From: ankleand@mtl.mit.edu (Andrew Karanicolas)
Subject: Re: valve/tube amp biasing
Originator: tjk@csd4.csd.uwm.edu
Organization: MIT Microsystems Technology Laboratories
Date: Tue, 28 Jul 1992 12:52:41 GMT

In article <9207270640.AA11297@spider.ento.csiro.au> peterca@ento.csiro.au writes:
>I have found three approaches to the biasing of valves and would welcome
>comments on the relative merits of each approach.  I am new to valves
>having sofar made one preamp and done some tweaks on a pair of Leak TL12s. 
>I have been told that a worthwhile tweak is to improve the regulation of
>the bias voltage especially on the output valves of the power amp.  The
>methods I have heard of and my impressions/naive ideas follow:
>1) cathode resistor to jack up the voltage at the cathode plus capacitor
>bypass.  Appears to be used on older equipment.  Seems easy as no

Well, most vacuum tube power amplifiers are push-pull arrangement.
In some cases, a single cathode resistor is used for bias.  In this case,
the point at which the two cathodes are connected, corresponding to
each respective output tube, is connected to a resistor.  The other
end of the resistor is at ground.  The cathode point is at incremental
ground during small signal operation and no bypass capacitor is needed
in principle.  However, for large signal class AB/B operation, the
cathode point will change since one of the output tubes is cutoff when
the other is conducting.  For class A operation, the cathode point should be
constant for large signal swings because the signal current is much
smaller than the bias current, hence no device is cutoff.
In other power-amp designs, separate cathode bias resistors are used
for each tube with bypass capacitors across each resistor.  I prefer
the single resistor method as it is simplest.  A bypass capacitor is
not recommended in this case as the polarity across the single
resistor reverses depending on which tube is heavily conducting.
However, I don't like electrolytic capacitors and the single
resistor method is much preferred for class A power amplifiers where
the cathodes are at incremental ground for a balanced condition.

>adjustment is required.  Possibly the sound quality is dependant on the
>quality of the bypass caps which are necessarily electrolytics to have
>large enough capacity even though film type bypasses can be added.
>2) Negative DC supply added to the signal going to each valve, cathode
>connects to ground via a small resistor, I presume to allow measurement of
>the current through each valve. Constant readjustment of the bias on each
>valve sounds like a nuisance.  Is it? This appears to be used on all more
>recent designs I have seen.  Is this because it sounds better?

This is a good approach but the price is complexity and cost.

>3) Common cathode power zener or zener plus power transistor to replace the
>cathode resistor of 1 above.  The cathodes of a push-pull amp output stage
>are joined together and connected to earth via a zener which jacks up the
>cathode voltage to provide the -ve bias for the grid.  The current through
>the zener should be fairly constant for a class A amp because as one of the
>valves is turning on the other is turning off by the same amount. 
>Nevertheless a smaller, quality cap can be left in the normal bypass
>position just in case.  This idea was published in a mid-60s article by Reg

What?  Using _semiconductor_ devices in vacuum tube amplifiers? :) :)

>Williamson and reprinted recently in Audio Conversions, issue 9.  The
>claimed advantage was that type 1 amps would give a continuous power output
>closer to their peak rating.  This seems a very good idea and I am tempted
>to replace the type 1 arrangement in my TL12s with this one but am
>concerned that for an old idea I haven't seen it used-is there some problem
>which is not obvious?  Or does everyone do this and I hadn't noticed?
>Peter Campbell
>C/O Div. Entomology, C.S.I.R.O.
>Box 1700 Canberra A.C.T. 2601
>Australia  (peterca@ento.csiro.au)

You could connect the cathodes of the push-pull arrangement to the
positive terminal of a regulated solid-state power supply.  Since the 
response of the power supply falls off with frequency, a good bypass 
capacitor across the supply would keep the cathodes at incremental
ground for high frequencies.  Again, the price is complexity and cost.

The most bizarre output stage biasing I have seen is in the output
stage of a Roland FAX-400 receiver.  Instead of using resistors, the
filaments are used.  I was servicing the unit once and I thought the
connection was rather strange.  Sure enought, the cathode connects to
the filament and the other filament connection goes to ground.  Kind
of a strange feedback arrangement if you think about it :)  It seems
to work pretty well though.

-- 
Andrew Karanicolas
MIT Microsystems Technology Laboratory
ankleand@mtl.mit.edu

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