Author Topic: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion  (Read 8475 times)

Offline ghostinthemachine

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!
« Last Edit: May 21, 2009, 03:53:22 PM by ghostinthemachine »
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Offline ghostinthemachine

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #1 on: May 22, 2009, 10:21:38 AM »
Is this topic too boring for the forum? No sex drugs and rock 'n roll?
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Offline iondb

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #2 on: May 22, 2009, 10:36:09 AM »
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.....

Offline ghostinthemachine

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #3 on: May 22, 2009, 10:50:57 AM »
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!
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Offline Prince

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #4 on: May 22, 2009, 11:18:12 AM »
 ;D

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

 8)
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Offline ghostinthemachine

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #5 on: May 22, 2009, 11:22:22 AM »
I think the best place to start would be with a simple dual triode line-stage circuit (to be used as a simple pre-amp in a system - between cd-player and power amplifier).
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Offline Prince

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #6 on: May 22, 2009, 11:27:06 AM »
I think the best place to start would be with a simple dual triode line-stage circuit (to be used as a simple pre-amp in a system - between cd-player and power amplifier).

-Ja, start small and work your way up. I have not done this type of thing for years. So. I'm very rusty with my electronics and electrical eng stuff.

 ;D

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Offline Ampdog

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Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #7 on: May 23, 2009, 03:44:21 AM »
There is other tube wizards that can contribute as well - Family Dog, Schalk, Mars to name a few...

Yes, I was just going to say ..

Thanks for the implied compliment, Gostinmac, but you may have answered your own need by the other thread you opened about the books scanned in by Mr Millet - a monumental task that must have been.

In a forum like this such an endeavour is perhaps rather large, if to be of any practical use. The brief series in AVSA was originated on another forum and appeared compliments of it: http://groups.yahoo.com/group/vintage_tube/, of which F_D is the owner. It was intended more for explanation of amplifier basics than a design brief, which was why no component values were given. (The final simple amplifier actually exists and shematics have been sent to a few enthusiasts round the world as a starting project.)

I know many of the books cited on Millet's site, and might give a few indicators as to which ones will be good to begin with. (They are all good, but not all provides a direct route to tube amplifier design.) Few would dispute that the 'bible' is, of course, Radio(tron) Designer's Handbook Edition 4, edited by F. Langford-Smith. (It has been a standard pre/post graduate volume at many universities.) But it is a huge work of 1500 pages, also fully covering r.f. Valve amp design is just a small part of it. Then I understood Morgan Jones is very good (have not seen one). As you mention the price is there; on the other hand little expense compared to what the hobby could cost in the end.

Anyway, I have been pressed for a compact set of amplifier designs using components available locally - we will have to see. (There already is a bewildering list of good/poor books on the subject, not to mention the internet, and I am certainly not planning to add to the latter kind.) I will go through Millet's list and suggest  a few.
« Last Edit: May 23, 2009, 04:32:27 AM by Family_Dog »
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Offline Shonver

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Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #8 on: May 23, 2009, 09:12:32 AM »
shematics have been sent to a few enthusiasts round the world (hic) as a shtarting project.

Bit early for a papsak, hey?
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Offline ghostinthemachine

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #9 on: May 23, 2009, 09:56:42 AM »
Mmmm... sometimes it feels like I am seeking "The Philosophers Stone"

I have downloaded plenty of those books on P Millets site and will most certainly study them good and well but I just thought we can get a casual discussion going (mostly in layman's terms) on what the function is of the basic parts in tube amplifiers.

It is very frustrating you know... wanting to understand this wonderful technology. Its like deciphering a foreign language (arabic or chinese) with the aid of a mutli-language instruction sheet on the back of a washing-powder box (or something like that)...  ??? :-[ :-X

:-[ :-[ :-[
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Offline Hi-Phibian

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Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #10 on: May 23, 2009, 12:22:59 PM »
I would say that a line stage is the place to start. Plenty circuits everywhere for power amps but available trannies dictate design/build choice more then preferred circuit layout for most DIYers I would assume.
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Offline Ampdog

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Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #11 on: May 23, 2009, 08:25:21 PM »
Bit early for a papsak, hey?

Well, I do declare......

Here is a man, not only reading my post at all but even in great detail (for which I am gratified), and furthermore, most singularly, finds for the slight slip a reason of grand culture, instead of as most of the male examples of the species might have done, in the sexual abarration contained therein.

Unique indeed ....
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Offline Ampdog

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Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #12 on: May 23, 2009, 09:23:30 PM »
Mmmm... sometimes it feels like I am seeking "The Philosophers Stone" ........  I just thought we can get a casual discussion going (mostly in layman's terms) on what the function is of the basic parts in tube amplifiers.

I think the main reason for such a sensation is that one has not started from the beginning. One would first have to comfortably understand how basic components operate, beginning with what a thermionic tube is, then how it is made to 'amplify' electricity using resistors and capacitors and what each of the latter two is doing/not doing. The fatal temptation in amplifier design (of which hi-fi amplification is a smaller specialist field) is to do the 'tweaks' before the basics are understood. (The classic tragic example of what can happen is the bad name 'earned' by NFB, because a certain lazy brigade started off at the end, leaving them floundering in misconceptions as they vainly tried to 'patch up' poor designs with layers of NFB.)   

From Hi-Phibian
Quote
I would say that a line stage is the place to start. Plenty circuits everywhere for power amps but available trannies dictate design/build choice more then preferred circuit layout for most DIYers I would assume.

I sense that this is different from what Gostinmac wanted; perhaps following on that - function of components before application to particular stages. (That is how I read his "basic parts of tube amplifiers" - components, not functional stages.) The function of a stage does not really depend on whether 'glass/sand' is used. Available transistors are fortunately more than enough not to limit audio design among themselves. But then I take it that the thread title mercifully rules out dealing with the vagaries of those little three-legged bugs!

For the former purpose (function of ...) something at least as complete as the first seven pages of the RCA TUBE MANUAL is required - can that be off-loaded only? The author of this thread will have to point out what he wants, and one must point out again that such a tutorial is going to be bulky, with many diagrams.

The purpose should not be to make physicists of DIY'ers, still the pitfall mentioned in the beginning should be observed.  "Ag, we don't need to know all that; we only want to design something like a Leak or a Mullard 5-20". ...... yah, sure: "Welcome, said the spider to the fly."

As Hi-Phibian said, many circuits exist (also poor ones) - do we start off with some off them and explain at the hand of which what each part is doing, or does one start as per RCA TUBE MANUAL's beginning or similar?
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline ghostinthemachine

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #13 on: May 27, 2009, 08:42:02 AM »
Okay... I sense a bit of "fear" in the air. Some might think this thread is going the same route as that other one that caused such a moerse commotion a few months back (me and an oscillator circuit for a turntable motor...) - well that is not my intention here.

I just merely want to "get the news out there" that a tube circuit is not that difficult to understand. I'll try and post my broken ideas on how it works (in accordance to what I read in the technical manuals).
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Offline ghostinthemachine

Re: Push-pull, Class A/B (and/or B) VALVE/TUBE Amplifier - Discussion
« Reply #14 on: May 27, 2009, 09:19:49 AM »
Okay, lets get things straight.

Who would like to see a description of how a vacuum tube amplifier works?

I ask this before I go, like a fool and post a long possibly boring, possibly senseless post of ideas.


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