Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Grantorino on February 28, 2026, 03:41:34 am

Title: Marshall JCM 2000 Combo - Buzz that increases with volume - Total recap needed?
Post by: Grantorino on February 28, 2026, 03:41:34 am

Hello again EL34 World.

I’m working on a Marshall JCM2000 DSL401 combo (year 2000) with a buzz present on all channels. The buzz increases with both channel volume and master volume.

Initial findings:

The main reservoir capacitor (C32, 100µF/450V) was visibly bulging and has been replaced.

Bias was previously set low at 0.31 VDC at TP2. Spec is 1.25–1.45 VDC. It is now set to 1.35 VDC (~33 mA per EL84).

R31 had been changed from the schematic value of 15kΩ to 2.2kΩ.

R36 had been changed from 10kΩ to 3.3kΩ.

Both resistors have now been restored to stock values.

C94 (47pF/500V) had browned and drifted in value and was replaced.

Current condition:

Plate voltage on pin 7 of the EL84s is ~384 VDC and consistent across all tubes.

Known good preamp tubes have been substituted (including V1 and V4).

Buzz remains present on all channels at approximately 200 Hz.

Buzz increases with volume controls.

I’m suspecting ripple in the downstream HT supply nodes and considering replacing the remaining electrolytic filter caps in the preamp supply chain.

Does that seem like the logical next step, or is there something in the DSL401 known to produce this behaviour that I should check first?

Thank you.

Title: Re: Marshall JCM 2000 Combo - Buzz that increases with volume - Total recap needed?
Post by: stratomaster on March 01, 2026, 04:18:18 am

You can measure the AC at each node, provided your meter is capable of AC on DC measurements, to confirm or disprove your suspicions without having to replace anything.

Title: Re: Marshall JCM 2000 Combo - Buzz that increases with volume - Total recap needed?
Post by: Grantorino on March 01, 2026, 04:43:54 am

Thanks for your reply Stratomaster.

I think it is time to invest in a decent multimeter. Mine can do AC, but the highest resolution setting is 200 V AC.

Title: Re: Marshall JCM 2000 Combo - Buzz that increases with volume - Total recap needed?
Post by: stratomaster on March 01, 2026, 07:14:49 pm

You shouldn't see more than 10Vac or so on the dirtiest node.  But given the limited voltage capacity, I'd be inclined to think you're meter is incapable of the required measurement.