ECL80 circuit required.

[Biggles]

Hi all, to save me a bit of looking on the web or through my service sheets, has anyone got a circuit for an ECL80 single ended amplifier? The tricky bit is the use of a common cathode for the triode and pentode sections/negative feedback etc. I could probably knock something up but it would be better to have a "proper" circuit. One with tone controls would be even better. Cheers in advance.
Alan.

[G6Tanuki]

Google throws up plenty.

This forum has a few: https://www.vintage-radio.net/forum/...d.php?t=121846

It's really a 'junior school' rather than lower-sixth-form project.

[GrimJosef]

https://www.vintage-radio.net/forum/...d.php?t=121846

EDIT: Beaten to it !

Cheers,

GJ

[Sideband]

I always liked the little ECL80. It's even more fun trying to get them to work in push-pull using one triode as an amplifier and the other as a phase splitter.

[Boater Sam]

There many posts on here about ECL80 amplifiers if you use "Search".

[G6Tanuki]

In the past I used an ECL80 as crystal-oscillator and driver for a HF transmitter using a 5B/254M (miniature-807) output. An ECL83 did "Series-gate" modulation with an EF91 mic amp. The result was nicely compact and sounded good.

[Biggles]

Thanks everybody. I have a little spare chassis but the mains transformer is a bit on the small side. However, it will support two ECL80's for a stereo amp for one of my valve tuners. A product of an overactive project-seeking mind when I spot potential parts just waiting to be used. Two ECL83's would be too much for the transformer so ECL80's it has to be. The triode bias on that circuit from a previous thread on the forum uses a neat potential divider I see. I was wondering how to bias the triode properly, without using the much larger bias of the pentode. Probably introduces a bit of negative feedback at the same time.
Alan.

[Biggles]

Just a quick follow up from this request. I have constructed a stereo version of the circuit suggested by Top Cap, from the Champion Record Player. I was actually pleasantly surprised by the performance. I modified the circuit by reducing the value of the output transformer top cut parallel capacitor from 0.01 microfarad to 1000 picofarad. This increased the frequency response up to 20KHz. Reducing it further increased the prospect of instability so 1000pF was chosen as a sensible in-between value. Considering I am using contemporary 3W max RS output transformers, the response is within 3dB between 50Hz and 20 Khz. Sounds fine and 1.5 Watts REAL music power is quite loud into efficient speakers. Good enough for me and the modest little amp.
Alan.

[Sideband]

Well done on completing the little project. For all it's limitations, the ECL80 can give a good account of itself as an audio amplifier. There have been push-pull circuits published but these need care in construction to work properly and some neutralisation to prevent it becoming an oscillator!

One vintage TV I have (Ferguson 45) uses an ECL80 for audio output and it is one of the better sounding TV's in the collection.

[Biggles]

Thanks. Every now and again I just seem to find a few components and a little project pops into my head. A lot of folk probably ask why bother building such a "pointless" circuit, but after all it's a challenge and fun into the bargain. At the same time, the amp performs well and one and a half watts is more than loud enough for casual listening. Uses NOS genuine Mullard valves too. And it cost me nowt! I wonder if I can somehow make the ECL80 "sought after". I have loads of them.
Alan.

[Herald1360]

Hmmm, must be possible to do an "ECL80" superhet. Give it push-pull output for an added decoupling challenge

[julie_m]

Here's how I dealt with the common cathode problem:
Attachment 84081
The HT PSU is not shown, but it was a simple affair with a HEF4047BE, two power MOSFETs, a hand-wound toroid and some ultra-fast diodes.

[Sideband]

Originally Posted by Biggles

I wonder if I can somehow make the ECL80 "sought after". I have loads of them.
Alan.

Well it was a fairly universal valve so the possibility of an all ECL80 superhet is not so far fetched.....it was used as a mixer/oscillator in one TV though exactly why is known only to the designers. Whether it would function as a mixer/oscillator at MW/LW remains to be seen. However if it did you could use one for mixer/oscillator, one for I.F amp (pentode) and detector (diode strapped triode) and then (for fun) a push-pull output stage using two of them....!! Well they used EF50's and EF91's in all stages of some home-brew designs. Anyone fancy a challenge?

[Biggles]

I think they must secretly breed when in storage. Just like the EF91. I seem to have collected loads of them too over the years. Don't use them for anything. Oh and the EB91 seems to do the same thing. So there is the challenge. ECL80 mixer/osc. EF91 IF amp, EB91 detector, ECL80 AF amp and power output.
Alan.

[Herald1360]

That's somewhat less of a challenge but a far more "sensible" lineup. Far too much like using the proper tools for the job. Decent AGC still a bit of a challenge, though.

[AC/HL]

I've seen circuits with the 6K7 in every stage when the surplus market was awash with them. Valves are very adaptable if not called on for specific performance.