[Amps] Tuning amplifiers

Gary Schafer garyschafer at comcast.net
Sat Jan 30 17:40:24 PST 2010


It is easy to do. WD7S has this feature on his "triode control boards".
You simply use a diode detector at the input and a second detector of
opposite polarity at the output of the amp. It is coupled right to the plate
of the tube thru a voltage divider so as to produce approximately the same
voltage on the output detector as you have on the input detector. Feed both
detectors to a sensitive zero center meter.

Tune the amp up for full power and then adjust the meter circuit (a pot in
one leg) for zero meter reading.

>From there on it is a simple matter of tuning for zero on the meter. You can
even tune up at a low power level from there on. No need for full power tune
up!
This is how the tune meter works on the Collins 30L1 and 30S1.

Theoretically the meter can also serve as a linearity meter too during
normal operation. If you drive the amp too hard the input voltage will be
greater than the output voltage ratio and the meter will come off center as
you talk. But in normal operation the meter swings a little as the amp is
not perfectly linear to start with.

This all works with just measuring the voltage and at any power level
because an amp has a particular amount of gain and that gain is constant
over the whole power range. The efficiency of the amp will change
drastically from low power to full power but the input to output gain should
remain constant, if the amp is linear.

73
Gary  K4FMX


> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Fuqua, Bill L
> Sent: Saturday, January 30, 2010 4:16 PM
> To: Carl; ka4inm at tampabay.rr.com; amps
> Subject: Re: [Amps] Tuning amplifiers
> 
> I kind of like the tuning method used by the 30S1. Perhaps a variation
> of that would be useful.
> That is, knowing the gain of the amplifier when properly tuned, measure
> the ratio of output power to input power.
> With proper drive ,even pulsed, you can fine tune to that ratio.
>   Does anyone make a power gain meter?
> Some old designs in QST  looked at the ratio of input voltage to output
> voltage directly at the tube. This ratio would be correct when
> the plate was properly matched to the load.  I recall one that actually
> varied the bias to maintain linearity on high peaks. That is sort of
> a quasi-daf amplifier.
>    Speaking of projects I still have to get back to my 4-1000A amp and
> 30S1 and Desk KW.  But where does anyone find spare time
> these days?
> 
> 73
> Bill wa4lav
> 
> ________________________________________
> From: amps-bounces at contesting.com [amps-bounces at contesting.com] On
> Behalf Of Carl [km1h at jeremy.mv.com]
> Sent: Saturday, January 30, 2010 3:58 PM
> To: ka4inm at tampabay.rr.com; amps
> Subject: Re: [Amps] Tuning amplifiers
> 
> With GG triodes, tetrodes, or pentodes, the grid current is the most
> sensitive tuning indicator.
> 
> For grid driven tetrodes or pentodes the screen is the most sensitive
> tuning
> indicator as well as the most easily damaged by mistuning.
> 
> Carl
> KM1H
> 
> 
> ----- Original Message -----
> From: "Ron Youvan" <ka4inm at tampabay.rr.com>
> To: "amps" <amps at contesting.com>
> Sent: Saturday, January 30, 2010 2:59 PM
> Subject: [Amps] Tuning amplifiers
> 
> 
> > Anyone:
> >
> >   I am confused by some of the things I am reading here about tuning,
> > but I have long heard triodes and tetrodes are tuned differently,
> > I have never figured out why or what, could someone enlighten us and I
> > assume pentodes are tuned like tetrodes, right?
> > (I have been tuning tube amplifiers for over 40 years now, normally
> > with a swept test signal, frequently with -20 dB of drive.)
> > --
> >    Ron  KA4INM - He who attacks the fundamentals of the American
> > broadcasting industry
> >                  attacks democracy itself.        -- William S. Paley,
> > chairman of CBS
> >
> > _______________________________________________
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> > Amps at contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> 
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