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What is the consensus opinion on needing to re-bias when switching a pre-amp tube?

devinb

Member
Messages
2,030
I'm looking for a little more gain out of my Valvetrain 205 TB, Rich suggested a 12ax7 a while ago, I'm thinking about running by my favorite guitar shop this afternoon, I'm curious what the consensus opinion is on biasing when dealing with a pre-amp tube? I've read opinions in both directions, and the manual that came with the amp doesn't say one way or another.
 

hamfist

Member
Messages
1,595
huh ? The global agreed concensus by those who know about guitar amps and tubes is that pre-amp tube changes do NOT require re-biasing.
As Twangbanger states, they are cathode-biased. I'm no tech, but I've not heard of one that wasn't.
 

aleclee

TGP Tech Wrangler
Staff member
Messages
13,185
No need, no point since the bias on preamp tubes is not adjustable.
 

Rod

Tone is Paramount
Gold Supporting Member
Messages
22,355
You DON"T bias pre amp tubes....
 

StompBoxBlues

Member
Messages
19,951
Preamp tubes don't need it, which is GREAT news for you, because you can buy a couple different types of preamp tubes, even the same types vary, and try them out and see if you get one that has more gain, or the right sound. You can trade them in and out.

Ask the guys at the shop, or better yet, go on the net and read up (keep in mind, this is PREAMP tubes 12ax7, 12AT7, 7025...there are several that are interchangeble, but even keeping in one...say 12AX7 I think there is a good deal of variance on how much gain they have, so you could get a dud, or a champ of a tube...)
 

rockon1

Platinum Supporting Member
Messages
13,188
Yep virtually all pre amp tubes are cathode biased. John Philips pointed out one Mesa which wasnt but I cant remember which it was. Bob
 

Gordon

Silver Supporting Member
Messages
842
Good question, you've gotten the answer about ten times over.

Swap preamp tubes as you would like, do not do the same for power tubes.
 

John Phillips

Member
Messages
13,038
Yep virtually all pre amp tubes are cathode biased. John Philips pointed out one Mesa which wasnt but I cant remember which it was. Bob
The Studio Caliber series phase inverter tube.

It still doesn't need adjustment though (what did you expect in a Mesa! ;)).

The only time you might want to adjust bias for tonal reasons is in something like a Marshall 2203, second gain stage - the cathode resistor is unusually large and a popular mod is to reduce its value.

Apart from that, the only bias issue in any 'preamp' tube application I know of is the reverb driver in a Silverface Fender - which technically is a power stage anyway, even though it uses a 12AT7 - where the stock bias setting is unnecessarily hot and kills the tube fairly quickly. Raising the value of the cathode resistor is a good idea.
 




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