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 91 
 on: February 17, 2019, 03:24:55 PM 
Started by n4joy - Last post by n4joy
I was rag chewing on my Lettine 242 6 meter transmitter and the other station reported that my audio suddenly became distorted.  All is fine with mic gain turned down all the way, but I begin to hear a high pitched whine and oscillation when the gain is turned up and to my usual level.  I replaced the 12AT7 speech amp with a NOS and swapped all tubes including the final--no change.  The whine does not occur when the mic is unplugged but the gain is turned up to full.  Everything appears to be good inside the rig--no out of tolerance resistors and all caps were recently replaced.  I thought maybe RF was getting into the rig, so I transmitted into a dummy and still had the same problem.  Tube compliment is two 6V6s driven by a 6V6 for the modulator, 6146 final, two 5763s, 12AT7 speech amp, and a 5U4 rectifier.

Could the issue be with my driver transformer or modulation transformer?  I don't have a tube tester, so I cannot rule out a bad tube despite swapping with ones I suspect are good.

 92 
 on: February 17, 2019, 03:09:55 PM 
Started by Chris P. - Last post by Chris P.
Does anyone have an old Jefferson catalog? Looking for the specs on a filter choke with the number 466-001-186

Thanks

 93 
 on: February 17, 2019, 03:00:06 PM 
Started by K1JJ - Last post by K1JJ
We never had the AM Window Drake AM mods drawn out in a schematic.

Bob, WA2POW is modifying his T4X and was nice enough to send me his CLEAN, redrawn before and after modification schematics. We only had the description in the past, so these mod schematics will be helpful to those folks in the future.

I wanted to get it into the our archives and the Google, etc.,  search engines.

Description:
https://www.amwindow.org/tech/htm/t4.htm


T4-X and TR4, etc., schematic mods before and after below:

(click on the BLUE .jpg LINKS below for proper up/down orientation)

Tom, K1JJ

 94 
 on: February 17, 2019, 02:04:03 PM 
Started by KK7UV - Last post by KK7UV
Thanks very much for the tips.    I will surely consider blowing air into the chassis and exiting up thru the socket holes (I plan to mount the 813s with their bases sunk 1.25 to 1.5 inches into the chassis box).   Will an axial fans provide enough air or should I look into squirrel-cage type blowers?  

 95 
 on: February 17, 2019, 01:45:22 PM 
Started by PA0NVD - Last post by PA0NVD
There were various persons, including the son of my neighbor, that saw a black panther with a kitten near the house. That is approx 200 - 300 meters down the property in a forest near lake Arenal.  A while ago there were also sightings of a puma and most probably a bobcat (according the description of the persons that saw them) Unfortunately, it did cost the life of a few chickens, a dog and a calf. So people has scared them away, killing is forbidden in Costa Rica. (fortunately)
Costa Rica, pura vida Grin

 96 
 on: February 17, 2019, 01:40:03 PM 
Started by KK7UV - Last post by K1JJ
Aha!   Thanks!    I hadn't seen the PS schematic before.


Hi Steve,

Looks like the guys have answered your questions well.

I agree that when going above about 1600-1800 VDC HV, a Variac and separate screen supply is the best way to go with 813s due to the moderate waste of power and heat when tapping the HV supply as Don pointed out. Also, the Variac gives you exquiste control of screen current on the fly as you change HV settings, plate loading and other things that influence screen current.  No need to regulate the RF final screen voltage since the screen current is at steady state (~80mA) when dead carrier or under full modulation.

However, if the modulators are run with screen voltage as tetrodes (instead of triodes as shown) then we definitely need a regulated modulator screen supply.

An RF final screen resistor limits you to one basic optimized setting. A screen resistor HV tap works great in smaller rigs and also eliminates the need for the self-modulation screen choke, so it has its advantages too.  A matter of preference in many cases...

Consider blowing some pressurized air through the 813 bases and upwards. Especially for the modulators running class AB2, this can increase the tube dissipation rating from 125 to maybe 160 watts. I see a difference in color from dull red to gray when I do.  I have also used lantern chimneys, available on the web to do the job. Blown 813s (2X2) is quite the rig and will do 1KW carrier @ 140% modulation at 3KV when pushed to the insane limit.  Running at 600W carrier is a very FB setting.

T  

 97 
 on: February 17, 2019, 12:49:19 PM 
Started by KK7UV - Last post by KK7UV
Aha!   Thanks!    I hadn't seen the PS schematic before.


 98 
 on: February 17, 2019, 12:36:10 PM 
Started by KK7UV - Last post by KK4YY
There's a 15H choke in the screen lead so it looks like it's a "self-modulated" screen. The 400V would need to be filtered. Regulating it would probably be overkill.

Alternatively, replacing the choke with a dropping resistor from the modulated B+ supply could be used to modulate the screen. The value of the resistor will determine the screen voltage. If your target is 400V at 80Ma, then subtract the screen voltage from the plate voltage and divide by .08 to find the resistor value. Example: (2000V - 400V)/.08=20000 ohms. This resistor will dissipate 128 watts(!) which would come from the 2KV supply.

Using a self-modulating choke, a separate 400V supply at 80mA would use only 32 watts.  So, there would be 96 watts less heat in the shack and an over-all smaller power supply.

If you use a self-modulating choke, make sure its insulation is rated for 2000V (have fun finding one) or you can mount it insulated from the chassis (ground) as a work-around. The 10H filter choke, used in the 400V power supply, is cleverly mounted in the negative lead so it's not subjected to high voltage. You can't do that with the self-modulating choke.

Nothing is ever easy!


Don

 99 
 on: February 17, 2019, 11:47:06 AM 
Started by KK7UV - Last post by KC2ZFA
http://www.amwindow.org/tech/htm/813/813ps.htm

 100 
 on: February 17, 2019, 10:53:08 AM 
Started by KK7UV - Last post by KK7UV
That schematic does not show how the 400v is derived, which is why I asked.

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