Audio and Video Talk > Valves / Vacuum Tubes

Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion

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Curiosity got the better of me a while ago and I simply had to investigate the basic function of valve based amplifiers by all means possible. With a little bit of Googling here and replies from experts there I now have a better understanding how tube amplifiers work and how they can be designed. I am by no means an expert like honorable member Ampdog but strive to gain more knowledge. (I profoundly enjoyed Prof Potgieters' writings about power amplifiers in the AVSA a few editions ago)

I have also noticed one cant always reinvent the wheel but you can improve it some way or another.

A basic valve power amplifier can be broken down into the following sections.

INPUT stage (usually doubling as phase splitter)
PHASE SPLITTER (when input stage as utilized as buffer)
DRIVER STAGES (separate stages for each power valve bank, usually two each feeding a signal to the output valves 180 degrees out of phase from each other)   
OUTPUT STAGE (in push-pull amplifiers this would consist on a minimum of two valves, driving an output transformer)
POWER SUPPLY (basic HT & LT supply with no frills filtering - in some cases a Negative supply is also needed for biasing the valves in the output stage)

Output stage valves can be multiplied for higher potential.
Output stage valves can either be cathode biased or biased using a Negative supply (in reference to ground).
Output transformer can be UL tapped (I'm still studying its advantages)
Feedback supplied from secondaries of output transformer to smooth out any response anomalies.

If we may, for means of education of those interested in this matter (including myself), discuss how a circuit can be drawn up to produce a functioning amplifier.

I call upon Prof Potgieter to lead this discussion if he do not mind as he is most experienced in this field.

I think this can be most vital to those wishing to construct their own valve based amplifiers. It is easy to copy-build an amplifier from a schematic but its parts function is not always self-explanatory.
Questions might be raised such as:
"Why is bias needed?"
"How much gain is needed in the driver stage?"
"How do I choose the appropriate output transformer?"[/li][/list]

I am fully aware of wonderful books such as Morgan Jones' Valve Amplifiers but unfortunately these publications are not always in reach.

What else can be added at this stage? Maybe a basic parts list - common components that would make up an amplfier.

Lets say we want to use the following:

A pair of KT88 beam tetrodes
A pair of 12AU7 triodes for the driver stage
A 12AX7 for the input and phase splitter

Class type: A/B
Biasing: Cathode biased
Input: Single-ended
Estimated power potential: 45 watts

The field is now open for play... let the games begin!

Is this topic too boring for the forum? No sex drugs and rock 'n roll?

I am also interested in learning more about tubes!!!!!!!!!!!!!!  I have assembled a tube amp, but still don't know jack about them.
It will be nice to go in stages with this eg: input stage, how to build it, test it and then go to the next part of the amp.....

Yes Jakkie! Now were talking...

There is other tube wizards that can contribute as well - Family Dog, Schalk, Mars to name a few...

Lets put this forum to good use for a change!


Ja, a good introduction to tube amps, might just convince a digital junkie like myself to attempt building one!!



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