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Danko 7th October 2005 07:06 PM

oscillating PP EL84 amplifier
Hi Everyone!

I have built this schematic:
The output transformer is not 4 and 8 ohm, it's secondary is 15ohms. So I reduced the 24kOhm to 12kOhm in the feedback. When I power it up, after the warmup, it oscillates about 500kHz.
Near the ECC85 tube there are 1uF/400V and 100nF/400V foil capacitors. Why? :-) Where should I decouple it?

What should I modify to replace the ECC85 with E88CC, or ECC82 ?


kopite 7th October 2005 07:48 PM

Try disconnecting the feedback, it does not need it to run ok. Usually pp EL84's give about .7% distortion without feedback. You can also remove the bypass cap from the shared cathode resistor. I can't see the caps you are talking about on the circuit.
The layout and the way you have built it may be contributing to the oscillations.

Danko 7th October 2005 07:58 PM

Those capacitor's aren't on the schematic.
The "layout" is only a "spiderweb" layout. I soldered the resistors and capacitors (except the elkos) on the ECC85's socket.

I will remove the feedback, and see what will it result. Thanks!

Jaime 7th October 2005 10:03 PM

You proved to interchange the plate exits of tranfo?

Danko 7th October 2005 11:44 PM

No, but I changed the output trafo's secondary order.
With this, the oscillation frequency reduced to something in the audible range, but it still oscillated. Then I removed the feedback. The oscillating stopped. Of course stopped :))
I put a speaker on the trafo, and there was a looots of noise. I thought, it was coming from the computer, but not. When the input was shorted, the noise was still there. After this, I noticed, that the grids were glowing. It's not good! :)
I turned it off, waited some time, to cool down everithing. I turned ON. There were some blue glowing inside the output tubes. I changed them to brand new 6P14P (russian version of EL84), and they were also glowing blue inside.

What is that blue stuff? I think it means, that the vacuum in the tube is not such big. But the 6P14Ps were new (NOS).

Why was the amplifier so noisy? Becouse of the glowing grids?

Johan Potgieter 8th October 2005 12:39 AM

Hi Danko!

Your experience is very normal for EL84s in pentode mode, depending on what you load the output with. If you try working this circuit without output load you would certainly get the kind of instability at 200Khz - 500KHz that you had. What did you use as load? A loudspeaker?

To begin with try get 15 - 30 ohm high wattage resistors as load so that you know you have a docile load. (By the way, for 15 ohms output you should have increased the feedback resistor, not decreased it.) But a rough calculation show that you have very little feedback anyway - about 6 dB. I am also concerned about the high level of noise you get without feedback. The input signal without feedback for full output is of the order of 0.25V peak, which is not sensitive. You might want to check the whole business through and clear this up (without feedback) before you go further. Especially red grids (or anodes) are NOT a good idea; get this sorted out immediately.

The blue glow in especially power tubes is normal, unless it is excessive. There is not too much difference between the ECC85 (mu of about 50) and the ECC88 (mu of 30). ECC85 is usually an RF tube so not much graphs are available; I would risk saying you can exchange directly - check the anode voltages. I would not go to ECC82/12AU7 (mu of 17) - you will get too little amplifiaction there.

With respect to Kopite; I would not run any pentode output circuit without some form of feedback. It is not just the distortion. Pentodes are very fickle with varying load, and loudspeakers are atrocious loads. The use of screen taps (ultra-linear) are far better, but then you already have the output transformers and cannot change now.

Hope this helps!

rdf 8th October 2005 03:51 AM

Aren't there really three feedback paths in this schematic, or am I reading it wrong? Between the two input cathodes, drive correction to the second grid of the EEC85 and a final global loop. Try losing one of the internal loops maybe?

dhaen 8th October 2005 06:12 AM

Re: oscillating PP EL84 amplifier

Originally posted by Danko
snip... The output transformer is not 4 and 8 ohm, it's secondary is 15ohms. So I reduced the 24kOhm to 12kOhm in the feedback. ...snip

You should have increased the value of feedback resistor. Try 47K.

rdf 8th October 2005 06:30 AM

The feedback's connected to the 4 ohm tap though, not the 16.

dhaen 8th October 2005 06:32 AM

But Danko's first words were:

The output transformer is not 4 and 8 ohm, it's secondary is 15ohms.

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