Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on April 19, 2024, 09:46:46 am
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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
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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.
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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
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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
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1K screen resistors did not fix the problem. Cathode voltage drops a bit but not enough.
/Leevi
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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.
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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
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Here's the actual Marshall schematic. No voltages though.
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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