Help with my el509 amp almost done but with one problem.

[NotInTheMafia]

Hello all
i recently built a PP el509 amp, but i am having a problem with it.

When a put my meter across TP 1 & 2 i cant get the voltage difference to less than 0.8v. The voltage from tp1&2 w.r.t. gnd hits 0.7v when the b+ hits about 300v before this it is about .3v
only goes up as i crank up the b+. I checked and rechecked the output section and cant find an error, i get the 150v on pin 3 and the -bias voltage in pin 1

Its just the TP volts thats the problem.
any help would be appreciated.
Thanks
--Tom

[NotInTheMafia]

Sorry, should have said i can NOT get the voltage diff to less than .8v between the TPs. also bias supply uses either 48v zener or 53v zener because all i have is 12 and 5v zeners to string togeather. i used the only 2 51v zeners i had in the screen reg circuit.

[Martin Hayes]

Perhaps on a pair of EL509s running as close to maximum dissipation as these would be in this circuit, a voltage difference with respect to ground of 0.7v between their cathodes is too much.

But is it realistic to expect to get the difference to zero when the dual gang 10K pot (P1) used to adjust the grid bias is joined at the hip? I don't get why that is. I would consider getting another pot of the same value and voltage rating and rewire the circuit so you can trim each tube's bias separately. The tubes are going to differ from each other, particularly as they age, and in the circuit as it is presently configured, there is no way to accommodate tube asymmetry.

[Martin Hayes]

I see from the schematic that the dual gang bias trimmer is wired to work in opposition, so that the voltage at one of the wipers will be rising as the voltage at the second wiper will be dropping.

Have you checked that this is the case?

[NotInTheMafia]

Everything works correct, untill i get the rail voltage up to about 420v, then the cathode voltage difference gets to out of wack, both tubes will sith with their cathodes at around a volt or more above ground when i turn the voltage up much more.

Think im gonna wire with 2 pots seperate and change the resistor value so i can increase the voltage ranges. The pot must be turned all the way in order to maintain ballance, after 425volts i cant compensate any more. Mabey need to find closer matched tubes??

I still cant figure out whats up with the Voltages at TP1&2, they should be no more than .3v above ground but they sit so high. All other voltages are correct.

Thanks
Tom

[Martin Hayes]

The EL509 datasheet states that the g1 should see no more than 100K resistance to the bias voltage source. Since R16, R17 are already 100K, you should consider lowering this value if you want to gain more "spread" in the P1 pot by increasing its value (beyond 10K).