Ampeg V4 1977 Troubleshooting Weird voltages

[vmazz]

Got a Ampeg v4 1977. Customer complained of intermittent sound, then it went silent. Checked preamp tubes ,power tubes are new 7027. When I crank volume all the way I get sligh signal passing briefly. Also bringin it up on current limiter, shows some kind of short (bulb glowing dimly) Voltages are very low at node A should be around 530v, we have 386v . Replaced recitifier diodes. No change. When in standby I get -522v at screen taps( this is ultralinear design). Worried of transformer short. Ideas?

[clintronics]

The way I would do this, is remove the output tubes. Put in a sine wave at the input and use scope to check the path of the signal to see where it drops out. Do you have audio on the line out jack?

[Jon Snell]

There is and external amp socket. This works both as slave out and power amp in.
I would check the power section by inserting a signal at that point. I will know where the fault lies after that simple check; Power or Preamp.
The dim lamp will glow due to heater and HT current.

[Helmholtz]

Also bringin it up on current limiter, shows some kind of short (bulb glowing dimly) Voltages are very low at node A should be around 530v, we have 386v . Replaced recitifier diodes. No change. When in standby I get -522v at screen taps( this is ultralinear design). Worried of transformer short. Ideas?

On bulb limiter all secondary voltages are lower than normal, because the lamp takes away from the input voltage.
The more current the amp draws the lower the voltages in the amp.

There are no signs of a bad PT.

[Helmholtz]

Please post schematic.

[g1]

Is this the one with a 'distortion' control and 6AN8 tube?

[vmazz]

Yes this is the one with 6an8 and distortion. Iv ehad issues with my signal generator. Im sure the issue is with maintaining the bias. It keeps dropping to -90v . Replaced both resisistors in bias circuit, it cam back to -50v for a minute and we had sound. But it drifted back down.

[The Dude]

What happens to your B+ voltage when this happens? I'm wondering if you're losing your ground via the standby switch. Have you checked the switch itself? Maybe just jumper across the standby switch temporarily to see if it fixes the problem.

[vmazz]

What happens to your B+ voltage when this happens? I'm wondering if you're losing your ground via the standby switch. Have you checked the switch itself? Maybe just jumper across the standby switch temporarily to see if it fixes the problem.

Great idea. I thought the switch might be the culprit .The amp is back on the road unfinished. I wll relay to the owner-next shop.

[g1]

You spoke of screen taps and ultralinear design, but that is not what this is. It runs the screens off a separate PT winding. Screens run at a relatively low voltage (338V) which is how they get away with only -30 is volt bias.
Is it possible you mixed up nodes A and E in post #1 ?

[vmazz]

You are right. I assumed ultra linear. But Im certain I didnt mistake the nodes. The problem was not overall low b+ but is the bias voltage dropping negative.

[pdf64]

That standby looks to leave the output valves momentarily unbiased when standby is flipped from standby to operate mode, as their HT feeds will charge up much faster than their bias supply. So inducing a massive surge in cathode current, potentially degrading it.

[Helmholtz]

That standby looks to leave the output valves momentarily unbiased when standby is flipped from standby to operate mode, as their HT feeds will charge up much faster than their bias supply. So inducing a massive surge in cathode current, potentially degrading it.

Bias supply stays charged during standby as it's always fed from the PT (just as the other 2 supplies).
The standby switch just lifts the ground connection.

Contact resistance of the standby switch adds a negative voltage to all 3 supplies.

[pdf64]

Ah, yes, thanks.
That’s different to the earlier versions then?
https://www.ampegv4.com/images/schematics/V4.jpg

[Helmholtz]

That’s different to the earlier versions then?
https://www.ampegv4.com/images/schematics/V4.jpg

Definitely.
Look at the PT secondaries of post #7.