6B4G SRPP

[GK]

I’ve got a box of low power (5Wrms) PA speaker transformers with tapped primary impedances ranging 500-5000 ohms and secondary taps for 4,8 and 16 ohms which I bought a while ago for about 5 bucks a pop. I’ve tested them out and they work rather well – they are surprisingly flat out to 20kHz and quite happily reproduce asymmetrical audio signals with little distortion.
Problem is they can’t take the specified 60mA quiescent plate current of a single-ended 6B4G without saturating and they are not suitable for push-pull.
I’ve got a dozen or so 6B4G’s, so I’m thinking of making a stereo amplifier using a pair of them for each channel, SRPP connected with a regulated 500V plate supply (I have a few dozen 12E1 series pass tetrodes, so a few of these will come in handy here). The output will be capacitivly coupled to the transformer primary, thus circumventing the DC saturation problem.

So who do I calculate the optimal (Pout max) load resistance for a pair of 6B4G’s in SRPP then?

PS
And yes, I do know that this amplifier will be ridiculously inefficient with a rather low power output, but I don’t care.


Cheers,
Glen

[gingertube]

Search the articles on SRPP at tubecad.com - lots of good info.
Cheers,
Ian

[GK]

Thanks for the link. I’ve had a quick peruse of a few SRPP-topic blogs on that site so far, but they don’t address what I’m trying to do. I’ll keep searching.

Anyway, attached below is a basic diagram of what I plan to do. An individual floating regulated DC supply powers the heater of each tube.


Cheers,
Glen

[Tweeker]

A 6B4G SRPP should be able to manage 5+ watts out.

The totem-pole Amplifier. Taub&Millman, Pulse and Digital Circuit, Cap.3, Linear Pulse Amplifiers, pp. 99-101, McGraw-Hill, 1956. This has the math you need.

[Tweeker]

Is a 590V supply possible, you have cathode bias to account for. Theres also a fixed biased variant you could rather use.

[Tweeker]

For a 500V supply and cathode bias I come up with this:
Vbias=34V, Ik=67ma, Rk=510ohms, mu=4.2, gm=5350mhos.
Anode=216V, plate dissipation=14.5watts.

Rk=1/gm+2Rl/mu

Optimum load =~680ohms.

[GK]

Originally posted by Tweeker
A 6B4G SRPP should be able to manage 5+ watts out.

The totem-pole Amplifier. Taub&Millman, Pulse and Digital Circuit, Cap.3, Linear Pulse Amplifiers, pp. 99-101, McGraw-Hill, 1956. This has the math you need.

Originally posted by Tweeker
Is a 590V supply possible, you have cathode bias to account for. Theres also a fixed biased variant you could rather use.

Originally posted by Tweeker
For a 500V supply and cathode bias I come up with this:
Vbias=34V, Ik=67ma, Rk=510ohms, mu=4.2, gm=5350mhos.
Anode=216V, plate dissipation=14.5watts.

Rk=1/gm+2Rl/mu

Optimum load =~680ohms.

Fantastic! Thanks for that. You're right about the plate voltage. I also made an error on my scribble - the anode of the lower tube idles at +250V, not the cathode of the top tube. With cathode resistor biasing and a +500V supply, both tubes effectively run with a plate voltage of 205V. This should still work fine, but in order to get the most out of the tubes, I'll run +545V with fixed bias on the bottom tube. That will give me 250V across each tube and will do away with the need for a large bypass capacitor.

Now I can log-off and start calculating the load impedance

Cheers,
Glen

[Tweeker]

For 60ma bias resistor is actually around 583ohms, and anode is at about 215V. 35V is across the bias resistor, which is lost from the anode voltage.

The fixed bias variant requires some other changes, linked article details. I think a mixed bias version might not push-pull as evenly.

[the_manta]

Hi !

Tom Schlangen, a German guy recently built a Totem Pole amplifier. It can be seen here: http://f23.parsimony.net/forum45451/messages/202764.htm

Frequency response is amazing !

Reagrds, manta

[Tweeker]

Id use the lowest expected case for the load, ei with a 8 ohm nominal speaker, something more like 7ohms.

Edit: gm is is 5350micromhos at indicated conditions, not 5350mhos. Used RCA 2A3 data, 6B4G might be slightly different.