Recently did a repair on an older 20-20 amp (about 10 yr) and found some of the circuit had been altered on the newer models. I will outline the changes made as they are what is on the only current available schematic.
1) input downstream of 4k75 schematic shows 10k & 47k to ground and older amps used an 8k2 1/4w here. Value
wise 10k & 47k works out to 8k2. They're located one each lower side of the V1 socket.
2) on the bias supply (E) downstream of the 33k resistors schematic shows 330k resistors and older amps used
220k. These are the top 2 resistors between V3 & V4 and between V5 & V6. (1 for ea el84)
3) Between the A & B supplies on the PS board the schematic shows 1ea 220r and 10ea 330r in series. The older
amps had 16ea 220r in series, with 5 in front and 11 to the rear of the end 30uf filter cap. 3k52 either way.
4) Between the B & C supplies on the PS board the schematic shows a 56k and the older amps had a 47k5. This is
the end resistor after the row of 11ea 220r on the rear side of the PS board.
5) On the main DC supply rail, on the downstream side of the standby switch before the A tap, the schematic shows
a 120r 10w resistor that the older amps did not have. I believe it is an Ohmite 40J120E.(pretty sure)
6) There is one other maybe if needed factory change and that is the 250pf cap on the upstream side of the PRSNC
pot should be changed to a 500pf if the amp has oscillation issues.
The older version of the amp I found that with the amps original values the high voltage readings were consistently 10 to 20 volts above the listed values. This all came about due to V6 EL84 redplating even with new tubes. It ended up being a socket contact issue that is a result of the limited space available when changing the power tubes and not being able to rotate them equally as you remove and install them. A stainless dental pick is the tool to use to tighten them up followed with a little DeOxit on the sockets and pins. Talking to the factory guys the value changes are to lower the DC rails a tad as the EL84s run pretty darn hot in the older models.
EL84 plates, 396-398 versus 380. Grids were within 2-3 volts. Bias less than .5v off.
12AX7 V1 plates 159-162 versus 148. Cathodes within .13-.16v.
12AX7 V2 & V7 PI's plates 262-264 versus 250 and 281-282 versus 258.
Supplies
A- 402 versus 390, B-326.3versus 326, C-253.2 versus 242, D-2.32 versus 2.5, E- (-)14.43 versus (-)14
Will post the differences after a few changes. REMEMBER, DON'T LET EM LAY NO BOOJIE WOOJIE ON THE KING OF ROCK AND ROLL.
1) input downstream of 4k75 schematic shows 10k & 47k to ground and older amps used an 8k2 1/4w here. Value
wise 10k & 47k works out to 8k2. They're located one each lower side of the V1 socket.
2) on the bias supply (E) downstream of the 33k resistors schematic shows 330k resistors and older amps used
220k. These are the top 2 resistors between V3 & V4 and between V5 & V6. (1 for ea el84)
3) Between the A & B supplies on the PS board the schematic shows 1ea 220r and 10ea 330r in series. The older
amps had 16ea 220r in series, with 5 in front and 11 to the rear of the end 30uf filter cap. 3k52 either way.
4) Between the B & C supplies on the PS board the schematic shows a 56k and the older amps had a 47k5. This is
the end resistor after the row of 11ea 220r on the rear side of the PS board.
5) On the main DC supply rail, on the downstream side of the standby switch before the A tap, the schematic shows
a 120r 10w resistor that the older amps did not have. I believe it is an Ohmite 40J120E.(pretty sure)
6) There is one other maybe if needed factory change and that is the 250pf cap on the upstream side of the PRSNC
pot should be changed to a 500pf if the amp has oscillation issues.
The older version of the amp I found that with the amps original values the high voltage readings were consistently 10 to 20 volts above the listed values. This all came about due to V6 EL84 redplating even with new tubes. It ended up being a socket contact issue that is a result of the limited space available when changing the power tubes and not being able to rotate them equally as you remove and install them. A stainless dental pick is the tool to use to tighten them up followed with a little DeOxit on the sockets and pins. Talking to the factory guys the value changes are to lower the DC rails a tad as the EL84s run pretty darn hot in the older models.
EL84 plates, 396-398 versus 380. Grids were within 2-3 volts. Bias less than .5v off.
12AX7 V1 plates 159-162 versus 148. Cathodes within .13-.16v.
12AX7 V2 & V7 PI's plates 262-264 versus 250 and 281-282 versus 258.
Supplies
A- 402 versus 390, B-326.3versus 326, C-253.2 versus 242, D-2.32 versus 2.5, E- (-)14.43 versus (-)14
Will post the differences after a few changes. REMEMBER, DON'T LET EM LAY NO BOOJIE WOOJIE ON THE KING OF ROCK AND ROLL.
