Larry,
One of the features of the 20A that motivates me to use this as an
AM rig is the stability of the VFO, and heterodyne scheme. No more 160 / 40
meter VFO which gets multiplied for 160-10 meter coverage. I hate that
feature on most commercial AM rigs. Also the tune, or Cal function is handy
for precise zeroing of the received frequency. The variable level is also
nice to get the spot signal level similar to the received station. US Amers
often have trouble spotting, and then staying on frequency. The 20A solves
these problems quite nicely. You could use a 20A as a carrier exciter (no
modulation) to a big plate modulated rig.
I have run some SSB with my 20A recently. Everyone is amazed at the
quality of this thing on SSB. I guess part of it is the phasing set-up, and
part of it is the linearity of the 7591's. There are some major shortcoming
of the 20A on SSB however. The carrier null circuit is not the most stable.
Mine is worsened because the 1K pots (2) are worn out, and need to be
replaced. The adjustment is too coarse for me. Also, if you put a variac on
the 20A power, the carrier null moves around with line voltage. A 5 volt
change in ac line voltage will move the carrier null significantly. Maybe my
20A is still busted somehow. Does yours do the same?
Here is what I plan to do:
* Replace the carrier null pots with 1K Cermit 10 turn pots. Mouser
has some that are chassis mount.
* Identify the source of drift.
* If the drift is filament voltage to the 6U8 / 6Ba7, then I can run
that off regulated 6 Vdc with a 7806 regulator. If I go that way, I might
also regulate the filament to the 12AT7 audio stage in the 90 degree phase
shift network.
* If the drift is plate voltage (more likely) then I am thinking about
making a LV B+ regulator. That 1K 20 watt resistor in my power supply (part
of C-L-C-R-C mentioned below) could get replaced with a series regulator
tube like a 6AS7, or 6DQ6. This tube should go where the 5u4G once went.
Maybe I can find a 5 volt filament tube for this task. Did they make any 5
volt filament sweep tubes? Since I also solid stated the bias rectifier, a
5651 reference tube could go where the 6AL5 once was. I would need another
socket for a dual triode (6SN7? 6SL7?) control tube. Maybe that could go
where the VOX anti-trip module went.
Back to the external B+ booster. My first attempt was using a tiny
115v/115v 15 va isolation transformer. It works fine, but the B+ boost is
only a little over 100 Volts loaded at 150 ma 7591 total plate current. This
is a compromise, and could be easily mounted inside the 20a chassis. Put a
diode across the output (back biased), and a switch to the AC primary to the
transformer. The switch could be labeled low B+ / Hi B+.
When I key up my 20A on am without the high B+ on, the scope shows
severe clipping on the mod+ peaks. This is because the carrier is set too
high. Instead of backing off the carrier, I turn on the 200V Kepco (has two
6550's in a series regulator), and the Modulation peaks now run up to 120+%
with no clipping. This is a huge "bang for the buck" modification,
especially if you are trying to drive a grounded grid linear amplifier.
Good luck with all your projects...
PS How did your 20A modulation transformers fail?
73,
Jim Candela
WD5JKO
> -----Original Message-----
> From: Larry Szendrei [mailto:[EMAIL PROTECTED]
> Sent: Tuesday, December 24, 2002 11:03 PM
> To: Jim Candela
> Subject: Re: CE20A mods for QRO
>
>
> Wow, Jim, thanks for all the info! You've given me a lot to chew on, and
> yes, it is quite helpful.
>
> I've only run my 20A on AM once, just a couple of weeks ago on 160M. But
> everyone in the QSO was on their way out, so I didn't get much in the
> way of reports. I don't remember if I had the 'scope on at the time
> (should have!).
>
> I run mainly AM here, but have lots of other rigs I can use on AM that
> are better suited to the mode. My main interest in the CE20A is for
> sideband and RTTY. I've run my functional stock unit on 160M - 15M SSB
> with no problems - my VFO-458 doesn't have the 10M converter in it, but
> I plan to build one someday.
>
> Youv'e got me fired up to fix and start playing with the other CE20A -
> but I've got several other, unfinished projects I need to wrap up before
> I embark on any new adventures. Plus a guitar amp and PA amp that need
> fixing. You know how it is, I'm sure.
