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Hello, I need a suggestion regards the value of resistor / cap for a quad of EL34 cathode biased, please. OT 1.9K, B supply 450V. Thanks
Let me know if is possible th run it in common mode with a single big power resistor placed somehow external on a heatsink ? The PT spec rated as 400mA for HT winding.
Last edited by catalin gramada; 03-07-2019 at 10:25 PM.
"If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."
A single, common, cathode resistor "smooths-out" possible conduction differences between the banked tubes (think "mean" value here), while separate cathode resistors can be individually "trimmed/optimized" to accomplish the same/better results.
Last edited by Old Tele man; 03-09-2019 at 12:36 AM.
...and the Devil said: "...yes, but it's a DRY heat!"
Be a "smarty trousers" and run
330 Ohms with 220uF/63V bypass on each tube but instead of returning these to 0V then
For each push pull pair add a common 68 Ohms unbypassed under the 330 Ohms to 0V.
That introduces some "conduction balance" and is good to supress some odd order harmonic distortion because of the common mode feedback it introduces.
Be careful that the common (to 2 tubes) 68 Ohms is on a push pull pair and not on 2 tubes on the same side of the push pull.
And be aware that this is a "cork sniffer" HiFi trick. You may not want that in your Git Amp.
You could take this a step further and make the shared 68 ohm resistance a 100 ohm power rheostat. It would only need to be a three watt wire wound pot (no bigger and not much more expensive than than a regular pot). That way you have an actual bias adjust and some balancing ability.
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If you want a Marshall sound you'll probably bias at 70%.
You've got 450 V +B. So you want 156 mA total quiescent current for all 4 x EL34.
From the following datasheet
we get that you want the grids at about -28VDC.
So 450-28 = 422.
You want 422 volts across the EL34's and 28 volts across Rk.
28 = Rk * 156mA
Rk = 179,49 OHMs
If you want individual cathode resistors, multiply by 4.
And Bob's your uncle.
A problem - the curves you are using are for a screen grid voltage of +250V.
The data Dave H linked is more suitable in this case.
The "rule of thumb" for the 3rd harmonic cancellation scheme is to use a common resistor equal to 13 to 15% of the single tube cathode bias resistor , hence 68 Ohms as nearest preferred value.
There are idle currents of 2 tubes in that so it effectively is the same as 2 x 68 = 136 Ohms in each cathode (as far as developing bias voltage goes).
465 - 136 gives me the 330 Ohms for the individual resistors as the nearest preferred value.
That is how I arrived at the values in my post. The common resistor needs to be unbypassed. the individual resistors should be bypassed, 220uF is middle ground suggestion but certainly you could try 100uF or 470uF.
Similarly - for EL84 amps (270 Ohms individual cathode bias is standard) I use 39 Ohms Common and 180 Ohms as the individuals to get an effective cathode bias resistance (per tube) of 258 Ohms
I occasionally use 2 x 390 in parallel = 195 Ohms instead of the 180 for slightly colder bias.
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