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B&W 5100B screen voltage




 
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Author Topic: B&W 5100B screen voltage  (Read 657 times)
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KK7UV
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« on: February 14, 2022, 06:04:46 PM »

The PA is a pair of 6146 in parallel.  The manual schematic shows +250v for the screens.  When I looked at a couple different 6146 datasheets for Class C plate-modulated RF service, they say 250v is the max, and the Typical Operation example for 600v plate voltage it shows 150v for the screen.  Yet, in operation with 250 screen volts I am getting the manual-specified screen current and output power.

What am I missing?  Did B&W really intend the screens to be operated at max voltage?

The screens 'self-modulate' with a series 20H choke off the LV (270v) supply if that is important.

Steve
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« Reply #1 on: February 14, 2022, 09:58:24 PM »

The PA is a pair of 6146 in parallel.  The manual schematic shows +250v for the screens.  When I looked at a couple different 6146 datasheets for Class C plate-modulated RF service, they say 250v is the max, and the Typical Operation example for 600v plate voltage it shows 150v for the screen.  Yet, in operation with 250 screen volts I am getting the manual-specified screen current and output power.

What am I missing?  Did B&W really intend the screens to be operated at max voltage?

The screens 'self-modulate' with a series 20H choke off the LV (270v) supply if that is important.

Steve

Sure, when tubes were cheap and plentiful you could replace them often.

B&W used a lower plate voltage and a higher screen voltage and I do not think that is a proper design for tube longevity. Running 6146 screens at max Vsg is nuts.

HeathKit used a higher plate voltage in their Apache with a 175V screen voltage. Below are the operating parameters for my Apache:

RF Power Out at Keydown* in AM Mode (120VAC input): 120W
Final Vp* = 750V
Final Ip* = 250 mA
Final Icg* = 4.0 mA
Final Grid Bias* = -78V (Measured at junction of R20, L15)
Final Screen Voltage* = 175V Measured at 6AQ5 plate, Pin 5.
Modulator Screen Voltage VsgM* = 370V; Switched by Main Relay in AM mode and should supply Speech Amps and Driver.
Modulator Grid Bias* = -36 volts
HVB+ = 770V*
LV* = 370V*


Phil - AC0OB
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KK7UV
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« Reply #2 on: February 15, 2022, 10:20:26 AM »

Might this be as easy as inserting a ~4k resistor in series with the 20H choke to drop 100v at the 25mA combined screen current?

 Or, alternately just taking the screen voltage off the VR-150 line that feeds the VFO-Buffer stages? or will the screen modulation on this line cause issues with the VFO-Buffer stage?



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« Reply #3 on: February 15, 2022, 06:22:54 PM »

Might this be as easy as inserting a ~4k resistor in series with the 20H choke to drop 100v at the 25mA combined screen current?

Is the actual measured screen voltage 250V in AM Mode at Keydown? Are you using AM and CW only, no SSB?

If the Vsg really is 250V at Keydown in AM Mode, then a 2.7k 5W could be inserted in series in between the screen supply from P301 Pin 3 to the junction of R319 and R320. SW302 should be in the NORM position.

Note: In transmitters of this vintage with SSB adapters, the Final's screen voltage is usually above +220V in SSB mode since the Finals are biased as Class AB1 Linear Amplifiers.

Or, alternately just taking the screen voltage off the VR-150 line that feeds the VFO-Buffer stages? or will the screen modulation on this line cause issues with the VFO-Buffer stage?

Not a good idea since your VR circuit is not capable of supplying all of the needed current.

Phil - AC0OB
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KK7UV
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« Reply #4 on: February 15, 2022, 07:07:35 PM »

250 is the actual measured keydown voltage on the screens in phone mode.
In key-up during CW, it is 290.
I don't use the SSB.

Putting the 2.7k resistor where you indicate will bridge across the 20H screen choke and meter.  Did you mean P301 pin 1?
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« Reply #5 on: February 16, 2022, 02:06:06 PM »

250 is the actual measured keydown voltage on the screens in phone mode.
In key-up during CW, it is 290.
I don't use the SSB.

Putting the 2.7k resistor where you indicate will bridge across the 20H screen choke and meter.  Did you mean P301 pin 1?

Since I have no idea of your parts placement or modified schematic, you will have to somehow put it in series between the 2X0V? supply and your choke.

Phil
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