Cathode biased EL-84

[AlNewman]

So I have a Trace Elliot amp here, push pull, cathode biased with a bypass cap.  Both 100 ohm screen resistors blown.  Looked like maybe a couple rum and cokes had migrated to the pins of one tube.

I replaced the resistors and started checking voltages, which weren't too far off of the schematic.  B+ was 365, but the current draw on the 120 ohm cathode resistor was 94 mA or something, so 30 wattsish. I happen to have some 5 watt 330 Ohm resistors on hand, so using that it brought the cathode current down to about 18.5 watts @ 395 volts.  So I figure around 250 ohms is my sweet spot, as far as current is concerned, but still well above maximum voltage, as well as maximum cathode resistance as per datasheet.

I'm wondering how important the cathode resistor maximum of 130 ohms is and why?  As well as does the fact that it's bypassed with a 100uf cap make a difference on maximum voltage/current draw or maximum cathode resistance?  And finally, what's the lesser of 2 evils, high voltage, or high current?

I need to order new electrolytics, which I hope will lower the voltages some, but I have replaced the bypass cap already.  All other components seem within spec.

Thanks in advance for any wisdom.

https://el34world.com/charts/Schematics/files/Trace_elliot/Trace_elliot_velocette.pdf

(if you can read this, you have better eyes than me, but it is helpful)

[sluckey]

I'm wondering how important the cathode resistor maximum of 130 ohms is and why?

I see 130Ω listed as typical value for shared cathode resistor. But I've never seen it called a MAXIMUM value. 130Ω seems to run the tubes hotter than I like. I like to use 150Ω if the plate is over about 325V.

[tubeswell]

Any resistance that gets a cathode voltage between 10V at idle to 12ishV at full power works

[bmccowan]

Interesting vid - working on a Gibson Goldtone/Trace Elliot EL84 amp:
https://www.youtube.com/watch?v=ddO-SSCX1T0
I recall that these amps had high failure rates.

[AlNewman]

Well, hopefully the voltages settle down a bit with new caps.  11.5 volts is around where the grids were set, and it was over 30 watts.

That's a good video.  I guess I should up the screen resistors as well.  I'm a bit surprised he went with 5 watt resistors on the screens, as the small metal films they used probably saved the rest of the amp.  The screen resistors were the only part that failed. 

This one isn't branded Gibson, it's from 96, but yeah, not designed very well.  Had the main power wire fall out of one of those rinky dink connectors while I was taking the board off. 

[dwinstonwood]

I actually went with this Vishay/Dale 1% 160 Ohm in my EL84 AC15:

https://www.mouser.com/ProductDetail/Vishay-Dale/RS010160R0FE12?qs=%252B%2F1MMOhkBTe%252BHftPPz4KBA%3D%3D

But, I'm using NOS tubes and want to go easier on them.

[bmccowan]

I was also surprised that he used 5W screen resistors. Seems common, but common is not always good, eh?
I picked up a couple of Trace Elliot PTs many years ago. I heard too late that many were failing. But the ones I used have held up fine. 

[AlNewman]

Well, I'm not sure what the rating on those resistors were, but even at 1/4 watt @ 100 ohms, the screens would have had to be pulling 50mA to blow.  Unless I'm getting my math wrong.  I have some 1 watt 820 Ohm that I'll drop in, should be fine.  Like I said, it looks like a drink was spilled.

[AlNewman]

So I'm kinda thinking out loud here, but as it sits, there's a 1k resistor to the screens, and a 4.7k resistor to the preamp.  Right now the 12ax7 are running about 270V, could I drop the screen resistance and maybe the preamp resistance and feed more power to the preamp tubes?  I know they don't take much current, but every little bit would help.

[sluckey]

I would leave the 1K and 4.7K dropping resistors alone. The 10K dropper feeds the preamps and it is dropping about 20V. Take it down to 5K to drop only 10V. This will increase the preamp supply approximately 10V. I doubt you will notice a difference but it may give you a warm feeling.

[AlNewman]

Ok, I'm losing you now.  Which 10k dropper?  Is that right off of the front end of the AC power?

I'm trying to reduce plate voltage, so maybe I wont hear a difference, I'm looking for reliability.

[kagliostro]

May be this of your interest AlNewman ?


Franco

[sluckey]

Ok, I'm losing you now.  Which 10k dropper?  Is that right off of the front end of the AC power?

I'm trying to reduce plate voltage, so maybe I wont hear a difference, I'm looking for reliability.

It's on the schematic you posted. You said you wanted to "feed more power to the preamp tubes". What I suggested will do that.

