69' Marshall Metal Panel Bias Issues

Donni

Member
Messages
8
I have a 69' Marshall 50 Watt Metal Panel that seems to have a bias problem. The B+ voltage is 385 volts, but I can't seem to get the El-34's above 30-32ma maximum. It does have a non-original power tranny, (1202-324 which was a later 70's Marshall transformer that supposedly puts out about 450 volts) so I'm kind of wondering where to start looking for the problem. I've tried 3 different sets of El-34's and they all yield the same approximate readings. From what I remember, I used to be able to bias at about 46ma or even higher. Whats wrong?
 

SatelliteAmps

Member
Messages
6,168
Sounds like the bias resistor is either the wrong value, or it has drifted. If you are having the same biasing issue with three separate sets of tubes, then it probably isn't the tubes.

Any idea what value your bias resistor is?
 

Donni

Member
Messages
8
The resistors values are both 220k and they are reading 219k and 223K respectively, so they look pretty close. Could it be a leaky coupling cap?
 

SatelliteAmps

Member
Messages
6,168
Both 220k? There should be one 220k before the diode. There is also a 47k after the bias adjustment that could have drifted. And yes, the cap could also be an issue.

The 220k's I think you are referring to are after the bias, probably going to pin 6 of the output tubes?
 

Swarty

Member
Messages
1,130
If there is a 47k that is the problem as it should be 27k, but this is the resistor to focus on. You need to lower the bias voltage and lowering the value of this resistor will do it. Also a problem if 220k resistors go to pin 6.
 

SatelliteAmps

Member
Messages
6,168
My mistake, it is pin 5 that the two 220k's go to, not pin 6.

Just double checked the schematic, and it is a 47k for EL34s. A 27k would be for a pre-1968.
 

Donni

Member
Messages
8
Thanks for all the tips. What I did find is the feedback resistor is reading 4.3K and it is color coded as a 47K. So it's way out of spec.
Would that affect the bias?
 

HCStraub

Member
Messages
335
Thanks for all the tips. What I did find is the feedback resistor is reading 4.3K and it is color coded as a 47K. So it's way out of spec.
Would that affect the bias?
Did you take it off the board to measure it? If you measured it on the board hooked up, the cap in the circuit will throw off your reading. If you did take it out of the circuit to measure it, it is a bad resistor and should be replaced.

My guess......you need to change the bias range resistor.
 

s2khawk

Member
Messages
379
thats crazy I had a 69 metal panel 50 I never should have sold, and it had 600VDC plates and sounded like god under water...amazing VH tones with no pedals but you had to use NOS EL34's obviously, what a brutal amp with tone! I'd consider putting a proper high end PT in there with much higher VDC, its a thought at least...
 

SatelliteAmps

Member
Messages
6,168
The bias resistor that we are talking about should be a 47k. Should be attached to one leg of the bias pot, and the other side should go to the pair of 220ks that you measured before. Unsolder one side of the 47k and check it. If it is way off, replace it. If it is correct, then there are two options. One is to check down the line back to the power transformer to see if there are any other parts that are out of spec, or you can adjust the 47k resistor to get into the right bias range. If it is close, I would probably try a 39k in it's place. Also, there should be two cap's (I think 8uf, but I didn't look at the schematic) in the bias supply that you might want to check as well.

600vdc is pretty high. I prefer the under 400vdc ones. Just sound a bit warmer to me.
 




Trending Topics

Top Bottom