Vibro Champ Voltages

[caesparza]

Hi guys,

I'm doing maintenance to a 75 Vibro Champ. As usual I got high dissipation for the 6V6. I'm trying to address that issue but the results are kind of weird. I replaced the 470 cathode resistor with a 1K 5W one and the 25/25 cap with a 47/50.

The thing is that I get a voltage drop across the resistor of about 72.8 which is very high for the cap I used. And it's out of specs according to the schematic

I took some readings with the stock components, before the change:

PT red wires to rectifier 296.8 VAC
PT green wires to lamp and heaters 5.43 VAC

Filter section voltages 261.4, 278.5 289.2

Pin 1 V1: 190.6
Pin 3 V1: 1.2
Pin 6 V1: 187.5
Pin 8 V1: 1.12

Pin 1 V2: 130-143 value oscillates.
Pin 3 V2: 1.22
Pin 6 V2: 279.5
Pin 8 V2: 131-143 value oscillates.

Pin 3 V3: 269.3 (plate)
Pin 4 V3: 280 (screen)
Pin 8 V3: 55.7

Plate voltage on 6V6 is 55.7, already high.

I know that rising the voltage drop I'll drop the current on the tube. But looking at the numbers I guess I should rise the plate current a little bit too. I don't get why the voltage drop on the cathode resistor is too high on this one.

Any pointers on that?

Thanks a lot guys.
Carlos.

[caesparza]

I took measurements with the original components for 6V6 dissipation.

This is what I got:

Cathode Resistor: 251.8 ohms (almost half the value that the schematic says).
Voltage drop: 20.3V (within specs).

Current would be around 80.6mA.

Plate Voltage: 248.1V (really low).

Tube Dissipation: 20 watts aprox.

This makes me wonder, how can I get almost the correct BIAS voltage with cathode resistor value drifted almost to half the value. I need to increase plate voltage to get the correct tube dissipation.

Probably what the amp needs is a recap job.

[pdf64]

What is the Vdc at the 6V6 control grid terminal 5?
If not 0, then suspect the coupling cap of leaking dc or the grid leak being open.

[caesparza]

What is the Vdc at the 6V6 control grid terminal 5?
If not 0, then suspect the coupling cap of leaking dc or the grid leak being open.

It reads 4.10v

[Justin Thomas]

Ouch. Check that coupling cap & grid leak!

Justin

[caesparza]

Hi Justin,

Thanks for your kind answer. I'll check the coupling between the stages to ensure everything is working properly, I have a doubt, do this issue influence the voltage over the plate on the power tube? I know it changes the operation conditions for the tube but the voltages seem to be low after they go to the filter caps.

Thanks.
Carlos

[pdf64]

Do not doubt that coupling caps passing dc will cause tubes to draw way too much current and that HT voltages will sag significantly as a consequence.

[caesparza]

Do not doubt that coupling caps passing dc will cause tubes to draw way too much current and that HT voltages will sag significantly as a consequence.

Great, I'll order the parts and let you know how the thing goes.

Thanks a lot.

[pdf64]

Bear in mind that the power tube cathode cap looks to have been subject to significant overvoltage, so consider replacement.

[caesparza]

pdf64 I have a doubt, as I understand, to check a coupling cap for leakage, you need to lift the negative leg and measure voltage on it, isn't it?

I understand that there shouldn't be any DC voltage after a coupling cap. But I just have that doubt. If you could clarify that to me that would be much appreciated.

All the best mate.
CE

[pdf64]

If you’ve got a regular digital multimeter it will have an input resistance of about 10M ohms, and then yes, it would be fine to do as you describe.
You also need to check that the power tube control grid leak is good, eg its 220k path to 0V is good (amp de-energised).

[caesparza]

Let me check if I understood the last bit of information you gave me. Right between 220K resistors, there should be -34volts (that is the BIAS voltage for the tubes), according to the schematic. You mean that the voltage on the grids after the 220K should be 0 as well?

[pdf64]

What amp are you looking at there?
Doesn’t seem to be a vibrochamp. If it is, could a schematic link be provided.

[caesparza]

It's a 75 Vibro Champ.

There are a couple of shcematics on the internet. I'm asuming this one is the correct, since the 6V6 has the small cap between pin 5 and 8. The other one is the AA764 (earlier that same year).

[pdf64]

So where’s the negative bias voltage? This is a cathode bias power amp.
Are you getting mixed up with the Princeton?