70s Ampeg V4B Blows Fuses

[vintagekiki]

V2, V3, V4, V201 now for some reason are not lit at all? Not able to give rest of measurements.

Tubes V1, V2, V3, V4, V201 still not lit, heater voltage (6.3VAC) somewhere is interrupted.
Check do you have on preamp PCB, voltage (6.3VAC) on pin4&5 and pin 9 (preamp tubes), which comes from pin2 and pin7 tube V5 (6L6GC) (PCB red right)

Not clear on exactly what your asking here so I'll give you vac and vdc for every tube you mentioned.

Try to find why preamp tubes V1, V2, V3, V4 not lit (don't have heater voltage 6.3VAC between pin 9 and pin 4)
Check do you have on preamp PCB, heater voltage 6.3VAC from tube V5 (6L6GC)
When count the pins on tube socket, counts from the copper side.
Heater voltage (6.3VAC) for tubes V1, V2, V3 and V4 not measured in relation to gnd, but between pin 9 and pin 4 (or 5)
When all the tubes are lit, you can perform the any measurements.

[ca7922303]

Possibly the hum balance pot is bad so measuring to ground is giving poor results. Put one probe on each of the pins mentioned and measure AC volts. For example, V5, one probe on pin2, other probe on pin7.

There is no hum balance pot and wiring on this one. Bought it that way .I'm thinking I'm going to have to have one with as noisy as this one is right now .Can I use a typical pot instead of the one fliptops offers. Also, how would it be wired to the board?

[ca7922303]

Try to find why preamp tubes V1, V2, V3, V4 not lit (don't have heater voltage 6.3VAC between pin 9 and pin 4)
Check do you have on preamp PCB, heater voltage 6.3VAC from tube V5 (6L6GC)
When count the pins on tube socket, counts from the copper side.
Heater voltage (6.3VAC) for tubes V1, V2, V3 and V4 not measured in relation to gnd, but between pin 9 and pin 4 (or 5)
When all the tubes are lit, you can perform the any measurements.

V5 pin#2 & #7(6.3vac), V201 pin#4/5 &#9(5.6vac), V1/V2/V3/V4(5.6vac). Also R51 now(400/420vdc).

[ca7922303]

V5 pin#2 & #7(6.3vac), V201 pin#4/5 &#9(5.6vac), V1/V2/V3/V4(5.6vac). Also R51 now(400/420vdc).

Also, V3 #1(399vdc), v201 #10(396vdc), V1/v2 pin#3(0.4mv), pin#8(1.9v), V3 pin#3(85v), pin#8(2v), V201 pin#3(115v), pin#4(117v), pin#6(2v). Vdc with black probe on ground.

[vintagekiki]

There is no hum balance pot and wiring on this one.

I'm not g1 but I can help.
On place of V4B hum balance pot, install a 100 ohm pot which used in Fender amps as hum balance.
The ends of the hum balance pot (100 ohm), soldered to pin 2 and pin 7 tube 6L6GC, and the middle of the hum balance pot (100 ohm) to gnd.

[Helmholtz]

A 100R hum pot needs to be rated for at least 0.5W.
A 500R pot only requires 0.1W.

No functional difference.

[vintagekiki]

V5 pin#2 & #7(6.3vac), V201 pin#4/5 &#9(5.6vac), V1/V2/V3/V4(5.6vac). Also R51 now(400/420vdc).

Do tubes V1, V2, V3, V4 and V201are lit.
How much is the heater voltage (6.3VAC) on the preamp PCB for tubes V1, V2, V3 and V4 between pin 9 and pin 4.
How much is the heater voltage (6.3VAC) on the mid EQ PCB for tube V201 between pin 1 and pin 12.
The voltage at V5 (pin 2 & 7) is OK

1)
https://www.banzaimusic.com/fender-hum-balance-100-ohm.html

[ca7922303]

Do tubes V1, V2, V3, V4 and V201are lit.
How much is the heater voltage (6.3VAC) on the preamp PCB for tubes V1, V2, V3 and V4 between pin 9 and pin 4.
How much is the heater voltage (6.3VAC) on the mid EQ PCB for tube V201 between pin 1 and pin 12.
The voltage at V5 (pin 2 & 7) is OK

V1/V2/V3/V4 are lit and warm and are 5.6 vac between pins 4/5 & 9, can post V201 between 1 & 12 shortly.

[vintagekiki]

V1/V2/V3/V4 are lit and warm and are 5.6 vac between pins 4/5 & 9, can post V201 between 1 & 12 shortly.

It is illogical that at tube V5 between pin2 & 7 you have a voltage of 6.3VAC, and at tube V1/V2/V3/V4 between pins 4/5 & 9 you have a voltage of 5.6VAC.
Tube V5 is connected with tubes V1/V2/V3/V4 on preamp PCB with only 2 wires.
Check do you have on the left side of the preamp PCB, between the points where writes pin1 and pin12 filaments of V201 (red left) voltage 6.3VAC.

[ca7922303]

It is illogical that at tube V5 between pin2 & 7 you have a voltage of 6.3VAC, and at tube V1/V2/V3/V4 between pins 4/5 & 9 you have a voltage of 5.6VAC.
Tube V5 is connected with tubes V1/V2/V3/V4 on preamp PCB with only 2 wires.
Check do you have on the left side of the preamp PCB, between the points where writes pin1 and pin12 filaments of V201 (red left) voltage 6.3VAC.

V201 between pins #1 &12(5.6vac).

[ca7922303]

V201 between pins #1 &12(5.6vac).

Checked again with new dmm batteries and V5 is 5.6vac and and v201/1,2,3,4 are all 5.6vac.

[vintagekiki]

Checked again with new dmm batteries and V5 is 5.6vac and and v201/1,2,3,4 are all 5.6vac.

Now heater voltage is OK.
If I understood you correctly, now all the tubes in V4B from V1 to V8 are lit and heated.
When on the V4B Power ON and Standby ON measure the voltage on both sides of the R51 (2k2)

[ca7922303]

Now heater voltage is OK.
If I understood you correctly, now all the tubes in V4B from V1 to V8 are lit and heated.
When on the V4B Power ON and Standby ON measure the voltage on both sides of the R51 (2k2)

R51(400/420vdc).

[vintagekiki]

R51(400/420vdc).

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?

[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