From mgarvi–(at)–anix.com Sat May 13 10:50:55 CDT 1995
Article: 1210 of alt.guitar.amps
Path: geraldo.cc.utexas.edu!cs.utexas.edu!swrinde!sgiblab!uhog.mit.edu!bloom-beacon.mit.edu!panix!not-for-mail
From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: A few questions
Date: 13 May 1995 03:06:57 -0400
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In <3oulse$gb--(at)--ewshost.loc3.tandem.com> pau–(at)–verest.tandem.com (duffy_paul) writes:

>Standby switches, power on/off and tube life.
> As far as using standby for power on/off, I would guess there
> is no point in using standby on power up, and that on power
> down turning the amp off without going into standby dumps
> the power supply charge through the output tubes because the
> bias goes away first? I have always used the standby before

Good question about power-down. I doubt that many people think
about this. Fortunately, the bias circuit usually has very low
current drain, so the electrolytics usually hold charge for a
while.

On power-up however, conventional wisdom (and I think the original
reason for standby sw’s) is that the cathode will get ‘stripped’
(burlesque theme goes here) if plate supply is applied before
the cathode is warmed up. I never questioned this, but I just
heard someone say that it’s never been proven (???).

One of the famous errors in standby switches (which Marshall
seems to make once in a while) is putting the bias supply on
the same circuit as the standby sw. The bias will sometimes
not get up (down, actually) in time and lots of current flows.

>Bias
> If you adjust the bias positive, more standby current will flow.
> Suppose you ran the bias so that about half the headroom of
> the amp was gone, you’d get a saturated tone, right? But,
> would the output transformer melt down? Just thinking of ways
> to get tone at lower volume. But I don’t want to destroy my amp.

See tech books which explain class A and class AB. Usually this
is harder on the tubes than the xfmr but if a tube shorts out, it
can take the xfmr down. When you say ‘adjust the bias positive’
you probably mean ‘toward ground’ cause very few output tubes
require positive bias. It’s usually somewhere betw -25 to -55v.

You may get better tone at low volume if you heat ’em up a bit more.
I know several guitarists who claim this helps at high volume, too.

> I biased an amp by starting out with visible zero crossing
> distortion and reducing bias until the zero crossing was smooth.
> Is this the normal way to bias an amp? This amp had a pot
> for bias adjust. If you want to add bias to run the amp hotter

There are references for this in Aspen Pittman’s ‘Tube Amp Book’.
Check out Ken Fischer’s section. He’s pretty knowledgable, but
do *not* start poking around in there if you don’t know what you’re
doing. 450v hurts, at the very best.

Regarding crossover distortion, it’s a gradual effect. Turning up
slightly past the point where it disappears is the usual method,
but expect some residual xover to remain, simply cause the tubes
are not quite ‘meshed’ in terms of operational characteristic
until current goes up a bit more.

I’ve heard of the opposite approach, too. I would not do this
if I were you, but someone actually said ‘turn down the room
lights and crank the pot until the plates start to turn orange.
Then back it down a bit’. Yikes.

> what is the best way to do this in a calculated way? Is there
> a tone improvement by running hotter in an amp that has an output
> transformer that was designed for straight Class AB?

My preferred approach is to break the connect betw output tube
cathode and ground. Insert 1 ohm resistors (or even 10 ohm)
betw cath/gnd. Then you can measure volts across the resistor
and figure current according to ohm’s law. Of course this
methode lets you set current exactly but tells nothing about
how this particular set of tubes is operating at that current.

Mark Garvin

 

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