PY 81 as power rectifiers?

[Amie]

Yes Terry, my intention was to use the resistor anyhow, in the experiment I put selected values between the second winding and chassis (OV) to observe the effect, as in post #26, I then tied the capacitor across the best resulting resistor and the results were inferior. So shunting the induced (winding 2) audio to ground isn't such a great idea.

And yes it went black and started to rain here just before five, its definitely not bikini weather any more....

Diagram to follow...

Amie.

[Kalee20]

And yes it went black and started to rain here just before five, its definitely not bikini weather any more....

Diagram to follow...

Yep!

My rule of thumb is, if it's too bright and sunny to see cathodes glowing, it's non-electronics weather.

Look forward to diagrams  . I'm interested in the OP transformer exercise, I need to make a couple in the near future!

[Kalee20]

I've got a query about Post 26, you said 4mA was needed, how did you settle on this figure, what made you think it was 'right'? Sounded best, for instance?

As the two windings are equal, yes I'd agree that the ideal current would be expected to be same as Ia of the output valve.

However, more current means lower value resistor, so although the magnetisation is optimised, you are also shunting away more of the audio in the resistor.

And, efficiency is going to be crummy... The circuit effectively wastes the compensation current. It's drawn from the HT supply but contributes nothing to power output. This is why designers either go push-pull, or design the output transformer so that it can cope with steady DC current without saturating.

Looking forward to next post!

[Amie]

Looking forward to next post!

Hello kalee 20

As more current was drawn from the second winding the frequency response improved. At the point 4mA was being drawn it was -3db at twice the bandwidth of the unmodified circuit, it also seemed to be the point that provided maximum output versus total current consumed, I think that's the trade off,  sorry it's a little unquantified, I was just using a sweep function generator and 'scope.

200volt supply and 20ish mA consumption....I worked that out as 15% efficiency for the output I was getting. However, if the current that was being used for balance were decoupled, it could power the rest of the set, the design I have needs only 100 to 160volts to work successfully.

This really is all about core saturation I think, a good method could be made of it, but a proper gapped transformer would win every time.

I have no data on the hum cancelation issue in this circuit as yet...

I've included the diagram, its the Grundig one and I butchered it in MS paint. I've added the values I used....I hope it can be read, its a bit scruffy.
   
Time for some beauty sleep, I'm past bed time curfew  

Amie.

[Kalee20]

First... the circuit diagram is fine, easy to read, it helps a lot.

200V and 20mA is about what I would expect using the ECL80. That's 4W. The ECL80 is rated at 3.5W max anode dissipation, and 0.5W for the screen-grid, the triode, and a bit of balance current, all adds up fine.

Circuit values all look OK. You could probably dump the 47pF capacitor between the anodes. It just adds a bit of high-frequency roll-off.

When you say the frequency response improved, I take it that this was at the low-frequency end? Because core saturation reduces transformer inductance and this hurts LF response. (Fantastic news that you have access to a signal generator and a 'scope.)

You might also try adjusting the bias. If you use more bias the ECL80 pentode will pass less current. That will reduce available output power of course, but it will also give the transformer an easier time, so you may get back as much as you lose.

Connecting a capacitor across your balance-adjust resistor is virtually equivalent to connecting the capacitor directly across the balance winding. And by transformer action it's then equivalent to connecting said capacitor directly across the transformer primary, which you can see would just shunt away your audio, as you have found. What you actually want is just the opposite - a big choke in series with the resistor, or even better, a constant-current source such as a pentode valve. (But if you use a valve, then you might as well drive it rather than have it just passing steady current, in which case you have push-pull!)

Carry on the good work... in the meantime I will have a look as I am sure I have a few small output transformers that I removed from dumped TV's years ago, which would be 'near enough' for the circuit... and would be interesting to compare performance against the mains transformer.

[pwdrive]

To avoid using a separate bias supply you could use cathode bias (potential divider)

Lawrence.

[Terry]

To avoid using a separate bias supply you could use cathode bias (potential divider)

Unfortunately, because of the common cathode, this would provide way too much bias to the triode section as well.

However, by connecting HT- to the point marked -3V (Bias) and connecting a 70Ω (or near miss) resistor across the 10µF capacitor would provide grid bias to the output pentode (only) as in the Grundig circuit.

Varying the value of this resistor will vary the bias, if required.

[pwdrive]

To avoid using a separate bias supply you could use cathode bias (potential divider)

Unfortunately, because of the common cathode, this would provide way too much bias to the triode section as well.

However, by connecting HT- to the point marked -3V (Bias) and connecting a 70Ω (or near miss) resistor across the 10µF capacitor would provide grid bias to the output pentode (only) as in the Grundig circuit.

Varying the value of this resistor will vary the bias, if required.

As I said, cathode bias potential divider....(triode grid return resistor to the divider point)

Lawrence.

[Kalee20]

The triode is biased by contact potential using the 20MΩ resistor direct from grid to cathode. It's not my favourite method of biasing a valve, but it does work and for now I'd counsel Amie not to fix (yet) what ain't broke.

At the moment we don't know how Amie is deriving her bias (battery maybe, with trimmer potentiometer? Or a bench power supply?) but for experimental purposes, it presumably works. Refinements such as replacing with a resistor are for the future, but may also be awkward depending on the HT source negative rail!

[Terry]

As I said, cathode bias potential divider....(triode grid return resistor to the divider point)

Sorry Lawrence, I hadn't taken into account the significance of your potential divider reference.