Help with HD 212-130/2475-130 Bad Hum

SteveFundy

Hello all. I am trying to correct a bad hum & 2 power tubes redplating. I am unfamiliar with this type of bias circuit. I thought one of the 25uf/20V caps might be bad, but C43 tests good out of circuit & I also tested what I believe is C42. Also good.
The tubes are 6L6 and the 2 furthest from the DB-2 are redplating, the other 2 are normal. Transistor bad? Any insight would be greatly appreciated!
Steve

Music Man Equipment
MM 2-10-65

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11 years 2 months
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mm210 Fri, 02/22/2019 - 06:20

FIRST off, that amp was NOT designed for 6L6GC tubes. It was designed for 6CA7's or EL34's. I'm assuming since you say DB2 that it's the solid state driver so put the right tubes in it, follow the procedure outlined in the tech notes section of this site for amps with SS drivers and THEN see what it does. Mike

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Steve Fundy
Music Man Equipment
HD 212-130 Solid State Driver

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6 years 3 months

SteveFundy Fri, 02/22/2019 - 14:42

Thanks Mike. Yes I see per the schematic that the amp is supposed to have EL34's. It came to me with 6L6GC. I just replaced 6L6 with EL34 and attempted to adjust bias as per instructions. I was able to get 25mV across one 3.9 ohm, (one closer to rear of amp) but the other is 2.4 V. Way off. The 2 tubes were glowing bright red. I tried more adjustment but not even close. I turned the amp off so as not to do any further damage. OK, any ideas? Thanks, Steve

Music Man Equipment
MM 2-10-65

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11 years 2 months
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mm210 Sat, 02/23/2019 - 07:19

First guess is driver transistor is toast. Replace both with at the same time.

Full Name
Steve Fundy
Music Man Equipment
HD 212-130 Solid State Driver

Member for

6 years 3 months

SteveFundy Sat, 02/23/2019 - 23:23

That was what I had thought. I measured the resistance (in circuit) and one of them had pretty odd measurements. I will see if I have any or track some down on internet. Thanks Mike.