Bias Wrong?

kam4ff

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So a year or so ago, I had my tubes swapped in my 2203. It came back sounding better than it had going in, but still not great. Well the other day I decided it was sounding too weak, and thought maybe I’d just swap the preamps, and try 6ca7s in the power amp. I set up my bias probe on the old el34s, and it only registered 20ma. Put in the new 6ca7s and brought it up to 38ma, and the amp sounds so much better. So much more life.

Anyone hear of ma dropping over time or is it likely my past amp guy made an error?
 

Cjsinla

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You put in new preamp tubes and the bias went up to 38ma?
 

kam4ff

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No. I put in new pre amps, still didn’t dig the sound, put it new 6ca7s and biased it up to 38. But I checked the el34s bias before I swapped them and they were at 20
 

Cjsinla

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Did you try biasing the EL84’s up to 38 before you put the new tubes in? Tube values can drift over time, not sure that they could go from 38 to 20. I recently learned how to bias and checked 2 of my Marshall’s and they were both at 30 and they had both been serviced by the same guy. Not sure if the values had drifted but I do remember him telling my something about 60% or 70% but it was all Greek to me at that point, damned if I can remember exactly. Whatever he had done they both sounded great, I assume there is a little leeway there. Not sure about setting an amp to 20 however.
 

Matt_Krush

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If those EL34's were at 20ma, that's way too low.
I tried the 6CA7's, they were ok, but I like the KT77's best so far.

You went to 38ma...this based on calculation from plate voltage or ear?
 

Sean French

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What year 2203 and what is it's plate voltage?
20 ma is way too low regardless.
IME , those amps sound their best between 60% and 65% max dissipation.
 

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