RE: Have you tried a know, good set of EL84?

Posted by Jim McShane (D) on July 14, 2016 at 11:19:22

In Reply to: RE: Have you tried a know, good set of EL84? posted by Eli Duttman on July 14, 2016 at 10:41:06:

Or you can simply install a new production Tung-Sol or Genalex - problem solved!

BTW, if it was my amp I'd remove the 18 Ohm resistor and install a 10 Ohm 1 watt resistor at EACH EL84 tube cathode so the cathode current goes through the resistor to ground. Then you can measure the voltage at EACH tube's cathode and use Ohm's Law to calculate the voltage:

Cathode current = voltage at pin 3 divided by 10.

So if you wanted 30 ma per tube you simply use the bias and DC balance pot to adjust so you see 0.35 volts at pin 3 of each tube. You'll have to go back and forth with the bias and DC balance pots to get both tubes right since they interact. Set the DC balance pot at midpoint then adjust the bias pot until you see close to .30 volts at the cathodes. Then adjust the DC balance so the tubes show equal voltages at the cathode. Finally a last adjustment of the bias pot should get the tubes spot on - if not repeat the sequence once more and you are set.

If you can't get enough bias current then either the tubes are tired (put some EL84M Sovteks in there, they are VERY rugged and can survive plus they sound excellent), the plate or screen voltages are too low, or the heater voltage is low.

PLEASE read my FAQ post on this topic, the link is below. Item #4 is particularly important.

Take a look and let me know what questions you have. Do this resistor change once and life becomes MUCH easier both now and in the future!

Tube Problem FAQ (Open in New Window)

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Follow Ups Full Thread

Topic - insufficient range on bias pot scott 299b - thermionic emitter 22:39:37 07/12/16 (24)

Here is another idea and someone may have suggested it but - Michael Samra 14:38:59 07/14/16 (0)

RE: insufficient range on bias pot scott 299b - DaveV 19:06:01 07/13/16 (16)

RE: insufficient range on bias pot scott 299b - thermionic emitter 19:26:19 07/13/16 (15)

RE: insufficient range on bias pot scott 299b - Michael Samra 14:41:12 07/14/16 (0)

There is an easier way to do this. - Michael Samra 19:52:39 07/13/16 (13)

RE: There is an easier way to do this. - DaveV 20:39:24 07/13/16 (12)

RE: There is an easier way to do this. - thermionic emitter 22:10:36 07/13/16 (11)

RE: There is an easier way to do this. - DaveV 01:03:57 07/14/16 (0)

Have you tried a know, good set of EL84? - FenderLover 23:48:34 07/13/16 (9)

RE: Have you tried a know, good set of EL84? - Eli Duttman 10:33:08 07/14/16 (1)

I saying trying another set of tubes... - FenderLover 18:15:56 07/14/16 (0)

RE: Have you tried a know, good set of EL84? - thermionic emitter 09:12:34 07/14/16 (6)

RE: Have you tried a know, good set of EL84? - Eli Duttman 10:41:06 07/14/16 (5)

RE: Have you tried a know, good set of EL84? - Jim McShane 11:19:22 07/14/16 (4)

Server Not Found - Triode_Kingdom 12:01:37 07/14/16 (3)

Got it - Nice Article Jim! (nt) - Triode_Kingdom 13:27:12 07/15/16 (0)

RE: Server Not Found - Jim McShane 17:57:13 07/14/16 (0)

RE: Server Not Found - Eli Duttman 12:52:10 07/14/16 (0)

The 6p14p tubes look like they are running hot in photo - Michael Samra 09:54:27 07/13/16 (2)

His bias seems too cold, not hot - FenderLover 17:56:37 07/13/16 (1)

RE: His bias seems too cold, not hot - Michael Samra 19:42:09 07/13/16 (0)

RE: insufficient range on bias pot scott 299b - FenderLover 00:07:57 07/13/16 (2)

RE: insufficient range on bias pot scott 299b - thermionic emitter 10:38:45 07/13/16 (1)

RE: insufficient range on bias pot scott 299b - Palustris 12:54:22 07/13/16 (0)

Follow Ups

RE: Have you tried a know, good set of EL84? - Jim McShane 11:19:22 07/14/16 (4)

Server Not Found - Triode_Kingdom 12:01:37 07/14/16 (3)

Got it - Nice Article Jim! (nt) - Triode_Kingdom 13:27:12 07/15/16 (0)

RE: Server Not Found - Jim McShane 17:57:13 07/14/16 (0)

RE: Server Not Found - Eli Duttman 12:52:10 07/14/16 (0)

RE: Have you tried a know, good set of EL84?

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