PY 81 as power rectifiers?

[pwdrive]

With a good valve something would start to get Thomas Cooked with the 3 volt bias shown in the schematic:

http://www.r-type.org/pdfs/ecl80.pdf

Around 440 ohms or thereabouts seems to be the going rate for the pentode cathode bias resistance, split to 220+220  and that should give the triode a ball park bias and keep the pentode happy.

Lawrence.

[Amie]

Hello all.

Seems like the discussion has turned to the shortcomings of the ecl80 valves.

The reason that I used one is:

The Grundig set used another (albeit larger) common cathode type, and I wanted to make a comparison.

I have ten NIB.

Its a small valve, and I don't need a ghetto blaster.

Heater consumption is modest and 6.3Volt.

Thomas Cook

I was confused about the talk of 3volts bias, and where that came from? then I realised that I put that on the diagram! It was a vestige from the Grundig literature.

What I actually did was provide the bias from the GB output of dads old bench supply that will give a referenced adjustable, low current negative supply. I just lazily adjusted it for anode current, and accepted it.

As eagle eyes Lawrence spotted it, I switched on the project just now, and noted the value of bias.

It's -6.7volts.

Well done.

Oh, and I don't much like this biasing set up either, I don't think it would be long term stable. At least in the Grundig set it would self adjust to a degree virtue of the current through the 70 ohm resistor, but its just experimental.....

Thanks,
Amie.

[Amie]

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.

I tried removing that capacitor, the circuit has marginal stability (nothing to do with the rats nest wiring... I'm sure)
I think the expression is squealing like a, (sorry I'd be banned if I wrote it).....!

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.)

Very much across the board improvement, less distortion, "entertainment" quality rather than "telephony", but yes, more bottom to the sound, can't be precise but it extends quite flat from 120Hz (say) to about 8KHz

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.

I will try that next tinker time.

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!)

Yeah, got that, my intention was to go, winding, dropping resistor, decoupling/smoothing capacitor, rest of circuit

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.

I wanted to do the same for comparison, but haven't found a suitable victim!

Thanks,
Amie.

[Kalee20]

It's a cute little valve, easy on the heater power as you say.

The common cathode makes it awkward to use sometimes though. In your circuit you have made things easy for yourself, with the external pentode bias, and contact-potential bias for the triode, and taking the cathode directly to circuit 0V.

There is a fair bit of gain present, depending how much of a rats nest your wiring is! Long wiring on the pentode anode is bad news as is unscreened wiring on the triode grid. Coupling between the two will be positive feedback so it is easy to make it hoot.

It is good for a watt of audio, which with a decent speaker is highly useable!

Mullard developed a circuit using two in push-pull for 3W. And they had another, rather quirky circuit which uses the ECL80 pentode as a phase inverter (working at very low current), and the triode as an oscillator... the grid-leak bias developed in the triode was used to bias two push-pull PL81 output valves without wasting voltage in cathode resistors. But that's all going off-topic!

[Kalee20]

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.

I wanted to do the same for comparison, but haven't found a suitable victim!

Thanks,
Amie.

Hi Amie,

I found the small single-ended output transformer I had in mind - salvaged from an old TV years ago, and I've never used it. It's a neat little transformer, just 2 terminals primary, 2 terminals secondary. If you want it, PM me your address and I'll stick in the post.

Here are vital statistics:

1) Rpri = 348Ω so will drop only 5V at 15mA anode current. Not bad.

2) Connecting to a Variac and applying 50Hz AC, Vsec = 1.95V at Vpri = 100V. So turns ratio = 51:1 thus a 3Ω loudspeaker will give 7.8kΩ load to the output valve. Should be fairly good for the ECL80.

3) Again with the Variac, Ipri = 39.2mA @ Vpri = 100V. So reactance = 2.55kΩ @ 50Hz thus primary inductance = 8.1H. (I also measured at 1V primary, got 0.552mA implying 5.8H. This is pretty typical for a small transformer with an air gap. If the laminations had been interleaved, inductance would be much higher, but far less constant. I also tried to measure at 200V but the current started rising disproportionally, indicating core saturation was kicking in at about 60mA).

Now 8H and 7.8kΩ load gives a low-frequency 3db cut-off of 155Hz so if you like deep thumpy bass, you'll be a bit disappointed. On the other hand, I'd bet it will do better than the mains transformer you currently have. And it was good enough for TV viewers in the 1960's and 1970's.

It should do you fine.

[Amie]

It should do you fine.

Thanks.
Got to say I don't know if the mains transformer would be too much worse, But, that's with balance current, which is a waste, unless it can power something else.

I was sure Dad had some little gapped tranformers that were spares made as replacements..... But I cannot find them, PM. Sent.

Incidentally fixed bias, I've upgraded to a very bad idea, the anode current is not in the least stable, if I used the ECL 80, that would have to be considered....

Amie

[Kalee20]

[quote pid='82935' dateline='1569485816']
PM. Sent.

Incidentally fixed bias, I've upgraded to a very bad idea, the anode current is not in the least stable, if I used the ECL 80, that would have to be considered....

Amie

[/quote]

PM hasn't come through (I know my PM's work as I've used them in past).

The ECL80 anode current should be pretty stable with fixed voltages. If it's not, it is quite likely that the valve is oscillating at an ultrasonic frequency! But 'scoping will find that.

[Kalee20]

Just has now. Hopefully will post tomorrow

[Amie]

My bad, sorry. The system only works if the PM is sent to the right Kalee!

Amie.

[Amie]

Hi Everyone!

So, I've learned a lot.

TV boost diodes make perfectly good HT rectifiers.

You can use a power transformer as an output transformer for a valve amplifier, particularly useful for push pull.

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. Maybe it would be possible to combine both, but, it seems an either/or situation to me, and balance of AC at 50Hz and DC together in a power transformer would make me want to go back to building a PP amp.

I've tried several valve types, thanks to a couple of the members here who are super guys and took the time to send me some that I wanted to try (Boater Sam & Ppppenguin) the best for the PP version have been the PCL83,  though that's a bit much for what I wanted here.

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!

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.

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

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

Lawrence.

The penny finally dropped on this one Lawrence! I've incorporated it into the recent version, thanks.

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.

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....

Amie.