Mark iii with EL34 - Bias%?

Hi, I have a Dynaco Dynakit Mark III amp with weak and unmatched 6550 output tubes.
At the moment I can't get here a pair of new 6550 / KT88, but I have a new pair of EL34.
So I would like to use it as mark ii.
I tried to change the resistor that connecting the bias pot to ground from 18K to 7K7 and took some measurements on the amp. Without output tubes the palate voltage is 520V. With -50V at the wiper of the bias pot the plate voltage is 470V but the EL34 draws only 10mA so the bias seems to be too cold. For a higher plate dissipation value I was needed to reduce the bias voltage at the wiper to -30V and the plate voltage was 370V and the tubes draw 48mA which is 17.76W or 70% of 25W.
Does it seems to be normal? or maybe this amp designed to run on cooler bias and 70% is too much for it?

You misunderstand Peter , he is saying that he would like to reduce the plate voltage to 370V for EL34.

@smilan , IIRC , the ST70 runs a smidge over 400V (415V on my ST70) on EL34 anodes . 70% is not too much , I reckon 75% .

415V x 45mA = 18.5W total dissipation per plate .

To drop 55V from 470V @ 45mA x 2 tubes = 90mA

55V/0.09A = ~600 ohm resistor , make it 680 ohm for a standard value (it will drop 61V) , for 409V on anodes

55V x 0.09A = 4.95W , so make it a 680 ohm 10W resistor to be safe . Insert in series between choke and 20uF half of can cap .



Then , for lowering bias I would install a 100k pot between the 2 100k resistor going to EL34 grids , so with first pot you can set bias level and 2nd pot you can balance between the 2 tubes . In order to achieve measuring output tubes individually you need to remove the 11.2 ohm resistor and have 2x 10 ohm cathode resistors , one between cathode and ground per EL34 . Those 5w wire-wound cement resistors should do fine . Reading 0.45v over each 10 ohm resistor will equal 45mA per tube .


On my amp I have bias level and balance 2w pots mounted to the top plate , with a banana socket behind or next to each EL34 . Each banana socket is wired to the junction between each EL34 cathode and each 10 ohm resistor . See EL34 datasheet curves to approximate neg grid voltage required . The 100k ohm balance pot should drop a couple of volts . Re-install the neg grid voltage resistor you removed (swap back to 18k) . EL34 requires lower grid voltage than KT88 .

Don't forget to connect pin 1 to pin 8 on the EL34 tube sockets if you are going to use them in place of a KT88 / 6550 which has this done internally and pin 1 is free.  Make sure before you do that pin 1 has not been used for another purposes.

sayntjack wrote:Don't forget to connect pin 1 to pin 8 on the EL34 tube sockets if you are going to use them in place of a KT88 / 6550 which has this done internally and pin 1 is free.  Make sure before you do that pin 1 has not been used for another purposes.

Yes ! I forgot about that G3 should be connected to cathode , on KT88 connection is internal .




Grid voltage should be in -30V region , would be good to have some wiggle room either way

Two more changes I would make to run EL34 instead of KT88 ...

Change the 2x grid resistors from 100k to 220k , and then , to keep phase-margin/step-response the same as before , change the 220nF caps to 100nF/630V .

The 6800ohm resistor at TP5 will also require lowering to either 5600ohm or 4700ohm to correlate with what measurement should be at TP5

Schematic updated with changes to 6800 resistor at TP5 to 3900 ohm .