Bias test points on a Fender Twin Reissue

[endserenading81]

Hey guys,
I just replaced all of my tubes in my Fender Twin Reverb Reissue. I don't want to pay somebody to turn the bias knob to the correct position. Does anyone know where to measure the bias with a multimeter?
Thanks,
R

 

[ocnor]

 

[timthetortoise]

Google is your bestest friend

 

[dementedchord]

there is no "right position" for the controll persee' it's the right one for that particular amp using those particular tubes... there are 2 ways to judge this one is using whats called a bias probe that tells how much current the tube draws at idle... the other which is more accurate actually is to find the point where one tube shuts off and the other on... using an oscilloscope looking for whats called crossover distortion..

 

[endserenading81]

huh

Thank you Demented, obviously Ocnor likes to pay people for something he can rather LEARN to do on his own.
r

 

[ranjam]

..... obviously Ocnor likes to pay people for something he can rather LEARN to do on his own.

He can also start a relationship with his repair tech, who could often throw in a bias check freee if Ocnor buys his tubes from the same tech. How bad can that be?

It seems innocent enough, but if you have no electronics experience, and have to ask how to bias a tube amplifier..... oh, boy. You should keep your fingers out of your amplifier. Start with repairing cords, graduate to learning how to use a multimeter, and then teach yourself all about Sovtek tubes, which are likely in that Twin. Learning to bias with Sovtek tubes is like learning to swim in the English Channel. Start with your wading pool, and graduate to bigger ponds.

 

[ocnor]

Thank you Demented, obviously Ocnor likes to pay people for something he can rather LEARN to do on his own.
r

Dude I learned to bias my own amps while you were learning your ABC's from Big Bird.

It seems innocent enough, but if you have no electronics experience, and have to ask how to bias a tube amplifier..... oh, boy. You should keep your fingers out of your amplifier.

That's exactly right. You can easily get electrocuted F'ing around inside a tube amp.

 

[soundchaser59]

I had a red knob Twin last year.

Seems to me if your manual gives instructions on how to set the bias adjustment, then it is obviously something that Fender thinks you can do yourself.

If you cant find the bias points (mine were clearly factory labeled on the back of the amp) and the manual doesn't tell how to do it then you have two possibilities:

1. Fender doesn't want you to do it, or.....

2. it is self-biasing and you dont have to worry about it.


Only your spandex-clad big-hair dresser knows for sure.....

 

[soundchaser59]

You can easily get electrocuted F'ing around inside a tube amp.

I know I'm not supposed to take seriously anything posted in this place, but I have to point out as the Angel's Advocate here that I really hate it when people say this without any qualification. With only 100-some posts, we dont really know if the OP will take this statement the way it was intended. Heck, I dont even know what was intended, but I'm brave enough to guess.

Yes you can easily get electrocuted, but let's be clear that you have to take the chassis out of the cab and expose all the inner guts components where you can actually touch flesh to cap wires and transformer terminals, etc. If you dont remove the chassis from the amp, if all you ever do is change tubes (or set bias adjustements) you would have to try really really hard to play the fool and get shocked.

We now return to our regularly scheduled buffoons....

 

[dementedchord]

IIRC... the evil twin is the only fender that has externally accessable bias points...

 

[ranjam]

Dude I learned to bias my own amps while you were learning your ABC's from Big Bird.

Unless you're well into your 60's, I think you have it bass ackwards. Even when I was in High School, one of the issued text books was an RCA Receiving Tube Manual. My college years and CET Certification took me all the way to 1979. Still mostly tubes on those tests, too. I didn't write any of this down, but I believe I was building my own tube amps, and a few for other local players, about 1987 or 1988 at the latest. I was already into my 30's for certain.
But that's nowhere near the point. The point was even though it seems easy, I can still take my own gear in to get repaired somewhere else, and chat with techs, shop in the store, see who I know shopping, etc. I know I'm lazy, but at my age I'm allowed. And it's never expensive, or I would do it myself. And the other point was Sovtek tubes have an awful QC. They're all I can buy in my teeny weeny little town, so I learned how to make do. If you aren't experienced enough to do all that I do with Sovtek tubes, pay someone to do it for you. It's cheap, and you have a relationship with your tech.
But this is all moot, as the original poster is not using Sovtek tubes. And soundchaser 59; even though the red knob Twin has it all layed out for you on the back of the chassis, this is absolutely asking for trouble if you use Sovtek tubes. But NOS GE's should be fine. If you have to ask what's so bad about Sovtek and biasing by a common 1-ohm resistor................

 

[endserenading81]

Eurotubes

I am using JJ's.
r

 

[ocnor]

Unless you're well into your 60's, I think you have it bass ackwards.

If you look more closely at my post you'll see that my Big Bird comment was not directed at you.

 

[ranjam]

If you look more closely at my post you'll see that my Big Bird comment was not directed at you.

Then please accept my apologies, and forget everything I said about my real age.