Problem with Dr Tube Bias mod repair for Marshall DSL100

[greengriff]

Thanks. I'll do it ASAP.

[greengriff]

I could actually get to the legs of R71. Voltage was 465 on one side of it, and 475 on the other. Screen voltages: V5=456, V6=458, V7=457, V8=459. Today the anode voltage was 464.

[Helmholtz]

How do you measure idle currents?

10V drop across OT primaries seems rather high.
Measure DCRs of both halves of the OT primary. Make sure filter caps are discharged and short the speaker output while measuring.

[nickb]

I could actually get to the legs of R71. Voltage was 465 on one side of it, and 475 on the other. Screen voltages: V5=456, V6=458, V7=457, V8=459. Today the anode voltage was 464.

That seems very reasonable to me. That's very different from the number for the screen voltage that was reported earlier.

[greengriff]

How do you measure idle currents?

10V drop across OT primaries seems rather high.
Measure DCRs of both halves of the OT primary. Make sure filter caps are discharged and short the speaker output while measuring.

I have a socket that goes in between the valve and the valve base. It breaks the connection to pin 3 and allows me to run a multimeter in series with it. I will measure the OT halves ASAP.

[greengriff]

That seems very reasonable to me. That's a very different from the number for the screen voltage that was reported earlier.

Yes it was completely different! No idea why. Today the amp biases properly within range as well. I have no clue as to what's changed!!!!

[g1]

That seems very reasonable to me. That's a very different from the number for the screen voltage that was reported earlier.

Yes it was completely different! No idea why. Today the amp biases properly within range as well. I have no clue as to what's changed!!!!

Likely the low screen voltage was why it wouldn't bias up before, as was suspected.
Restoring the screen voltage allowed normal idle current. I would think there may be a poor solder connection at R71, and the fault may return.

[greengriff]

Thank you. When I have time I'll dismantle the amp and remake the connection just in case. Shall I still measure the DCR of the OT as suggested by Helmholtz?

[g1]

Thank you. When I have time I'll dismantle the amp and remake the connection just in case. Shall I still measure the DCR of the OT as suggested by Helmholtz?

Sure, when you have it opened, why not. Then do the math to solve for idle current and see how close it agrees with the number from the bias test points. It will be good exercise to familiarize yourself with the technique. Then you can use it in other amps that don't have the cathode resistor test points.