'79 JMP 50. Old tubes cause low bias voltage?

[1971SuperLead]

I would greatly appreciate anyone's help! I have read a few books about amplifier repair but I'm certainly still a novice.

I have a 1979 Marshall 50 watt head with old GE 6550's with a ton of miles on them (20 years?) and I just bought a BiasPro bias probe kit to see why my old amp is not sounding golden like it used to. It is low in volume and does not sing like she used to. It does have a Mike Morin extra pre amp tube installed.

Anyways, I hook the head up to a variac to make sure it's getting 120 volts. I've connect a 4x12 cab to the output. I have installed the bias probes and reinstalled the old GE 6550's and I read around 15 dc milli-volts on one tube and 18 on the other, roughly. I turned the bias all the way up and I can get only 30 milli-volts. The probes have a 1 ohm shunt resister built inline so using Ohms Law I convert the milli-volts to milli-amps and read 15 and 18 milli-amps as my bias amperage.

So. can it be that the old tubes just build up resistance over the years and are causing the current drop or is it possible the original P/T is dying (How can I tell? Where do I read the voltages from and would it be AC or DC?) or can it be something else?

Thanks in advance,
Robert

[reverendbk]

If the amp is all original it is time for new filter caps. Power supply components drift in value and need to be replaced, and this includes the bias supply. Measure resistances and change out old caps.

As for measurements, I like to measure DC from each filter cap (NUMEROUS WARNINGS ABOUND).
Then output tubes pin 3 (plate) & pin 4 (screen)
Preamp tubes pin 1 & 6 (plates)

Try putting in some new tubes, re-bias and remeasure. Be careful and good luck.

[booj]

It's time to replace them tubes! They should be drawing somewhere about 50 ma. each give or take depending on your H.V. supply. I'm thinking for 450 volts and 24 watts dissapation ea. comes out to 53 ma. 500 volts and 24 watts would be 48 ma.
there are various threads about cap replacement. Absolutely change the bias caps.

[1971SuperLead]

Does the age of the tubes effect the draw?

Thank you for responding. I'm just curious as to whether the age of the tubes is what could be limiting the current draw. I turn the bias all the way up and it only goes to around 25 ma each. So as the tubes get old they build up resistance? Do the filter electrolytic caps build up resistance as they age?

I'm new at this. I've read 7 books on all this stuff by Hunter, Weber, Fisher, Mitchell and Pittman and I did soldering in aerospace for years but I'm still trying to figure out your math there and this other stuff.

Thanks again,

Robert

[Enzo]

You are assuming they used to draw a lot more. I don;t assume that. As tubes wear their current drifts, but not usually to half.

You want to check on the health of the whole amp. what is the B+ voltage, and is it getting to ALL pins 3 and 4 of ALL ppower tube sockets. Part if that is also finding out if all the screen grid resistors are OK.

Then, just what is the range of bias voltage your system can present tot he tubes? If it adjusts from -80 to -70v or something silly like that., then the bias supply has a bad resistor.

And don;t assume that just because these days everyone wants to bias their tubes to the edge of redplating that they always ran tubes that way. 15ma would be perfectly normal for something like a stock Peavey 5150, and that thing will blow the hair off your head. Your readings sound cold to me too, but don;t get too caught up in that detail.

[1971SuperLead]

Thank you,

So i need to check b+ voltage and the voltages on 3 and 4 on the power tube sockets.

Anything else?

Thank you

Robert

[Jazz P Bass]

Bias value

It's time to replace them tubes! They should be drawing somewhere about 50 ma. each give or take depending on your H.V. supply. I'm thinking for 450 volts and 24 watts dissapation ea. comes out to 53 ma. 500 volts and 24 watts would be 48 ma.
there are various threads about cap replacement. Absolutely change the bias caps.

Curious how the 53 ma value was obtained.
Plate dissipation/ plate voltage x 0.7 is what I have always used.
24/450 x 0.7=37.3 ma.

[booj]

Curious how the 53 ma value was obtained.
Plate dissipation/ plate voltage x 0.7 is what I have always used.
24/450 x 0.7=37.3 ma.

I figure plate dissipation in watts. We're talking DC volts and DC amps. Where does the .7 come in? Are you thinking 70%? Wouldn't that be 70% of Max. idle dissipation?
I figure "normal" plate dissipation (at idle) for a 6550 basically by this chart which may or may not be everyones idea of correct.

If 30 watts is max diss., then 70% would be 21 watts, so I guess Aspen was thinkin' 6550's could handle 80%, or 24 watts or 53.3 ma. at 450 volts on the plate.
Anyway, that's how that was obtained.

[Jazz P Bass]

6550 bias

I stand corrected.
Thanks for the insight.

John G