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jrme 23rd February 2010 01:05 PM

Configuring an AB class amp to work in pure A class

I have always been using solid state amps. In my experiences, I have reached the conclusion that pure class A sounds better than the others. Then, I am currently using pure class A amps since a long time.

Now I have bought a vintage tube amp, Pioneer SM-83, which is working in AB class. I am wondering if it is technically possible to make modifications on it, in order to bring it to work in pure class A only.

Do you have any experiences in transforming an amp in such a way?

In advance thanks.



Arnulf 23rd February 2010 02:46 PM

Of course it is possible - adjust the load (change the output transformer) and adjust the bias point so that both tubes always operate in conduction (neither ever goes into cut-off).

Existing transformers might work, depending on how far into class (A)B the output tubes are biased.

richwalters 23rd February 2010 03:31 PM

In my 1950's service book, the cue to class A tube amps was to set the quiescent current so the anode +screen is at the max quoted dissipation figure (often red hot anodes). In reality this is somewhat true as the anode heating power is reduced when driven hard. However, it is also the quickest way to shorten tube life. Others may have other ideas.
Some of the cheap mass produced early radiograms did operate at very high quiescents = high dissipation and often close to being unstable.

DougL 23rd February 2010 04:04 PM

In general, you need to lower the Voltage and increase the current in order to change from class AB to A, and not destroy the output tubes. Also changing the load can help.

It may be enough to run the 8 ohm speakers on the 4 ohm tap, and modify the bias and power supply.

I would recommend the TubeCad PP calculator to give you a simulation of your output before you make changes.


jrme 23rd February 2010 05:19 PM

Great! Thank you very much for your answers, I will try the ways you explain.



m6tt 23rd February 2010 06:31 PM

Another way of thinking of it is that you need to put the idle current at or slightly above (toward current) the center of the loadline, and the waveform cannot cause cutoff on negative cycles. Equal headroom up and down. Unfortunately, this is point is usually very close to the max dissipation line. This makes the loadline critical, since the idle point needs to be inside the max dissipation line, requiring a lower load and higher current/less B+.

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