Reducing high gain noise on my VHT Special 6 Ultra.

[tonequester]

Greetings to ALL.

I am seeking suggestions for reducing the noise(hum/buzz) on the high gain settings of my Special 6 Ultra(mod friendly) amp. The clean channel is very quiet, but anything beyond that has too much noise. I wish I knew a better way to give a schematic link, but you can view it at: http://www.vhtamp.com/products/the-spec ... ltra-combo
I am going to add a choke(Hammond157M) but there is no real consensus as to whether this will help or not. I have Googled this, and some sources say yes, and some say no. Some say it will alter tone positively, by introducing "sag", and others say no. I have ordered it, and will install it in hopes that it will be a positive thing.
I contacted VHT about the noise, and they suggest changing R values for R1,R42,R43,R48,R49, but they did not explain why, nor did they let me know the OTHER effects this might have on tone and output power. I emailed them again, but I haven't yet received an answer. Outside of the noise, I am close to having the tone that I want after re-tubing. The stock "no-name" Chinese tubes sucked. I replaced the first position 12ax7 with a 12ay7(Electro-Harmonix) to preclude excess pre-amp distortion. I changed the 2nd 12ax7 to a Tung-Sol, and changed the 6V6 to a Tung-Sol 6V6GT per the advice of my tube vendors tech. I'm trying to minimize pre-amp distortion, yet make it easier to get output tube distortion.
I would appreciate it greatly if anybody has suggestions for reducing the high gain noise, or could tell me about the resistance changes that VHT suggested. I am afraid that if I just arbitrarily change the R values, my tone and output power might suffer. To be truthful, if I would have realized just how much gain this amp had stock, I might have settled for the cheaper Special 6, with just volume and tone control. The Ultra has lots of bells and whistles. One, the Texture switch is a joke. I can't hear the effect at any level of gain or volume.
Anyway, if anyone has some ideas, I thank you ahead of time.
tonequester.

[mrn1ngstr]

Have you considered changing the 12ax7 to 5751?

[vibratoking]

That link doesn't work?

[mrn1ngstr]

Remove the period at the end.

[Colossal]

Here is the schematic. Now we can see something.

I see a Fender Champ with an effects loop. It has a variable bypass cap (Depth Control) on the output tube which will effect the gain and local feedback on the output tube. I would probably set it and forget it. Let's see, there is a Zobel filter on the output presumably to take some icepick out. Not sure I see any value in that unless it is a low cost output transformer with Chinese steel or they are trying to smooth it out a bit when the Watts control is way down.

There are some options to rearrange the circuit. You could insert the fixed tone stack in between the Ultra and Clean stages and just get rid of the Clean input. That will make it similar to a Princeton.

Reducing R1 will reduce the amount of resistor noise in the signal path when in the Ultra channel but should not make a mind-blowing difference. Reducing R42 and R43 will reduce the preamp voltage, not something I would do if you want clean. To make it more Fender like, get rid of C52, C5, R5 and C9 (and move the tone stack as I mentioned).

Looking at the circuit it is not particularly high gain and is pretty well filtered. You could certainly add another 47uF 500VDC cap in parallel with C41.

You could also parallel V1A and V1B for a single gain stage (run as a common anode mixer) or run V1A as the input stage and convert V1B into a cathode follower. That would be cool. If you want to sacrifice the effects loop, lots could be done with four triodes.

[tonequester]

First, thanks a bunch for your replies guys.

mrn1ngstr, You are the third person to suggest the 5751 tube to me. I'll have to check around for a decent deal on one. I did a very quick search and the price I found was $95.00 if I am remembering correctly. However, I believe that this was a nos tube. Can you tell me what is so special with this tube, for my needs here ? vibratoking. I apologize for the link to the schematic not working. It was the url showing while I was looking at the schematic. Perhaps this is not the way to post a link ? I AM definitely a newbie to forums, and even pretty much to computers, other than searching the net. If there is a special way to post links, please don't hesitate to tell me what the proven method is.
mrn1ngstr, the resistors that you told me NOT to remove if I wanted Clean, VHT told me to remove to lower the noise problem. I need to play clean at times so thanks to you, I'll leave them alone. I will lower the value of R1. I really don't want to Fenderize the amp, so I'll pass on the caps for now. I'm, also going to take your tip on paralleling a 47uf cap with C41. It's funny, you state that the Depth control is pretty much a "forget and set" device/circuit. VHT claims the opposite in their literature. They specifically say that it is NOT a forget and set control, it matches the amp to the guitars pickups and shapes the bottom end. I find that it sounds fine wide open for the most bass, which is not muddy at all.
Your comments on the Zobel network are very interesting, because I count this feature(texture control) to be the one big lemon on the amp. I have actually NOT ever heard an effect with this 3-way switch, nor could my brother. VHT says it's most effective at high volumes, but with the amp totally cranked, there is no audible effect. I read that the Zobel is mostly used for stability in an amplifier, I don't know how this is supposed to contribute to tone, but mine does NOT ! I may double the stock values of "C" in this Zobel. The stock values are 2.2uf and 3.3uf, plus a bypass position.

Once again, I truly appreciate all of your time and effort to help me out with my problems. Best regards,..tonequester

[Colossal]

mrn1ngstr, the resistors that you told me NOT to remove if I wanted Clean, VHT told me to remove to lower the noise problem. I need to play clean at times so thanks to you, I'll leave them alone. I will lower the value of R1.

