Marshall 2061x issue

[Leevi]

I built a clone of the following Marshall circuit and met an issue with biasing. The amp is running too hotPlate voltage: 373Cathode resistor 150RCathode voltage 13.3V That setup gives dissipation 14.7W which is too high. By increasing the cathode resistor it will run cooler but is 14-15V on cathode any more optimal? Another problem is that B+ will increase close to 400V.I don't like hot voltage drop resistors either.
I think the best solution would be a some kind of voltage drop system by using power mosfet (like Vvr).Any ideas?/Leevi

[sluckey]

What are the specs for your PT?

I suggest installing two 1K screen resistors and call it done. Almost everyone abuses EL84s, especially with cathode bias.

[Dave]

I agree with Sluckey. But if the PT has 5-volt secondaries, it would be easy to add a tube rectifier. That would give you some options.


Dave

[Leevi]

What are the specs for your PT?

I suggest installing two 1K screen resistors and call it done. Almost everyone abuses EL84s, especially with cathode bias.

230/325-0-325/6.3/5,  Marshall has 318-0-318,  I'll try the 1K screen resitors

it would be easy to add a tube rectifier.

Unfortunately there is no room for that

/Leevi

[Leevi]

1K screen resistors did not fix the problem. Cathode voltage drops a bit but not enough.
/Leevi

[sluckey]

That schematic clearly shows B+ is 399VDC with that 318-0-318 PT. You had to know your voltage was gonna be high too. I've built several 2xEL84 cathode biased amps, Vox, Marshall, Matchless, and have found that a 275-0-275 or 300-0-300 (with tube rectifier) is an ideal PT voltage for these amps.

[Leevi]

That schematic clearly shows B+ is 399VDC with that 318-0-318 PT. You had to know your voltage was gonna be high too. I've built several 2xEL84 cathode biased amps, Vox, Marshall, Matchless, and have found that a 275-0-275 or 300-0-300 (with tube rectifier) is an ideal PT voltage for these amps.

Yes, I also have built lot of el84 amps and fully agree with you. I made a mistake and trusted too much on the schematic. Marshall runs the tubes too hot. I'll drop the voltage with mosfet.


/Leevi

[sluckey]

Here's the actual Marshall schematic. No voltages though.

[Leevi]

Yes, they have not stated voltages but when reading different forums the high dissipation seems to be common.


https://www.mylespaul.com/threads/2061x-bias.344559/


https://www.marshallforum.com/threads/2061x-bias-my-2%C2%A2.42622/


I added a voltage dropper and got the bias to recommended level.


/Leevi