Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fkadi on March 05, 2025, 03:20:30 am
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Hi,
I want to put a pair of new "matched" output tubes in a 1979 Fender Twin Reverb Ultralinear 135W. I do not want to make any modifications to the amp but I want to make sure that the tubes are balanced. I understand that the output Tube matching pot can be used but what is the exact way to do so precisely?
Any info would be appreciated.
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- Set the main bias pot to max 'cold';
- Set the balance pot in the center;
- Now turn up the main bias pot until the idle current in one half of the OT hits your target bias current (the other half should now be less than or equal to this figure, depending on tube matching);
- Now adjust the balance pot until current in both halves of the transformer are equal (i.e. the measurement in [3] will cool off a bit, and the other side will hot up a bit, until they are matched).
Schem:
https://el34world.com/Forum/index.php?topic=30328.0 (https://el34world.com/Forum/index.php?topic=30328.0)
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- Set the main bias pot to max 'cold';
- Set the balance pot in the center;
- Now turn up the main bias pot until the idle current in one half of the OT hits your target bias current (the other half should now be less than or equal to this figure, depending on tube matching);
- Now adjust the balance pot until current in both halves of the transformer are equal (i.e. the measurement in [3] will cool off a bit, and the other side will hot up a bit, until they are matched).
Schem:
https://el34world.com/Forum/index.php?topic=30328.0 (https://el34world.com/Forum/index.php?topic=30328.0)
Thank you for the reply, but I am not sure I understand. On the amp there is no main "bias pot" it has the "Output tube matching" pot only.
Can you clarify please? Thanks
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Ah I see, the bias is factory set. In that case simply adjust the balance matching pot until the idle current in each half of the OT is equal.