Got my amp rebiased.. weak OT?

[suislidE03]

Picked up my Laney from a guy here in town and said he had trouble biasing my amp. I dont understand much about bias still, but he said he usually biases amps by reading the voltage, and said there was a big drop in the negative voltage. blah blah blah.. when setting by the voltage, he was getting a reading of like 7 ma or something, and that when he used a different formula, he had to set the amp for 50% instead of 70% like you are supposed to set for. So basically he was saying the amp has a weak transformer, and he set the bias really cold, basically to where the tubes will last forever. Anyone comprehend what he was trying to say?

 

[LesPaulRules]

Re: Got my amp rebiased.. weak OT?

It could be the output transformer, though it could also be the tubes (even if there new, one of them might not be that great).

This can get very involved, but simply. When biasing an amp, you are basically adjusting an amps bias for optimum sound and tube life. This bias (voltage) is usually set somewhere between cold (50%) and hot (70%). Hotter usually means a warmer, punchier sound, but the tubes will wear out quicker. Colder usually means the tubes will last longer, but the sound will not be as warm, full, etc. (Look up the terms: Crossover distortion and maximum dissapation on the net. This will help you understand what's going on a little more).

The above are only some general comments. I have heard amps that were biased out of what's considered the normal range sound very good.

 

[kmcguitars]

Re: Got my amp rebiased.. weak OT?

Did he say your plate voltage (on pin 3 that connects to the OT) was high? If your plate voltage it to high it will cause the Neg. voltage on pin 5 to be excessive making the cathode current difficult to read. In simple terms did he say you need a new OT? That's what I would comprehend.Even though your OT is working, It's not working optimally. Here is some techno mumbo jumbo from Doug Hoffman on this subject.

The input control grid is able to repel/hold back or attract/allow electrons to flow across the vacuum gap between the cathode and the plate. The input control grid is between the cathode and plate physically. The electrons are repelled back to the cathode or attracted away from the cathode by the control grid voltage. When the electrons see the higher voltage of the plate, they are attracted to the plate, not the control grid. They pass right on by the control grid and make a bee line to the plate.

The higher the negative voltage level (more negative) at the control grid the more it can hold back or repel the electron flow from cathode to plate. The lower the negative control voltage is (less negative), the more the electrons are allowed to flow from cathode to plate. Of course the AC control grid voltage is always fluctuating up and down and so the current flow through the tube is always fluctuating up and down following the sine wave of the control grid voltage.

 

[More-Gear-Than-Skill]

Re: Got my amp rebiased.. weak OT?

If you trust the guy, and the amp sounds good then I say "cool, rock on"

But if you're curious about whats going on in there, perhaps take it to another reputable tech. Do a bit o reading, and pick his brain when you get it back.

Biasing must be done with love...