>
> Regarding the 10K resistor accross the mixer tank - isn't it's purpose
> to "de-Q" the tank, so that the next stage doesn't oscillate in TPTG
> fashion? I'm surprised you could get away with removing it. Yep, I would
> expect you got more signal with it gone.
>
> I don't think I'll worry about an external supply. I'll be happy with
> ~40W PEP if I can get that after changing finals.
>
> Gonset GSB-201, eh? My other vintage SSB rig is a GSB-100.
>
> 73 and thanks for taking the time to answer in such detail.
>
> -Larry/NE1S
>
> Jim Candela wrote:
> >
> > Larry,
> >
> > I chose the 7591 because I have several, and they are
> about the same Gm as
> > a 6AG7. That means the drive requirement is similar for Ab1
> operation. I did
> > have to increase the G1 bias, and to fill out the AB1 range, I
> removed the
> > 10K resistor across the 6BA7 mixer tank circuit (it's hidden up
> top), and
> > the available drive increased noticeably. The mixer circuit has
> significant
> > modifications from CE between the early rigs, and the late
> 20A's. Mine is
> > the old version. The newer version has cathode bias on the 6BA7, and
> > significant mods to the 12BH7 cathode follower. I see why
> because mine has
> > reduced output drive to the 7591's as you go up to 10 meters. With the
> > 6AG7's in there, I could get 1/2 watt output on 10 meters, and
> the 7591's
> > didn't change that. My VFO has the optional 10 meter circuitry, so drive
> > from the VFO is fine.
> >
> > I have had no instability unless I have so much RF in
> the shack that I get
> > RF burns from the D-104 screen. I run mine so far out of the case, and
> > without the screen above the final cage. No tendency to oscillate, or be
> > unstable. Both the 7591, and 7868 have two G2 connections at
> the base. This
> > allows for redundant G2 bypassing. I am very happy with the changes thus
> > far. I can now drive my Gonset GSB-201 to 150 watts output on
> AM, whereas
> > before 30 watts out is all I could muster when the 20A was stock..
> >
> > With the 6AG7's on 160, 80, 20, I could get about 12
> watts out, or about 3
> > watts AM. On 40 meters, the VFO must triple, so drive can be a problem.
> > There is a CE endorsed mod to the VFO to help out there. With
> that mod, 40
> > meters comes in the same. The output on 15 meters is a little squirrelly
> > (rich in spurs). I haven't perused fixing 15 meters however.
> >
> > Well back to the 7591. I was actually looking at the
> 7868 with envy because
> > of the very low Cgp, but the Novar socket requirement stopped
> me. Surplus
> > sales of Nebraska has a nice ceramic Novar socket that isn't
> too expensive.
> > The 7591 is electrically similar, but with Octal socket. You must rewire
> > from the 6AG7 pinout. It's pretty easy... The 7591 is the same size as a
> > 6V6, so the size is kind of nice. There isn't much room in
> there, so a fat
> > tube like a 6DQ6 may be a challenge to remove (if it fits at
> all!). I also
> > like the linearity of the Hi-Fi audio tube. The EL-34 you mention is a
> > possible contender, and it is skinny too. A grid driven sweep
> tube may not
> > be very linear, or have enough gain. I don't know though,
> because I never
> > tried them.
> >
> > I currently have my 20A P/S solid stated. The capacitor
> input pi filter is
> > still there. I added a 1K 20 watt resistor, and another 40 uf 450 volt
> > capacitor following the Pi C-L-C filter, so I know have a
> C-L-C-R-C filter.
> > I run all the low level stuff off the 1K feed. This gives me
> about 300v B+
> > (key up), and 275 (key down max carrier). The output from the
> C-L-C is 400v
> > (key up), and 375v (key down max carrier). The output from the
> C-L-C Pi also
> > goes to the 7591 screen grids (G2). Here is where I get crazy.
> I then take
> > an external 200v (rack mount Kepco regulated) P/S, and add it to the pi
> > C-L-C output. This gives me 575 volts for the 7591 plates. This is just
> > above the max rating for the tube, so no big deal. The 7591's
> give me about
> > 30 watts carrier output on 160-80 meters at 375 v B+, and about
> 45 watts at
> > 575 v B+. Since the HV B+ increases some at reduced load, I can
> get close to
> > 60 watts PEP output from my 20A. That means that 15 watts AM is
> possible. I
> > like to keep it below that for pos+ peak headroom. If you add
> an external
> > P/S like I did, put a diode across it's output (back biased by
> the 200V).
