Re: re Vibroverb RI Class A ?

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

> 
> Lets think about the output impedance problem that occurs when you run a 
> Vibroverb with just one of the 5881 output tubes.  Normally with the two 
> 5881's the plate to plate primary impedance is going to be 4200 ohms.  
> Assuming that the output transformer is wound for 2 10" 8 ohm speakers 
> wired in paralell for a load of 4 ohms nominal we find that the turns 
> ratio for the stock transformer would have to be 16.2 to 1.  Running just 
> one of the 5881's in the output doubles the plate to plate impedance to 
> 8400 ohms.  The problem with this is that running the stock speaker 
> configuration the output tube is now running into a 4200 ohm load when 
> it wants to see 8400 ohms.  

oops!

There is one fatal flaw in this scenario.  You've assumed the output is
parallel.  You sound like you know your theory, so I assume you've simply
overlooked the fact that it's push pull.  

I'm sure the rest of this is obvious to you, but I'll cover it for the 
sake of the group.

Removing one tube does nothing to the impedance, as each push pull stage
uses half of the primary.  The turns ratio is still exactly the same. 
We've only disabled what can be looked at as a separate amplifier stage. 
Only one half of a push-pull amplifier stage is active at any instant. 

Removing half of the push pull amplifier does not move it to class A bias. 
This would have to be done elsewhere, and is probably not a good idea for 
hardware designed to be run at class B bias.  Class A bias is biasing the 
tube to always be passing 50% of the maximum current.  This allows it to 
amplify both positive and negative half cycles.  Class B is when the tube 
is biased just at cutoff (zero current), and amplifies only the positive 
half cycles.  By using one tube fed 180 degrees out of phase, and feeding 
the opush pull transformer 180 degrees out of phase, we have an amplifier 
that amplifies the negative hald cycles.  Put these two together and you 
have an amplifier that amplifies both positive and negative hald cycles 
very efficiently.  Class B is a lot more efficient than class A, because 
you're not dissipating the 50% idle current.

The whole purpose of removing half of the push pull amplifier is to 
get a distorted output ("raw" class B amplification.)  Just because an 
amp is single ended does not automatically make it class A :-)

> ps while I thought about the above post before i sent it the possibility 
> of errors exists.  Corrections accepted with a smile.  After all it's not 
> a contest.  Is it? 

nope - it's a discussion forum where we ALL benefit.  The more knowledge
we ALL have as a group, the more harmonica becomes recognized as a serious
instrument.  Yes, anyone can play one.  But to really play one well
requires incredible skill, practice, determination, ability, and maybe 
even anatomy.  Many vocal techniques also apply to playing harmonica.  Ask 
a good vocalist about singing from the diaphragm, or properly opening the 
mouth to resonate the voice, etc.  I theorize this is why so many of the 
better harmonica players are also better than average vocalists.

 -- mike curtis
wd6ehr@xxxxxxxxxx