(fixed) bias question

[Fragle]

hi all

i was discussing about the (dis)advantages of having el34 vs. 6l6 tubes in a laney amp, and a question popped up whether those laney amps are fixed bias or not.
on the back there's a switch called 5881/EL34 which basically adjusts to those types of tubes. now, afaik the only way to do this is switching between two different resistors that set the bias for the selected type of tube. the question is, if there is a resistor that sets the bias "about right", it is not possible to bias the amp, is it? at least not without replacing those resistors with a trim pot, or am i wrong? i guess i am wrong, because i recieved this mail from laney (the question was whether i should have the amp biased after a tube change or not)

"Hi there,
While it is not essential for the running of the amp (once you have set the switch for the correct type of valves that should be OK), we do recommend you get the amp rebiased to get the best performance from it.

Thanks, "

now, i guess it is possible to have it biased [img]/images/graemlins/laugh.gif[/img] can anyone explain how that works?

greets
fragle

[JerryP]

Re: (fixed) bias question

You should have the amp biased when ever you change power tubes. Which Laney model is it? The ones I've worked on had a bias pot.
Jerry

[Fragle]

Re: (fixed) bias question

its a GH 50L, and it does not have a bias pot. only that el34/6l6 switch

greets

[JerryP]

Re: (fixed) bias question

The GH50L does have a bias pot. Take another look.
Jerry

[Dreamland_Rebel]

Re: (fixed) bias question

you don't need a bias pot to re-bias it. a tech can just change out resistors for ones with different values to adjust the bias. thats how they rebias mesas.

[Fragle]

Re: (fixed) bias question

jerry, where is the bias pot located??

[JerryP]

Re: (fixed) bias question

[ QUOTE ]
jerry, where is the bias pot located??

[/ QUOTE ]

It's been a while since I had one of those apart so I can't remember. Can you take a pic and I'll point it out to you. It should be on the board down in the area of the power tubes. It's just a small little trim pot.
Jerry

[Fragle]

Re: (fixed) bias question

unfortunately, i cant take pic at the moment.

but well, how does it work? are there two different pots (one for el34 one for 6l6) and the switch on the back basically switches between those two or how does it work....


greets
fragle

[JerryP]

Re: (fixed) bias question

There's one pot. You set the switch to which tube type you're using and then use the pot to dial in the bias.
Jerry