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-   -   Cathode resistor for EL84 P-P Triode wired (https://www.diyaudio.com/forums/tubes-valves/366543-cathode-resistor-el84-triode-wired.html)

 itsikhefez 21st January 2021 04:49 PM

Cathode resistor for EL84 P-P Triode wired

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Hi,

I've searched extensively for a concrete answer here and couldn't find one.
I've built an EL84 P-P in UL (7.6K OPT)
It is cathode biased and has a 270ohm resistor per tube, B+ around 310-315V, giving 35-39mA bias per tube.

For converting this amp to Triode (without replacing the OPT), do I need to increase the cathode bias resistor from 270ohm to 560ohm (per tube)?

A high-level explanation on the reasoning would be helpful too :)

Going off of the attached Mullard EL84 datasheet.

P.S Amp is a Tubelab SPP but question is generic.

 Steve Morley 21st January 2021 05:23 PM

Dou you mean a Tubelab SPP (Simple Push Pull)?

 itsikhefez 21st January 2021 05:31 PM

Yes, silly me!
Corrected

 TG 21st January 2021 07:58 PM

Quote:
 Originally Posted by itsikhefez (https://www.diyaudio.com/forums/tubes-valves/366543-cathode-resistor-el84-triode-wired-post6498177.html#post6498177) A high-level explanation on the reasoning would be helpful too :)
The datasheet gives you Rk values for ultralinear ("distributed load") mode in class A and for triode mode in class AB. Hence the difference in Rk (and in quiescent current) for the same plate and second grid voltages.
If you're OK with like 3-4W output in the class A triode mode, you can keep Rk intact.
If you want more watts, you'll have to bias the amp for class AB, increasing the Rk and decreasing the quiescent current.

 Steve Morley 21st January 2021 08:14 PM

Itsik,
Be careful running upwards of 35mA per EL84. You are getting close to their dissipation limit. Not so bad with the Russian tubes, generally speaking, but the JJ EL84 has a bit lower limit. If you are thinking of running triode it might be safe to increase Rk a bit even if you don't go as far as 560 ohm. Morgan Jones ran 560 ohms on his EL84 in triode for about 5 watts output.

S.

 Steve Morley 21st January 2021 08:25 PM

DIY TUBE BIAS SCHEME

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The DIY Tube Dynaclone (no longer available) used an adjustable cathode bias scheme to optimally balance the current flow through the output tube pairs. I've used this on a few EL84 amps and it could be helpful for cranking back current flow in triode mode too.

S.

 itsikhefez 21st January 2021 08:27 PM

Thanks both.. TG, that clarification is very helpful.

Steve, I may not be calculating it correctly. What I have been doing is dividing the voltage across the cathode resistor by 270.
Most tubes measured around 10.5V, which would lead to 39mA

 Steve Morley 21st January 2021 10:30 PM

That's correct, 10.5/270= 0.039A
To calculate power dissipated by the tube subtract Vk (10.5 volts) from your B+ (310 to 315) times 0.039A= 12 Watts.

It would be more accurate to measure the voltage at the anode of the tube (it will only be a couple of volts lower than B+) then subtract Vk. But B+ will be close enough.

Most Russian-made EL84s will dissipate approx. 16 watts (14 anode + 2 G2). So if you are running your EL84s at about 12 watts you are OK but personally I wouldn't go much higher.

JJ EL84s are rated at about 14 watts (12 + 2). These would be run a little too hard in your rig if they draw more 35mA. It's not serious but you should keep an eye on it.

Cheers, S.

 Diabolical Artificer 22nd January 2021 08:19 AM

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Quote:
 For converting this amp to Triode (without replacing the OPT)
The required Z pri for triode connected valves are usually higher, see attached. For your B+ etc a Z pri of 10k is recommended. Using a less than optimum Zpri will alter power out,THD etc.

Re choosing the right Rk, you could just temporarily wire in a 500r pot and tweak it whilst monitoring Ik, power out, THD etc. If you wire the pot in series with a 100r fixed resistor you can use the 100r as a "sense" resistor to measure Ik. Make sure the pot is capable of handling the power and wire it as a rheostat, EG wper connected to one end tag.

Andy.

 kodabmx 22nd January 2021 09:30 AM

Not just any 500R pot... a 5W WW rheostat would be needed if you don't want the pot to smoke.

026TB32R501B1A1 CTS Electronic Components | Mouser Canada

They are so much cheaper than I expected, why not just use one per tube :D

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