70s Ampeg V4B Blows Fuses

[ca7922303]

This is OK. 6.3VAC coil is "floating" because it is not grounded.
If you don't have a hum balace pot, connect 2 resistors 100 Ohm/0.5W in a seria.
Medium resistors connection solder to pin8 (c) tube V5 (6L6GC)
Ends resistors connections solder to pin2 (f) and pin7 (f) tube V5 (6L6GC)

I have a hum balance pot on order. Thanks for the info

[ca7922303]

The OL reading from 2 & 7 to ground on power tubes is because the hum balance pot is missing. I just wanted to verify that the person that removed it didn't already install resistors there. So you should replace with a hum balance pot or use resistors like vintagekiki described in post #307.

V201 pin3 voltage is grossly out of whack. It is not due to the slight difference of R203 being 7.5K or 10K, but a bigger problem. Does R203 still measure 10K? What does R202 measure? If they are both good, remove the tube and measure resistance from pin3 of the socket to ground. Also check R204.
V1 pin3 and V201 pin3 voltages are the only ones that are grossly out of whack. The others are close enough for the amp to work.

The solder joints on the preamp tube sockets are a problem in these amps and cause lots of intermittent issues. When I work on a V4 or V4B I resolder them all. For something like V201, after you verify all resistances are good, you can still have a problem because of the solder at the tube socket. That's why I suggested checking resistance directly from socket pin3 to ground (from tube side with tube removed).

R203(9.86k), R204(229k), R202(120k)burnt in half. V1 pin#3 to ground(1.0 ohms), V2 pin#3 to ground(1.0 ohms), V3 pin#3 to ground(48.5k), V4 pin#3 to ground(1m), V201 pin#3 to ground(110k).

[g1]

The OL reading from 2 & 7 to ground on power tubes is because the hum balance pot is missing. I just wanted to verify that the person that removed it didn't already install resistors there. So you should replace with a hum balance pot or use resistors like vintagekiki described in post #307.

V201 pin3 voltage is grossly out of whack. It is not due to the slight difference of R203 being 7.5K or 10K, but a bigger problem. Does R203 still measure 10K? What does R202 measure? If they are both good, remove the tube and measure resistance from pin3 of the socket to ground. Also check R204.
V1 pin3 and V201 pin3 voltages are the only ones that are grossly out of whack. The others are close enough for the amp to work.

The solder joints on the preamp tube sockets are a problem in these amps and cause lots of intermittent issues. When I work on a V4 or V4B I resolder them all. For something like V201, after you verify all resistances are good, you can still have a problem because of the solder at the tube socket. That's why I suggested checking resistance directly from socket pin3 to ground (from tube side with tube removed).

[ca7922303]

Input #1 is louder than input#2, but both are barely audible over the extremely loud hum. V5,6,7,8 pin#2 to ground(OL), pin#7 to ground(OL). Tried this from chassis point and from points listed in blue on VK's post and still get OL and no continuity on continuity test.

Is there anything to the pin#2/#7 to ground on power tubes reading OL and no continuity?

[ca7922303]

V1 / V2 (12AX7)
# 3 (0.3mv)
Check R13 (68k)
# 8 (1.9v)
Voltage is OK

V3 (12DW7)
# 1 (396v)
Voltage is OK
# 3 (85v) Check R19 (1k) and R20 (47k)
# 8 (2v) Voltage is OK

V201 (6K11)
# 10 (398v)
Voltage is OK
# 3 (117v) on schematic 22V
# 4 (117v) on schematic 140V
# 6 (?) on schematic 1.26 to 2V
Check all resistors in the anodes and cathodes of the 6K11 tube.
If the resistors are within limits, it is most likely a 6K11 tube.

V201 #6(2v), R13(72k), R19(1k), R20(47k), R201(1m), R205(1m), R206(3.3k), R211(47k), R208(56k), R209(47k).

[ca7922303]

Remember that there is an error on the schematic for V201 pin3 voltage.

So your saying that when I change the 10k to a 7.5k, the voltage reading will drop from 117v to 5v? Is this causing issue with output signal being low and distorted possibly?

[g1]

Remember that there is an error on the schematic for V201 pin3 voltage.

