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GE tubes should work fine with the “stock” cap setup.?

Steve Wedge, W1ES

Time flies like an arrow. Fruit flies like a banana.

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On Tue, Apr 8, 2025 at 08:39, n4buq <n4buq@...> wrote:

The schematic I have shows 12.5pF.? I'm not looking at it at the moment, but I think the one that was in there is (was) a 12pF so I presume a 12pF or 13pF should do fine. I'll need to get one ordered.

My tubes are GE.

Thanks,
Barry - N4BUQ

Yes, it needs to be the value shown in the schematic, in parallel with the variable. Substantially higher values are not recommended and the original values should be used if you’re not using RCA tubes.?

I’m just getting up and so don’t have the manual in front of me and can’t remember the exact values.?

Steve Wedge, W1ES

Time flies like an arrow. Fruit flies like a banana.

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On Mon, Apr 7, 2025 at 19:53, n4buq < n4buq@...> wrote:

Regardless of how I go about it, with C77 in place, C76 minimizes the output at fully open and with C77 disconnected, C76 minimizes the output at fully closed.

With one leg of C77 disconnected, I started checking its value and I kept getting very weird values: either several microfarads (yes with a "micro") or somewhere around 140 picofarads.? I decided to de-solder the other end and checked it again and it went complete to "Unknown or damaged part" on my T7 component checker.? Apparently it was on the verge of completely bad and de-soldering finished it off (short lead on one end).

I don't have a proper replacement yet but I do have a 27pF 500V SM that I might throw in there just to see if changes things much the same way the original cap did.? I presume it will still be too much extra capacitance but it might be interesting to see, with it in circuit and even though it's about double the original capacitance, whether C76 gets the output closer to a full null when it's at or nearly fully open.

I presume a disk ceramic is still the appropriate cap to use for C76?

Thanks,

Barry - N4bUQ

Steve,

I'll run through it that way.? As I said, all I did was monitor the output at the antenna jack with no dummy load.? Since the controls didn't seem to affect what I was seeing on the scope.? Perhaps it was mostly due to there not being a proper load resistance.? I'll report back - hopefully tomorrow.

Thanks so much!

Barry - N4BUQ

Just to check, the transmitter should be tuned for a dip at the highest frequency or where specified by the procedure. You then leave the plate and load controls as they are, then disconnect the screen supply. Then, after disconnecting the screen supply, go into transmit with the minimum drive to see a reading on the scope. The transmitter remains connected to the dummy load. Adjust the neut cap for minimum amplitude. Reconnect the screen supply and verify that max power out coincides with plate current dip.?

Steve Wedge, W1ES

Time flies like an arrow. Fruit flies like a banana.

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On Sun, Apr 6, 2025 at 17:01, n4buq < n4buq@...> wrote:

Yesterday, I disconnected the screen grid supply to the PA tubes and monitored the output at the antenna jack with my scope.? While I can see a reduction in what appears to be the spurious oscillation, that occurs at the point where C76 is fully open (which might explain why it was in that position when I saw it earlier).? I find this odd and likely a problem.? Unlike needing to change C77 just a bit for some of the tube brands, it appears there's already way too much there.

I disconnected C77 and that didn't have much, if any, affect on the value I'm seeing across C76.? That reading is somewhere around 40 nF which I think may be very much out of line as to what it should be.? Changing the band switch, varying the tune control, and varying the plate circuit caps had little, if any, affect on that capacitance value.? Disconnecting the plate connections also didn't change it much, if any.

Does that reading make any sense?? I'm thinking it doesn't and something else is very wrong but I'm at a loss to explain it.? Is what I'm seeing completely abnormal?

Thanks,

Barry - N4BUQ

Mike,

I've read where there are some circuits that can have individual circuitry for each band that allows for "tweaking" the neutralization on each of the lower bands.? Of course, the Drake circuits don't have that but I presume it is possible.

I'm currently only using the watt meter in my MN-2000 for CW as I don't have a mic wired up for the TR-4 yet.

Thanks,

Barry - N4BUQ

Barry and Steve,

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Neutralizing an RF amplifier is occurs ONLY on the frequency

it is performed on. This typically is done on 10 meters on an amateur

radio transmitter/transceiver final amplifier.

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Once the neutralizing adjustment is performed on 10 meters, do NOT

make any changes to the neutralizing circuit on any lower band. You

will not see the same maximum RF output at maximum PLATE current

dip on any lower band. This is NORMAL and should NOT mean you have

to make changes to the neutralizing circuit on the lower bands.

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Follow the manufacturer's transmitter loading/tuning instructions

in the operator's manual.

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The Drake MN-2000 watt meter is an "average watt meter". Do not

expect to see "SSB PEP" RF levels on the MN-2000 watt meter.

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73

Mike W5RKL

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