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Suppressor Grid Mod GU81M Tx




 
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Author Topic: Suppressor Grid Mod GU81M Tx  (Read 352 times)
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GW0FZY
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« on: July 14, 2020, 02:51:18 PM »

Hi Folks,
Looking for a bit of advice please. I have a PA from an old Russian R118 transmitter which I have re-built into an AM transmitter. The original valve line up was a GU50 driving a GU81M which was suppressor grid modulated by a small speech amp ( a single AF pentode and a double triode, Russian jobs) with a 1:1 audio transformer (which is faulty). As I had no info or circuit diagram available I have re-built the whole thing by adding a crystal oscillator and a new speech amp consisting of an EF86 with a ECL82. The audio is coupled to the suppressor grid via a .1 uF cap with a choke in the anode of the ECL82. The biasing of the SG comes from the main bias supply to the GU81m grid 1 which runs at -220v or there about regulated by a string of zeners. The SG bias voltage is varied by means of a 50k pot which normally runs at about -170v.
The GU81m runs at an anode voltage of 2.5kv and a G2 voltage of about 700v. After a few stability issues in the audio amp I managed to get about 120w of reasonable mod with the audio sounding OK if not a bit scratchy!
The problem I have, is that for some reason when measuring the Suppressor grid voltage while transmitting the SG bias voltage seems to slowly decay becoming less negative ,with the RF output slowly increasing and the corresponding mod level decreasing. This occurs after about 10  seconds or so. I have check/changed the de-coupling caps at the suppressor grid but it still does it. If I could keep the volts negative it would work fine. Incidentally the suppressor grid does not appear to be drawing any current and the G1  bias voltage remains at the same level . I'm wondering if this problem is due to a build up of positive volts so to speak, from the audio output or whether it could be due to the fact that the SG bias comes off the same supply as the G1 bias? Would appreciate any advice
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GW0FZY
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« Reply #1 on: July 14, 2020, 03:20:22 PM »

Hah, think I may have just solved this....stuck a 16mH signal choke in series with the bias supply and the volts are staying put!!
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DMOD
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« Reply #2 on: July 14, 2020, 09:26:13 PM »

Since you redrew the schematic it would be nice to post it since most of us on this side of the pond have not seen the circuitry.

Phil
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GW0FZY
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« Reply #3 on: July 15, 2020, 01:36:00 AM »

Thanks, yes I'll draw the schematic of the suppressor grid modulator in case someone else wants to try it
73  Justin
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KI4YAN
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« Reply #4 on: July 15, 2020, 03:38:55 AM »

GW0FZY, do you know of a suppressor grid modulated GU-50 transmitter out there?
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GW0FZY
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« Reply #5 on: July 15, 2020, 06:04:11 AM »

Yes I do know someone who has done just that... Mike G4EJM who comes on our AM net on 3615khz uses a 3x GU50 ‘S which are SG modulated. He uses a very similar arrangement to the one I use. I have attached the circuit diagram for those who might want to try it. For Mike’s gu50 transmitter he did not use a choke but a 100k resistor on the suppressor grid. Very nice audio to be fair...


* 5D51268F-759D-4B23-A0A7-5B620C81C81F.jpeg (100.45 KB, 640x480 - viewed 63 times.)
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WU2D
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CW is just a narrower version of AM


« Reply #6 on: July 15, 2020, 08:06:57 PM »

I made this little QRP rig back when Frank GFZ had the PW AM Net.

I used a carbon mic and a step up transformer - the ART13 mic transformer actually.

The components to the left are a negative cycle loading idea- untested.


* 837_AM_REV2.jpg (290.98 KB, 2460x1525 - viewed 46 times.)
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GW0FZY
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« Reply #7 on: July 16, 2020, 02:49:29 AM »

That might work with a GU50....potential one valve transmitter! might give it a go

Justin
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