Power Scaling: Fixed Bias or Cathode Bias ?

[yZe]

How can i tell when my amp has either a Fixed Bias or Cathode Bias ?

Seems like I have to know this before i decide which kit to install:

http://www.londonpower.com/psk-faq.htm

I plan on doing this to a both a red Bear MKE 60 and MKE 120

both are P2P based on a JCM 800 circuit

What does fixed bias and cathode mean to the laymen ?

[Old Tele man]

...fixed bias (usually) has a potentiometer (often hidden) to "adjust" the so-called fixed bias; power tubes may/maynot have cathode resistors (usually 1 or 10 ohm) for "reading" cathode current.

...cathode bias has NO adjustment and ALWAYS has a cathode resistor, but may/maynot have a cathode capacitor; and, occassionally there may be separate cathode RCs for each power tube.

PS--the schematics can also "clue" you to the existance of the above components.

[TheAmpNerd]

PS--the schematics can also "clue" you to the existance of the above components. OTM Does that mean we actually might have to read and (hopefully) understand those little squiggly things which represent an amps circuitry?

[Old Tele man]

...read, no? just guess CORRECTLY 100% of the time and maybe you'll sometimes be possibly not wrong.

...rough translation: when in ROME (electronics) speak (read?) as the ROMANS (techs) do.

[yZe]

Is it possible if you guys to tell me which bias?

Schematics are on this page. Hit ctrl F and type red bear - press enter
http://www.triodeel.com/schindex.htm

Here's the :
pics of a 120 (http://www.acplink.com/chassis/c_full.aspx?id=262)

[trdlasvegas]

Their Fixed Bias.

-Tony

[yZe]

Their Fixed Bias.

-Tony
Thanks, Tony. If you don't mind me asking - How did (or do) you know?

[TheAmpNerd]

Thanks, Tony. If you don't mind me asking - How did (or do) you know?

The classic JCM800 circuit is fixed bias (with a pot in there).

I assume those were your pics posted. If you look at the
bias circuit you can see (as best as I can tell) a silver pot.

It would be my recommendation that you have a tech
install the PSB for you. No offense, but if you have a
difficult time reading a schematic and therefore not
knowing which type of bias arrangement your amp
is, it would be well worth the money not to fry
yourself, the amp or components.

The time you save will be well worth the investment of having
a power scaled amp and your pleasure of playing through it.

Then when you get a little champ amp, buy a smaller
scale box and tweak that first and learn from it.

[yZe]

Actually, I am looking for a tech. i posted that i am looking for a tech in Tampa to install it in the Amp forum.

I am just trying to educate myself as to the how and why's so i know that some tech isn't taking me for a ride.

I was just curious for educational purposes only

So, in light of the fact that I am in no way attempting or ever have intended to install this myself; would it be presumptuous of me to ask how I can tell where and what the bias circuit is in the first place from the schematic?

[trdlasvegas]

Because the schematics you listed show the all the output tubes cathodes connected directly to ground, no cathode resistor. Also there is a extra winding on the transformer producing a bias voltage. (It's called -V on those schematics)

-Tony

[Old Tele man]

...another "hint" is the existance of a negative voltage (-Vg) supply, because negative voltage is not used anywhere else in a vacuum tube amp, except as bias voltage for "fixed-bias" operation.