From reaike–(at)–x.netcom.com Wed Jan 10 11:16:18 CST 1996
Article: 7732 of alt.guitar.amps
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From: reaike–(at)–x.netcom.com(Randall Aiken )
Newsgroups: alt.guitar.amps
Subject: Biasing output tubes
Date: 10 Jan 1996 04:57:15 GMT
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In <4cumln$4e--(at)--anyon.sr.hp.com> daver–(at)–r.hp.com (David Roach) writes:
>
>David Taylor (dtaylo–(at)–yberramp.net) wrote:
>: In article <4crojp$4g--(at)--anix3.panix.com>, moskowi–(at)–anix.com says…
>: Good post on biasing, but the problem with using any kind of voltage
>: measurement method is they (methods) are all based on the assumption
>: that a given output tube or set of output tubes fall within exact
>: design specifications.
>
>: Unfortunately we all know this just ain’t so.
>
>: I hate to throw a wet blanket on everyone’s do-it-yourself
>: sensibilities, but IMHO the only completely accurate way to bias an
>: amp is with an Oscilloscope, an Audio Oscillator and a dummy load
>: resistor. Too many times I’ve taken amps that were biased to
>: multimeter readings and put them on the scope only to find that they
>: are noticeably too cold or hot.
>
>: With today’s huge disparity in tube output and quality, biasing by
>: any other method seems like such a crap shoot.
>
>
> I’ve been setting the idle current, then hooking up a ‘scope
> and oscillator to check the result, and have had a hard time
> seeing either crossover distortion or top-to-bottom half
> assymmetry at any reasonable bias adjustment. What exactly are
> you going by on the ‘scope reading to make a judgement of “too
>cold or too hot”?
> If indeed there are reliable guidelines for idle current for
> specific tube types, it seems like a good idea to use them
> rather than a seat-of-the-pants opinion of what the ideal
> output waveform should be. Obviously, setting an arbitrary
> bias *voltage* is not the way to go, since different sets of
> tubes behave so differently,
> but idle *current* gives a good, reliable indicator of how hot
> the tube is running.
>
> Dave R.
>

First of all, there are only two things to worry about in biasing a
guitar amp. The first and foremost is not to set the bias current so
high as to exceed plate dissipation at any portion of the tubes
operation, as indicated by that nice warm red glow of melting plates
and the sound of a cash register ringing up a new set of tubes. The
second is to not set the bias current so low as to sound bad, which
generally means approaching or exceeding class B operation as indicated
by the onset of crossover distortion. Any point in between these two
is fair game and is subject to personal taste.

The _only_ truly accurate method of consistently setting the bias is to
measure the quiescent plate current and set it to a point within the
acceptable range for the plate voltage the tube is operating at and the
desired class of operation. Remember, however, when measuring the
quiescent current via a cathode resistor that you are not only
measuring plate current, but screen current as well, since the cathode
current is the sum of the plate and screen grid currents.

Biasing by negative grid voltage is highly inaccurate because the same
grid voltage can produce drastically different plate currents in
different tubes of the same type.

Biasing by the crossover distortion method as indicated in the Pittman
book and other places is also extremely inaccurate; the point at which
crossover distortion appears is very hard to detect and is subject to
changes with load impedance, and, in particular, with grid drive if the
phase inverter is AC coupled to the output tube grids (as it is in
almost all guitar amps). When driven into the positive grid region at
clipping, the output tube grids act as a forward biased diode and clamp
the positive peaks of the grid waveform. As the input signal level is
increased, the clamping action forces the average value of the grid
waveform downward, effectively increasing the negative grid bias. This
results in more crossover distortion, even if the amp is biased higher
into class AB. Because of this clamping effect, the amount of
crossover distortion that you are trying to “bias out” will change
depending upon how far into clipping you set the grid drive.

Randall Aiken
reaike–(at)–x.netcom.com

From SRSN81–(at)–rodigy.com Wed Jan 10 11:16:51 CST 1996
Article: 7749 of alt.guitar.amps
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From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Newsgroups: alt.guitar.amps
Subject: Re: How to bias a 1972 Twin?
Date: 10 Jan 1996 05:53:07 GMT
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dtaylo–(at)–yberramp.net (David Taylor) wrote:

>I hate to throw a wet blanket on everyone’s do-it-yourself sensibilities,

>but IMHO the only completely accurate way to bias an amp is with an
>Oscilloscope, an Audio Oscillator and a dummy load resistor. Too many
>times I’ve taken amps that were biased to multimeter readings and put
them
>on the scope only to find that they were noticeably too cold or hot.

