[AMRadio] Re: AMRadio Digest, Vol 38, Issue 24

D. Chester k4kyv at charter.net
Mon Mar 26 13:28:55 EST 2007

> From: "Jim Candela" <jcandela at prodigy.net>

>   At the following link is a schematic of an AM rig that uses similar
> tubes. You could replace the 6AG7 with a 6V6 or 6K6, and replace the two
> 6LF6's with four 6DQ6's. This rig is good for 25 watts AM, and doesn't 
> need
> a modulation transformer. The downside is the Class A series modulator, 
> and
> the need for a +1100v B+. It sure is simple though, and should be easy to
> home brew.

I wouldn't recommend replacing a 6AG7 that is used as a straight-through rf 
amplifier with a 6V6 or 6K6.  6AG7's are too easy to find, and the 6AG7 is 
extremely well shielded internally, and rarely requires neutralisation.  The 
6V6, 6K6, 6F6, 6L6, 41, 42, 47, 59 and similar tubes were designed to be 
used as audio output tubes in broadcast receivers.  Even though they are 
pentodes or beam power tetrodes, the screen and suppressor grids in those 
tubes do not provide very good internal shielding, since that was not 
required in audio amplifiers. These tubes may become unstable when used as 
rf amplifiers where the input and output frequencies are the same.  This 
instability can be eliminated by neutralising, but it is cumbersome to go to 
the trouble to neutralise tubes in low power intermediate rf stages.

This is exactly why the audio output tubes listed above used to be so 
popular as crystal oscillators.  There is enough internal feedback due to 
the poor shielding that they can easily be made to oscillate without 
additional rf feedback circuitry - just put the xtal in the grid circuit, 
and tune the plate to the crystal frequency, and it should go into stable 
oscillation near the resonant point.  The screen and/or suppressor grids 
provide just enough shielding to limit the crystal current to a safe value. 
If a well-shielded tube such as the 6AG7, 802, 807, etc. is used, it is 
sometimes necessary to add a small coupling capacitor between plate and 
control grid, or use some of the more exotic oscillator circuits that 
provide the feedback external to the tube.  The coupling capacitor need be 
nothing more than a "gimmick", a couple of  short pieces of hookup wire 
twisted together.

I once had a most mysterious problem with a transmitter - it would go into 
(very stable) self-oscillation on 160m with nothing but the filaments on, in 
standby mode with the HV power supply completely turned off by cutting off 
the a.c. voltage to the HV power transformer.  At first I noticed it as an 
unidentified carrier that kept popping up near my operating  frequency.  It 
was very stable, so I assumed it to be someone tuning up near-by, but they 
never ID'ed.  This drove me up a wall for a couple of weeks, as I thought it 
must be someone local, since the s-meter did not move to indicate any QSB. 
One time I just happened to adjust one of the tuning knobs on the rf 
exciter, and the carrier went away.  Upon further investigation, I 
discovered that turning the tuning knob changed the frequency of the 
carrier!  It would have made a good VFO, it was so stable.  But how could 
the stage go into oscillation with no DC voltage on the tube?

The offending stage used a 6V6, as a buffer stage to the external VFO.  The 
power supply used a 5R4 rectifier tube.  I started to measure voltages, and 
discovered that the plate of the 6V6 stage had about +15 volts.  I could 
short out the B+ line to that stage, and the oscillation would stop.  I 
pulled out the 5R4 rectifier, and the oscillation would stop, but shorting 
out the transformer winding did not kill the DC voltage or the oscillation. 
Therefore, it was not due to the plate transformer picking up stray 60~ a.c. 
by magnetic coupling to another transformer.  After some head-scratching, I 
finally figured out what was happening.  I killed the DC voltage by opening 
a relay in the a.c. line to the transformer, but the filament of the 5R4 
stayed on all the time.  With no transformer voltage, there were enough 
stray electrons randomly hitting the rectifier tube plates from the heated 
filament, to generate about 15 volts DC.

My solution to the problem was to change the 6V6 buffer stage to a 6AG7. 
That took care of the oscillation.  That 6V6 stage must have been very prone 
to self-oscillation.  I still use that rf exciter unit to drive my lower 
power homebrew transmitter, usually tuned to 40m.

Don k4kyv


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