70s Ampeg V4B Blows Fuses

[vintagekiki]

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

V1, V2 (12AX7)
# 3 (0.3mv) on schematic 2.1V
R13(72k) 68K OK
Check the wiring of the ultra low switch in the cathodes of both tubes, as it acts as a short circuit.

V201 (6K11)
# 3 (117v) on schematic 22V
R201(1m) OK
R202 ?
R203 ?
R204 ?

# 6 (?) on schematic 1.26 to 2V
R205(1m) OK
R206(3.3k) OK
R207 ?

# 4 (117v) on schematic 140V
# 10 (398v) OK
R208(56k) OK
R210 ?
R211(47k) OK
If the value of the resistor is within the limits, and the voltages V201 are not adequate, it is most likely that tube V201 is bad.

V3 (12DW7)
# 1 (396v) OK
# 3 (85v) on schematic 200V
R19(1k) OK
R20(47k) OK
V3 (12DW7) is bad

[ca7922303]

V1, V2 (12AX7)
# 3 (0.3mv) on schematic 2.1V
R13(72k) 68K OK
Check the wiring of the ultra low switch in the cathodes of both tubes, as it acts as a short circuit.

V201 (6K11)
# 3 (117v) on schematic 22V
R201(1m) OK
R202 ?
R203 ?
R204 ?

# 6 (?) on schematic 1.26 to 2V
R205(1m) OK
R206(3.3k) OK
R207 ?

# 4 (117v) on schematic 140V
# 10 (398v) OK
R208(56k) OK
R210 ?
R211(47k) OK
If the value of the resistor is within the limits, and the voltages V201 are not adequate, it is most likely that tube V201 is bad.

V3 (12DW7)
# 1 (396v) OK
# 3 (85v) on schematic 200V
R19(1k) OK
R20(47k) OK
V3 (12DW7) is bad

R202(120k) burnt, split in half, R203(9.86k), R204(229k), R207(0.48m), R210(6.7k), V201 pin#6(2v).

[g1]

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).

V4 pin3 1M to ground? Is R28 bad again? What wattage resistors are you using for replacements? Suggest using 1watt type.
R202,203, and probably R28 need to be replaced.

Recheck pin3 to ground resistance for both V1 and V2. Try with ultra lo switch in both postions. It doesn't make sense that V2 pin3 to ground is 1 ohm as you had reported proper voltage there earlier.

V3 #3(85v)

Is V3 a 12DW7 or some other 12**7 type?

[ca7922303]

V4 pin3 1M to ground? Is R28 bad again? What wattage resistors are you using for replacements? Suggest using 1watt type.
R202,203, and probably R28 need to be replaced.

Recheck pin3 to ground resistance for both V1 and V2. Try with ultra lo switch in both postions. It doesn't make sense that V2 pin3 to ground is 1 ohm as you had reported proper voltage there earlier.


Is V3 a 12DW7 or some other 12**7 type?

V4 is (1m) to ground with red probe in tube socket #3 pin, black lead to chassis. V4 is (1.5k) to ground with red probe on solder joint on #3 pin. R28(1.48k), using 1/2 watt resistors, V1/V2(3.1k/1.0 ohms) ultra lo switched, V3 is a 12AX7 temporary, waiting on a 12DW7.

[vintagekiki]

R202(120k) burnt, split in half, R203(9.86k), R204(229k), R207(0.48m), R210(6.7k), V201 pin#6(2v).

R201 (1m) OK
R202 (120k) burnt, split in half on schematic 560
R203 (9.86k) on schematic 7k5
R204 (229k) on schematic 220k OK
Because bad resistor R202, you have on V201 #3 (119v) #268 instead on schematic 22V ¹⁾


# 6 (2v) on schematic 1.26 to 2V OK
R205 (1m) OK
R206 (3.3k) OK
R207 (0.48m) on schematic 470k OK
this 1/3 part of the tube is OK

# 4 (117v) on schematic 140V
# 10 (398v) OK
R208(56k) OK
R210(6.7k) on schematic 6k8 OK
R211(47k) OK

Replace
R202 (560)
R203 (7k5) or 2pcs 15k in parallel, or any 6k8 to 8k2.

When you replace all bad resistors, measure DCV again at tube V201 (6K11) at pin 2, 3, pin 5, 6, pin 10, 4.
Based on the value of the DCV tube V201 (6K11), we can talk about compactron correctness.
Since the signal from both channels (J1, J2) passes through the mid eq tube V201 (6K11), V4B will most likely somehow work.
All provided that the tubes V1, V2, V3, V201 are new ,or in OK condition (tested at least on some other correct amplifier).

What do you hear on the external amplifier?

