The Red Light District - another PP EL84 amp

[TheGimp]

That could well be.

Based on what y'all have said, if I build screen reguators, I should be able to adjust the cathode current, but will probably need to change the cathode voltage source as well.

I'll up the cathode bias voltage source to 10V and should hit 50mA of plate current around 220V on the screen based on this data sheet I've been using (or ignoring as the case may be).

[TheGimp]

Well, I thought I posted a long dissertation about testing with RLD bias, LED ref Vreg bias, zener bias, and cathode bias, but the post apparently contained majik smoke that got away.

Basically, low screen voltage was my problem. All four bias methods worked well. I drove the plate current up till I could hardly see a feint glow to the seam of one tube in the dark at 14.75W plate dissipation per tube. Stout little buggers.

The amp has been running fine all evening.

Back to documentation of what I've done.

Next will be adjustable screen supplies, I didn't make it out today at lunchtime to buy the parts I needed so it will wait till I get the parts.

I'll take this back to the 6P1P thread since it will diverge from RLD bias now.

Thanks all.

[albertli]

SY,
I have my RLD singing for me for almost 2 years now.
Lately I open it up and recheck all the voltage to make sure everything is working ok and I found couple problem hope you can tell me how to make it work right.
1). The Maida screen supply ( direct copy from the schematic provided ) can't be adjusted to desired voltage, but only working between 298v to 320v. I double check there is nothing burnt. Can I lower the value of the 56K 3w and or other resistor ?
2). The output tubes on one channel seems working not properly, one of the tube obviously shows dimmer than the other one. Is it the result of aging after heavily use? and is it possible to put a balance pot in between the two cathode to make the adjustment?
Thanks in advance

Albert

[SY]

I'd replace the output tubes. The JJs I was using still seem to be singin' along, but I can't vouch for the longevity of others. I didn't find a need for this, but you might want to put a 1k resistor in series with the screens to get longer life. For balancing currents, use separate regulators- they're cheap and compact.

If your adjustment range on the screen supply is that small, you probably DO have something burned. Check the pass transistor for collector-to-emitter shorting. If that's the case, both it and the 317 should be replaced. Also check the zener.

[jackinnj]

Originally Posted by SY

If your adjustment range on the screen supply is that small, you probably DO have something burned. Check the pass transistor for collector-to-emitter shorting. If that's the case, both it and the 317 should be replaced. Also check the zener.

...and when you put it back together, make sure that the pass transistor and lm317 are heat sinked (insulated too). I find it helpful to keep things like zeners mounted above the board so that air can circulate -- when a zener dies it goes short (like a tantalum cap) and it can cause a cascading series of events.

[SY]

What Jack said.

[albertli]

Thanks

[jackinnj]

Letters of Note: We all feel like that now and then

[Lingwendil]

Originally Posted by jackinnj

Letters of Note: We all feel like that now and then

?

[SY]

He was just commenting on my current signature line.