaps you would have been better served by focusing your discussion in only one or two threads and bumping them instead of eight separate redundant threads. If over a period of weeks you have failed to get an adequate answer here, then perhaps it is time to move on to another topic.

TM

[V2]

Just a few general comments and questions:

- why do you want to avoid preamp distortion and have only power amp distortion? Don't be afraid of preamp distortion.
- you definitely need more than one preamp gain stage to overdrive an output tube (in response to another one of your threads)
- the easiest power tube to drive with a tiny preamp signal is the EL84. Much easier than the 6V6
- when you tried the EL84 with a converter, it sounded fuzzy. THAT is your EL84 distorting. Power tube distortion isn't always nice. It can be avoided, but EL84 amps take some clever design (which is often not the case in EL84 amps).
- local NFB: in various places of the posted schematic you'll find a cap between the plate (output of preamp gain stage) and grid (input of preamp gain stage). The output is fed back into the input, but the (polarity of the) signal is inverted.
- if you want even less preamp gain with two or three stages, (i) put the tone stack as early as possible, e.g., after stage one; (ii) use voltage dividers to reduce the signal between stages

- maybe start with a blank slate and built a blackface circuit into your SE 6V6 power section. I've done it and it can be clean (Fendery). You could design it for whatever amount of gain you want.

[LOUDthud]

A symmetrical squarewave sounds hollow, is that the problem here? I noticed on the schematic that every preamp tube has a 1.5K cathode resistor. Try changing the cathode resistor on V1A, V1B and V2B to anything you have between 820 and 2.7K. Go higher than 1.5K on one stage and lower on the next. That will move the squarewave away from symmetry.

[tonequester]

Greetings.

Thanks for the clarification about my posts. I'll try to watch that. I must re-iterate that I'm not convinced that pre amp distortion is bad. I just suspect that it is dominating my VHT amp. That EL84 sounded so bad mainly because I selected a Mullard Re-issue(made to break up early). Hell, it started breaking up on the clean channel, which is only 2 gain stages. It still broke up when I subbed a 12at7 in V1. As far as designing my own amp, fat chance. Man, I joined this forum to glean some expertise. You have to have knowledge before you can spread it. I hope that, in part, if I hang around here a while, I may someday be able to do my own design.
Until that day comes, I want to thank ALL of you for the tips and tweaks that you have given me to try. I truly do appreciate it. I'm printing this whole topic, so I don't miss a thing. I will probably do each thing suggested here. That's the main reason I bought this "made to mod" amp. I just didn't expect it to sound so crappy stock. I may make it worse, but I'll be surprised if I do ! Best Regards..............tonequester.

[gui_tarzan]

I read somewhere (here I think) that the PI actually distorts more than people think and should be replaced when power tubes are replaced. Is that true?

[tictac]

No it's not true.... Torres Newsletters say that so they can sell you new tubes.

Treat the PI tube like the other preamp tubes and change it only if it goes bad...

TT

[printer2]

Well the amp is not designed to be a dirty amp. The input 12AX7's with cathode 1.5k resistors and 100k plate are pretty much center biased. The supply voltages are reasonable given the range of the amp. I would change the 220k plate resistor on the effects return to 100k. Other than that just set the volume (VR1) so you are not overloading the preamp tubes and (VR2) turned up all the way, you should have enough voltage to get the output tube into distortion.

[tonequester]

Greetings.

Thanks for the advice printer2. I don't know what effect on the overall tone of the amp the effects loop plays, but I'm willing to change that resistor. I've been setting volume to full, and adjusting the drive control to try to get the overdrive right. This is what has confused me. I have always heard that to get output distortion, the volume control should be pretty well maxed. The drive, or gain control adjusts the pre amp gain. With this VHT, even on the Ultra channel, if the volume is wide open and the Ultra control is off there is zero sound. As you bring up the Ultra channel it begins to break up, but it is NOT a pleasing tone. Even a three note E Major chord sounds dissonant. I,m going to try replacing the 2 X 12ax7's with 2 X 12at7's first. If the problem persists, I'll make the change to the effects loop as you suggested. Thanks again to ALL who have replied..........tonequester.