70s Ampeg V4B Blows Fuses

[ca7922303]

So the question now is there a signal at ext jack? I think that answer is a yes. Now you need to trace the signal past v3. Look at c10. The signal rides on the DC voltage out of the plate of v3. The other side of c10 will have signal voltage as the DC voltage is filtered away by c10. Just focus on this next step or we will get lost again. Build a signal tracer if you don’t already have one. Something simple like this.

http://musicfromouterspace.com/analo...naltracer.html

look up videos about tracing audio signal tracing on YouTube

C10(246v/15mv).

[vintagekiki]

Yes, I'm using four tubes, and i've got signal when plugged into external amp jack, but no preamp out.

The fact that the signal is plugged to external amp jack (J4 or J5 V4B) and with four power tubes (6L6) you got the signal in speaker which is connected to (J6 spkr V4B), shows that the signal passes through ¹/ _² V3, V4, V5-8.

V4B power amp for now is OK. For repair the V4B preamp, so feel free to remove tubes V5-8 (6L6). Mark tubes V5-8 to know that they are usable.
Connect external amp jack (J4 or J5 V4B) to input jack of any external amplifier.

Connect the signal from the generator (1kHz 10-50mV) to the input jack (J1 or J2 V4B).
Set pot VR101 and VR102 to max, other pot to 12 o'clock.
What do you hear on the external amplifier?

To be continued¹⁾

[ca7922303]

The fact that the signal is plugged to external amp jack (J4 or J5 V4B) and with four power tubes (6L6) you got the signal in speaker which is connected to (J6 spkr V4B), shows that the signal passes through ¹/ _² V3, V4, V5-8.

V4B power amp for now is OK. For repair the V4B preamp, so feel free to remove tubes V5-8 (6L6). Mark tubes V5-8 to know that they are usable.
Connect external amp jack (J4 or J5 V4B) to input jack of any external amplifier.

Connect the signal from the generator (1kHz 10-50mV) to the input jack (J1 or J2 V4B).
Set pot VR101 and VR102 to max, other pot to 12 o'clock.
What do you hear on the external amplifier?

I can hear the signal thru the external amp, but it is faint in comparison to the loud hum I continue to hear all along since ive had this one.

[g1]

With no signal applied, check DC volts at V1 pins 6, 8, 1, & 3. V3 pins 1 & 3.

[ca7922303]

With no signal applied, check DC volts at V1 pins 6, 8, 1, & 3. V3 pins 1 & 3.

V1 pin#1(396v), pin#3(0mv), pin#6(175v), pin#8(2v). V3 pin#1(410v), pin#3(99v).

[vintagekiki]

I can hear the signal thru the external amp, but it is faint in comparison to the loud hum I continue to hear all along since ive had this one.

Check preamp continued from #257

It is assumed that you have previously checked tubes V1, V2, V3 on another amp and that they are as correct as possible (usable)
Check the shielded connection cable from the external amp jack (J4 or J5 V4B) to the input jack external amplifier.

- Measure the voltage at pin 1 (V3).
- Measure the voltage at pin 10 (V201).
- Does the loud hum change its tone color with regulation bass, mid and treble (VR105, VR104 and VR103) pots.
What do you hear on the external amplifier?

If the signal described during check preamp (#257) does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted replace V3 (12DW7) first (tube V1 and V2 for now is OK). If you do not have 12DW7 for temporarily check, can be used 12AX7.
What do you hear on the external amplifier?

If and in addition to replace V3 (12DW7) signal does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted, replace the compactron tube V201 (6K11).
What do you hear on the external amplifier?

If the signal still does not pass to the external amp jack (J4 or J5 V4B) check (replace)
- C7 (.1), C9 (.47), C8 (.01);
- C205 (.1), C201, C202 (.01);
And that's all.

[g1]

V1 pin#1(396v), pin#3(0mv),

Something is wrong here. Check resistance from pin3 to ground.

