PY 81 as power rectifiers?

[Kalee20]

What I seem to be settled with is the ECL80, despite its shortcomings, a pig to bias, and it's liability to take off at the least excuse, it has a low heater consumption, is not OTT for my application, and, I kind of don't like to be beaten by a dumb valve!

Interesting you find it takes off at least excuse. I have found the same, using the pentode as an RF amplifier and the triode as a grid-leak detector, on medium waves (sometime I will finish this project).

As Kalee20 kindly sent me a small SE transformer to suit my needs I built another version with the ECL80, with no balance feature needed it could be compared with the mains transformer version.

Its rather good!

The frequency response is better, though not going down quite as far as the mains transformer version it appears.

Frequency response will be 3db down at 72Hz, that is due to your 2.2nF capacitor from triode anode to pentode grid, plus the 1MΩ grid resistor of the pentode. It will also be down 3db at 155Hz due to the output transformer's inductance (or lack of!). The two together do not give a good bass response.

On the other hand, the treble response could well be better with the proper output transformer. Mains transformers keep the primary and secondary windings well apart, for safety reasons. This really hurts the treble response. The output transformer does not need to have such separation, so the coupling from primary to secondary is better. Glad you like it

Also it produces approximately a third more power for three quarters of the current consumption....

That is probably because the component values you have used are not optimum values. If it works, fine! But for more current, you ought to get more power. (But you do need to have the right output transformer, in that case).

I will tidy this up for some glamour photos to post when I start putting all the elements of the project together.... Hopefully!

It will be the most complex thing I've built....

Can't wait for the photos, I'm drooling already  ...

A neater circuit ought to work better, anyway. But it has to be the right sort of neatness. People have been known to 'tidy up' a circuit, and find it does not work as well... sometimes instead of components tacked across from point to point with short leads, they lay them out in neat rows and the extra stray capacitances upset everything. So neaten-up with caution, and throughout the dressing-up, keep trying to make sure it still works.

So, I've learned a lot.

Congratulations! There's lots more to learn. My supply of 'things to learn' shows no sign of running out, and I've had a career in electronics

[Terry]

Congratulations on your progress so far - you are doing very well indeed!

Recent experiments with the balance circuit earlier in the thread using a half wave power supply (as I intended to use), show that it CAN be able to null hum, but that feature isn't coincident with the standing current balancing that I was aiming for.

I think the reason for that is that the two windings on the mains transformer are the same whereas in the Grundig circuit that you borrowed the idea from, the ratio between the two sections will have been designed to null both at the same time. Not much you can do to improve on that but full marks for trying.

The frequency response is better, though not going down quite as far as the mains transformer version it appears.

Yes, as Kalee20 has already pointed out, 2n2 is rather a small value for an audio coupling capacitor - I would be inclined to experiment with 10n being the minimum and increase that if the bass response is still lacking

I'm including the update on the attached diagram, one erratum is the cathode capacitor is 470uF not 47uF as the diagram, its needed to stop + feedback.

That's very big for an AF cathode decoupler! Particularly as you seem to be experiencing HF rather than LF instability.

It might be worth putting another resistor in series with the triode grid leak, 100k, say, or whatever comes to hand, and adding extra decoupling there instead. It would be much more effective than the current arrangement.

Also, setting the triode bias to 50% of the pentode figure seems a bit high to me - perhaps others might like to comment?

Keep up the good work - you really are doing very well!

[Amie]

Thanks guys.

I will try your ideas on the coupling capacitors, the 470uF was at hand but it seems the trouble was because of the cathode resistors, I will try some different decoupling and possibly a grid stopper for the triode too, it may help. Also if I get the grid biased spot on it may help.

Is there any reason a preset pot couldn't be used to set the bias empherically (sounds better than experimentally)

Thanks,
Amie.

[boater sam]

You have a PM Amie.
Sam.

[Kalee20]

I'm including the update on the attached diagram, one erratum is the cathode capacitor is 470uF not 47uF as the diagram, its needed to stop + feedback.

That's very big for an AF cathode decoupler! Particularly as you seem to be experiencing HF rather than LF instability.

I will try your ideas on the coupling capacitors, the 470uF was at hand but it seems the trouble was because of the cathode resistors, I will try some different decoupling and possibly a grid stopper for the triode too, it may help. Also if I get the grid biased spot on it may help.

