Sound Gets "Spitty" With Master on 10

[Jag]

I'm messing around with a Vibro Champ and am wondering what is causing some ugly breakup with the volume on 10.

What I've done is remove the vibrato circuit and used one half of the 12AX7 that used to be for the vibrato as an additional gain stage. I don't have the amp or my schematic drawing in front of me right now so I can't quote the components exactly. The added stage is based on Ken Fischers "Universal Gain Stage" from the first Gerald Weber book.

The cathode resistor on the new stage is 10k unbypassed and there is a voltage divider on the output (2x220k) to bring down the gain. From there the signal goes through a .022uf coupling cap and a 250k Master Volume pot into the stock Champ output section. That's the basic setup.

Everything before the added gain stage is stock Champ. In otherwords the signal goes through one half of the first 12AX7 to the standard tone stack and through the other half of the first 12AX7. From there the signal goes into the added gainstage out to the master volume and power amp.

The amp really sounds great. Very touch sensitive. Clean and responsive when backing off the volume on the guitar and crunches nicely with the guitar full out.

The only issue is, if I turn the master volume above about 8, the output gets kind of spitty. It breaks up in a way that sounds similar to digital clipping or crackly. Is this the result of grid rectification? Am I slamming the output tube too hard? Should I pad down the input to the power amp a little bit.

Thanks

I should add. This occurs with the regular volume set to 10 inaddition to the master being at 10. In otherwords, when I run the amp "full out". If I leave the master on 10 and bring the regular volume down (I can't remember how low) the "Spitty" sound will also quit.

[Don Symes]

I should add. This occurs with the regular volume set to 10 inaddition to the master being at 10. In otherwords, when I run the amp "full out". If I leave the master on 10 and bring the regular volume down (I can't remember how low) the "Spitty" sound will also quit.

I'd say you answered your own quesion, there. 'Grid rectification' or not, you're hitting your 6V6 too hard. You can either just not dime it any more or fiddle your 220k/220k divider down juuust enough to stop the spit when you do dime it - reduce the lower one to about 180k, I'd say.

[Jag]

Thanks alot Don.

I was inclined to do just that but I didn't want to do it "just because it works". I wanted to be sure that was the right solution and I wasn't just masking a different problem.

I've been modding and buiding tube amps for almost three years now and have been pretty happy with the "stock" mods and tweaks. Lately I've been trying to find things that are a little different from the standard stuff and messing around with my own ideas. I've realized that there are a few general symptoms that I have not yet experienced and don't know how to identify. Grid rectification is one, blocking distortion is another. I know the concepts but actually tying real world experience to the concept is a little nebulous sometimes.

I've been able to talk with Kevin O'Conner on the phone a number of times and he's always told me I could call anytime. He's a great help but I don't like to bother him too much. I used to call Gerald Weber and Dan Torres alot in the early days too. They also were always very helpful, although I know some people have "issues" with them. One thing I've learned is that every amp designer has their way of doing things and sometimes they can have VERY strong opinions. :-)

My next big area to study is going to be transformers, especially OTs. I know how they do what they do but I'd like to really understand the theory so I can be confident that my builds sound as good as they can. Can anybody recommend some good resources on the subject?

I really appreciate all the help you guys provide.

Jag

[Arthur B.]

Another idea would be to use the second 12ax7 as cathode followers to buffer the existing gain stages. You would get extra gain from the unloading effect of the cathode followers, but won't suffer as much blocking distortion.

[Jag]

Thanks for the reply Arthur.

This Champ is my test amp which I use to try out various things. The first thing I did to this amp was convert the preamp to a Marshall style with the typical Marshall tone stack. I don't really like that sound, that's why I've set it up this way. I hadn't really thought about adding a CF at the end of the preamp path with this configuration. I've been thinking about what I could do with the "leftover" half of the second 12AX7, I think I'll give your suggestion a try. I'll let you know how it goes.

I prefer the way the typical Fender style tone stack sounds being earlier in the signal path. To me it seems like the tone controls are a little more subtle there and the top end is a little smoother. Actually, I'd like to try a circuit where the Treble control is early in the path and the Mid and Bass controls later. I think that would suit my preferences the best.