PP Amplifier output impedance matching

[Amie]

Hello everyone.

I need a bit of clarification please, I'm experimenting with a project that requires a small audio power amplifier, I've decided to investigate a self splitting class A pushpull output.

Because for no better reason than it's unusual, and I want to learn.

Here's the Q.

In a pushpull output, does the transformer need to present an Impedance on its entire primary to match the individual R.a. of the valves used, or does it need to match the R.a. in each half of the winding?

The texts I've so far read on the subject have been written with ambiguity.

My intention is to use modern production ('cheap' China) twin triodes, in the design, so 12AU7s for this part of the job, even if it means paralleling the sections up to lower the r.a.

I understand that using triodes will mean less critical impedance matching.

Thanks.
Amie.

[pwdrive]

If you haven't already seen/read it have a look at this:

http://www.tubebooks.org/tubedata/RC25.pdf

It's one of the editions of the RCA Receiving Tube Manual, scroll down to book page 28, it goes through triode Class A power output both single ended and push pull for power valves and shows how to determine anode to anode load etc. 

It might or might not help....

Lawrence.

[Ed_Dinning]

Hi Amie,. a good subject to study and all the answers will be on the tube books site, even if a lot of digging will be required.

The approximate way is to look in a valve data book. This will give a value for Ra (anode load impedance) as well as Rl (load inpedance). This is normally lower, and is the figure about which your output transformer is designed to match to the loudspeaker. The valve data books then show the anode to anode load impedance for a pair of valves in push pull. This figure is usually twice that of a single ended system.
If you parallel valves to reduce impedance further then ohms law applies; 2 valves in parallel 1/2 impedance, 3 in parallel 1/3 impedance.

Note that this applies to valves operating on sensibly the same DC conditions. If the Dc conditions change then the values of Ra and hence Rl will also change. The tubebooks website will explain how.


Good luck, Ed

[ppppenguin]

The 12AU7/ECC82 seems a rather unlikely output valve. Single ended or pp. But then the EF80 has been used for audio output in some TVs and that's even less likely.

In pp the transformer is less critical as there is no net DC in the primary. A mains transformer can often work well.

[Mike Watterson]

It will certainly be more power than a pair of DL96 O/P valves.
There are other cheap valves, inc the Russian version of the ECH81.
A triode preamp and Pentode / Hexode O/P can be compared with vice versa.
Generally a Pentode is better than a Triode for O/P. The exact matching is only critical for squeezing full power. PP is good as you can use 115-0, 115-0 mains TX as primary and then a secondary to suit loudspeaker load and impedance. The voltage ratio SQUARED is the speaker impedance to Anode impedance ratio.

[ppppenguin]

Another way to do pp output on the cheap is a pair of PCL8x TV valves. A pair of PCL805 , the later version of PCL85 , should deliver a good few watts in pp. The PCL86 is even more capable but may be more expensive. The main disadvantage is the odd heater voltage.

[Amie]

Thanks all for your comments.

So, I've been led to believe that to provide a suitable load for the output valves, anywhere between Ra and 4Ra is required for Impedance matching from the full primary of the output transformer, depending on which author you believe.

The RCA manual agrees with the 4Ra, and a small commercial amplifier design that I stumbled upon using those small triodes has a primary of 25K with the specific secondary load.

I had a transformer and speaker in mind for the job but the combination would only give a 12K primary load, half what's needed for it.

I could go with using two valves with both sections in each in parallel?

Use a lower supply?

Use different valves?

I've just been told that the PL81 is an excellent choice, but I don't have any of them!

I have had some successful experiments with PL36's at lower voltages, with a 0-115 0-115 to 0-6 0-6 transformer, but the project would double as a room heater!

Amie.

PS  ra=anode impedance vs. Ra=anode load, where I've referred to them. 
PPS if I get something repeatable and made with available parts I'll post in the homebrew forum.....

[boater sam]

A good and sensible question, I was confused for a long time myself as to whether the impedance of each half winding had to match one valve and reading the literature there still seems to be some uncertainty.
The perfect matching is an irrelevant unless you are looking for the "perfect sound" beloved by audiophools. A mismatch of 50% will make little or no effect to the ear which is an imperfect organ and gets worse as we age.

Could I suggest PCL83 valves as they are cheap as chips being TV valves of which there is a glut and the heaters are an easy to satisfy 12.6v?

Best of progress with your experiments, very encouraging.
Sam.

[Ed_Dinning]

Hi Amie, plenty of P series valves available but sometimes strange heater voltages, let me know if you need any mains transformers for these voltages, or use a 24v transformer with a suitable dropper resistor, also a good range of oddball valves you are welcome to.
Cheers, Ed

[Amie]

Hi Mr Sam.

The PCL83 sounds interesting, they would work in with the project well with the heater voltage of 12.6, the triodes could have the bases wired for this voltage too.

Probably would have to look for new ones, (I don't think I have any) SH would have had a hard time if they have been used for the intended purpose?

I'm not really looking for a great deal of power, a couple of watts would be more than enough. May have plumped for a single ended design if I could have found an old gapped transformer, dad had several new ones meant for replacements in TV sets (probably not great quality) but I've not found them amongst his treasures, yet.

I've been having fun with this self split circuit though, the lash up produced a nice sound. Uncle Greg has been helping with LT spice simulations, that's been helping visualize what's happening, and avoiding what doesn't work.
I do have EL84s (new production ones) and a couple of transformers for those, but that's totally OTT! I may end up saving that idea for a guitar amplifier.

Mr Dinning.

The project, a VHF radio, (I've got the signal stages working well), can make use of two ECC81s a PCF80, or PCF82, the heaters are a bit awkward on the last two. Your suggestion of a transformer/dropper resistor may be the best way of using odd P series in a series chain....

Thanks for the offer of help. I'll be sure to shout if I get stuck...I have to ask permission to PM....

Regards,
Amie.