[AMRadio] FS: restored Gonset G-76


Jim Candela jcandela at prodigy.net
Thu Feb 9 11:36:39 EST 2006


Brett,

   If you want to go Ab1 without that driver
transformer, consider sticking with the 6DQ6's where
the g2 voltage might be 150 volts (OA2). Sure these
won't be the cleanest tube out there, but they fit,
and will provide the power you need. Heck a little
inverse feedback from the modulated B+ might do fine,
or better yet sample the modulated RF, rectify &
filter the carrier out, and use that for negative FB.
That will clean up the 6DQ5 non-linearities as well.

   One highly overlooked tube is the 7591. These are
super linear in Ab1 audio, and about the size of a
6V6. The maximum rated plate voltage is 560 volts,
with a 19 watt Pd rating. I run two of these in one of
my 20a's as Ab2 RF output tubes, and on 80M I get 70
watts RF output, or about 80 watts PEP. I use about
650 on the plate with no trouble so far for two years.
These tubes are in production (EH - fat bottle, JJ-
skiny bottle), and can be had for about $25-30 for a
matched pair. I recall the RCA tube manual saying 45
watts audio @ 1.5% THD with the plate voltage at 450,
and g2 at 400. The AC drive is pretty low at about 19
volts peak. Check Ebay.

Jim
--- Brett gazdzinski
<brett.gazdzinski at verizonbusiness.com> wrote:

> Jim,
> The receive audio before any of my mods was ok for
> what it was.
> Yes, I likely have some problem, the driver circuit
> likely.
> 
> I just did not think I was going to get good results
> pushing
> power through that silly driver/output transformer.
> 
> I also thought about an IC chip (8 ohm
> output)driving the 
> grids of the modulator tubes, but there is no space
> to fit
> a good 8 ohm to grid transformer in the case.
> 
> The AB1 setup would eliminate the little transformer
> and give better fidelity.
> The extra power is of no concern to me, the home
> made
> (not by me) power supply is quite robust.
> 
> As far as voltages go, the G76 has a 260 volt low
> voltage 
> supply, or VR tubes off the high voltage would 
> work for the screens, and maybe a zener diode setup
> for bias?
> 
> I would have to experiment with whatever tube I
> used, 
> KT90's are too tall, I have some 6CA7's, not sure if
> I have any 
> of the other hifi tubes about.
> There are bigger versions of the 6CA7, 7720? or
> something 
> that might work and fit.
> 
> I should try seeing what can be done with the stock
> bits though,
> the mike gain is way too high, the distortion
> problem may or may
> not be related to that....
> 
> Not looking for hifi here, just plenty of clean
> modulation with
> a smidgen of low end....
> 
> But stuffing AB1 modulators in sounds like a lot of
> fun...
> 
> Another thing to look into is the RF output, how
> does the screen
> get modulated?
> Its got that VR tube along with a clamp tube (like
> the DX100).
> 
> Brett
> N2DTS
> 
>  
> > Brett,
> > 
> >     You might poll other G76 owners about the
> audio
> > issue. My two cents seems to concentrate on what
> you
> > said where the receive audio was bad (before the
> IC
> > mod), and the transmit audio is low unless you
> drop
> > the carrier way back, and even so you seem to
> indicate
> > distortion is apparent on the modulation envelope.
> > That 6CM6 audio driver / output is likely the
> culprit.
> > I would look hard at the DC operating point of
> that
> > tube, and also consider the possibility of a turn
> to
> > turn short in that output / interstage
> transformer. If
> > you scope out that area, I think you will find the
> > 6DQ6's faithfully reproduce whatever they are
> driven
> > with, and I bet the distortion is seen on the
> drive to
> > the modulator grid2 (grid one at ground).
> > 
> >    It has been about 10 years since I went through
> my
> > G-76's, but I recall the driver distortion as
> fixable,
> > and the 6DQ6's were really nice as is by Gonset.
> Full
> > modulation at about 65-70 watts out with a nice
> scope
> > pattern is what I remember. Maybe my recollection
> is
> > putting this in a better light, but if so, not by
> > much.
> > 
> >    If you go for the EL-34 approach, consider the
> two
> > additional supplies needed, (abt 300v G2, and -25v
> > g1), the higher class AB1 idle current (about
> 15-20
> > watts heat per tube), and the lower efficiency of
> AB1
> > versus class B (more heat, and power supply draw).
> It
> > would be nice and clean though, and I guess that
> is
> > why your tempted to go that route. If you go for
> it,
> > keep us informed as the group will want to know.
> > 
> > Regards,
> > Jim
> > WD5JKO
> >  
> 
>
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