[AMRadio] FS: restored Gonset G-76


Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Thu Feb 9 11:07:36 EST 2006


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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