[ARC5] BC375 and Dave's Excellent Tube Adventure, Part One

[Kenneth G. Gordon]

On 25 Oct 2013 at 8:59, David Stinson wrote:

> 375 and Dave's Excellent Tube Adventure, Part One
> OK... these two used 1625s are expendable so let's push this thing.
> But the beam plates are connected to the cathode inside. Won't that
> keep it from working?

No. What it does when used as an oscillator is prevent "true" ECO action. It 
couples the signal from the cathode directly to the "plate" or screen in this 
case. It doesn't prevent it from working as an ECO: it just makes it far less 
stable.

> Who knows.... we'll find out. Change the bias?
> No way. Any irreversable circuit change is absolutlely forbidden. If
> it doesn't work, it doesn't work.

No need. It'll work.

> Time to mount the 1625 sockets in the 211 bases.
> Tie cathode to pin 7.... 
> Plate lead with parasitic choke to screen on pin 3.
> Grid to pin 4..... man, that's close to the plate lead.
> Better dress that as far away in the 211 base as possible....
> There! Two of them ready to go. 
> 
> First- I'll tune-up the rig on 3890 KC with the 211s so 
> I'll be somewhere close. 70 Watts out. Good.
> Off with the power and in with the 1625s.
> My primary supply is 27 volts... Switch the Primary Source switch
> inside the rig to "24" Volts and, since the fil current is different,
> that gives me something close to the 12 volts. Nice, red heaters!
> 
> (Angle on my sholder; "But that's 1000 Volts on the plates, boy!")
> Hmph, David! Is you a man or is you a mouse? Go for the gusto! If it
> blows-up, you can do like a government contractor; Hide it!
> ........(cring...) *CLICK!*...... 70 watts out. (un-Click)......
> hmmmm...... *CLICK!*...... 70 watts out. No red plates. No smoke. No
> fireworks. Hmmm! OK.... OK.... Let's see about neutralizing. Switched
> to "voice" (Angel on my sholder: "Wait you dummy! Don't you know about
> plate modulation?? You're really pushing your luck!").


OHHHH, Yes !!!! :-)

> "Fortune favors the foolish!" *CLICK!*
> ...... (whisper...) "hello? hello test?" Audio,
> but FMing, of course. Needs to be neutralized.
> "Hello hello..." (fiddle fiddle...) "hello hellooooo"
> (fiddle fiddle fiddle) .... getting "cocky"...
> "hellloooooo HELLOOOOOO....HELL.." "**BOOOM!!**"
> (flash flash sizzle sizzle.... as one 1625 takes wing to Tube
> Heaven...).

Too much plate (and possibly screen) voltage. Doesn't the 375 use 
something like 1100 VDC on it? Maximum rate plated voltage, even in ICAS 
service, is 750 VDC.
 
> ummm.... so.... I guess I should write this up and tell everyone
> how it's gone so far, right? 
> Even if it does make me look like a DOOFUSS, right??
> Naaaaaaaaaaaa!

Oh, YES!! :-)

Good on ya.

Ken W7EKB