[Milsurplus] Considering GO-9 Transmitter Power Options.

[Richard]

Interesting project.  I have a TBW and chose to use 800 cycle power 
rather than re-engineer the transmitter, and it works fine. I would 
grid-block the 837's and put some fixed bias on the 803. As is the 837's 
grids are floating when key-up, they will cut off,  but not as cleanly 
as with high fixed bias.  Including the 803 may work too - try it.  
Using two filament transformers looks easier than combining them.  Using 
the 803 is the better choice as it's more stable than the 813.  Beam 
power tubes are known as prolific parasitic generators. (The 803 was 
probably thought of as old-fashioned)  High voltage keying with FET's is 
an interesting idea, but would have to be after filtering; primary 
keying at 60 cycles would sound like a bell if you had adequate filter 
capacitance.

Richard, AA1P

On 9/1/19 3:28 PM, David Stinson wrote:
> Been considering how to revive the GO-9 HF transmitter.   The original 
> power supply has been stripped, so that's a good reason to go with a 
> new 60-cycle supply.  The transmitter module itself has been somewhat 
> modified, but recoverably.  The "Man With the Golden Screwdriver" 
> removed the 803 and socket, replacing them with an 813 PA.  Simplifed 
> original-circuit schematic attached.
>
> Left Red Circle:
> K201 on the left is a relay contact in the original power supply. In 
> the original design, when the radio is keyed, this contact is grounded 
> to complete the grid circuits of the MO and the Buffer, turning them 
> on.  A relay contact in the power supply then keys the B+ and B++ to 
> the transmitter.
>
> Right Red Circle:
> In the modded circuit, he lifted theombining them. PA grid ground 
> connection at the ground end of Grid Current meter M502, opening the 
> PA grid circuit, then connected these two "red" points together.
>
> In original configuration, when the transmitter is keyed, both these 
> points are grounded and the PA's bias is developed from drive.   In 
> the modded version, the most likely idea was to apply -250V +/-  bias 
> voltage to cut the tubes off on key-up, and ground them for normal 
> operation on key-down.  This isn't necessarily a bad idea; many 
> transmit circuits work that way.  I still don't understand replacing 
> the 803 with the 813.  The 540V supplied to the PA Screen is well 
> above the 813's rated maximum of 400V.  The 803 Screen can take up to 
> 600V.   The 803 suppressor requires about +40V or so.  The 813 is a 
> beam power tube, so that's kinda weird.   The 813 can deliver a little 
> bit more power for a given power supply, but not enough to 
> foozle-around with the design. With 1900V B++, the 803 should deliver 
> about 200W.  With the 813, assuming the circuit will work with it, 
> about 250W; not enough additional to justify the mod, IMHO.
>
> To key the circuit as originally intended,  we'd need to switch the 
> 540V and the 1900 V.  This is not hard anymore- there are MOSFETs 
> off-the-shelf that can switch 3000V, even 4000V at Amps for $14-$15 
> these days which are plenty fast to cover CW. Regulating the 540V to 
> stabilize the MO is child's play now.  I have NOS 803 and socket, so 
> putting that back original is a given.
> Your opinions, please:  Key the B+, or cut-off bias keying? Keying the 
> bias can also be done with FETs, so we don't have a fraction of a 
> second of unbiased tubes during the relay's throw-time.
>
> The Green Circles:
> These are the isolated filament circuits for MO/Buffer and for the 
> PA.  The transformers in the diagram were actually located in the 
> seperate power supply module; not the transmtter itself.  I don't see 
> any way to wire these to a common DC filament source, do you? I can 
> provide both correct-voltage/current transformers, but it's always 
> nice to condense and eliminate extra power busses when possible.
>
> TNX ES GL OM DE Dave AB5S
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