KVH
Member
- Joined
- May 3, 2020
- Messages
- 44
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So this amp was plugged into a cabinet that had issues with the 16ohm input jack. Basically it was operated without a load.
One of the HT fuses was blown and the 1.5k grid stopper was cooked. Fuse replaced and new resistor in place. Input jack PCB on Cab re-soldered and now reads 13.6 ohms.
Pin 5 to ground on tubes 1 and 4 read 85vdc. Tubes 2 and 3 have the 1.5Ks mounted on PCB
and read the same. Adjusting bias pot lowers to 71V. This is with all power tubes out of amp. This amp was made just when Marshall was transitioning from EL34s to 5881s.
R31-34 are 1K
R28 is 22K
R2915K
I've read 42vdc to 56vdc is what I should be getting depending on which tubes it's set up for.
Where should I go from here?
Also on the EL34 schematic R28 is listed as 47k and VR1 as 22K but it's hard to read.
One of the HT fuses was blown and the 1.5k grid stopper was cooked. Fuse replaced and new resistor in place. Input jack PCB on Cab re-soldered and now reads 13.6 ohms.
Pin 5 to ground on tubes 1 and 4 read 85vdc. Tubes 2 and 3 have the 1.5Ks mounted on PCB
and read the same. Adjusting bias pot lowers to 71V. This is with all power tubes out of amp. This amp was made just when Marshall was transitioning from EL34s to 5881s.
R31-34 are 1K
R28 is 22K
R2915K
I've read 42vdc to 56vdc is what I should be getting depending on which tubes it's set up for.
Where should I go from here?
Also on the EL34 schematic R28 is listed as 47k and VR1 as 22K but it's hard to read.