2204 1R bias problem

[goldenGeek]

Meaning it still makes a loud noise unless you short pins 1 and 8 to ground? One power tube, either, or both?
That suggests that the ground connection at the tube socket is poor. I can't see, but it there a solder lug there? Can you take those bolts out and clean the mating surfaces?

Was it working fine before you first installed the 1R's?

That's right, there is noise/motorboating unless one of the tubes is grounded, it does not mater which one. It's enough to bypass the resistor with a wire from pin 8 to 1 (as the resistors now hang between 8 and 1).
I can try to clean the bolts/surfaces, but I also tried to solder the resistors (when hanging from pin 1 and 1 and 8 was connected) to a wire feeding over to the second filter cap ground (not connecting via the lugs at the socked base). Same issue there.
It was working before I installed the 1Rs, but I also replaced the filter caps and the bias circuit previous to that. There was some trouble with one red plating tube, so I installed the 1Rs to make biasing easier (my bias probe only has one socket so its a pain, litterally, to check both tubes)

(I posted the voltage chart in the prevoius post, I see this one got to page 2)

[martin manning]

Where are you grounding the cathode to make the noise go away? If you can short it to the chassis at the tube socket and stop it, the existing ground must be bad.

[martin manning]

BTW, it looks like the bias supply is connected directly to the PT secondary and not through the standby switch, so you have bias voltage when in standby, correct?

[bepone]

I can't seem to find anything very suspect.. the B+ is rather on the lower side, but as my wall voltage is 238V I have the amp set to 240/250V. I have also tried the 220/230V setting, but same results regarding the noise.

The noise seems to be some kind of "motorboating", but the only way to make it go away is to ground one of the output tube cathodes (it does not matter which tube). I have tried with different tubes, both EL34 and 6CA7.

Skjermbilde 2022-04-19 kl. 12.33.18.png

voltages at the supply are very low.. i never seen that in marshall.. something is missing in this story, 1R doesnt matter if is connected or bypassed.. where are those electrolytic caps and are you sure that you return all the connections ok? recheck all the grounds (- poles) and other what you touched

[bepone]

compared to secondary voltages... looks like this model is with low anode voltages? strange, maybe can be put to 220V selector to increase voltages little bit.

if there is no 1R resistor suspicious is 44mV at the pin 8, so it is not grounded.. or your ground connection from the capacitors is not ok..cathodes from output tubes must go to the supply capacitor directly not over the chassis i would check this route.. or add permanent wire because chassis use for returning of the curents must be avoided

[goldenGeek]

compared to secondary voltages... looks like this model is with low anode voltages? strange, maybe can be put to 220V selector to increase voltages little bit.

if there is no 1R resistor suspicious is 44mV at the pin 8, so it is not grounded.. or your ground connection from the capacitors is not ok..cathodes from output tubes must go to the supply capacitor directly not over the chassis i would check this route.. or add permanent wire because chassis use for returning of the curents must be avoided

44mV is WITH a 1R, sorry for not stating that. (I shorted the resistor on the opposite tube when measuring this to cancel the noise).

[martin manning]

It's enough to bypass the resistor with a wire from pin 8 to 1 (as the resistors now hang between 8 and 1).

That 1 ohm shouldn't make any difference. Maybe the socket is not making good contact with the tube pins? Have you cleaned and adjusted the socket tension?

[goldenGeek]

It's enough to bypass the resistor with a wire from pin 8 to 1 (as the resistors now hang between 8 and 1).

That 1 ohm shouldn't make any difference. Maybe the socket is not making good contact with the tube pins? Have you cleaned and adjusted the socket tension?

I cleaned but forgot to adjust tension. The owner needed the amp today so I jumpered one of the 1Rs and it’s working okay for now. He’s well aware that it could get worse but he was okay with it. He’s gonna drop it off in a few weeks for further investigation. However, if the cause was dirty/loose socket/pins, why does it not matter which 1R I bypass? This is by far the weirdest amp so far

[martin manning]

The owner needed the amp today so I jumpered one of the 1Rs and it’s working okay for now. He’s well aware that it could get worse but he was okay with it. He’s gonna drop it off in a few weeks for further investigation. However, if the cause was dirty/loose socket/pins, why does it not matter which 1R I bypass? This is by far the weirdest amp so far 😅

Good question, and yea, weird.

[goldenGeek]

Where are you grounding the cathode to make the noise go away? If you can short it to the chassis at the tube socket and stop it, the existing ground must be bad.

I did not see a couple of messages, sorry.
I dont ground to "ground" , I just short pin 1 to pin 8 (since the 1R is between 1 and 8 at the moment)

[goldenGeek]

BTW, it looks like the bias supply is connected directly to the PT secondary and not through the standby switch, so you have bias voltage when in standby, correct?

I did not see a couple of messages, sorry.
That seems to be correct (though, I dont have the amp at the moment)

[goldenGeek]

voltages at the supply are very low.. i never seen that in marshall.. something is missing in this story, 1R doesnt matter if is connected or bypassed.. where are those electrolytic caps and are you sure that you return all the connections ok? recheck all the grounds (- poles) and other what you touched

I did not see a couple of messages, sorry.
I did try that also, I have the readings in my studio, can post later. The readings seemed more like the voltage chart I found online, but the heater voltages was quite high I think, about 6.8V. My wall voltage was 238V at the moment of measurement.