Re: re Vibroverb RI Class A ?

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

> 
> Even if the amp isn't biased for class A it will run class A at lower 
> volumes.  The reason being that the tube won't get driven into cutoff at 
> lower volumes and will pass current for 360 degrees of the cycle.

Absolutely true.  Most push pull stages are biased class AB1 or AB2.  
when operated at full output, the negative going output is clipped.  when 
operated at a low volume, it is possible to run entirely within the idle 
current envelope at what amounts to class A.

> I was 
> thinking in pairs of tubes then switched to singles.  Hence the error.  

I assumed as much.  An easy oversight to make, especially in view of the 
many tube circuits using parallel tubes, e.g. high power RF, etc.

> With a center tap transformer the turns ratio would remain the same.  I 
> still think that if you  just pulled the tube it would work at lower 
> volumes but better yyet you could change the first preamp tube to a 12at7 
> from a 12ax7 and lower the preamp gain ensuring that the remaining output 
> wouldn't be driven into cutoff.  Would I do this?  No.  Buy a Princeton 
> or Champ is the obvious solution. 

I'd have to dig out some very old tube manuals to check on the 12at7 vs 
12ax7 thingie (it's been a while, folks :-)

But I think the reason folks yank one of the push pull output tubes is so 
that it operates in the nonlinear region, producing distortion.  This 
would also imply that a lower gain preamp would not have the desired effect.

But if one wanted a CLEAN amp, what you're saying is absolutely true.  
And there ARE reasons to run a class A single ended amp in "clean" mode.

And of course I agree about buying a Champ if that's what you're trying 
to do (convert a push pull amp into a single ended class A)

One other option - I haven't looked into this seriously, but one _might_ 
be able to bias a Champ into class B to get more distortion and power.  
But let me check the power supply ramifications first before making a 
recommendation!  I believe less overall power would be dissipated in 
everything, but I might be wrong here, too!  Maybe a switch to reset the 
bias for class AB1?  Of course, the driver would have to be capable of 
swinging more than double the drive voltage for straight class A.  Class 
B would gain some efficiency, although running it at class AB would eat a 
little.  But you wouldn't want to run it straight class B.  Soft passages 
would definitely sound broken up.

And of course, the little speaker may not be happy with more power!

> BTW There is some Little Walter stuff 
> where he sounds awful.  It's on the Chess records release of Super Blues.  
> It was recorded just before he died.  Bo Diddley, Muddy Waters, and LW 
> together for the first time.     

Excellent point.  And you're absolutely correct.  Not everything Walter
did was up to snuff.  During his last years, he was noted for having
substance problems.  Substance abuse will ruin even the best players.  but
that's a whole 'nother topic. 

but when he was sober and clean, everything he did was excellent or 
better.  (I suppose I should qualify my blanket statements, especially 
with size 13-EEE's 8-(L)

A question to the group: is there a lot of interest in this topic?  I know
it's not everyones cup of tea, but is there a reasonable "audience"?  Is
it reasonably understandable?  (I know a lot of it is highly technical,
but both of us have tried to make it understandable to "the average bear",
as Yogi is fond of saying :-) Do you amplified harp players feel there is
benefit in it?  Or should we just move it off to a private discussion? 
Public or private responses are fine.  And thanks in advance.


 -- mike curtis
wd6ehr@xxxxxxxxxx