> > That way if the 200V is off, you still get the 375-400V to the
> 7591 plates.
> > You don't want to ruin these tubes by running with G2 on, and the plates
> > off. I adjusted the bias (g1) for about 60 ma idle current.
> This takes about
> > 18-20 volts of bias to get there.
> >
> > On the subject of pos + expansion, my 20A, and every
> other one I have heard
> > on AM sounds a little "clicky" whenever the neg- mod peak goes
> beyond 100%.
> > Instead of just cutting off the carrier like a plate modulated rig, the
> > phase reverses 180 degrees, and we make nasty DSBRC. I have found a way
> > around this. On the rear of the 20A there is a RCA phone jack for audio
> > output. This same signal is used on AM to drive the upper balanced
> > modulator. The impedance is low, and the 1n34a diodes are non
> linear. This
> > results in asymmetrical modulation where the neg- peak hits
> 100% when the
> > mod+ peak is only at about 50%. It looks like [EMAIL PROTECTED] on the
> scope, and on the
> > air whenever you crank the audio just a little bit to get
> through QRN. Here
> > is what I do. Take a low drop diode, and put the cathode at the
> RCA center
> > pin, and return the anode to ground through a 15 ohm resistor.
> This seems to
> > balance the circuit, and sine wave testing shows symmetrical
> audio up to +/-
> > 80%. Beyond that the mod + goes further to beyond 100% (depends
> on carrier
> > output power), and the mod- clips at about 85% without ever hitting the
> > baseline causing DSB. The difference in your station monitor
> will be night
> > and day. You can now compete with the plate modulation rigs. In
> fact if you
> > cut the power in half (7 watts in my rig), the dam thing looks like a
> > ultra-modulated rig on the scope. On my 20A I wired the diode
> into the mode
> > switch (there are extra contacts) so that it is only there for
> AM. I started
> > out using germanium diodes. I have some 1n270's. Two in
> parallel did pretty
> > good, but the diode knee is sloppy. Maybe more in parallel
> would be better.
> > I abandoned them when I found some 1n5818's. These are Schotty,
> and one is
> > all I need.
> >
> > There is room to explore this subject further..and I
> welcome your comments.
> > please share them with the group.
> >
> > Well Larry, I hope this helps!
> >
> > Regards,
> > Jim
> >
> > > -----Original Message-----
> > > From: Larry Szendrei [mailto:[EMAIL PROTECTED]
> > > Sent: Tuesday, December 17, 2002 9:24 PM
> > > To: Jim Candela
> > > Subject: CE20A mods for QRO
> > >
> > >
> > > Hi Jim,
> > >
> > > I read one of your posts on your mods to the CE20A to increase output
> > > power. I have two 20A's, one which works fine that I'll keep
> stock, and
> > > one I need to repair (open primaries on both "modulation
> transformers")
> > > that I would like to experiment with to increase the output as you've
> > > done.
> > >
> > > I'm interested as to why you chose the 7591A - I know it's a
> > > single-ended high transconductance beam power tube capable of
> some poop,
> > > but it's also quite rare and expensive. I was thinking of a
> pair of 6DQ6
> > > sweep tubes (moderately high Gm around 7000, or so, as I
> recall, and it
> > > shouldn't be too difficult to adapt the CE20A for this configuration,
> > > since the plate tank components are above chassis already),
> or maybe the
> > > EL34.
> > >
> > > Did you have to add neutralization? If so, how did you do it?
> I think CE
> > > could get away without neutralization with the 6AG7's because it is a
> > > "well screened" pentode, but Ggp is typically high enough to require
> > > neutralization with the beam power tubes. Do you have any
> problems with
> > > instabilities on the higher bands?
> > >
> > > Thanks, and I'll be very interested to hear the details.
> > >
> > > 73,
> > > -Larry/NE1S
> > >
> > > ---------------------------------------------
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> > > 1st month Free!
> > > Sign up today at: www.netzerolongdistance.com
>
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