[vintagekiki]

Measure DC volts at pin 3 and pin 8 tubes V1(again), V2, and V3.
Measure DC volts at pin 3, pin 4 and pin 6 tube V201.
Measure DC volts at pin 1 tube V3 and pin 10 tube V201.
Measure DC volts on both sides of resistor R51 (2k2)
thanks

[ca7922303]

Check preamp continued from #257

It is assumed that you have previously checked tubes V1, V2, V3 on another amp and that they are as correct as possible (usable)
Check the shielded connection cable from the external amp jack (J4 or J5 V4B) to the input jack external amplifier.

- Measure the voltage at pin 1 (V3).
- Measure the voltage at pin 10 (V201).
- Does the loud hum change its tone color with regulation bass, mid and treble (VR105, VR104 and VR103) pots.
What do you hear on the external amplifier?

If the signal described during check preamp (#257) does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted replace V3 (12DW7) first (tube V1 and V2 for now is OK). If you do not have 12DW7 for temporarily check, can be used 12AX7.
What do you hear on the external amplifier?

If and in addition to replace V3 (12DW7) signal does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted, replace the compactron tube V201 (6K11).
What do you hear on the external amplifier?

If the signal still does not pass to the external amp jack (J4 or J5 V4B) check (replace)
- C7 (.1), C9 (.47), C8 (.01);
- C205 (.1), C201, C202 (.01);
And that's all.

I am able to hear signal from external amp. Signal is extremely weak and drowned out by consistent hum i've had since starting work on this amp. Installed new V201, and have changed V1, V2, and V3 to known good tubes

[ca7922303]

Measure DC volts at pin 3 and pin 8 tubes V1(again), V2, and V3.
Measure DC volts at pin 3, pin 4 and pin 6 tube V201.
Measure DC volts at pin 1 tube V3 and pin 10 tube V201.
Measure DC volts on both sides of resistor R51 (2k2)
thanks

R51(567v/567v), V201 pin#10(567v), V3 pin#1(567v). V201 #3(40mv), #4(43mv), #6(0mv), V1, V2, V3, V4, V201 now for some reason are not lit at all? Not able to give rest of measurements.

[ca7922303]

Something is wrong here. Check resistance from pin3 to ground.

V1 pin#3 to ground(172 ohms) resistance.

[ca7922303]

Something is wrong here. Check resistance from pin3 to ground.

Also, found this wire loose. Could it causing some of the issues with preamp?

[ca7922303]

R51(567v/567v), V201 pin#10(567v), V3 pin#1(567v). V201 #3(40mv), #4(43mv), #6(0mv), V1, V2, V3, V4, V201 now for some reason are not lit at all? Not able to give rest of measurements.

Correction: preamp tubes lit up after installing power tubes only. Now measurements with all tubes installed. V1 #3(44mv), #8(1.9v), V2 #3(1.5V), #8(1.9v), V3 #3(85v), #8(2v). V201 #3(119v), #4(121v), #6(2v).

[vintagekiki]

R51(567v/567v), V201 pin#10(567v), V3 pin#1(567v). V201 #3(40mv), #4(43mv), #6(0mv), V1, V2, V3, V4, V201 now for some reason are not lit at all? Not able to give rest of measurements.

Since V1, V2, V3, V4, V201 are not lit at all, heater voltage (6.3VAC) for tubes are probably somewhere interrupted.
For the same reason you have an inappropriate voltage (567VDC) on both sides of the resistor R51 (2k2) and voltage (567VDC) at pin#10 tube V201 and pin#1 tube V3.
When all tubes lit, then measure voltages, now there is no purpose, because tubes are not lit, there is no consumption DC current, there is no voltage drop on R51 (2k2).
Find where is interruption is 6.3VAC for heater tubes V1, V2, V3, V4, V201.

[g1]

V1 pin#3 to ground(172 ohms) resistance.

Now measurements with all tubes installed. V1 #3(44mv)

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