In praise of smaller bottles

[Edward Huggins]

When it comes to the sound that can be produced from small audio valves, not much is ever covered in terms of amplifiers using the ECL80 or the ECL83. Both of these in simple Class A are capable of providing very good sound when paired with suitably sensitive loudspeakers. I once heard a DIY Stereo amp using 4 X ECL80s and at only 3 watts per channel these sounded so sweet and triode-like. I have a 1962 Philco record player using a UCL83. These are rated at just 2.2 watts, but it offers room filling volume and with no noticeable distortion, although probably at c.5%.

[Sideband]

ECL80's can be challenging to use because of the common cathode...it doesn't take much for them to perform as a very efficient cathode coupled oscillator! However with care and neutralising, a couple in push-pull can give a very good result with only about 3 watts. There was a TV produced (forget the make) that used a couple in push pull that sounded very good indeed.

ECL83's can give good results but with them (and the PCL and UCL versions), they have a tendency to go into grid current if the operating conditions are not ideal. They are a small valve with quite a high dissipation so can run very hot which doesn't help.

[julie_m]

My first valve project was a portable audio amplifier using a single ECL80. This brings out the suppressor grid to its own pin, but has a common cathode, which can make selecting an operating point awkward. (Mullard must have noticed this too, because all their later triode-pentode offerings seemed to have separate cathodes.)

Fortunately, I was using a 6V rechargeable battery to power the heater, and this gave just enough voltage to bias each of the grids negative with a separate adjustment pot. Selection of operating points was then done empirically. I plotted the pentode's cathode current against grid voltage with the actual intended anode load, picked an operating point that was right in the middle of the straightest section of the curve, and repeated the exercise for the triode section. (I used a bit of computer assistance here; in the form of a Velleman K8055 experimenter board and a breadboard circuit with a dual op-amp, to amplify the voltage across the sense resistance and give me a negative-going voltage from the positive-going DAC output, and a potential divider to measure the HT.)

The only bit that wasn't already in my parts drawer was the speaker transformer (an RS universal).

The power supply is slightly sacrilegious, with a CMOS IC driving a pair of chunky MOSFETs and a ferrite toroid that was a labour of love to wind, but it can shove out 140V at 35mA all day long (or at least it could, if the battery would last that long) without even getting warm. (It's actually the mark 2 PSU; the original plan was to use a mains transformer with centre-tapped secondary in reverse, but I ran headlong into the square-cube law.)

It was a fun learning experience, though, and wonderful to see all the physics I'd read about in textbooks physicking in real life!

It doesn't sound bad, either.

[Sergeauckland]

I think the issue of using small audio valves hinges on the 'small'. These valves produce only a few watts in push-pull, a lot less single ended, so there are vanishingly few loudspeakers made in the past 50 years that are efficient enough to make use of those few watts.

High efficiency horns such as Lowthers or Vitavox are larger and a lot more expensive than most people would countenance, and indeed, for some of us, their coloration is unacceptable, even with their great efficiency.

I remember how loud my Mullard 5-10s would go into Goodmans Axiom 10s but that was a pair of EL84s, not a single-ended EL84 or ECL80, even in PP. I fear those days have passed, so apart from some nice record players like the Pye Black Box, there's not a lot of interest in few watt amplifiers.

S.

I am amazed at the volume which can be produced by the output stage in the RA17, which is just a small RF pentode!

[Trevor]

I myself built a valve TRF that used EF91 in all stages including the output valve plenty of volume! At the time I used to think that the EF91 was rather like the 741 op amp and could be used in so many ways still have loads of them
Trev

[G6Tanuki]

In my youth I built a little stereo amplifier using a pair of EF91 as the preamps and a pair of EL91 as the outputs.

Purely because I had been given them.

The result was excellent as a headphone amplifier for listening to heavy metal at the sort of level which means you can still hear the music a day later.

From memory some early 405 line tellies used an EF91 as the audio output valve!

[Radio Wrangler]

Those comments all fit together:

RA17 used EF91/6AM6/CV138 as audio output. About 100mW in its performance sheet.
RA117 stepped up in the game and used EL91 for a full 1W output.

David

[kalee20]

There's lots of small valves that pack quite a punch.

The 6AQ5 for instance, is a 6V6 in a B7G bottle - 12W of anode dissipation, and it isn't even as tall as an EL84!

The only thing that bothers me, is that this sort of thing runs stinking hot, so although performance for size is impressive, it may not last as long as the big-brother version.

The ECL80 - Mullard published a design using two in push-pull, for 3W output, using 200V of HT and a common cathode resistor for all. That eliminates feedback through the common-cathode connection completely. It's a circuit which some day I'll try (I've already used the ECL80 single-ended).

[Valvepower]

Ah, small bottles… I was recently looking at using the ECL80 in some ‘odd ball’ amplifier projects. This was inspired by a friend saying about using an ECL80 in a headphone amplifier and also seeing the ECL80 being used as a see-saw driver/phase splitter in the Ferranti Model 435 radiogram!

