Different B+ on V4 & V5 ?

LPV

Member
Messages
700
I put a new pair of el34s in my traynor (modded to 2204) and noticed that the V4 plate is 425v and V5 plate is 385v . The OT is a Merren that I installed and I never noticed that they weren’t the same before (could have been, had brain surgery in 2016 and memories of 2015 are kinda vague :) ) . Been using it for 3 years like this, sounds great. But any idea what would cause this ? I just bias to the 385v (45ma) so that means that I’ve always got a tube running a little cold. About 10%.
 

HotBluePlates

Member
Messages
10,702
I put a new pair of el34s in my traynor (modded to 2204) and noticed that the V4 plate is 425v and V5 plate is 385v. ...
You're measuring actual plate current for both tubes?

Ifone tube drew more current, its plate voltage would likely be lower (voltage drop across OT winding resistance). Even if tube plate currents are identical, one side usually has a little less voltage because typically one side has higher winding resistance (normally this would be a few-volts different, not 10's-of-volts different).

So what do you have?
 

LPV

Member
Messages
700
You're measuring actual plate current for both tubes?

Ifone tube drew more current, its plate voltage would likely be lower (voltage drop across OT winding resistance). Even if tube plate currents are identical, one side usually has a little less voltage because typically one side has higher winding resistance (normally this would be a few-volts different, not 10's-of-volts different).

So what do you have?
Yes, measuring plate voltage at pin 3. Its not the tubes. Switched sockets, same thing.
 

HotBluePlates

Member
Messages
10,702
You're measuring actual plate current for both tubes? ...
Yes, measuring plate voltage at pin 3. ...
Gotta know the plate current as well. Then you can use Ohm's Law to understand what's happening.

Try this: Turn off the amp, measure voltage at pin 3 of the output tubes to confirm it falls to zero. Measure resistance of each half of the primary, from center-tap to pin 3, and make a note of the measured resistance and the plate voltage for that tube. Measure each tube's plate current by the OT Shunt method (or bias probe if your probe excludes screen current from the result). Post your notes of voltage, current & resistance here.
 




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