[AlNewman]

May be this of your interest AlNewman ?


Franco

Yes, that is a beautiful thing, thank you.

[AlNewman]

Ok, I'm losing you now.  Which 10k dropper?  Is that right off of the front end of the AC power?

I'm trying to reduce plate voltage, so maybe I wont hear a difference, I'm looking for reliability.

It's on the schematic you posted. You said you wanted to "feed more power to the preamp tubes". What I suggested will do that.

Ok, on the back end of the power supply. 
So in your opinion, would that reduce plate voltage?

[sluckey]

So I'm kinda thinking out loud here, but as it sits, there's a 1k resistor to the screens, and a 4.7k resistor to the preamp.  Right now the 12ax7 are running about 270V, could I drop the screen resistance and maybe the preamp resistance and feed more power to the preamp tubes?  I know they don't take much current, but every little bit would help.

Everything I said after this quote from you has been in response to this quote. You thought the 4.7K was the dropper for the preamp, but that's wrong. You wanted more power to the preamp so I pointed you to the correct dropping resistor to do that. Now you're saying you want to reduce plate voltage. Make up your mind.

[AlNewman]

I see it as the 1k is a screen dropper, and the 4.7k is the dropper for the pre amp.  But I also see the phase inverter as part of the preamp, because the current draw is negligible.
I recently did some work on a Traynor amp, where somebody had replaced the "screen" resistor from 390R to 1k, and the "preamp" resistor from 10k to 100k.  I replaced 2 resistors, and I realized the voltages on the power tubes had dropped by 40 volts or so. 
I was just thinking out loud, like I said.

[sluckey]

The 1K is the screen dropper. The 4.7K is the PI dropper. The 10K is the preamp dropper.

I think I'm not understanding what you want to do. That's OK. Someone else will get it.

[AlNewman]

It's ok.  I see what you're saying.  And I'll figure it out. 

I think I'll just set the wattage to within el84 specs.  And maybe raise the screen grid resistor values.  It isn't my amp to play with voltages anyhow, I'll just do my best with what I have.

I do appreciate your help.  Thank you.

[Bieworm]

FWIW I built quite a few 18 watt amps in various circuits. Over at 18watt.com we have a" kind of" consensus about the 18W EL84 PP power amp. We aim for following values with pleasing results:

B+
A: 1st 32uf filter cap/EL84 plates: 340 DCV
B: 2nd 32uf filter cap/screens:      320 DCV
screen resistors 1k for each tube
Cathode resistor: 180R
Cathode bypass cap: 2200uf

But all is actually up to your personal preferences. My plates run more often between 345 and 355 DCV and I use a 1500uf cathode bypass cap because it fits better on my boards...

[AlNewman]

FWIW I built quite a few 18 watt amps in various circuits. Over at 18watt.com we have a" kind of" consensus about the 18W EL84 PP power amp. We aim for following values with pleasing results:

B+
A: 1st 32uf filter cap/EL84 plates: 340 DCV
B: 2nd 32uf filter cap/screens:      320 DCV
screen resistors 1k for each tube
Cathode resistor: 180R
Cathode bypass cap: 2200uf

But all is actually up to your personal preferences. My plates run more often between 345 and 355 DCV and I use a 1500uf cathode bypass cap because it fits better on my boards...

I'd like to see 345 volts at b+.  I wonder if the lower value filter caps would make a difference.  These are 100uf.  So what kind of current are you getting across your cathode resistor?

[Bieworm]

FWIW I built quite a few 18 watt amps in various circuits. Over at 18watt.com we have a" kind of" consensus about the 18W EL84 PP power amp. We aim for following values with pleasing results:

B+
A: 1st 32uf filter cap/EL84 plates: 340 DCV
B: 2nd 32uf filter cap/screens:      320 DCV
screen resistors 1k for each tube
Cathode resistor: 180R
Cathode bypass cap: 2200uf

But all is actually up to your personal preferences. My plates run more often between 345 and 355 DCV and I use a 1500uf cathode bypass cap because it fits better on my boards...

I'd like to see 345 volts at b+.  I wonder if the lower value filter caps would make a difference.  These are 100uf.  So what kind of current are you getting across your cathode resistor?

I have 11.68V across the 180R cathode resistor, with 348V on the plates …gives me 85.6% dissipation and sounds great.
I don’t think your bigger caps will affect the voltages a lot.. but they will make your amp more sterile

[tubeswell]

Did you replace the EL84s (in case they were damaged? Tubes that have been run too hot can suffer permanent damage inside like melted deformed screen support rods  which put the screed grid out of alignment, creating odd tube current characteristics)