Class A Push-Pull Primary Impedance

[RF Burn]

I have been looking at the use of some neglected TV valves for use in the push-pull stage in an audio amplifier, however most data sheets only give a Class A single-ended primary impedance recommendation.

I am only interested in Class A PP at the moment.

Having attempted to work out how the necessary primary anode-anode impedance can be derived from the SE value I have arrived at the result:

PP Anode to Anode primary impedance = 1.414 (root 2) x the recommended SE primary impedance for a particular valve type.

So for a valve with a SE impedance of 5K requires a 7K CT transformer for PP, sounds about right?

Can anyone confirm this or have I got it wrong?

Adrian

[kalee20]

If a single-ended valve changes its Ia by 50mA while its Va changes by 250V, then the load needed is 5k.

If you have two of these valves in push-pull, driven in antiphase, then the current in one would decrease by 50mA while the current in the other increased by 50mA. Each half primary would see a 50mA change. However, as one anode voltage dropped by 250V, the other would rise by 250V. So total voltage change is 500V; current change is 50mA, giving you an anode-anode load requirement of 10k.

If you let yourself depart from Class A, you could reduce this load, 7k would not be unreasonable, and drive the grids a bit harder. At a certain level, you'd enter Class AB push-pull, the average current would start to rise, and you could get more power. But this is outside the scope of your request.

[Ed_Dinning]

Hi Adrian, there has been some work done on TV power valves for audio, but you should note that many of these valves were designed for pulse operation and the Vg/Ia curve is not very linear.
The alternative, that gives good linearity, is to put signal into G2 and bias on G1. A low impedance driver stage is then required but it seems to work well with low distortion. It does not effect the Rload the valve needs to see.
Quite a bit of stuff on the web about this mode of operation.

Ed

[trobbins]

Which valves are you interested in?

Philips liked to use horizontal deflection 6CM5/EL36 for vintage PA amps, and more recently many guitar amp diyers have furthered that with 6DQ6A and 6CD6G, as those valves are relatively more plentiful and cheap compared with standard types used in audio amps. I have a few vintage 6CM5 PA amps with up to sextet output stages and output transformers down to 1.2kohm PP. In class AB, they can generate a prodigious clean output power level.

But the details need to be addressed, including highish heater current ratings, setting a modest anode dissipation as datasheets often didn't include a rating for continuous operation (which is not synonymous with a significant class A power level), and a relatively low PP impedance.

[Argus25]

Originally Posted by RF Burn

I am only interested in Class A PP at the moment.



Adrian

Hi Adrian:

This question was all worked out by the good people at MIT:

So for maximum output the MIT staff recommended that the plate to plate load resistance in push pull class A operation, needs to be 2 times the individual tube’s plate resistances. In practice this theoretical value for the Rpp for tubes in push pull appears to be seldom used because rarely are tubes in push pull run in entirely class A as the potential power supply energy savings and increased power output possible in class AB is not realized. As the class of push pull operation shifts away from class A to AB and B, one tube has less and less of an effect on the load impedance seen by the other tube
as it remains cut off during the other tubes plate current cycle, which means the Rpp needs to be higher.


You can read about this topic on page 6 to 12 of this article:


http://worldphaco.com/uploads/UX-171...amplifier..pdf

In the early days (1930's) class A push pull was a recommended configuration by RCA, until it was realized that more power could be obtained with the same valves (keeping them within dissipation limits) and lower power supply consumption at lower volume, if the bias was altered toward class B.


Hugo.

[RF Burn]

Thanks for the replies! I admit to be totally confused by the information gleaned from data sheets and designs seen on the web, indeed the earlier Mullard/Philips data sheets gave data for Class A push-pull which worked out to actually be Class AB, maybe everything used to be A unless it was B?

So for Class A the PP Anode to Anode load impedance is twice the SE Anode load impedance. Got it!

Hugo, that UX171 amplifier info appears to be applicable to triodes only, as the internal resistance of pentodes/beam tets is very much higher than triodes.

I did find a web page that seems to tie in which published manufacturer's data for valves, and seems to cover about all aspects of PP output transformer design:
http://www.turneraudio.com.au/output...pp-calc-1.html

My original task was to design a reasonably good household amplifier using readily available and cheap parts i.e. nothing at audiophool prices.

Adrian

[kalee20]

Originally Posted by RF Burn

So for Class A the PP Anode to Anode load impedance is twice the SE Anode load impedance. Got it!

Absolutely! And for two valves in PARALLEL, the load required is half the single-valve load.

As valves aren't ideal - they have curves to their characteristics - don't be surprised if the manufacturers recommend somewhat different loads. But if the valves were ideal, with straight-line characteristics until they hit a zero of some sort, the ratio of double or half would be dead right.

In fact, if the valves were ideal, the optimum load for single-ended pentodes would be just Vak/Ia (with the proviso that gm is high enough that you can swing the grid to give from zero to double Ia without the grid ever becoming positive). In practice, about 0.9 times this ratio is about right, which allows for the fact that the characteristics have a 'knee' as the anode voltage swings down to 10% of the HT rail. This formula allows you to optimise your load if you want to under-run your valves to help them run cooler, or operate at HT voltages that makers haven't accounted for, etc.

Triodes are a bit more complicated because getting sufficient current flow, at low instantaneous anode voltages, without taking the grid positive, puts an additional limitation on the set-up.

Once you go push-pull, and consider Class AB, there are so many variables you can choose, that it is impossible to say what is 'optimum.' Luckily, you have not asked that question!