Amp Bias

[fingers]

Hello to all , total tube amp newbie here , i have an old Crate vintage club 60 its a combo amp with 2 10s in it , currently it has 2 el34s groove tube , 4 12ax7s sovtek preamp tubes and a 12au7 make unknown. I got it used but is in awsome shape , but i think these are stock tubes , i would like to change them myself without having to bring it to a tech , its gonna be expensive. Heres my question. On the back of the amp there is two slots for the bias , adjust and test , is the adjust the trim pot ? if i get a bias probe and mutimeter can i do this myself this way ? Any help please

[hasserl]

if i get a bias probe and mutimeter can i do this myself this way ?

Well you probably can. I would suggest you do a little more research on it first. I don't know the specifics on that amp, do you have a schematic for it? That's your first step here, hunt down a schematic so you have something to refer to, and you can post a link to it when you ask questions about it so others can refer to it too. Google is your friend.

Then I'd do some reading on what bias is and why it is checked/adjusted, and how to do it. There is a lot of info on this free for the taking out on the net. Again, google is your friend.

Even if someone were to come on here and walk you thru this step by step, you still should read up to find out what it is your doing and why. Spend a little time and effort, it won't cost you anything, but the rewards are well worth it.

[MWJB]

Crate's advice is, unsurprisingly, "refer to qualified service personnel"

First, measure the current of the tubes that are in there, they might be somewhere from 15 to 40mA per tube? Report back if you get crazy high, or low, readings.

If all OK - Set bias to 30mA per tube (or 30mV if your probes use a 1ohm cathode resistor). Make sure both tubes read about the same +/- 5ma, so say 28 & 32mA would be fine too. If you start seeing crazy currents turn the amp off adjust the trim pot & start again.

Bias probes like Weber's bias rite that can also read plate voltage would be good, to determine plate dissipation at idle & maybe better fine tune plate current.

Be aware that the trim pot will have been designed to work with the original brand of tubes, its sweep may not accomodate all brands of EL34.

[Jazz P Bass]

Crate Bias

The schematic indicates -50Vdc at test point 27. The bias voltage is what is indicated.
That is a good healthy negative bias. It does not tell what the actual current draw is.
That you will have to determine with a bias probe, 1 ohm 1% resistor or current shunt method.

[fingers]

Bias

Well thanks to the members who gave me usefull advice , i guess for now ill leave biasing to the amptech. I dont have a whole lot of experience in amp electronics but i have a genuine passion for it , im no kid , im a 37 year old man. Someday i hope to really put it to practice though. I wont bore everyone with my jibber jabber. Thanks again to those who helped .