Bogner Twin Jet Bias mod. From EL34 back to KT88's

1meanplexi

1meanplexi

Well-known member
Hey crew I hope all is well. I have a question for our tech guys. I have a Bogner Twin Jet that had originally came with KT88’s. It was modded to run EL34’s. I attempted to mod it back by piggybacking (in the 402 position off the bias pot) with the original resistor that was removed...well, the KT88’s I installed were on the verge of Red plating. The bias pot would only take it down to 122 MA’s!!!!! What the hell did I do wrong????? Pics included of the mod I did.







 
Looks like you have two 22k in parallel to make 11k. Looks like a 33k behind it. Without knowing the amplifier or the schematic, it looks like a simple half wave rectifier Marshall bias circuit layout.

It has the same resistor in it now that you added to it in parallel. In reality you need more negative voltage, so the resistor needs to go up in value, not down which you’ve done by adding in parallel.

The correct answer is to take it to a tech that can lift the board and do it correctly. I’m not going to advise you do anything else.

Besides the fact, if you can afford a bogner twin jet, you can afford quality work.
 
glpg80":2rnp2t51 said:
Looks like you have two 22k in parallel to make 11k. Looks like a 33k behind it. Without knowing the amplifier or the schematic, it looks like a simple half wave rectifier Marshall bias circuit layout.

It has the same resistor in it now that you added to it in parallel. In reality you need more negative voltage, so the resistor needs to go up in value, not down which you’ve done by adding in parallel.

The correct answer is to take it to a tech that can lift the board and do it correctly. I’m not going to advise you do anything else.

Besides the fact, if you can afford a bogner twin jet, you can afford quality work.
Click to expand...

Hi.

Thanks for the quick reply. My basis for this was from a thread I found here on RT from many years ago. I figured it out and she is a beast with those KT88's in it.
 
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