02-20-2018, 08:23 PM
Hey folks,
New to the forum but have had a V4B and V4 (mid-70's, not distortion model) for around fifteen years. I having the same problem with both amps and am at wits end. On the scope with a 1K tone, each amp outputs a clean 102 Watts. When pushed to their limit with a guitar as the source, one pair of power tubes slowly starts to red plate. This is consistent in both amps.
I have basically rebuilt the power sections of both amps with no change in performance.
-Recapped both amps - 2x 40uf were replaced with 2x100uf (*tried paralleling a 40uf cap with each 100uf cap as seen in some schematics with no change observed)
-screen grid supply cap was replaced with a 20uf 600v (also tried 100uf cap here with no change observed)
-Screen grids upgraded to 1k 5W
-Plate load resistors replaced with stock value (3.6 ohm)
-R55 470omh 7W replaced
-Control grid resistors R37, R38, R43, R44 replaced with stock value
-Bias supply caps replaced with stock values
-Bias network branch resistors R35, R36 (100k) replaced with stock value
-C11 and C13 replaced multiple times (orientation swapped, no change observed)
-All diodes in amp checked out (*fly back diodes removed with no change observed)
-PI and power tube sockets replaced in one amp
-Re wired power section of one amp
-Checked all traces in power section for breaks (Went to the length of recreating the traces in the power section of one amp in a point to point fashion with bus wire. I know this adds resistance to the circuit, it was done as a mere experiment to rule out broken traces)
At one point, I drove the output tubes with a Marshall 2203 OT and observed the same red plating issue
Plate voltage is 535
Screen grid voltage is 535
One amp had old Ruby 7027's, the other amp had newer JJ 7027's - changing tube order does not alter the problem
**I did mod one amp to run 6550's and this did not alleviate the issue
Interesting observations:
-the pair of tubes that red plate exhibit a blue haloing effect
-I attached a Fluke 187 to each pair of power tubes control grids to actively monitor the bias supply. The pair that does not red plate, the bias voltage remains consistent. The pair that red plates, the bias voltage fluctuates.
Any info or suggestions anyone could send my way would be so greatly appreciated!
-Mitch
New to the forum but have had a V4B and V4 (mid-70's, not distortion model) for around fifteen years. I having the same problem with both amps and am at wits end. On the scope with a 1K tone, each amp outputs a clean 102 Watts. When pushed to their limit with a guitar as the source, one pair of power tubes slowly starts to red plate. This is consistent in both amps.
I have basically rebuilt the power sections of both amps with no change in performance.
-Recapped both amps - 2x 40uf were replaced with 2x100uf (*tried paralleling a 40uf cap with each 100uf cap as seen in some schematics with no change observed)
-screen grid supply cap was replaced with a 20uf 600v (also tried 100uf cap here with no change observed)
-Screen grids upgraded to 1k 5W
-Plate load resistors replaced with stock value (3.6 ohm)
-R55 470omh 7W replaced
-Control grid resistors R37, R38, R43, R44 replaced with stock value
-Bias supply caps replaced with stock values
-Bias network branch resistors R35, R36 (100k) replaced with stock value
-C11 and C13 replaced multiple times (orientation swapped, no change observed)
-All diodes in amp checked out (*fly back diodes removed with no change observed)
-PI and power tube sockets replaced in one amp
-Re wired power section of one amp
-Checked all traces in power section for breaks (Went to the length of recreating the traces in the power section of one amp in a point to point fashion with bus wire. I know this adds resistance to the circuit, it was done as a mere experiment to rule out broken traces)
At one point, I drove the output tubes with a Marshall 2203 OT and observed the same red plating issue
Plate voltage is 535
Screen grid voltage is 535
One amp had old Ruby 7027's, the other amp had newer JJ 7027's - changing tube order does not alter the problem
**I did mod one amp to run 6550's and this did not alleviate the issue
Interesting observations:
-the pair of tubes that red plate exhibit a blue haloing effect
-I attached a Fluke 187 to each pair of power tubes control grids to actively monitor the bias supply. The pair that does not red plate, the bias voltage remains consistent. The pair that red plates, the bias voltage fluctuates.
Any info or suggestions anyone could send my way would be so greatly appreciated!
-Mitch