Hi Jim, Thanks again for the information-packed emails!
Regarding the "modulation xfmrs" the primaries opened up on both units. Don't know why. I never used this particular unit except to test it out when I first bought it (I bought both my CE20A's from the same guy, at the same time, at one of the famous Deearfield, NH hamfests in the 80's). I remember at that time I had audio, but the sideband supression wasn't nearly as good as in the second unit, and it resisted any attempts to adjust/improve it (maybe one of the 2 transformers was dead then, in which case it would be transmitting DSBSC, in which case, no wonder I was having problems nulling out the unwanted sideband!) so it sat on the shelf until I tried to fire it up last year, at which time I had no audio at all. When I looked into it, I found both primaries open. Interestingly, In my junquebox I had several (4 or 5) of these same transformers among a box of stuff given to me over the years. Among these, all had open primaries except one! So I have one spare xfmr, but need one more to fix the rig. Alternately, a matching set of hi-Z: 600 ohm output xfmrs would probably work too. My carrier null appears stable enough. It is a touchy adjustment, tho. Thanks also for the (later) Email on the 7591 sources. 73, -Larry/NE1S Jim Candela wrote: > > 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 > > > > > > > > --------------------------------------------- > > > > Introducing NetZero Long Distance > > > > 1st month Free! > > > > Sign up today at: www.netzerolongdistance.com > > > > --------------------------------------------- > > Introducing NetZero Long Distance > > 1st month Free! > > Sign up today at: www.netzerolongdistance.com > > --- StripMime Report -- processed MIME parts --- > multipart/mixed > text/plain (text body -- kept) > application/ms-tnef > The reason this message is shown is because the post was in HTML > or had an attachment. 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