Cathode resistor for EL84 P-P Triode wired

[cnau]

Originally Posted by itsikhefez

Which value resistors did you add to the secondary?

I used 120R on each leg but I needed to take down the voltage quite a bit. In fact I ended up at closer to 290V; gets me more life out of the tubes.

In your case why not just increase the "R" in you RC filter to get the B+ down to at least 300v as per max specs for voltage?

[itsikhefez]

No particular issue in increasing except some laziness as the chassis resistor is mounted with thermal epoxy glue. That said, if it were absolutely needed I'd do it.
That value was chosen during the mockup phase for about 315V B+, which results in minimally above 300V across the tube.
It's also below George's target voltage for the SPP, 320V.
The Russian 6P14P-EV can handle way more than that, but from what I've read, this is also fine for most EL84's out there.

Originally Posted by kodabmx

It's within the spec since it's running as triode and you can add plate and screen dissipation together but by all means try it and see if you like it.
FWIW: I used to run 6P1P at 18W and they last for about a year 24/7. I dropped it from 56mA to 40mA so they last longer.

I made some more measurements and indeed the 270R look fine... total dissipation is well within the spec.
The only change I will do is replace the CL-140 across the mains (good for 1.1A steady state current) with a CL-90. This will add some resistance across the primary and reduce some of the B+.
It will also likely last longer than the CL-140 there.

Lastly, I was able to run some measurements with both the 6P14P-EV and Hammond labeled Amperex Austria tubes. ECC81 are Mullards Blackburn. Wired as triode, still with gNFB from when it was wired as UL.
Max output was around 5W, frequency response pretty flat.

Attachments are FFT at 1W, 5W and FR at 1W output (2.83V across 8ohm resistor)

[cnau]

I wonder if you can rerun the FFT's with no feedback (in triode) for my own curiosity? I am not set up to do such tests and to me no feedback sounds better.

[cnau]

Originally Posted by cnau

I wonder if you can rerun the FFT's with no feedback (in triode) for my own curiosity? I am not set up to do such tests and to me no feedback sounds better.

I have decided to get setup to do this type of measurement so no need but thanks if you were considering it.

[itsikhefez]

cnau, sorry for not responding sooner.
If no physical changes to the board are needed, I wouldn't mind running it.
I'm not sure if having the feedback caps in the board affect anything if wiring the OPT directly to the binding posts.

That said, it's great that you are setting up for this. For measuring at 1W (2.83V) it only really requires a soundcard/interface and software.

[kodabmx]

If you can test with the loud speaker under power, you can put a small resistive divider on the sound card input to measure higher power without needing giant resistors.