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Biasing For Power Tube or Transformer
#1
I was revisiting my p1x the other day and had a few questions.

I have the Heyboer 20w OT on mine but was wondering about he biasing calculations. I know I read somewhere that the bias is set at 90% of maximum anode dissapation. Doesn't that exceed the OT specs for the 125ESE? I would imagine the 20W heyboer would handle this better though.

My other question is primary impedence for EL34 in single ended... Most of the spec sheets seem to suggest 2k or 3k but that is usually with ~250VDC anode voltage. Is the 4K tap in the 20 SE building block schematic suggested because of the higher anode voltage?

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#2
Quote:On 11/09/2015 @ 12:52, boblove wrote :
I was revisiting my p1x the other day and had a few questions.
:
: I have the Heyboer 20w OT on mine but was wondering about he biasing calculations. I know I read somewhere that the bias is set at 90% of maximum anode dissapation. Doesn't that exceed the OT specs for the 125ESE? I would imagine the 20W heyboer would handle this better though.
:
: My other question is primary impedence for EL34 in single ended... Most of the spec sheets seem to suggest 2k or 3k but that is usually with ~250VDC anode voltage. Is the 4K tap in the 20 SE building block schematic suggested because of the higher anode voltage?
:

Hi Bob

My understanding is that with a single ended EL34 power stage your average output to the load will not be anywhere close to 25W .. more likely 10-12 watts depending on anode voltage and biasing arrangement. I'm sure you must have seen Merlin's info and calculations regrading SE power stages already:

http://www.valvewizard.co.uk/se.html

As to your other question, I would say that yes the 4K tap is used due to the higher anode voltage. Although why not use the 6K tap for better center bias at the 370v anode shown for the EL34?

From what I can tell, if you wanted to back off a little and bias a bit colder then this will also suite a higher impedance, so 6K should be good.

Some time ago I purchased the 20W Doberman OT but never got around to replacing the 125ESE that came with the P1-X kit. This has motivated me to hook it up. I will try both the 4K and 6K taps and let you know which I prefer.

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#3
Quote:On 11/09/2015 @ 12:52, boblove wrote :
I was revisiting my p1x the other day and had a few questions.
:
: I have the Heyboer 20w OT on mine but was wondering about he biasing calculations. I know I read somewhere that the bias is set at 90% of maximum anode dissapation. Doesn't that exceed the OT specs for the 125ESE? I would imagine the 20W heyboer would handle this better though.
:

Datasheet says 11 watts from an EL34 in single-ended class A.

125ESE is a 15W transformer.
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#4
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