Then you blow up all the Marshalls you work on?

Funny, I’ve gotten a lot of scope biased amps across the bench that were
too cold. Also gotten many DIY biased “by ear” amps that were way too
hot.. At any rate, biasing for max output power isn’t the same as biasing
for best feel and tonality in a guitar amp. That’s why I think biasing by
idle current is a better method. It has served me well for some years now,
and in fact the Fender factory techs set all the old amps by this method
in years past (they used the transformer shunt method btw..) I got that
tidbit from an ex-employee. I have rarely (never?) set up a 6L6 type for
35mA and had it sound “bad”. The system works, regardless of what a scope
& load might say. What matters in this case is how the instrument and amp
communicate to the musician. IMHO of course..

Joe

From SRSN81–(at)–rodigy.com Wed Jan 10 11:17:29 CST 1996
Article: 7728 of alt.guitar.amps
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From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Newsgroups: alt.guitar.amps
Subject: Re: How to bias a 1972 Twin?
Date: 10 Jan 1996 05:46:11 GMT
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moskowi–(at)–anix.com (Len Moskowitz) wrote:

Thanks for the comprehensive post on biasing. I just had a couple
comments on it.

You wrote:
> Other Methods
>
> A third way to set bias is to use a test signal, typically a sine wave.

> Monitor the output waveform on an oscilloscope and adjust the bias for
> minimum crossover distortion.

I’ve got all sorts of fun test equipment, but I’ve always had better luck
doing the final tweak by DMM and playing through the amp. Setting the amp
for minimal cross over distortion in many cases leaves the amps set with
a very cold bias. Marshall amps should always have some crossover
showing according to some techs. (I myself endeavor to never say “always”
or never.. oops, did it again 😉 )

> GENERAL BIAS GUIDELINES (from Tremolu–(at)–ol.com)
>
> Currents Per Tube – Class AB1 Operation (most musical instrument
amps
> are designed to run in class AB1)
>
> 6L6 – 30 to 35 ma
> 6V6 – 22 to 27 ma
> EL-84/6BQ5 – 22 to 27 ma
>

Those all seem in line with the exception of the 6V6 & EL84 settings. I
would take the 6V6 down a notch or two and reccomend a working Iq of 15-
25mA, preferrably 15-20mA. As a general rule, I set Iq at the point
where the B+ times the Iq = 1/2 the max plate dissipation. In a Deluxe
Reverb for instance, 20mA Iq at 410Va yields 8.2W diss, which is a little
over 1/2 the max dis. I try to always use 6V6GTA’s to get that extra 2W..

As for the EL84, it is nearly always seen in class A or near class A
situations and as such its idle current will be much higher (also the B+
will be lower so the dissipation is not as great..) Most 15W amps using
cathode bias and EL84’s use a 130 Rk and bias at about 11 volts which is
equivilent to a current per tube of 42mA. The currents in an AC30 work
out to about the same.

As far as the Sovtek 5881 goes, I’ve had great luck with them at 35mA and
above. In tweed style amps I’ve had them out to 50mA with no problems
(B+ at about 400..) BF style amps with B+ around 460V, they will be happy
for years at 40mA per tube and the increase in sustain and feel is worth
it IMHO. They are very rugged tubes from my experience.

Thanks for sharing your thoughts on the subject.

Joe

From tremolu–(at)–ol.com Wed Jan 10 11:17:48 CST 1996
Article: 7741 of alt.guitar.amps
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From: tremolu–(at)–ol.com (Tremolux)
Newsgroups: alt.guitar.amps
Subject: Re: How to bias a 1972 Twin?
Date: 10 Jan 1996 03:51:34 -0500
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>>>Those all seem in line with the exception of the 6V6 & EL84 settings. I

would take the 6V6 down a notch or two and reccomend a working Iq of 15-
25mA, preferrably 15-20mA.

I get great tone and decent tube life with my settings. IMHO, your 15 ma
is too low.

Regards.

 

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