Bias adjustment on Marshall 2195 100W Lead and Bass

[Timbo]

I decided to check the output transistor biasing in this amp. It actually sounds fine, but R35 looks discoloured as if it may have overheated at some time. Both R35 and R36 measure around 0.3 ohms but the voltages are 0.1mV across R35 and 0.26mV across R36, which doesn’t seem quite right?? Also, the preset RV1 has no noticeable effect on these voltages, or the voltages on the bases of T12/13. I’ve checked the preset itself and it appears to be working it should. I’ve done a fair amount of work with valve amps, but not so much with solid state, so I’m not confident of my methodology especially since the amp seems to be perfectly happy - any thoughts welcome.

[Jon Snell]

The differences in quiescent voltage, (with no signal or load) is perfectly normal due to the Vbe voltages of the different transistors.
0.26mV across 0.3R equates to 78mA, I would prefer it a little lower at say 15mA or 4.5 - 5mV drop but as long as it is not getting too warm, it will be fine. Don't connect a signal or speaker when setting up the quiescent current or you will get odd readings like yours due to offset voltage that is always present in these amplifiers. Take a fresh reading with no speaker connected.
I would guess that the transistor, MJ2501, that feeds R36 has been replaced at some point. Wire wound resistors don't drift in value, the go open circuit after glowing for a while, otherwise they are usually OK. It looks like it got hot but not damaged.

[Timbo]

Thanks for your reply Jon. Now I’ve disconnected the load I’m measuring 0.0-0.1mV across both resistors. I’m using a Fluke 175, should I have something more sensitive for this kind of measurement? How do you calculate 78mA from 0.26mV/0.33Ohms? At the risk of making myself look a complete numpty I make that 0.86mA…..

[Helmholtz]

A 0.26 mV drop means a current of only 0.87 mA.

[Timbo]

Definitely 0.1mV…..

[Jon Snell]

Whoops! You are right, had a brain fade senior moment there.

We need no more than 15mA through the resistors idealy.
You had the speaker connected I presume.

[Timbo]

Originally I had a power attenuator connected. Now, with nothing connected I’m measuring 0.1mV = 0.3mA!?

[Timbo]

I guess the bias voltage is too low to turn on the power transistors?

[Helmholtz]

0.1mV is the same as 100µV which is close to nothing.
For a current of 15mA you need 4.5mV (without load).

[Helmholtz]

Please post schematic.

[Timbo]

Here it is:

Marshall-2195-100W-Lead-Bass-Schematic.pdf

[Jon Snell]

To be quite honest, most players won't notice the crossover distortion caused by too low a current value but if you set the current to between 10 and 15mA it will sound as it should.

[Helmholtz]

What is the voltage measured between the bases of TR12 and TR13?
How much can you vary it with RV1?

[Timbo]

It sounds ok to me, but this is more of an exercise to improve my own understanding of the circuit. Which components would you recommend that I should change in order to increase the bias and preferably make the bias preset RV1 work properly?

[g1]

If the bias circuit is defective, you need to locate the part responsible by troubleshooting, not just swap parts.
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