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This amp came to me with a 4AB3C1B reverb tank. But as you can see in the schematic this type of tank doesn't look like the right one. There is no impedence matching transformer like you would find in amps using this type of tank. It appears maybe that a '4FB...' style tank is needed like other Ampegs. I thought I was in luck to have a 4FB.....on hand, but I can't seem to get this going either.
First some background, this amp came to me with a tube gone bad, white in the glass envelope. I also find the primary on that end has gone open. I also find there are 2 47K plate resistors opened on each of the phase inverters, that is on each channel.
So I replace the output transformer with what I assume is the correct one from flip tops. THe original OT has a 3 wire on the secondary that the new does not have. In this respect I am not seeing a connection to the reverb circuit from the positive end of the OT to the 25uf/25 back to back with the same value cap and the 330 ohm resistor. When I clip that in the reverb still doesn't work.
To complicate the issue, the owner recalls clearly that the reverb was work with the 4AB.... tank, but I can't trust my own recollection of whether it worked before the OT installation or not, but I thought it did too.
One problem is the schematic doesn't match identically with the the amp at hand. The difference is that this particular one has the 7591s and a tube rectifier. Also there is a mix match of which channel is which.
This amp clearly has the reverb connected to V5a and the voltages are looking good; plates at 170-180vdc, cathodes at 1.8v to 2.2v.
Can anyone shed some light on this for me?
I have read what Enzo has written on this amp about the reverb being 'speaker driven' like a Hammond reverbs but I'm not clear what that's about.
Is there anyway that 4AB... tank could have work in this amp?
First some background, this amp came to me with a tube gone bad, white in the glass envelope. I also find the primary on that end has gone open. I also find there are 2 47K plate resistors opened on each of the phase inverters, that is on each channel.
So I replace the output transformer with what I assume is the correct one from flip tops. THe original OT has a 3 wire on the secondary that the new does not have. In this respect I am not seeing a connection to the reverb circuit from the positive end of the OT to the 25uf/25 back to back with the same value cap and the 330 ohm resistor. When I clip that in the reverb still doesn't work.
To complicate the issue, the owner recalls clearly that the reverb was work with the 4AB.... tank, but I can't trust my own recollection of whether it worked before the OT installation or not, but I thought it did too.
One problem is the schematic doesn't match identically with the the amp at hand. The difference is that this particular one has the 7591s and a tube rectifier. Also there is a mix match of which channel is which.
This amp clearly has the reverb connected to V5a and the voltages are looking good; plates at 170-180vdc, cathodes at 1.8v to 2.2v.
Can anyone shed some light on this for me?
I have read what Enzo has written on this amp about the reverb being 'speaker driven' like a Hammond reverbs but I'm not clear what that's about.
Is there anyway that 4AB... tank could have work in this amp?
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