2)
Measure resistors R14 and R15 (pin 3 to gnd) in two cases
a) ultra low sw ON
b) ultra low sw OFF

[g1]

V4 is (1m) to ground with red probe in tube socket #3 pin, black lead to chassis. V4 is (1.5k) to ground with red probe on solder joint on #3 pin.

Resolder socket pin3. This is what I was talking about when I said all the board mounted sockets in these amps usually need resoldering.

V1/V2(3.1k/1.0 ohms) ultra lo switched

V1 & V2 pin3 to ground should measure the same (around 3K) no matter where the ultra-lo switch is set. Inspect the switch for bent pins touching something they shouldn't, or incorrect wiring.

[ca7922303]

Resolder socket pin3. This is what I was talking about when I said all the board mounted sockets in these amps usually need resoldering.


V1 & V2 pin3 to ground should measure the same (around 3K) no matter where the ultra-lo switch is set. Inspect the switch for bent pins touching something they shouldn't, or incorrect wiring.

Which tube(s), should I resolder pin#3? Thanks.

[g1]

Which tube(s), should I resolder pin#3? Thanks.

V4 is the one having the connection problem at pin3.

[ca7922303]

R201 (1m) OK
R202 (120k) burnt, split in half on schematic 560
R203 (9.86k) on schematic 7k5
R204 (229k) on schematic 220k OK
Because bad resistor R202, you have on V201 #3 (119v) #268 instead on schematic 22V ¹⁾


# 6 (2v) on schematic 1.26 to 2V OK
R205 (1m) OK
R206 (3.3k) OK
R207 (0.48m) on schematic 470k OK
this 1/3 part of the tube is OK

# 4 (117v) on schematic 140V
# 10 (398v) OK
R208(56k) OK
R210(6.7k) on schematic 6k8 OK
R211(47k) OK

Replace
R202 (560)
R203 (7k5) or 2pcs 15k in parallel, or any 6k8 to 8k2.

When you replace all bad resistors, measure DCV again at tube V201 (6K11) at pin 2, 3, pin 5, 6, pin 10, 4.
Based on the value of the DCV tube V201 (6K11), we can talk about compactron correctness.
Since the signal from both channels (J1, J2) passes through the mid eq tube V201 (6K11), V4B will most likely somehow work.
All provided that the tubes V1, V2, V3, V201 are new ,or in OK condition (tested at least on some other correct amplifier).

What do you hear on the external amplifier?

2)
Measure resistors R14 and R15 (pin 3 to gnd) in two cases
a) ultra low sw ON
b) ultra low sw OFF

R14(0.8 ohms), R15(0.8ohms), R202(0.55k), R203(6.25k). R14/R15(3.08k) switching ultra lo.

[ca7922303]

V4 is the one having the connection problem at pin3.

Resoldered V4 pin#3 and don't see anything obvious about ultra lo switch.

[ca7922303]

Resolder socket pin3. This is what I was talking about when I said all the board mounted sockets in these amps usually need resoldering.


V1 & V2 pin3 to ground should measure the same (around 3K) no matter where the ultra-lo switch is set. Inspect the switch for bent pins touching something they shouldn't, or incorrect wiring.

V4 pin#3 to ground after resolder is (1.48k).

[g1]

Any sound into an external amp yet? Set 'ultra-lo' to position that gives 3K reading at R14&15.

[ca7922303]

Any sound into an external amp yet? Set 'ultra-lo' to position that gives 3K reading at R14&15.

Strong signal now thru speaker output. Greatly appreciate all the help. Is there any risk of blowing resistors if I switch ultra lo?

[ca7922303]

V1, V2 (12AX7)
# 3 (0.3mv) on schematic 2.1V
R13(72k) 68K OK
Check the wiring of the ultra low switch in the cathodes of both tubes, as it acts as a short circuit.

V201 (6K11)
# 3 (117v) on schematic 22V
R201(1m) OK
R202 ?
R203 ?
R204 ?

# 6 (?) on schematic 1.26 to 2V
R205(1m) OK
R206(3.3k) OK
R207 ?

# 4 (117v) on schematic 140V
# 10 (398v) OK
R208(56k) OK
R210 ?
R211(47k) OK
If the value of the resistor is within the limits, and the voltages V201 are not adequate, it is most likely that tube V201 is bad.

V3 (12DW7)
# 1 (396v) OK
# 3 (85v) on schematic 200V
R19(1k) OK
R20(47k) OK
V3 (12DW7) is bad

Got it going, thank you for all the great help.

[g1]

Strong signal now thru speaker output. Greatly appreciate all the help. Is there any risk of blowing resistors if I switch ultra lo?

More a danger to the tubes V1 & V2. You need to find and deal with the problem. Can you get in close enough to the switch to post some pictures?
Also, I think you still have the issue where the heater voltage on the preamp tubes is lower than it is on the power tubes. (5.6VAC vs 6.3V). It should be the same for all tubes.