The voltage shown on the schematic for V201 pin3 is wrong. It's not uncommon to find these kind of errors on Ampeg schematics. When you get the proper resistor for R203, the pin3 voltage will probably be somewhere around 5V.

[vintagekiki]

V5,6,7,8 pin#2 to ground(OL), pin#7 to ground(OL). Tried this from chassis point and from points listed in blue on VK's post and still get OL and no continuity on continuity test.

This is OK. 6.3VAC coil is "floating" because it is not grounded.
If you don't have a hum balace pot, connect 2 resistors 100 Ohm/0.5W in a seria.
Medium resistors connection solder to pin8 (c) tube V5 (6L6GC)
Ends resistors connections solder to pin2 (f) and pin7 (f) tube V5 (6L6GC)

[vintagekiki]

V1/V2 #3(0.3mv) #8(1.9v), V3 #3(85v), #8(2v), V201 #3(117v), #4(117v), #8(12mv), V3 #1(396v), V201 #10(398v).

V1 / V2 (12AX7)
# 3 (0.3mv)
Check R13 (68k)
# 8 (1.9v)
Voltage is OK

V3 (12DW7)
# 1 (396v)
Voltage is OK
# 3 (85v) Check R19 (1k) and R20 (47k)
# 8 (2v) Voltage is OK

V201 (6K11)
# 10 (398v)
Voltage is OK
# 3 (117v) on schematic 22V
# 4 (117v) on schematic 140V
# 6 (?) on schematic 1.26 to 2V
Check all resistors in the anodes and cathodes of the 6K11 tube.
If the resistors are within limits, it is most likely a 6K11 tube.

[ca7922303]

Are you trying both inputs, 1 & 2 (and their volume controls) ?
You still have a problem with V1 (pin3 voltage) but V2 voltages you measured earlier were good. So be sure to try #2 inputs.

The hum problem should be helped somewhat by the hum balance pot. But make sure there are not resistors installed in place of it. Measure resistance from ground to pin2 of a power tube socket. Then check resistance from ground to pin7 of a power tube socket.

Input #1 is louder than input#2, but both are barely audible over the extremely loud hum. V5,6,7,8 pin#2 to ground(OL), pin#7 to ground(OL). Tried this from chassis point and from points listed in blue on VK's post and still get OL and no continuity on continuity test.

[ca7922303]

V1/V2 #3(0.3mv) #8(1.9v), V3 #3(85v), #8(2v), V201 #3(117v), #4(117v), #8(12mv), V3 #1(396v), V201 #10(398v).

Also, have continuity on all ground points listed in blue.(

[ca7922303]

Since all preamp tubes are lit and heated, if you not difficult recheck following voltages (compared to gnd)
DC volts at pin 3 and pin 8 tubes V1, V2, and V3.
DC volts at pin 3, pin 4 and pin 6 tube V201.
DC volts at pin 1 tube V3 and pin 10 tube V201.
Check (see on attachment marked in blue) grnd connections from PCB preamp to chassis.
thanks

V1/V2 #3(0.3mv) #8(1.9v), V3 #3(85v), #8(2v), V201 #3(117v), #4(117v), #8(12mv), V3 #1(396v), V201 #10(398v).

[vintagekiki]

Same as before extremely loud hum, very low, distorted signal from v4b

Since all preamp tubes are lit and heated, if you not difficult recheck following voltages (compared to gnd)
DC volts at pin 3 and pin 8 tubes V1, V2, and V3.
DC volts at pin 3, pin 4 and pin 6 tube V201.
DC volts at pin 1 tube V3 and pin 10 tube V201.
Check (see on attachment marked in blue) grnd connections from PCB preamp to chassis.
thanks

[g1]

Same as before extremely loud hum, very low, distorted signal from v4b

Are you trying both inputs, 1 & 2 (and their volume controls) ?
You still have a problem with V1 (pin3 voltage) but V2 voltages you measured earlier were good. So be sure to try #2 inputs.

The hum problem should be helped somewhat by the hum balance pot. But make sure there are not resistors installed in place of it. Measure resistance from ground to pin2 of a power tube socket. Then check resistance from ground to pin7 of a power tube socket.

[ca7922303]

The voltage is slightly increased in the tolerance relationship of 10%.
Probably a consequence of old preamp tubes.
What you now hear on the external amplifier?

Same as before extremely loud hum, very low, distorted signal from v4b