The AM Forum

Just bought a Valiant I transmitter (factory-wired unit). Has good output (about 120 watts) and made several contacts in CW mode with excellent reports. However, in AM mode and in the MOD current position, the meter needle pegs to the extreme right of meter when keying my mic (D-104) and no AUDIO. After about 10-15 seconds of keying, I notice the 6146 modulators turning a bright red and getting hot. I unkey immediately so as to not cause any damage to the modulators. Oscillator, buffer, grid and plate currents exhibit good readings during tune-up. Only have problem I have is with the modulator current. I've switched out the modulators, bias rectifier and low voltage rectifier will known good tubes, but get the same results. Any help would be greatly appreciated.

73,
Will KF4IZE

The first thing to check is that you have proper negative bias on the control grids of the 6146 modulator tubes.  Around 46 volts is appropriate.  The symptom you describe is most likely caused by low or no negative bias on the modulator tubes.  It could also be caused by high screen voltage, but that is much less likely.

You may find a copy of the Valiant 1 manual here:
http://bama.edebris.com/manuals/johnson/valiant/

See pages 7 and 8 for proper measurement and adjustment of bias voltage for final and modulator sections.

On page 31 you will find tube socket voltages listed.

The schematic diagram is at the end of the manual, pages aren't numbered, but it is around page 52.

The modulators are 6146 V16 and V17, check the control grid voltage on pin 5 of each tube.

Make this measurement with AC power on, but not high voltage.  Should not matter whether the mode switch is set to AM or CW.  (In AM the screen supply is enabled on the modulator tubes when high voltage is on.)

If you do not see  a negative voltage between 40 and 50 volts, check the output of the bias supply V21 pin 4 or 5 where it connects to resistor R50, which is 1000 ohms.  You should see negative 265 volts with respect to chassis ground at either end of R50.

Also check for modulator bias voltage on the tap of 5000 ohm resistor R22.  You might try adjusting R22 (WITH POWER OFF) then carefully  measuring the voltage on the wiper contact of R22 with AC power on, but NO HIGH VOLTAGE.  Once proper bias is established, continue with adjustments per manual to obtain proper modulator resting plate current.
 
If no negative bias is found here, perform resistance checks as documented in the manual WITH POWER OFF and POWER PLUG DISCONNECTED and after shorting / discharging filter capacitors.

Edited to correct link to manual on BAMA site.  Note: If bias is correct at output of bias supply tube and resistor, check continuity of the secondary of the driver transformer feeding the 6146 grids.  An open secondary winding (infinite resistance) would not provide negative bias to the 6146 tubes, causing excessive dissipation and red plates.

Appreciate all the advice. Bias voltages on both the finals and modulators are reading lower than recommended (-50 and -35 respectively). Additionally, I measured -177 VDC on both sides of R50 (1K-ohm); should be about -265. The rig has the original bias capacitors (C92A, C92B) and LV capacitors (C98A, C98B) as well as all the .1 paper caps. My next move will be to replace all of these and hopefully this will resolve the pegged needle in the MOD current position. I will also replace C99 while I'm in there.  I had replaced the 22-ohm parasitic suppressor resistors for both modulators. Thanks again for all the help.

Will KF4IZE

I had the same thing with my Valiant restoration and with another that I was helping with. The bias filter caps were bad in both cases, and even though the voltages tested OK without a grid circuit load, the moment the tubes were operating, the bias dropped dangerously low. It's a safe move to replace all the electrolytics from that era, and after you do, you'll be in a much better position to know if anything else needs attention. Good luck!!

Have you replaced the Chernobyl resistor???


klc

R3 18K 2W to 18K, at least 5W