70s Ampeg V4B Blows Fuses

[ca7922303]

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.

Photos of ultra lo switch

[g1]

Photos of ultra lo switch

Is that one lug with no wire that's bent upward touching anything?

[ca7922303]

Is that one lug with no wire that's bent upward touching anything?

No, not touching anything.

[vintagekiki]

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

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

[ca7922303]

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

Wires on ultra lo switch (4 on schematic), all show continuity. Will post measurements for c106/112 shortly. Have to lift one leg if each cap to measure correctly.

[vintagekiki]

Wires on ultra lo switch (4 on schematic), all show continuity. Will post measurements for c106/112 shortly. Have to lift one leg if each cap to measure correctly.

Wires show continuity, check that are soldered where should (on schematic).
Is the wire from ultra lo sw soldered to +caps point C106/R107 and point C112/R108, or soldered to -caps C106, C112 (gnd)

[g1]

Please note that there's an error on the layout drawing for the polarity of C106 and C112. See red markings on this pic for corrections. The actual circuit board is probably marked correctly.

[vintagekiki]

Correction
mark C107 should be replaced with mark C112 .

[ca7922303]

Wires show continuity, check that are soldered where should (on schematic).
Is the wire from ultra lo sw soldered to +caps point C106/R107 and point C112/R108, or soldered to -caps C106, C112 (gnd)

Wire from ultra low switch is soldered to + caps point C106/R107 and point C112/R108.

[g1]

Wire from ultra low switch is soldered to + caps point C106/R107 and point C112/R108.

Are those 2 wires pinched to the chassis anywhere? Can you post a picture of the switch pointing to where those 2 wires are connected?

[ca7922303]

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

C106(0.208 nf), C112(0.198 nf), black probe to lifted leg - side, red probe to + solder joint on board, cap test on dmm.

[ca7922303]

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

R14/15 with C106/112 one leg lifted now read 3.3k in both positions of ultra lo switch.

[ca7922303]

R14/15 with C106/112 one leg lifted now read 3.3k in both positions of ultra lo switch.

Is it possible that these two(2) caps could be left out of circuit and amp operate correctly?

[ca7922303]

Are those 2 wires pinched to the chassis anywhere? Can you post a picture of the switch pointing to where those 2 wires are connected?

They are not pinched to the chassis.

[vintagekiki]

Is it possible that these two(2) caps could be left out of circuit and amp operate correctly?

Values of C106 (0.208 nf), C112 (0.198 nf) indicate that these two (2) caps is bad. #341
The amplifier will work quite correctly without caps C106, C112, it will only have a shade less ultra lo.
If you do not have caps 6.8u, you are free to install caps 10u. Pay attention to caps polarity.
Just go ahead boldly. Soon V4B will be playing somewhere on stage.

[g1]

R14/15 with C106/112 one leg lifted now read 3.3k in both positions of ultra lo switch.

Back at post #270 I advised you to disconnect C106, which you said you did and it did not make a difference. When you cut the leg in post #271, was it some other cap than C106? Did you reconnect it?

Remove C106 and then re-check V1 pin 3 voltage.

Cut one(1) leg of C106, V1 pin#3(11mv).