[BC] Our Wild TECH Youth - more memories

[Bruce Doerle]

Any of you guys ever build any of the amps and stuff that used to be in the back of the RCA tube manuals when you where a kid?

>>> dynotherm at earthlink.net 07/09/05 1:04 PM >>>
On 9 Jul 2005 at 15:33, khcs at juno.com wrote:

> 
> Phil asked"
> 
> >You mean with miniatures featuring a 50C5 output, or the
> All-American 5 with Loctals? Somebody say what's a Loctal? Heh, heh. <g>
> 
> ------------
> 
> I don't recall any loctals in consumer stuff, but found lots in military 
> gear, which was subject to vibration.  

Philco used them in "table models."  IIRC Moto and possibly Delco used them
in car sets.



> The 50C5 was of course an octal, as was the 35Z4 rectifier.  

I beg to differ, those were both 7 pin tall envelope miniature.

I once had to whip up a PA amp overnight from my junkbox using 
2 50C5's and a 12AU7 with a little dropping resistor in the 
filament circuit and a single M-400 S-T diode for PS. Got over 
10 W clean audio out of it which was plenty into a couple of 
folded PA horns. It was for installation at a small outdoor 
skating rink were we were doing an afternoon remote. It worked 
fine as long as it was plugged in the right way so I hardwired 
it to the AC supply and put in a DPST switch to turn it off. 
First tune played - Eddie Cochran's Summertime Blues, or at 
least I think that was an Eddie Cochran tune - lot of years and 
many cobwebs between the ears since then. 

Believe it or not, the circuit just came back to me. The first 
half of the 12AU7 was a low gain pre with 10k plate resistor 
and 2.2k cathode bias. The second half was a split-load PI with 
51k plate and cathode connected to the 50C5 grids through 
0.25 blocking caps. The output stage had the screens tied to B+ 
and used 470 ohm cathode bias resistors bypassed by 10 MFD 
sections of a 10/10/30/40 quad can with the other two sections 
used for the PS. Put a couple of 470k drains on the grids, fed
the plates of the 50C5's through an open frame output xfmr. and 
it worked like a champ with more bass than the horns could use. <g>

Now, if only I could remember where I park the car when I go
to the mall. <g>

> Now, I guess all our younger readers will want an English 
> translation...

After what I just said, I know they will. <ggg> It was fun
sometimes though, wasn't it? ;-)


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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