Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on October 28, 2021, 01:38:02 pm
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Hi all,
A friend asked me to re-tube his Sovtek Mig-60, and I want to understand its biasing resistors. Instead of the more common 1 ohm resistors from cathode to ground, this one uses 3.9-ohm resistors. The schematic does not show voltages, but specifies that to bias the tubes properly, rotate the bias pot until each tube has about 82mv across that resistor.
Why use a 3.9 ohm resistor? And just so I can try to understand, would using a 1 ohm resistor show around 20.5mv on the cathode as opposed to 82mv with a 3.9?
I have a set of bias meters checkers (tubes plug into it, they plug into the socket) and a digital readout of ma is shown. Can I trust what this tells me, given that the amp has 3.9 ohm resistors from cathode to ground?
Thanks!
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Why use a 3.9 ohm resistor? And just so I can try to understand, would using a 1 ohm resistor show around 20.5mv on the cathode as opposed to 82mv with a 3.9?
Who knows why? Who cares? 82mV across a 3.9Ω resistor equals 21mA. Sounds reasonable and easy to do.
I have a set of bias meters checkers (tubes plug into it, they plug into the socket) and a digital readout of ma is shown. Can I trust what this tells me, given that the amp has 3.9 ohm resistors from cathode to ground?
Yes. Set your bias using the manufacturer instructions. Do the math to calculate current. Then use your bias meters to compare readings.
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BTW, this updated schematic says set the bias to 80mV.
https://el34world.com/charts/Schematics/files/Sovtek/Sovtek_mig60.pdf