Biasing question

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neilli

neilli

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I'm a complete newbie - I've done some reading and have an 'idea' of what and why (basically matching the voltage to the tube I think?), but have never actually done it. I guess I've had a fair few Mesas, and I've just put in new tubes and adjusted to any sound difference.

But my Friedman modified Marshall 87-X needs new preamp tubes, or at least I think it does because the tone kind of gets a bit muffled for a bit, then it's OK, then muffled again, and only on the BE/HBE channel.

I have it in my head that preamp tubes don't really need to be biased, it's just for power tubes? Am I completely wrong on this?
 
Changing preamp tubes doesn't affect the output tube bias. It's not a bad idea to check the bias even when you aren't changing output tubes. Did he add those banana plug test jacks on the back panel of your amp?
 
So preamp tubes don't get biased then?

And nope, there's no test jacks on the back unfortunately.
 
The preamp tubes are cathode biased. The cathode resistor sets their operating point. So plug and play. For the output tubes hopefully there is a 1 ohm resistor from pin 8 (cathode) to ground. You measure the DC voltage (in millivolts) across the resistor and that translates to how much current (in milliamps) the tube is drawing.
 
With regard to simple power-tube bias adjustments, I've found the inexpensive bias probe from Eurotubes, along with a basic multimeter, to be indispensable. This is especially noteworthy considering that I know precious little about electronics.

Also, at the risk of stating the obvious, make sure you know exactly what you're doing and exercise extreme caution when performing any work inside the amp: Some of the capacitors in there retain lethal levels of voltage well after the power has been switched off. My apologies if you're already aware of this.
 
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