After looking as the Ferranti I did think about triode connecting the pentode to see how it measures to see if its similar to the triode in the ECL80 From memory the triode in the ECL80 is not dissimilar to the ECC82… Personally, I feel fixed bias with the cathode anchored firmly to ground is the way to go with the ECL80.

I did pick up what I think is a Home Radio home brew build of the GEC Junior Micro amp using a pair of ECL83’s in push pull for output duties and an another tidder – the EBC80 – doing preamplifier duties. This amplifier in on the ‘odd ball’ amplifier round tuit pile along with a few others!

https://www.r-type.org/articles/art-192.htm

Talking EF91 and similar pentodes, I used a pair of triode connected E180F valves for preamplifier and EL34 driver duties in a recent single ended EL34 amplifier build.

Terry.

[G6Tanuki]

Originally Posted by kalee20

There's lots of small valves that pack quite a punch.

The 6AQ5 for instance, is a 6V6 in a B7G bottle - 12W of anode dissipation, and it isn't even as tall as an EL84!

The only thing that bothers me, is that this sort of thing runs stinking hot, so although performance for size is impressive, it may not last as long as the big-brother version.

The ECL80 - Mullard published a design using two in push-pull, for 3W output, using 200V of HT and a common cathode resistor for all. That eliminates feedback through the common-cathode connection completely. It's a circuit which some day I'll try (I've already used the ECL80 single-ended).

The 6BW6 - essentially the 6V6 in a B9A bottle - can be thrashed so a pair in AB2 push-pull produces 30 Watts of output!


The ELL80 being another good example of a very compact valve which ran murderously hot and so had a short lifespan.

[TonyDuell]

Wasn't there an HMV television set back in the 405-line era that used an RF pentode (EF80, EF91, something like that) as the audio output valve.

And I seem to remember an amplifier design in one of the magazines back in the 1960s (I think it was Practical Wireless) that used a PCC88 (yes, the RF double triode) as a push-pull output stage.

[Sideband]

Originally Posted by TonyDuell

Wasn't there an HMV television set back in the 405-line era that used an RF pentode (EF80, EF91, something like that) as the audio output valve.

Yes although the model escapes me. It used an EF80. A Derwent TV used an ECC82 as a push-pull output valve (actually I've used an ECC82 as an output valve...not pushpull, just as AF amp and output with good results).

More bizarrely, the ECL80 has been used as mixer/oscillator in a TV. Very odd seeing an ECL80 in a TV turret tuner.

[TonyDuell]

Originally Posted by Sideband

Yes although the model escapes me. It used an EF80.

According to the service manuals, the HMV1814 used a Z152 (equivalent to EF80) and the HMV1807 used a Z77 (equivalent to an EF91). Maybe others...

[GrimJosef]

Originally Posted by Sideband

... actually I've used an ECC82 as an output valve...not pushpull, just as AF amp and output with good results ...

The well-known Firefly guitar practice amp https://web.archive.org/web/20181221214218/http://www.ax84.c...oldprojects.html?project_id=firefly uses one in push-pull for a watt or so.

Hi-fi it ain't though.

Cheers,

GJ

[Valvepower]

GJ, thanks for the firefly post.

Interesting they have the output ECC82 configured a LTP cathode coupled amplifier. Also interesting the use of a cascode connected ECC83 for the booster input. I can see this kiddie goes up to 11…

An ECL80 would make an ideal candidate for lower power amp like this, and if it’s too loud one could configure the pentode as triode.

Yeah, Hi-fi it ain't… but, it could be awful fun though…

Regards
Terry

Originally Posted by GrimJosef

The well-known Firefly guitar practice amp
Hi-fi it ain't though.
GJ

[kalee20]

If you want small bottles, I've got a push-pull amplifier using a pair of 1p24b rod pentodes - about 2W output. It gets a lot of use!

Much smaller than these huge, ungainly ECL80 or ECL83 behemoths...

[Valvepower]

Ooh, those 1p24b rod pentodes are new one on me

I’d best get my ECL80’s on a diet

Terry.

Originally Posted by kalee20

If you want small bottles, I've got a push-pull amplifier using a pair of 1p24b rod pentodes

[G6Tanuki]

12BY7A is another enjoyable small bottle. Either use its twin triodes in push-pull to get 5 or 6 Watts of mono, or use the two halves independently for stereo.

For triode lovers there's always the 6C4 which is half a 12AU7. Good for a Watt or so in Class-A in a headphone amp.

[Jan Zodiac]

Push-pull output stages with ECL80 were used in the Philips TV sets from the early 50's (on the splitcarrier chassis'es). I did wonder why they used two of these valves to get 2 - 3W, when a single PL82 that was used later on, could easily give at least the same output power. But a quite neat little PP stage though.

Jan