Do power tubes in a Princeton Reverb need biasing?

[coyotemorgan]

Just bought a 76 Princeton Reverb and it needs new power tubes. I bought some JJ's matched at 31 mA. Do I need biasing. I read at Fender Amp Field Guide that the PR's are "fixed, non-adjustable", but I know these amps can get biased. Will it make a difference in this amp?

[Tim Armstrong]

Yep, the amp needs biasing.

Tim

[jjmantele]

Were they matched at 31ma in your amp? If yes, you probably have about 13watts idle on each based on around 420 volts on the plates. That would be a little hot but not crazy for a class AB amp. If they were running 31ma in a different voltage environment, you would need to determine what they are running in you amp and adjust (bias them.)

[Paul G.]

Fixed bias--the tubes themselves don't set the bias (as in cathode bias schemes), the bias is set by some external means.

Non-adjustable--some amplifiers have a pot to easily change the bias. Your Princeton reverb doesn't. The bias is checked then adjusted by changing some resistors. It is tedious, and usually if you are within the operating range of your tubes, it's left alone. You still have to make sure your bias current is within range.

It is no big deal for a bias pot to be added to these amps. I recommend it, as it will allow the best tone AND tube life to be easily achieved.

P.

[CancerLeoCam]

here's the scoop....

Fixed- that's it -fixed. You have to desolder and solder an appropriate resistor values to bring the bias where you want it. Think BF Princetons, PReverb, Brown Deluxes.

Adjustable- it may have a pot that allows you to adjust the bias as you turn the pot-no solder needed. Think AB763 BFDR, TR, SR, ProReverb, Showman.

Cathode- also fixed but found on many amps run @ Class A. You can spot 'em (big 1-2W'ers or more) as they are directly soldered on the power tube socket pins for the cathode or on the circuit board connected via wire to the cathode pins. Again, you have to make the adjustment via resistor values and soldering iron, gun, etc. Think Champ, Vibro Champ, Bronco, Reverb Tank.

[coyotemorgan]

I now get it...it's off the shop for the biasing treatment. Thanks for the clarifications.

[mark norwine]

There's no such bias method as "adjustable bias".

Twin reverb, Super reverb...those cited above....are all fixed bias. The pot is a convenience, but doesn’t change the bias methodology....it's still "fixed bias".

Fixed bias uses the application of a specific negative voltage applied to the grid so as to regulate plate current by keeping the grid "more negative" than the cathode.

Princetons, Boogies, et. al.....fixed bias, no pot.

Super reverbs, Twin reverbs, etc.......fixed bias, with a pot.

"Fixed" has nothing to do whatsoever with "components of fixed value" or components "fixed in place", etc.

-----

By contrast, cathode bias uses a cathode resistor, and employs ohms law to drop a voltage across it {as current is drawn}, thereby making the grid more negative. It's just another way of making the cathode positive with respect to the grid.

Cathode bias has nothing....nothing....whatsoever to do with class A. The method of biasing and the setpoint at which an amp is biased are unrelated.

mn

[trdlasvegas]

Guys, congratulations on all the technical explanations of bias schemes. But the question was:

"Do power tubes in a Princeton Reverb need biasing?"

A simple "yes" would have sufficed.

-Tony

[CancerLeoCam]

http://www.aikenamps.com/WhatIsBiasing.htm

an excerpt...

"There are two main types of biasing: fixed biasing and cathode biasing. Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot". It is no wonder the subject is confusing to people! "

I really don't think we need to get anal on this one. But hopefully you'll believe Randall.

[mark norwine]

Randy & I are great friends.....I can read his wit & humor in that statement.

I stand by my comments, and IEEE definitions stand behind me. If need be, we can drag Randy over here, too.....

[Tim Swartz]

I'm w/ Mark, but in the case of some '68/'69 Fenders (the bigger amps) I will coin the term "mixed bias"... (which I expect to be adopted in all text from this point forward). :D These amps use both cathode resistors and negative grid voltage to control plate current.

[mark norwine]

So true!

In fact, I think I have a lifetime supply of 150 ohm 10W resistors!

I can't count how many "Mixed Bias" {great term!!} Fenders I've brought back from the dark side......

mn

[Tim Armstrong]

Originally Posted by trdlasvegas

A simple "yes" would have sufficed.

-Tony

Originally Posted by Tim Armstrong

Yep, the amp needs biasing.

Cheers, Tim

[trdlasvegas]

Sorry Tim,
I saw your response AFTER I wrote that.

Nice to see the two of us were listening.

-Tony

[jazzguitar]

Geee, you all make it complicated!

To answer the original poster's question:

Yes, power tubes in a Princeton Reverb need to be biased like in any other amp!

Since the bias is fixed-non-adjustable, good 6V6GT's that meet the spec most often are plug and play, so an adjustment (which can be done by exchanging parts inside the amp) is not always necessary, but it is wise to have it checked anyway - each power tube should read about 15mA to 20mA.

[jjmantele]

Bias terms are confusing!

Fixed: A negative DC voltage is applied to the grid.

Adjustable: It is possible to adjust it w/o changing parts. I have adjustable cathode bias on 2 of my amps.

Cathode: There is a cathode resistor and no negative DC voltage applied to the grid.

Over Biased: The negative DC is too strong so the tube is idling too cool.

Under Biased: Opposite of above.

[mark norwine]

C'mon folks...

There is no such bias method as "adjustable bias".

What most fenders amps have is fixed bias, constructed with a variable resistor {potentiometer} in lieu of a specific resistor. This arrangement allows you to vary the amount of negative voltage applied to the grids {as properly defined above as fixed bias}.

Putting a pot into a fixed-bias amp doesn't change anything...it's still "fixed bias".

I've never seen a pot-adjustable, cathode-biased amp. But if you do, indeed have such, then what you have is "Cathode Bias", pure & simple.

No such thing as "Adjustable Bias".

mn

[jdfoosh357]

So which type of circuit allows for NO BIASING when changing the tubes?

The guy from Eurotubes told me that I didn't need to bias my Champ after I switch the tubes.

[Tremo]

Cathode bias is self-biasing.

Still, it should be checked.

[jazzguitar]

Originally Posted by jdfoosh357

So which type of circuit allows for NO BIASING when changing the tubes?

No circuit allows for "NO BIASING" because if you plug in a tube it will be biased. Maybe at a wrong value.

What you probably mean is which type of circuit allows for plug-and-play power tube changing ? "BIASING" does not necessarily mean changing anything. The word is sometimes used as "adjusting the bias" though, which is adding confusion.

Plug-and-play mostly is the case with cathode-biased circuits. All preamp tubes are cathode biased which is why preamp tubes are plug and play.

In our days of tubes of doubtful manufacture quality, fixed (i.e. non-cathode-biased) amps new power tubes require at least checking the bias to see if it is good. If so, nothing else is required.