70s Ampeg V4B Blows Fuses

[ca7922303]

120v, 2 power tubes installed, no red plating, but same hum and no output. Mid-range control is only one that has any affect on hum. And hum pot and wiring were removed by previous owner.

[ca7922303]

Any thoughts on where to go to get output? Appreciate the help.

[garytoosweet]

Any thoughts on where to go to get output? Appreciate the help.

Do you have a scope? No output is often solved by signal tracing and seeing where you loose the signal.

If not you can try tracing the signal with your meter set to AC Volts.

Here are some likely causes: http://www.geofex.com/ampdbug/onlyhiss.htm

[vintagekiki]

120v, 2 power tubes installed, no red plating, but same hum and no output. Mid-range control is only one that has any affect on hum. And hum pot and wiring were removed by previous owner.

Finally a positive shift.

Hum at mid-range control affects arises due to ferrite core L101 in mid-range eq. It's normal.
Instead removed hum pot, install hum balance with resistor.

[SoulFetish]

120v, 2 power tubes installed, no red plating, but same hum and no output. Mid-range control is only one that has any affect on hum. And hum pot and wiring were removed by previous owner.

first, I would make sure that the heater supply has a DC reference to ground in some way. If the previous owner removed the hum balance pot, they might have installed fixed 100R resistors to ground. If they haven't you can install two 100 ohm resistors from the 2 legs of the AC heater supply connected to ground, or reinstall the balance circuit.
If that isn't the issue, I would then eliminate the possibility of a bad tube causing hum. No need to make the repair anymore difficult than it needs to be. Start by pulling V1, and see if the hum goes away. If so, try another 12AX7 and see if that fixes it. If the hum is still there, pull V2 and repeat the same steps. Then go onto V3 if you need to, and so on.

[g1]

Agree with SoulFetish, if someone removed the hum pot circuit, they probably added resistors to replace it.
Solve the 'no output' problem first, maybe you will get lucky and it will fix the hum too. If not, you can chase the hum later.

The 'ext. amp' jacks on the back can be used as either a preamp out, or a power amp in. You can use it to find out if you have a preamp problem or a power amp problem.
Plug a signal into the 'ext.amp' jack. Do you hear sound now? If so, the power amp is ok.
Now plug your signal back in to the normal input. Use another cable to connect the 'ext.amp' jack into the input of some other working amplifier. Do you hear sound?

[ca7922303]

Appreciate that. Ran signal generator to ext. amp jack and am getting output. So preamp is at fault. I changed preamp tubes around,not the tubes. Any thoughts on the preamp?

[ca7922303]

Unable to hear signal from V4b ext. Jack to other amps input.

[ca7922303]

Only able to hear the hum.

[g1]

Had you found a burnt resistor around one of the preamp tubes earlier?

Whatever you smelt around the 6K11 probably got burnt and will have to be dealt with as well.

Start checking DC voltages at the 6K11 if you can't find the burnt component (possibly a resistor opened up). The DC voltages should be shown on the schematic.

[ca7922303]

Yes, R28 burnt earlier and I replaced it when I did c15,16,23.

[vintagekiki]

How much voltage is at the cathodes (pin3/8) tube V4 (12AU7) after replacing R28 (1k5) ?

[ca7922303]

Out of town right now, will get those measurements as soon as I get back. Thanks for your help.

[g1]

His power amp is working if he puts signal into the 'ext.amp' jack, so 12AU7 V4 is working ok. (post #97)
The problem must be before the 'ext. amp' jack.

[vintagekiki]

I changed preamp tubes around,not the tubes.

Replace (change) preamp tubes with new tubes (no like new, no around)
If there is no signal from either channel (one or two) for test, replace the tube V3 (12DW7) first and then V201 (6K11).
Since signal passes through the ext. amp jack, most likely defect is in mid eq tube circuit V201 (6K11)

Question
How much is voltages on the cathodes ...
V3 (12DW7) pin3, pin8
V201 (6K11) pin3, pin4, pin6