I am guessing you meant me (Colossal). I don't know why they would tell you that lowering the preamp voltages would lower the noise. Those resistors are referred to as dropping resistors, meaning that their value determines the voltage drop from one high voltage node to another. If you have a loud, buzzy amp...that kind of noise...that is something else altogether. There could be a ground issue or a leaking capacitor. Large value resistors <I>in</I> the signal path impart noise to the circuit. Lowering R1 will not change the amp from hissy to tomb silent, so don't expect that kind of result. But changing R1 to 10k from 68k and you might expect an audible difference or more top end and less roll off with your guitar volume. Do not be tempted to remove it altogether or your amp will be picking up Mexican radio.

I really don't want to Fenderize the amp, so I'll pass on the caps for now. I'm, also going to take your tip on paralleling a 47uf cap with C41.

Stock the amp is pretty close to a Fender champ which is a raucous amp with not much clean headroom. Adding more capacitance at C41 should stiffen the power supply so if your noise is actually power supply noise, this may improve it. Similarly, a choke will stiffen and dramatically reduce high voltage ripple which is a good thing if you want a quiet single ended amp.

It's funny, you state that the Depth control is pretty much a "forget and set" device/circuit. VHT claims the opposite in their literature. They specifically say that it is NOT a forget and set control, it matches the amp to the guitars pickups and shapes the bottom end. I find that it sounds fine wide open for the most bass, which is not muddy at all.

I can see their point specifically for an amp which has no EQ but as your results show, maxed out is fully bypassed and is going to produce the most thump. Personally, for my buck, if I were making a three knob amp, I would place control elsewhere. That's what I meant by set and forget. Nice to find what works for one's guitar, but not like I would be fiddling with it all the time.

Your comments on the Zobel network are very interesting, because I count this feature(texture control) to be the one big lemon on the amp. I have actually NOT ever heard an effect with this 3-way switch, nor could my brother. VHT says it's most effective at high volumes, but with the amp totally cranked, there is no audible effect. I read that the Zobel is mostly used for stability in an amplifier, I don't know how this is supposed to contribute to tone, but mine does NOT ! I may double the stock values of "C" in this Zobel. The stock values are 2.2uf and 3.3uf, plus a bypass position.

I don't see much utility in it but if it's not hurting anything, then, meh.

If you are looking for clean, clear, and articulate, then consider paralleling the first two triodes, stiffen the power supply, use the Ultra input (Clean input could just be a single triode input or dumped altogether). To run both V1a and V1b in parallel, you will halve Rp and Rk and double Ck. This has the advantage of reduced plate resistor noise.

[tonequester]

Greetings Colossol.

Thanks for ALL of your input. I just added the choke, and I would say that the noise level went down by a third. Besides the simple changes to R and C that you have suggested(which I will do), I am afraid that I am not capable enough to parallel the pre-amp tubes without good instructions. As far as VHT's advice, I don't want to come off like they are all jacked up, but I have found out that several things that they advise, are opposites of what I have been told on this and other forums. When they give advice, so far, they have not given any explanation as to WHY they want me to do something. Here on this forum, I get advice with explanation. I'll keep asking them about my problems with the amp, but I will check it all out on this forum.
It's obvious to me that you, and some others that have replied, have experience with such problems, and also have your theory down as well. I don't want to pester anybody, but I do want to learn as I do the mods that I have planned to do. It's bugging me that I am having to double the number of mods that I do, in order to fix things that VHT should have done a better job in designing.
You my friend, have impressed me and I am very thankful to have gotten replies from you that explain just what the changes are all about. I fear that VHT could very well get me to the point that I have no noise problems. However, I think that I would end up with an amp that is completely different tonally, if not worse.

Thanks again Colossal. Your A.O.K. in my book !

[dorrisant]

Here are some mods that you could use to tweak it a bit...

http://robrobinette.com/Special6UltraMod.htm

http://robrobinette.com/images/Guitar/S ... ed_Mod.jpg

Channel/Boost Switching
http://www.instructables.com/id/VHT-Spe ... od-inc-fo/


I did some of them like the variable bias mod. I put in a pot in place of the cathode resistor so I could try out EL34s, 6L6s and such. I had a KT66 in there for a while. Got the EL84 adaptor and I'm diggin' that for now. I also did the tone stack mod verbatim from the link above... It worked great but I had to reverse some of the pot wiring when I got done... Maybe I just did it wrong first... It's really cool with scooped mids!
Anyway, I also put some higher quality caps (original values) in the signal path. Can't really remember off hand what was changed exactly but I really liked the way the amp sounded when I got it , I just wanted to be able to sweeten it up a bit.
I've thought about making the boost variable... Maybe.
The 5751s... Gain of 70 where the 12ax7 is 100. EBay shows about $19-$20 shipped... You can get new production JJs or Sovtecs for cheap through AES and such.

http://www.tubesandmore.com/search/node/5751

https://tubedepot.com/t/tubes/preamp-tubes/5751

Hope I didn't bore you!
Tony

[Jerryz1963]

has Jan Phillips for 40 something. "other" NOS for 40, and a sovtek for under 20. It'll be a cold day in hades before I pay 100 bucks for a preamp tube

[Tone Lover]

There are certain mullurd longplate tubes, tested to be ultra quite and 100 % that i would pay $100.00 for, but I have been spoiled and have friends that let me test there rare tubes in my amps.
So I have a differant perspective . Now finding that tube in that condition is another animal altogether.
Bill

[tonequester]

Greetings Gents.

Hey, you guys never bore me. Hell, I have too much to learn. I appreciate all of the info given me on this post, including the tube data. You "youngsters" are tops in my book. I hope I can return the favors some day.
I wish you the very best with YOUR projects !!!

tonequester