ECL86 Stage

[AdrianPH]

One of the small projects is to complete a small audio stage for my home made TV.  So as I have a ECL86 valve (used) in one of my boxes, I thought I would doone using that.

I managed to get a few transformers off Ebay a few weeks ago that are meant to  be for the valve, so today I started. First make up a mild steel chassis for it, decided not to use the copper PCB as I did for the rest of the project, I think this would always be useful.  Ran out of welding gasses at the end of it all but got it finished just.

Will start to build up the components on some tag board, I also plan to fit a notch filter at 10,125Hz on the input clean up a bit of my TV audio.

       

Adrian

[Murphyv310]

Hi.
The ECL86 will make a pretty decent amp. These valves are rather expensive these days and a bit harder to find though.
Are you adding negative feedback?

[AdrianPH]

I was thinking of following this article, without the treble and bass sections http://www.r-type.org/articles/art-003g.htm

As I say I will also be sticking an L/C in the input as the output from the TV audio stage is not that clean.

Adrian

[Diabolical Artificer]

I'd advise finding your own FB R/C values or use no NFB at all. I built a similar amp using PCL82's and found though the amp tested better with NFB, it sounded better without.

Andy.

[AdrianPH]

There will be changes to the circuit as it will not have a very high HT probably be around the 220 Volt range to match the HT rail of the TV I will also be reducing the Ia down from the 36mA spec to about 15 to 20mA. When I had an external amp on the TV it was only about 200mW.

It will be a get the spec sheets and figure a bit out, then stick it in my DC valve tester and confirm any operating points, it my way of learning a bit more.

[Kalee20]

Reducing the Ia is a good idea! The ECL86 packs a lot into its bottle, and runs hot at full whack. At your suggested numbers, it should have a long and happy life.

Of course, at these different values, the optimum load will be different, but that is relatively straightforward.

[AdrianPH]

Well I have managed to get somewhere, I did a bit of static testing on the two ECL86's I have both down with the best one providing 30mA Anode current rather that 36mA at 250 Volts and -7 on the grid, good enough for me.

I reduced the HT to 225 Volts (anode and screen) and worked out what I would need to get around Ia = 15 mA with the better valve, it also turned out to be -7 volts on the grid, or close anyway. Ig2 was = 3mA at this level So this set the cathode resistor to 390 Ohms, most other values as per the Data sheet on the valves.

I did have a major issue of self oscillating at something is excess of 20KHz greatly influenced by where my hand was in proximity to the transformer above the chassis, the only way I could stop this was with a snubber arrangement across the primary, started off with a 0.01uF going as a last test before I hit my night-time deadline of 0.01uF in series with 330 Ohm.

I do have feedback on board and the response is reasonable to say 5KHz and then starts to drop to half level around 9KHz, the feedback values seemed to have very little difference to the response.

Peak power before distortion gets to be harsh is at 1.6 Watts in to 3 ohms speaker this is with a 300mV peak to peak signal input. I have yet to add a 10,125Hz LC filter for the TV.

One thing that did surprise me and I would appreciate thoughts on this, the audio transformer has standard interleaved E & I laminations and I was expecting it to have all E laminations through the bobbin with I's at one end with a paper or card spacer, obviously not?

Adrian

[Diabolical Artificer]

"I did have a major issue of self oscillating at something is excess of 20KHz" Did you connect the ground bus to the chassis? Could just be it being a SE Class A amp, I had similar issues with the PCL82 amp.It's a good idea to do a manual frequency sweep from 20hz to 100khz looking for an increase in THD. At the frequency where THD starts to increase fit a snubber. If no THD meter use Soundcard Scope which is free PC software that has FFT and an accurate THD meter.

3dB down at 9khz is dissapointing, could be the tfmr,well most likely the tfmr,but worth scoping the first stage just in case, though not sure what you can do if this is limiting HF response.

A lot of SE tfmrs have as you say E I lams but with thin paper fitted between the lams every 10 lams or so, it's often impossible to see it.

[AdrianPH]

OK the problems have now been resolved. I did scope the first stage but determined that was not the issue, in the end I changed the transformer for another out of the three I bought and it made a major difference.
I had to increase the feedback capacitance from 390 pf to be 3n3 otherwise I had more gain at higher frequencies than lower, no more snubber across the primary.

I did have to fit a 39pF from the triode grid to ground, I found it stable when I had a scope probe connected but remove the probe and it went off at around 30KHz soon as the scope lead was connected again it stopped. This would not be an issue when I have an volume control and coax to connect it to the IF sound stage as there will be the capacitance of the lead, but it does for now.
The power output at 5% distortion is 1 Watt and the difference from 500 Hz and 20KHz is it drops 1.2dB at 20KHz, 10 KHz it is 0.3 dB down.

As to the other transformer, no idea what the issue could be, shorted turns or a mains transformer maybe?

Adrian

[Diabolical Artificer]

That's a lot better HF performance Adrian. The new tfmr is probably better made with several layers of interleaved primary and secondary windings to reduce leakage capacitance and it probably has a higher inductance too. Well done.

Andy.