Fender Pro Reverb Tube Biasing

[quontos]

Does anyone have data concerning the tube bias for the output tubes
( 6L6GC) for a Fender Pro Reverb. It has a fixed bias supply and only
allows adjustment to equalize the 2 output tube currents. I get a bias
reading of 26 to 27 ma per tube by inserting a 1 ohm 1% resistor in
the cathode circuit and measuring the IR drop ( 26-27 mV). I found
data to suggest a idle current of 35 ma to be more in the ballpark.

BTW... The amp sounds very nice, The owner dropped it off at the music
store I do work for and asked to have the bias checked. It appears he
had replaced the output tubes on his own and then brought it in to
have the bias set. ( A little like installing sparkplugs in your car
and then taking it to the dealer to have them gapped...<grin> )

All insights, opinions, and anecdotes appreciated...!

Thanks

fazelock

 

[Jerry G.]

If the distortion is low, and the maximum power is in specs for the B+ level
being used, then I would not be too concerned about this. When referring to
the tube specs, take care of the B+ and screen voltage, before determing the
bias current. If you the output tubes are a bit under biased, the lifespan
of the tubes will be increased a bit.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
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Electronics http://www.zoom-one.com/electron.htm
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"quontos" <dan_sands@yahoo.com> wrote in message
newsioo50tf56fi706jgessc5e9f5qfiq8ona@4ax.com...


Does anyone have data concerning the tube bias for the output tubes
( 6L6GC) for a Fender Pro Reverb. It has a fixed bias supply and only
allows adjustment to equalize the 2 output tube currents. I get a bias
reading of 26 to 27 ma per tube by inserting a 1 ohm 1% resistor in
the cathode circuit and measuring the IR drop ( 26-27 mV). I found
data to suggest a idle current of 35 ma to be more in the ballpark.

BTW... The amp sounds very nice, The owner dropped it off at the music
store I do work for and asked to have the bias checked. It appears he
had replaced the output tubes on his own and then brought it in to
have the bias set. ( A little like installing sparkplugs in your car
and then taking it to the dealer to have them gapped...<grin> )

All insights, opinions, and anecdotes appreciated...!

Thanks

fazelock

 

[Merlin Zener]

On Sat, 20 Mar 2004 15:25:35 +0000, quontos wrote:

Does anyone have data concerning the tube bias for the output tubes (
6L6GC) for a Fender Pro Reverb. It has a fixed bias supply and only
allows adjustment to equalize the 2 output tube currents. I get a bias
reading of 26 to 27 ma per tube by inserting a 1 ohm 1% resistor in the
cathode circuit and measuring the IR drop ( 26-27 mV). I found data to
suggest a idle current of 35 ma to be more in the ballpark.

BTW... The amp sounds very nice, The owner dropped it off at the music
store I do work for and asked to have the bias checked. It appears he had
replaced the output tubes on his own and then brought it in to have the
bias set. ( A little like installing sparkplugs in your car and then
taking it to the dealer to have them gapped...<grin> )

All insights, opinions, and anecdotes appreciated...!

Feed it a low signal [like so it puts out about 10% of full output or so]
and look at the output on a cro [using an 8ohm dummy load, of course...]
If there's no crossover distortion [ie: no gap between the positive half
and the negative half of the signal] then the bias is not too low.

Personally, I like to see Fenders running cool - and Marshalls running
hard. [LOL]
Of course, this depends a bit on the type of music the owner likes, and
the kind of sound he's after.

At least your owner is actually *aware* of tube bias - unlike the plenty
of shredder idiots who pay through the nose for Groove Tubes and then
complain that their amps don't sound any better...

 

[Asimov]

"Merlin Zener" bravely wrote to "All" (22 Mar 04 03:57:20)
--- on the heady topic of "Re: Fender Pro Reverb Tube Biasing"

The 6L6GC is usually biased between 30 to 35mA but I'm pretty sure one
would be hard pressed to find much difference with the bias at 27mA.
There is quite a bit of non-linearity in a musical instrument
amplifier. It is not a thing of sonic beauty like an old Macintosh!
About the worst that can happen is that the overall circuit gain is a
tiny bit less but I bet the difference would only be measurable using
a precision a.c. voltmeter.

Another method of setting idle current in class AB1 is drive the tube
at 1/2 its power rating. It is more complicated because it means
knowing the B+ and using the tube maximum power spec to calculate the
required current. Although I have always used the recommended current
method like most, I think the 1/2 rated power seems to make sense.

Keep in mind however, that the lower the bias idle current the cooler
the tube runs at the expense of overall gain. The main problem to
watch out for is audible crossover distortion at low power levels.
This is typically heard as a slight fuzz-like sound as the string
vibration dies out and at high power it isn't heard. This implies yet
another method of setting idle bias using a distortion analyzer at a
convenient output power level of say 1 watt. This is perhaps the most
accurate method but requires more instrumentation and so is probably
overkill for a musical instrument amplifier.


MZ> From: "Merlin Zener" <merlin@localhost.localdomain>

MZ> On Sat, 20 Mar 2004 15:25:35 +0000, quontos wrote:

Does anyone have data concerning the tube bias for the output tubes (
6L6GC) for a Fender Pro Reverb. It has a fixed bias supply and only
allows adjustment to equalize the 2 output tube currents. I get a bias
reading of 26 to 27 ma per tube by inserting a 1 ohm 1% resistor in the
cathode circuit and measuring the IR drop ( 26-27 mV). I found data to
suggest a idle current of 35 ma to be more in the ballpark.

BTW... The amp sounds very nice, The owner dropped it off at the music
store I do work for and asked to have the bias checked. It appears he had
replaced the output tubes on his own and then brought it in to have the
bias set. ( A little like installing sparkplugs in your car and then
taking it to the dealer to have them gapped...<grin> )

All insights, opinions, and anecdotes appreciated...!

MZ> Feed it a low signal [like so it puts out about 10% of full output or
MZ> so] and look at the output on a cro [using an 8ohm dummy load, of
MZ> course...] If there's no crossover distortion [ie: no gap between the
MZ> positive half and the negative half of the signal] then the bias is not
MZ> too low.
MZ> Personally, I like to see Fenders running cool - and Marshalls running
MZ> hard. [LOL]
MZ> Of course, this depends a bit on the type of music the owner likes,
MZ> and the kind of sound he's after.

MZ> At least your owner is actually *aware* of tube bias - unlike the
MZ> plenty of shredder idiots who pay through the nose for Groove Tubes and
MZ> then complain that their amps don't sound any better...

.... I try to use my music to move these people to act... - Jimi Hendrix

 

[quontos]

On Sat, 20 Mar 2004 23:27:50 -0500, "Jerry G." <jerryg50@hotmail.com>
wrote:

If the distortion is low, and the maximum power is in specs for the B+ level
being used, then I would not be too concerned about this. When referring to
the tube specs, take care of the B+ and screen voltage, before determing the
bias current. If you the output tubes are a bit under biased, the lifespan
of the tubes will be increased a bit.

Thanks for the responses.... I balanced the tube currents and sent it
back to the owner with the excerpt from a tube biasing website that
was pretty informative, explaining how this model amp did not have
provisions for varying the bias currents and what it would take to
adjust them. The tube " sound" is very subjective, so I hope he is
satisfied, as I don't care to spend hours splitting hairs with an amp
that has great power and sound just the way it is. The boys at Fender
did a mighty good job with that model, IMHO...

Thanks again...