John,
Thanks for the input. Yes I have heard of this method. It seems that
Gonset
makes a feature out of the Class A Heising modulator in that they encourage
the modulator to be overdriven, and over modulation in either direction is
impossible. They put a .005 uf capacitor across the Heising reactor to roll
off the highs. Since it is single ended, the distortion products are
dominated by "even-order" products whch gives it that sought after "tube"
sound the audiophiles pay dearly for with a SE 300B making 8 watts at 5% THD
for a mere $1500 per mono-block channel.
I don't like that approach, and I would rather be clean & linear right
up
to the overload point. I usually use some form of negative cycle attenuation
(not loading) to keep the carrier power above zero though. See file I
attached on NOS Yahoo user group under files, "Super modulation, another
mans approach".
http://groups.yahoo.com/group/newoldstock/
It seems that with class A Heising modulation, the same thing is
accomplished, only a little more abruptly. I don't want to over modulate,
and chop up the carrier, but I would like to get the limiting (-) modulation
from about -85% to -95%. Since the modulators (two 5881's in parallel) use
cathode bias, I was thinking about making a change there. I could go fixed
bias (I worry about stability), or put the cathode resistor from HV
transformer CT to ground in effect making a fixed bias, and at the same time
lowering the 6146 B+ by the I^2R drop of the Ct to ground resistor (seems
tricky to get bias right between rcv & xmit), or I could return the cathode
resistor to a negative voltage like minus 50 to - 100 volts adjustable to in
effect define the maximum (-) modulation percentage. Or I could do exactly
as you suggested, and run a real nice 15 watt output. For now I will leave
it alone.
Thanks for the suggestion,
Jim Candela
WD5JKO
PS Check out my QRO Central Electronics 20A files, and power supply
schematic at the NOS Yahoo site.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of John Coleman
Sent: Monday, June 30, 2003 8:17 AM
To: amradio@mailman.qth.net
Subject: RE: [AMRadio] Schematic Needed
Hey Jim
I remember Otis showing me a way to get 100+% Modulation from
Hiesing the circuit. But it requires that you drop the output a little. He
would place a resistor between the modulation choke and the final supply
point. Then bypass it for audio with a large capacitor. This would couple
lower the plate supply voltage to the final but would allow the same
modulation voltage to appear on the final. I know it's a cheap fix, but you
may not have thought of it and you have to be willing to lose 20% of the
plate supply to the final to gain 20% modulation.
John, WA5BXO
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of Jim candela
Sent: Sunday, June 29, 2003 11:43 PM
To: amradio@mailman.qth.net
Subject: RE: [AMRadio] Schematic Needed
Charlie,
Please send me the diagram, and I will send you back some food
stamps. My
address is:
Jim candela
3305 Kissman Dr.
Austin, Texas 78728
I got my Gonset G50 cleaned up and transmitting last week. The AM modulation
is AWFUL looking on the scope, but that appears to be how those yahoo's
designed it. Looks like some simple modifications will make it tolerable. A
storm of clip leads, and R-C changes seemed to make it linear at up to +/-
90% modulation at 20 watts carrier output. In stock form it was linear to
only +/- 30%, and limited to +50% at the same time as -85%. Pretty crappy. I
added plate to grid RC on the 5881's ( 1meg - .033 uf 600v). This cleaned it
up a lot, but I needed more peak power. I then lowered the cathode resistor
to get more peak modulator power. The lower value cathode resistance only
kicks in during transmit ( 350 ohm rcv, 190 ohm xmit). The modulator's
tubes in parallel, class A Heising modulation seem to modulate up to +120%
now at 22 watts RF output (100% at 25 watts), and I am limited to -85% due
the Heising circuit limitations. Today I found a way to cool the modulators
a little, and maintain nearly the same peak power. I put a 7K 5 watt WW
resistor in series with the modulator screens. The screens are not bypassed,
so this drops the screen voltage some, and is another source of negative
feedback. Still plenty of gain from my D-104. The scope pattern looks real
nice now, and shows perfect symmetry up to > 80% modulation.
The receiver is another story. It works, but has audio motor boating
(when tuning in the 50.4 MHz spot (signal), and tunes like there is
regeneration somewhere. I replaced all the P/S electrolytics earlier this
week. The receiver might want to kick my butt for a while. I have a real
nice FET preamp for 6 meters. I may hook that up someday once I know if this
receiver is OK or not.
I hooked the G50 to my 80 meter inverted V, and I can copy all kinds
of
stuff (some cordless phones) including some amateurs on SSB, and CW. The
built in VFO spot function acts as a BFO. This is not the best, but kind of
cool for now.
I ordered a ku4ab 6 meter loop, and I just received a D-104 Mic element
(crystal) last week from Omnitronics. I just might have a 6 meter station
coming together. 50.4 Mhz AM is the goal. Something tells me that 6 meters
AM will be one of those bands where most CQ's go unanswered...
Thanks,
Jim
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED]
Sent: Sunday, June 29, 2003 9:00 PM
To: amradio@mailman.qth.net
Subject: Re: [AMRadio] Schematic Needed
BTW Jim, forgot to ask in previous e-mail...what is the problem with your
G-50 Modulation?? Surely you dont want more than 90%??
73 again. K0NG.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED]
Sent: Sunday, June 29, 2003 8:57 PM
To: amradio@mailman.qth.net
Subject: Re: [AMRadio] Schematic Needed
Hello Jim:
I cant scan a schematic for you but can copy mine and send it via post.
Let me know if you have not received one yet. My modulation on the G-50
has been described as "excellent" using a D-104 microphone. The ones I
have heard sound good too, but not many lows. The receiver sensitivity
in mine is worse than I think it should be, even for its time. I think it
was around 6 uV for only 6 dB S+N/N ratio. Have not worked on it lately.
My dial drive slips also but hate to tear out the reduction drive (lazy).
Good Luck es 73 DE Charlie, K0NG.
Quoting Jim candela <[EMAIL PROTECTED]>:
>
> Hi Fellow AM'ers;
>
> I am restoring / modifying a Gonset G50 six meter transceiver. I
have
the
> schematic, and documentation from the Bama ftp sight. The schematic there
is
> of poor quality, and reading component values is next to impossible. Does
> anyone have a quality print of this schematic? A good print scanned would
do
> as would a 11 X 17 paper copy.
>
> I got the transmitter going pretty decent. The stock modulation
really
> stinks, so instead of fixing it Gonset made it a selling feature. With
only
> minor changes it is now approaching "decent".
>
> I am having problems with the receiver, and the squelch circuit.
This is
> where I really need a readable schematic.
>
> Anybody out there with G50 on the air experience?
>
> Regards,
> Jim candela
> WD5JKO
>
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