In this application, 470μF is quite reasonable. If it's to hand... use it. Modern 470μF capacitors are not expensive and not big, either, even if 'traditionally' smaller values were used.

There is a good reason here for going big, however. If you have insufficient decoupling, there will be feedback from the pentode to the TRIODE, because of the common cathode, and this feedback will be positive. You will get motorboating. Without the capacitor, the circuit forms a cathode-coupled multivibrator. Snip it out and try it!

Is there any reason a preset pot couldn't be used to set the bias empherically (sounds better than experimentally)

None whatever! Sometimes you can't beat a good twiddle with a screwdriver. I strongly encourage it - you will get to hear, and recognise, the sound of different types of distortion caused by under-biasing and over-biasing. But I'd suggest something like the sketch, so you don't need to use a preset pot which is rated for the full cathode current.

[Terry]

I will try your ideas on the coupling capacitors, the 470uF was at hand but it seems the trouble was because of the cathode resistors, I will try some different decoupling and possibly a grid stopper for the triode too, it may help. Also if I get the grid biased spot on it may help.

Ah, I obviously misinterpreted your previous post - I thought you meant that you couldn't stop the instability unless you used such a high value.

There is a good reason here for going big, however. If you have insufficient decoupling, there will be feedback from the pentode to the TRIODE, because of the common cathode, and this feedback will be positive. You will get motorboating. Without the capacitor, the circuit forms a cathode-coupled multivibrator. Snip it out and try it!

Agreed - which is the reason I suggested the additional decoupling at the earthy end of the grid leak as it will be considerably better at isolating the triode from the pentode.

Agree also about the adjustable triode bias suggestion. I would suggest that Amie implements both suggestions together to cover all bases.

[pwdrive]

Hum cancelling....So...What if anything, went wrong ?:

https://www.radiomuseum.org/forum/grundi...guage_id=4

Lawrence.

[Amie]

Hum cancelling....So...What if anything, went wrong ?:

https://www.radiomuseum.org/forum/grundi...guage_id=4

Lawrence.

Thanks for that link Lawrence, in the first instance the little Grundig sets audio output stage reminded me that I had read Bernhardt Kainkas article (http://www.b-kainka.de/roehren/gegentakt.htm ) on self split and balanced class A circuits, that allowed the use of interleaved mains transformers.

Nothing really went wrong, I was trying to balance out the standing current in the mains transformer, to stop saturation, that worked OK, then I tried a half wave rectified supply rather than the bench PS to deliberately introduce ripple to test the hum cancelation theory, and that worked too, but not at the same "balance" setting. 

When I tried a proper gapped transformer, that had been kindly donated  the circuit was improved regards efficiency, and  sound quality.

With the mains transformer I used with its equal windings the current in the balance side had to be around 25% (4mA) of the anode current for it to work, not what I was expecting. The hum cancelation needed only about 500uA, increasing above that made the hum worse....

As others have said, the ratios of the windings have to be in proportion to the ripple on the supply AND the respective currents.... It work well if you get it right, but, makes things worse if you get it wrong. Also I don't think both effects could be reconciled without resorting to push pull output....

Or at least I'm not clever enough to do it

Amie.

[Kalee20]

Nothing about being clever enough to do it, if a thing ain't possible it ain't possible! Almost certainly this one isn't, as you are trying to balance two different things at the same time. You can't, without increasing the AC impedance in the 'balance winding' circuitry over and above the DC resistance, and that implies adding a choke. And if you're going to add a choke, then you might as well use it for smoothing...

However, drawing 25% of the current through the balance winding and finding an improvement is a bit surprising. It's possible that ultrasonic oscillations exist (driving the valve highly non-linear)  but drawing current and putting a load on the balance winding is applying sufficient damping to stop it, so you get a sudden big increase in quality. Terry's suggestion of grid stoppers is a good one: for maximum effect, the grid stopper resistor needs to be connected closely to the grid pin (10mm max of lead) with the other lead of the resistor being as long as you want (within reason). This won't hurt things, but can help. It's the work of a few moments to add 47kΩ or so of resistance directly in series with each of the two grids.

Looking forward to the next instalment! 

[pwdrive]

Hum cancelling....So...What if anything, went wrong ?:

https://www.radiomuseum.org/forum/grundi...guage_id=4

Lawrence.

Just to be clear, I meant what went wrong in the radiomuseum article insofar that his calcs didn't stack up to what he thought they might.

Lawrence.