[Milsurplus] [ARC5] Ben Hall's Tube Adaptors: Get You Some'o These!

[AKLDGUY .]

> IMPORTANT:  Whatever your application, the Cathode
> of the miniture tube needs to be connected to the Heater pin
> on the 4-pin base that is electrically nearest to ground.
> In the BC-230, connect the MO and PA cathodes to Pin 4. If you don't, the
tube cathodes will be filament voltage above
> ground and it will mess-up stage bias.

Yes, that's right, except that with the original directly heated VT-25s, the
pseudo 'cathode' was at the mid-point potential of the filament, as pointed
out in the manual. So there was always an additional 3.15 V bias present
on those RF tubes anyway.

> I'm back to fiddling with the bias on the modulators again.
> "Stock" bias runs the 6AQ5 modulators too hot. Still investigating.

Much the same thing applies. The VT-52 modulators had additional bias
due to the filament voltage of the two RF tubes PLUS their own mid-point
half voltage from centre of filament to pin 4. Thus, these mod tubes had
6.3 + 3.15 V (9.5 V) in addition to the grid bias derived from the drive to
the PA stage.

Whichever pin you decide to connect the 6AQ5/5763 modulator cathodes
to, it's most important that you connect both to the same pin. Nothing
could be more out of balance than connecting one cathode to pin 1 and
the other to pin 4. That would totally unbalance the parallelled modulator
pair.

73 de Neil ZL1ANM


On Sun, Jan 25, 2015 at 1:18 PM, David Stinson <arc5 at ix.netcom.com> wrote:

> We've been working on substituting affordable trasmitting tubes for those
> that have been abducted by the audiophile crowd and thus, priced out of
> reach for we mere mortals.
> Been concentrating on substituting for two VT-25s and two VT-52s in the
> BC-230 transmitter. So far have worked with 6AQ5 (7 pin) and 5763 (9 pin).
> Ben Hall, KB5BYB rose to the challenge and created
> adaptors for this task and kindly sent me two sets.
>
> Folks- I am impressed!  These are very high-quality and
> obviously a "labor of love."  Outstanding work, Ben.
> The bases can be assemble for either 7-or-9 pin sockets.
> Assembled them with no trouble and they work a treat.
>
> http://home.netcom.com/~arc5/SCR-183/7pinsubs.jpg
> http://home.netcom.com/~arc5/SCR-183/7pinBC230.jpg
> (Note to Hue:  That's the transmitter you sent.)
>
> I installed a capacitor across the filament pins
> so both heater and cathode connections would be at the same RF and audio
> potentials.
> http://home.netcom.com/~arc5/SCR-183/9pins.jpg
>
> IMPORTANT:  Whatever your application, the Cathode
> of the miniture tube needs to be connected to the Heater pin
> on the 4-pin base that is electrically nearest to ground.
> In the BC-230, connect the MO and PA cathodes to Pin 4.  If you don't, the
> tube cathodes will be filament voltage above ground and it will mess-up
> stage bias.
> Once these are installed tuning and neutralization will need to be
> tweeked, of course.
> They lowered the stage losses as well.
> With the transmitter running on the original dynamotor in the complete
> SCR-183 set, he 6AQ5s are doing
> between 6-7 watts out on 40*
> (one "hot" tube is giving me between 8-9)
> and the 5763s are making 5 watts out on 30 meters.*
> The 6AQ5s work best when running 80 and 40 meters
> "normally."  The 5763s don't do so well there for some reason but work
> fine when doubling in the final.  Maybe it's because 5763 was designed as a
> VHF tube?
> I'm making contacts on 30 meters using the 5763s.**
> I think there's a better 9-pinner out there.  Still looking.
>
> The $40 price tag for a set of four of these is a good deal.
> If you try to buy ONE VT-25, you're certainly going
> to pay a lot more than that.  Might get an empty tube base for that.  And
> try to get all these parts together for
> your project; your time is worth more than that.
>
> Ben- I think you should offer these to the Audiophiles.
> I'm betting for say, $60 a set, they'd buy them like hotcakes.
> Just need to convince one of the Audio "Gurus" to try them.
>
> If you ever intend to get that -230 or -223 or any other
> such rig running, get some of these.   Excellent investment.
>
> GL ES 73 OM DE Dave AB5S
>
> * The 40 rig has a non-destructive change to make it   xtal control.  The
> 30 rig has a 5 MC coil pack with
>  a broken PA/Antenna coil wound to double in the final
>  and feed an antenna tuner.   Chirps a little but works well.
>
> ** (NOTE for those working on these transmitters:
> I'm back to fiddling with the bias on the modulators again.
> "Stock" bias runs the 6AQ5 modulators too hot. Still investigating. For
> the 30- and 40-meter rigs which are only used for CW, I disconnected the
> MOD plates and the two tubes are there just for filament string ballast.)
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