Pages: [1]   Go Down

Author Topic: Improved johnson 500 modulation with screen modulation changeI  (Read 279 times)

N4MQ

  • Member
  • Posts: 577
Improved johnson 500 modulation with screen modulation changeI
« on: September 27, 2022, 05:45:46 AM »

The modulation on the 500 in my estimation is weak due to a fixed screen supply. The video shows the change I added and the resulting effect to the carriers envelope.  The single part revision modulates the screen voltage to allow the tetrode 4-400 a greater dynamic operational range.  Should be easily added to other rigs using tetrodes for better AM modulation.

Yes I have been spending lots of retirement time on the bench, loving it. N4MQ ENJOY

https://youtu.be/yXZBU3OPZdc
Logged

W3SLK

  • Member
  • Posts: 86
Re: Improved johnson 500 modulation with screen modulation changeI
« Reply #1 on: September 27, 2022, 06:13:38 AM »

I like that. I'm curious to see what the ramifications are for adding that. Most people, (myself included) would have used an audio choke in line with the screen. I wonder how applicable that could be to say a 4CX250B or 4CX300Y. They don't like being modulated without some modulation on the screen.
Logged

W5IR

  • Member
  • Posts: 8
Re: Improved johnson 500 modulation with screen modulation changeI
« Reply #2 on: September 27, 2022, 11:06:29 AM »

It looks like you may be over modulating. I see the carrier is being cut off on negative peaks.
Logged

N4MQ

  • Member
  • Posts: 577
Re: Improved johnson 500 modulation with screen modulation changeI
« Reply #3 on: September 27, 2022, 12:13:29 PM »

Correct, as i demonstrated it whistling and talking the gain was up for testing.  The carrier should not be clipped off at all. I use a scope to monitor and set that gain level. In checking i went on ocassion wayyy over 100% to monitor unwanted side effects.  At high overload, the 500 regulators actually light to moderate grid voltage . 

100% modulation is a slippery slope to be avoided for these reasons.  At least the modulation is improved as shown, Woody.
Logged

N2DTS

  • Member
  • Posts: 1368
Re: Improved johnson 500 modulation with screen modulation changeI
« Reply #4 on: October 27, 2022, 07:10:59 AM »

Crazy to handle those clip leads with 1200 volts on them!!!!
Surprised you are still alive, or did you wear gloves?

A choke (12 H or more) in series with the screen supply also works to let the screen self modulate.
Also a voltage divider between modulated and unmodulated plate voltage also works well.
Logged

N4MQ

  • Member
  • Posts: 577
Re: Improved johnson 500 modulation with screen modulation changeI
« Reply #5 on: October 28, 2022, 04:55:51 AM »

The HV was switched by the transmit switch, no HV when not transmitting.  The voltage was 2200 v and twice that on modulation peaks.  HV is serious, use your head and only one hand if working around it!!!

When I restored my thunderbolt, I set it up to apply HV only on transmit, saving all that wasted power in the bleeders ( about 300 watts) to keep the choke in continuous conduction.  Using solid state relays, I used had a two step soft start to reduce stress on the magnetics.

Enjoy W  :)  ;) D Y
Logged
Pages: [1]   Go Up