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 on: June 13, 2018, 02:16:59 PM 
Started by WB4YVO - Last post by PA0NVD
Hi Jim
Indeed nice find. I love the KT77, in UL mode these tube perform excellent with very low distortion.
The choke saturation can be solved by finding a balanced output transformer e.g. for 2 x KT88 or EL 34. There are plenty. Center tap to HV, one side to the modulator and the other side to the PA. Than the DC magnetization is almost canceled and you can modulate the transmitter with less iron and less distortion. Simple to try temporarily with the iron externally wired with clip leads.

 on: June 13, 2018, 01:49:47 PM 
Started by WB4YVO - Last post by WD5JKO
  A scope looking at the modulated envelope would be most helpful. My last comment about the G50 modulator earlier shows that in my case, the Heising choke was saturating, and audio below 500 hz was progressively more and more distorted as the frequency goes down. Distortion worsened, and upward modulation was limited.

   Besides the good stuff already mentioned by PA0NVD, a simple test would be to load the D-104 cartridge with something like a 100K load. This will attenuate most of the lows, and if the audio Heising reactor were saturating, getting rid of the lows should improve things a lot. It would sound like a stock Apache though!

   Lightly loading the RF stage should also help with modulation capability.

   Funny discovery in my G-50. The modulators were original Genelax KT-77's instead of 6L6's. What a find!


 on: June 13, 2018, 10:59:05 AM 
Started by KD6VXI - Last post by K1ETP
Tom Vu,

Looking at your pair of 4X1 linear, I see you still have a good stock of "GTO Blue" spray paint.


 on: June 13, 2018, 09:41:38 AM 
Started by KD6VXI - Last post by KF7WWW
I didn't know that one, seems quite a bit more powerful. You ever used it?

I have one that is down on emission, so I canít give you accurate results.. I do know that it seems to run better with more HV..

 on: June 12, 2018, 10:45:47 PM 
Started by WB4YVO - Last post by PA0NVD
No, should not be downward. Measure if the screen voltage is ok. See if the grid current is sufficient, low drive level may be a cause.  A too heavy load may be a reason as well, tune for a deeper dip until the power goes approx 10% down.
An other cause may be low emission of the tube. If you raise the filament to 7 - 7.5 Volts and the modulation improve (becomes positive), the tube doesn't have sufficient emission. Raising the filament should have just a minor improvement of both power and modulation. A substantial improvement indicates low emission, both for indirect and thoriated filaments.

 on: June 12, 2018, 10:28:40 PM 
Started by KD6VXI - Last post by K1JJ
I wonder how much the better alignment effects imd?

I don't think we are talking about enough misalignment to affect the transfer curve linearity much.  Though, I'll bet if a grid or screen had a hole blown in it, the IMD level deterioration could be easily measured... :-)

BTW, here's a picture of the filament choke I used.  I ran out of room underneath the 4X1s' sub-chassis  and didn't want to restrict air flow, so mounted them outboard but added shielding later on.

The whole thread is here:

Where do we get the time and motivation to build these rigs?


 on: June 12, 2018, 09:31:04 PM 
Started by WB4YVO - Last post by WB4YVO
Ok , Found some more bad solder joints.. Mainly Ground connections..  so now all seems to be good..

Wired up a D-104 Microphone.. I now have modulation..

But its downward..Is that normal for this radio? or does it require special tuning?

I remember back in my CB days in early 60's..  had to do some special tuning to get upward Modulation..



 on: June 12, 2018, 09:08:58 PM 
Started by KD6VXI - Last post by KH6KK
Yes, the 4PR-1000A is the same tube, but the elements are "blueprinted" -  IE, the spacing and alignment tolerances are closer to spec. A lot more work than just slapping them together on a factory jig. Labor intensive.  This allows higher voltages reducing the chance of arcing due to misaligment.  For example, if the screen is slightly leaning in respect to the plate, etc. We have all seen tubes with misaligned structures... I've seen my share when I look closely. I have a few 4PRs here and use them in RF service.

Interesting on the better vacuum.  I've not heard that but it makes sense.


Hi, Interesting to read as I have an Eimac 4PR1000A that has a slightly tilted anode, looks straight from one angle, but off from other angles. Just a slight amount only. I guess it got bumped in shipping.. Lights up fine, and no measurable shorts but never tried applying HV to it thinking it was damaged. Your post gives me hope and I think I'll try it next time. Thanks!  Cool

 on: June 12, 2018, 07:11:57 PM 
Started by KD6VXI - Last post by KD6VXI
Lol.  I like Clarence.  He's a character.

I'll take you up on the resistors if it proves necessary.  Definitely gonna have a hv fuse and surge R in this one!


 on: June 12, 2018, 07:10:29 PM 
Started by KD6VXI - Last post by KD6VXI
Bear, ultimately, it will be grommet mounted and then I use some rtv to seal.

Tom, I was under the impression x rays where a >10 kv thing.  This will normally be run on the 7.5kv tap, which is still pretty hot.   The other taps are their, well....  Because that's the transformer I got.

It's currently on 20 meters, but as soon as I hit a few hundred watts, my home alarm went off.  Nevermind it's not even turned on, was sitting in the 'off / home' mode!  Yes, quite disconcerting! Guess I need the antenna up in the air a bit more!

I've used the PR series tubes before.  Usually no difference in what we use them for.  I wonder how much the better alignment effects imd?


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