Shorted Choke?

[I.T.]

Would a shorted choke allow the screens to pull excessive current?

I have a stock 72 super lead that runs okay at lower volumes, but when the power section gets working to amplify signal, the screens glow orange and the indicator dims a lot. I have noticed that the amp is a little noisy too, and the tone isn't particularly musical when driven.

Replaced all filters (new 50/50uf F&T) and all screen resistors (1K 5w). OT is good. Tubes are matched and properly biased at idle. Idle voltages are relatively normal. Maybe the choke is bad?

[soundguruman]

No, most likely it's not the choke, but check this info:
Postby soundguruman » Thu Feb 10, 2011 3:23 am
Early Marshall amps do not have screen grid resistors. If you are using an old amp, you will need to install them. There is a difference between the old EL34 (original) and the newer ones, newer EL34, E34L, E34LS designs draw a lot more current.
Without the screen resistors installed, the amp will definitely blow fuses with newer tubes installed. Or the plates are gonna turn red when you crank the amp up.
And also remember that since newer tubes draw more current than the old ones, the bias circuit must be appropriately modified, to supply more bias voltage.
(Yes, the factory techs at Milton Keynes taught me this lesson, and it's a good one)
Now have a look at the original Marshall schematic and the newer schematics. See the difference? See how the screen resistors were added? That's what you need to do, just like on the newer models. Look at the bias circuit, it supplies more voltage for the newer tubes.
After adding the screen resistors, the amp must be properly biased. If the bias is wrong, this will cause too much current to flow also, and blow the fuse or make the plates turn red.

Now here is how the factory biases the amp:
Older model: 1 mv sine wave input should drive the amp to just barely clipping. Too much bias will prevent the amp from clipping with this much input.
Newer model: 1/2 mv audio input will drive the amp to clipping.
Yes, that' really how they bias it, at the factory.

If you do not understand how to do this modification to an older amp, and properly set up the bias, have an experienced Marshall tech do it for you. Remember, the mod MUST be done. Without this, you risk destroying your transformers.
The inside of a Marshall is a deadly shock hazard. Only trained experienced techs should be working on it, SAFETY FIRST in all amp repairs. Do not risk your safety to repair a Marshall, it's not worth it.

[demonufo]

Now here is how the factory biases the amp:
Older model: 1 mv sine wave input should drive the amp to just barely clipping. Too much bias will prevent the amp from clipping with this much input.
Newer model: 1/2 mv audio input will drive the amp to clipping.
Yes, that' really how they bias it, at the factory.

Are you entirely sure about this?

Last time I went to the service department (about 5 years ago) at Marshall in Bletchley (not Milton Keynes btw), they simply biased by negative voltage, and told me as such. No scope's or generators.

This was what got me into working on my own amps in the first place. I was gobsmacked that this was all they did.



And, uh, didn't the original poster already say his screen resistors were fine???

[soundguruman]

Obviously, the screen resistors or bias is not fine. It's time to check both. There's gotta be a bug....
Another interesting point is the mains voltage. Older Marshalls had 110 volt primary, but not 120. This results in an extra 50-60 volts, B+, for USA users, and is another reason why the bias needs to be so carefully checked.
And yes I am sure. Older Marshalls had no screen grid resistors. Better check. The efficiency of EL34 has been increased over the years by the manufacturers. The newer EL34 needs more bias compared to the original EL34.
And yes, the bias is checked according to sensitivity. The hotter the amp is biased, the more sensitive it becomes. Bias too hot and the fuse blows.