PP Amplifier output impedance matching

[boater sam]

Oh, please no Mr, just Sam, we are not so formal on this forum.

If you want any TV, P series valves I have thousands of "pulls" that are going to auction on Saturday. If you tell me what you can use you can have them for free, just £3 postage but be quick!
Sam.

[Terry]

I do have EL84s (new production ones) and a couple of transformers for those, but that's totally OTT!

Not necessarily, Amie!

If you can find an EF86 to go with one of your EL84s, this popular design is well worth building - as many others here will confirm.

You can tailor it to suit yourself - for example, if you want to keep the treble cut tone control but not the bass control, simply omit RV11 and C7 (as in the second version). I would also add an input socket and an Internal/External switch between the amplifier and the radio tuner for added flexibility of use in the future.

[pwdrive]

If you haven't already seen it, in these data sheets is shown power output versus distortion for two different sets of DC conditions for the 12AU7/ECC82 in push pull:

http://www.mif.pg.gda.pl/homepages/frank.../12AU7.pdf

Lawrence.

[ppppenguin]

Not back home until Friday but I'm sure I have plenty of NOS boxed PCL8x which you can have for price of postage.

[Terry]

Amie, something you should be aware of when building audio amplifiers is the avoidance of earth loops. These can cause all sorts of problems, particularly hum.

This advice comes from an idiot who once found a suitable chassis to build a Mullard 5-10 on and dived in with the soldering iron. When completed and switched on, the most appalling din emerged from the loudspeaker!

Much humbled by this experience, I carefully unsoldered all the bright new components I'd used and rebuilt it following the Mullard published layout which uses an earth bus bar - 16 swg (~1.7mm dia.) tinned copper wire - for all the earth connections, the bus bar only being connected to the chassis at one point.

The next time it was switched on, only a gentle hiss emanated from the speaker ...

[Kalee20]

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.

Easy part first - in Class A push-pull, you need the transformer to match double what each individual valve would require.

So if a valve single-ended requires 6kΩ, then two valves in push-pull would require 12kΩ anode-anode. This is because you have the same current as a single valve, but double the voltage swing between the two anodes. (If you had connected them in parallel, you'd need half the individual or 3kΩ, makes sense because you'd have the same voltage swing as a single, but double the current).

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.

It is unlikely, but it nevertheless works - I have done it!

Now, the harder part - what IS the load that a single-ended valve needs (so you know what to double for P-P)? Triodes are fairly uncritical so easy to match, but surprisingly hard to match properly.

If your signal voltage is fixed and very small, then the load for maximum power is simply the ra (anode resistance) of the valve. But that is an unusual case - generally, the signal is not fixed and if changing the load changes the power, you just twiddle the volume control to get back to where you were. The question then arises, what is the optimum load if each time you alter it and readjust the volume control up to the onset of distortion?

Triodes start to distort if the signal voltage is large enough that the grid voltage reaches zero in the +ve direction (because grid current flows and clips the input signal). They also distort in the -ve direction if the anode current reaches cut-off. Ideally, you want these two things to kick in at the same signal level.

It turns out that for a particular valve and a particular HT voltage, there is an optimum Ia (standing anode current) for maximum power, equal to Va / (4 x ra). More than this and the bias is so close to zero that you get grid current at small signal levels, so power is curtailed. Less than this and you are closer to cutoff before you even start applying a signal. At the optimum standing Ia, the optimum load is Rl =  2 x ra, for a single valve.

But that is not the whole story either. If you bias your triode for this optimum Ia, you may find yourself exceeding the anode dissipation. So you have to bias it farther back anyway (obviously giving Ia = Pa / Va where Pa = max power dissipation the valve can withstand). In that case, maximum power is reduced, but so is the standing current, and it turns out that efficiency is improved a bit. The optimum load is given by (Va/Ia - 2ra).

Once you have the single valve sorted, just double the load for push-pull.

One of the best textbooks for this is F Langford-Smith's Radio Designers' Handbook.

Once you have your optimum load, then matching to a speaker can be achieved with a small mains transformer if it has primary windings for 0-120V-240V. You use the 120V tap for HT and the 0 and 240V terminals for the anodes. If you need 12kΩ anode-anode and want to match to a 8Ω loudspeaker, then the turns ratio will be √(12,000 / 8) = 38.7:1 so your mains transformer will need this ratio. It is easy to see that a 240V in, 6V out transformer has a ratio of 40:1 so it will get you going and while not hi-fi, you will get decent results.

[Amie]

Hi

Thanks so much everyone for the tips and information, I've been hunting for the se gapped transformers, but no luck. I'm going to carry on with the pushpull idea for the project, and perhaps try the little triode pentodes.

Amie.

[Mike Watterson]

You can use a mains transformer for SE, rule of thumb is to use about twice the power rating so the bias current doesn't saturate the core. Works well using say a 6VA transformer for a 1W amp. Only use EI type, never toroid.
However PP is superior as you can be class AB or class B (more efficient) and the 2nd harmonic distortion in the O/P valve cancels, less iron is needed and no airgap needed. Gives more power than paralleling two class A tubes.

[Ed_Dinning]

Hi Amie, it is usually quite easy to dismantle a mains transformer an re-stack the lams so they are all the same way. Then add a layer or sellotape across where the E & I lams butt and you have your gapped transformer for SE use.

Ed

[Amie]

Hi Amie, it is usually quite easy to dismantle a mains transformer an re-stack the lams so they are all the same way. Then add a layer or sellotape across where the E & I lams butt and you have your gapped transformer for SE use.

Ed

Hi.

Id like to try this.

I looked at a small 6va transformer that wold be suitable ratio wise, that I have a few of, but it looks as if it has been coated in a lacquer finish. Is there any trick to get the fIrst few llaminations out?

Thanks,
Amie.