• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Poinz Musical Machine - which class is it?

Status
Not open for further replies.
Jump to Latest
D

dannoto

Member
Joined 2016
#1
Guys,

Sorry if this question might sound simplistic, but I am new to vacuum tubes and for my first project I am building Poindexter's Musical Machine. The original Push-Pull design using 5965s and 6V6s.
Which class of amp is it? Class A? Class AB?
How can I tell?

Any help is appreciated. Thanks,
 
#6
Probably not class A (few push-pull amps are) but the only way to be sure is to draw a load line, and that is a good exercise to go through because it lets you see what the optimal screen voltage would be if run as pentode.

Any push-pull amp at any operating point could theoretically be made class A with a sufficiently light plate load. Do you know the output transformer impedance? That would settle the question for sure.

But I'm almost certain it is class AB since Class A push-pull amps are rare beasts.
 
#9
dannoto said:
Poindexter's original design notes specify that 7.2K to 10K primary impedance output transformers can be used. For my implementation I selected 8K transformers.
Does this help?
Click to expand...

That's good. Do you understand what I posted about the amp running in Class A until the volume level rises enough to make the amp push the output tube into cutoff?
 
#10
dannoto said:
Poindexter's original design notes specify that 7.2K to 10K primary impedance output transformers can be used. For my implementation I selected 8K transformers.
Does this help?
Click to expand...

I'm very surprised that so many people are willing to weigh in and definitively answer without really looking at the design carefully and doing the analysis. 🙄

Let's do the analysis. It looks like the Musical Machine runs the output tubes as triodes and is not capable of driving the grid positive, due to the coupling capacitors, so clipping will be determined by when a tube hits the Vg=0 line. The schematic I found shows 305V B+ and -20V grid bias and a 10k output transformer.

I just grabbed a JJ 6V6 datasheet and drew a load line. First off, I noticed that at the specified bias and B+, we get ~46mA at idle. We also notice that this design is cooking the tubes a bit at slightly above their rated dissipation (actually the JJ datasheet specifies 10W limit for triode 😱). I hope you don't mind changing output tubes fairly frequently.

With a 10k transformer, each tube works into half of the primary turns, which would be a 2.5k load but since both are working in parallel, each tube sees a 5k load while both are conducting. If either tube cuts off, the other tube will suddenly see a 2.5k load. The question is, can we get to a point where that will happen at any signal level before grid current clipping?

With the 5k load line we get about -135V of plate excursion with +20V signal applied. With -20V signal applied we get +120V of plate excursion but we still have a little above 20mA of current flowing in the tube. We aren't even close to cutoff in one tube when the other is clipping due to grid current.

I did a load line for an 8k transformer and I got a similar result where Vg=0 clipping would occur in one tube before cutoff is reached in the other. Still almost 20mA flowing in the tube at clipping.

So the answer is yes, this is a class A amplifier when used with an 8k or 10k transformer, and by quite a bit of margin actually. With a more common transformer like a 4-5k, it might very well be class AB at this operating point. Again, you would want to draw a load line and do the analysis to be sure.

Edit: By the way, the answer would be completely different if these output tubes were run as pentodes, since clipping would occur at a much lower plate voltage. One tube would still be amplifying away while the other was in cutoff, since it would have more headroom to do so. What makes this design class A is the limited plate movement of the triode and the high load impedance that the 8-10k transformer presents.
 
Last edited:
#11
SpreadSpectrum said:
>
So the answer is yes, this is a class A amplifier when used with an 8k or 10k transformer, and by quite a bit of margin actually.
.
Click to expand...

Agree. If this is a triode mode amp, it'll never run outside of Class A with that bias voltage. You'd need to run the bias down to -30v to push out of A. But I wouldn't bother. It would start sounding rough at higher input levels which you would be tempted to use with this being a triode amp, And the drivers may not even be able to go that high anyway.
 
Last edited:
#12
I wonder how this design would fare with a CCS in the tail of the 5965 (a la Baby Huey). It could probably be powered by the -20V fixed bias for the 6V6s.

Actually I have a stash of 407A's with 20V heaters, too. Hmm...20V fixed bias, 20V heaters, 20V CCS supply. Curious.
 
Status
Not open for further replies.
Top Bottom