What does Fixed bias w/ adjustable pot means ??

[DesertRose]

To all the D.I.Y kings and techs

This is found on a user manuel of a 1980 Fender super twin reverb.
does that mean replacing of power tubes and carry on from there. No biasing needed as long as the replacement tubes are the same. E.G 6L6GC to 6L6GC regardless of brand

Thanks !

 

[marvar]

Re: What does Fixed bias w/ adjustable pot means ??

Nope, well, only if the replacement tubes are the same grade- example, if you use Groove Tubes, #5, and you replace them with #5, you shouldn't have to bias- if you replace them with a different value, you should re bias.
Tubes vary widely, even among the same brand.
You should have your bias checked anyway, because over time, it can drift.
To find out more about the difference in bias circuits, do an internet search.

 

[Pierre]

Re: What does Fixed bias w/ adjustable pot means ??

A fixed bias amp has a negative voltage grid compared to the cathode. It may STILL have a biasing pot to change this value though. The other way is cathode biasing.

 

[shredaholic]

Re: What does Fixed bias w/ adjustable pot means ??

Fixed bias is the type of amp that you need to rebias after changing tubes.

Cathode bias amps don't need rebiasing, as they have a cathode resistor that automatically sets the bias inside.

 

[DesertRose]

Re: What does Fixed bias w/ adjustable pot means ??

Ok one more question. I opened up the "tank" and I couldn't locate the bias knob. Its neither underneath the "tank". oh well .. because its DAMN heavy and I'm lazy to carry it down to my nearest local store which i in the city 30mins away.

Any better ways ??
Solution: I intend to get a SRS bias tool

 

[marvar]

Re: What does Fixed bias w/ adjustable pot means ??

Unless you have the proper tools and knowledge, setting bias is best left to those that do.
There are voltages inside your amp that can hurt or kill you. Plus the bias settings have to be calculated.

 

[DesertRose]

Re: What does Fixed bias w/ adjustable pot means ??

Ok. Advise taken
I just contacted my local tech to come over to my place and I'll watch him closely how he does it.

*sigh*

I've got 2 such amps due for tube change