New Ampeg V4-B Tube Biasing

[DavidEdenAria]

Keep in mind, if you bias at 70% and the wall volts happen to be say 118 wall volts supply, and the wall volts later on go to as HIGH as 125, you now have exceeded 70% dissipation.

Personally I have sold 100's of power tubes at the Gear Page, Plexi Palace and Metro amp forums.....I always strongly suggest biasing right at 60 to 65% to allow for wall volt variation and slight bias drift as the tubes wear.

The tubes will probably be fine at 75% in a class a/b push/pull amp (and sound GREAT!) but you are not allowing for bias drift or wall volt increases.

Most folks can't tell the difference between 60 & 70 dissipation.

 

[JimmyM]

Hello everyone. Would it be possible to have the V4-B schematics? I'm seriously in the market for one, but I would prefer an XLR DI out pre 'master' volume control. I would like to know if I could easily modify it. Thanks for any help!

Call Ampeg. They might be closed tomorrow but should be open bright and early Monday at noon EST/9a PST.

 

[davidchampoux]

Thanks Jimmy for the follow-up. Do you think it's possible to do this mod?

 

[beans-on-toast]

Do you think it's possible to do this mod?

If you turn the master all the way up and use the gain to control the level, you are effectively removing the master. Maybe this will give you what you need.

 

[davidchampoux]

Not really as the level on the di out is also gain dependent...

 

[davidchampoux]

I just want the DI signal level not affected by the master level pot, but I want it post eq, to provide my own colored tone (especially with the tube pre-amp!).

 

[beans-on-toast]

I just want the DI signal level not affected by the master level pot, but I want it post eq, to provide my own colored tone (especially with the tube pre-amp!).

I don't think that it would be easy. You want the signal feeding the DI to be buffered which is how it is done in the V4B and other amps. The SVT CL for example has the master pre-DI. The need to have it buffered so that there won't be impedance issues is the biggest problem. In the V4B, the master is integrated into the midrange circuit. Taping it off post midrange so that you would have all the tone circuits would include the master. One solution might be to move the master into the power amp but this creates problems as well.

 

[davidchampoux]

Depending on the circuit of the XLR out section (I suppose they use an IC for buffering and balancing, not a transformer), the Z might be high enough to be sourced somewhere just before the master volume without loading too much the circuit. It's what I suppose, or would like to try anyway... That's why I'm curious to see the actual schematics.

 

[beans-on-toast]

Loud isn't going to recommend modifying the amp but it never hurts to ask them. I think that if there were a cost effective solution to what you wanted to do, they would have implement it that way.

 

[davidchampoux]

Thanks a milion for that!

My guess is that they do post power amp DI to have the that particular color... A 3 way switch between pre eq/post eq/post power amp would have been fantastic, but that means higher $$ ...

I think it would be possible to lift R2 (in the XLR out section) and source it from TP3, or from the 'output' side of R93... At least, I would be curious to measure what's there in term of signal, and try it if I feel safe about it!

 

[beans-on-toast]

You'll never know how well any patch would work till you try it in the real world. I'm not a fan of a DI that isn't transformer based. Maybe if you used an active DI (such as a Radial that has buffering and a Jensen transformer) and patched that into the circuit.

 

[davidchampoux]

That would be a good idea. I'm also a lot more into transformers vs ICs ... A nice Jensen or Cinemag could do wonders, but would definitely requires additional buffering. My goal is to make the modification as simple as possible, and to allow to go back to the original config. NOTHING permanent.