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Old 27th Nov 2022, 8:36 pm   #1
Bazz4CQJ
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Default Stable ECL80 Amp?

I know from reading earlier threads on the forum that the ECL80, with its common cathode, has a tendency to show instability.

Attached is a circuit for using a single ECL80 for the output of an amateur bands receiver (Amateur Radio June 86). Would anyone like to offer an opinion as to the merits or de-merits of this design?

Thanks

B
Attached Files
File Type: pdf ECL80 SE amp .pdf (21.8 KB, 141 views)
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Old 27th Nov 2022, 10:01 pm   #2
Restoration73
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Default Re: Stable ELC80 Amp?

It does feature negative feedback;

http://www.r-type.org/articles/art-365.htm

But personally I would always use ECL82/6BM8 for this application.
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Old 27th Nov 2022, 10:28 pm   #3
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Default Re: Stable ELC80 Amp?

When I've used ECL80s in projects in the past, I've always just ignored the triode and used the pentode. I know there are lots of designs that claim to be stable, but I've never managed to make them work.
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Old 27th Nov 2022, 11:20 pm   #4
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Default Re: Stable ELC80 Amp?

The only merit I can see would be low power consumption, heater and HT drain. The audio output power should be fine for speech use.
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Old 27th Nov 2022, 11:30 pm   #5
Bazz4CQJ
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Default Re: Stable ELC80 Amp?

This is the Rx I'm working on https://www.vintage-radio.net/forum/...d.php?t=195648.

It orginally uses 3 off 6U8's (ECF82), but I didn't want to restrict experimention of the shared RF amp valve. Given the concern about the ECL80, but wanting to work within restricted HT and LT power, I think I could just use another 6U8, which should give about 380mW out - see https://www.cool386.com/6BL8_amp/6BL8_amp.html.

Thanks for your comments.

B
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Old 28th Nov 2022, 2:05 am   #6
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Default Re: Stable ELC80 Amp?

If the common cathode is sufficiently signal grounded by the bypass capa it or it should be OK.
It might be better, though more costly, to hard ground the cathode and provide separate grid bias supplies for the triode and pentode.
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Old 28th Nov 2022, 8:24 am   #7
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Default Re: Stable ELC80 Amp?

See page 92/93 of this book for some measures to prevent/deal with oscillations (most of these measures are incorporated in the schematic linked to in post #2):

https://frank.pocnet.net/sheets/046/...kIIIc_1953.pdf
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Old 28th Nov 2022, 10:29 am   #8
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Default Re: Stable ECL80 Amp?

There were many audio and frame output circuits in TV receivers in the 50s with no instability problems, I've made simple AF amps delivering 1.5w driving a 10 inch speaker with no problems. Pay attention to good layout and decoupling of the cathode and G2 and all should be well, the ECL80 also can withstand 1.2kv peaks on the Pentode Anode. It's a good little valve despite its shortcomings.
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Old 28th Nov 2022, 10:37 am   #9
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Default Re: Stable ECL80 Amp?

I used the ECL80 in many radios and amps as a 12 year old and always found it worked well and was plentiful and very cheap as many TV's used it in timebases as well as audio amps at the time
R24 in your circuit forms a potential divider with R25. Rather pointless. It would make more sense to fit it as a grid stopper which would help stability.
C21 coupling capacitor should be no more than 4n7 or even lower to help low frequency stability, also C21 should be fairly large for the same reason, say 220uf minimum.
Finally R15 the volume control would be better wired with slider to grid.
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Old 28th Nov 2022, 3:05 pm   #10
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Default Re: Stable ECL80 Amp?

I've never really had any issues with the ECL80 if you take a few sensible precautions.

Make sure the cathode is properly decoupled to ground both at audio and RF frequencies [put a small ceramic capacitor in parallel with the bypass electrolytic] or the thing can aspire to becoming a cathode-coupled multivibrator.

Also add low-value [10KOhm] resistors on the contol-grids as stoppers; put them as close to the valve socket and as short-leaded as possible.

If you still have problems, wire low-value capacitors [100pF] from each control-grid direct to the cathode [not to earth].

An ECL80 with the triode as xtal oscillator and the pentode as an output stage works OK as a low-power CW transmitter on 7MHz!
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Old 28th Nov 2022, 4:17 pm   #11
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Default Re: Stable ECL80 Amp?

Champion used the ECL80 in some of their audio amplifiers. The only problem I have found is when the cathode decoupling capacitor goes low value.
This causes audio instability, presumably because of positive feedback between the two sections, due to the common cathode.
When the capacitor is replaced, they perform perfectly OK.

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Old 28th Nov 2022, 6:03 pm   #12
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Default Re: Stable ECL80 Amp?

A curious thread, the initial replies all negative but followed by posts all positive.

I used to use ECL80's as "standard" valves for my Avo VCM, so have three which have barely been used. I'll have think about this; is 380mW output really enough for ears of my age .

Many thanks for your inputs.

B
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Old 28th Nov 2022, 9:30 pm   #13
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Default Re: Stable ECL80 Amp?

Great little valves when used carefully. They've even been used in push-pull. https://www.petervis.com/valve-circu...put-stage.html
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Old 28th Nov 2022, 10:27 pm   #14
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Default Re: Stable ECL80 Amp?

Bazz, 380 mW is sure enough output. Just make sure you use a decent sensitive speaker.
If we think back to the geranium radio days, some of those little radios made less than that but served admirably. Its a radio after all, not a railway PA system. I like your idea, especially from the power point of view. It doesnt need lots of HT, nor heater current.

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Old 28th Nov 2022, 11:02 pm   #15
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Default Re: Stable ECL80 Amp?

I'm not sure what these valves were designed for, but they were certainly mostly used in frame video circuits in 1950s 405 line TVs, which is why pulls still exist in huge numbers even today. Some manufacturers did try to use them for audio output, but they were an oddity in that role.
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Old 28th Nov 2022, 11:09 pm   #16
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Default Re: Stable ECL80 Amp?

Quote:
Originally Posted by Bazz4CQJ View Post
A curious thread, the initial replies all negative but followed by posts all positive.

I used to use ECL80's as "standard" valves for my Avo VCM, so have three which have barely been used. I'll have think about this; is 380mW output really enough for ears of my age .

Many thanks for your inputs.


B
Sometimes the negative replies can be from those that haven't actually made anything with these valves or thought why they may have a poor reputation.
We have on occasions have to suck it and see
I've even made little transmitters with them and the Pye V4 used them as a frequency changer, wonderful little underrated valves.
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Old 29th Nov 2022, 2:08 am   #17
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Default Re: Stable ECL80 Amp?

Quote:
Originally Posted by paulsherwin View Post
I'm not sure what these valves were designed for,
Some information on the intended applications was given here:

WW 195010 p.32 Mullard ECL80.pdf


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Old 29th Nov 2022, 2:16 am   #18
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Default Re: Stable ECL80 Amp?

More here, with some comment upon possible oscillation in the AF role:

ECL80 from Philips Book III.pdf


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Old 29th Nov 2022, 9:44 am   #19
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Default Re: Stable ECL80 Amp?

As can be seen in the data the Pentode section short duration pulse maximum voltage of 1.2kv, I've run them happily at 500v continuous HT.
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Old 29th Nov 2022, 10:39 am   #20
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Default Re: Stable ECL80 Amp?

Quote:
Originally Posted by paulsherwin View Post
Some manufacturers did try to use them for audio output, but they were an oddity in that role.
My Ekco T45 TV uses an ECL80 as sound output. Sounds good.
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