[Central-Electronics] 20A Power Supply gold

[Stan McIntosh]

Bob,



*>>Welcome to the Central Electronics family. *



Thanks.  This seems like a great place to be.  I’m glad that you dragged
the 20A back, just as I’m glad that the 20A dragged itself into my house.
As far as I’m concerned, it did worm its way in, and I’m really looking
forward to my time with this piece of gear.  As I mentioned before, the
deal that I was making with my friend was for a 7' over-engineered rack
containing a 2x813 cathode driven amplifier.  The modules take up the
entire rack:  power supply, meter bank, diagnostic module, blower module
with variac to control blower (the 813s have cooling chimneys), amplifier,
and cabinet vent module with fans.  The main point is that, even with all
that fun to look forward to in the amplifier, this 20A really has my
attention now.  The CE piece was a throw-in, but I would have been happy if
Tom had changed his mind on the amp and made the 20A the deal itself.



*>>To reduce the heat inside the 20A you can replace the 5U4 with "sand".  *



That sentence made me cringe, but you make an argument that I cannot
refute.  I have dozens of 6X5 and 5Y3 tubes, but you’ve made a case I just
can’t argue against.



*>>…with 115 VAC input and key up the B+ is about 460 volts.  Key
down shows ~ 380 to 390 VDC. *



Maybe I need to check my book at home, but the sheet I’m pulling up lists
300V as the maximum plate voltage (or am I misreading?).  Though millions
of tubes are still floating out there, we can’t go to the corner drug store
for a replacement.



*>>You can move one wire in the 20A and drop the B+ down to ~ 280 VDC under
load, yielding 6 to *

*>>8 watts RF output. *



Consider it done.  If there’s an argument for not moving the wire, I can’t
see it.  This also greatly simplifies replacing the electrolytic (yes, I am
afraid I am favor of the pre-emptive replacement.).



*>>Saves 15 watts heat, and reduces the load on the power
transformer or allows 15 watts more power *

*>>to go to something else - 458 VFO, or higher power from 6AG7's etc.  *



Uncle!  Uncle!  You win, Bob!  Reducing heat and transformer stress is the
big selling point.  The VFO is a double-edged sword.  On one hand, I really
don’t need to add another project to the bench.  On the other hand, Hey!
Let’s put another cool project on the bench!



(An hour or so passes with bits of surfing.)



 Okay, so we now have another project on the bench.  Le boxe du junque has
one of those National reduction drives with the paper scales, and Lew McCoy
published a design from 1962 that looks promising.  Already having a Heath
PS-3 also should help.  Actually, I have a couple of them, and one is
getting ready to get 'pimped up'  (it came without a chassis and with a
poorly-chosen, hacked-in meter, and I'll address both.).



*>>… to plug-in sand rectifiers.  *



Until just a few years ago, I could still buy those in town.  We had a shop
that had been in business since the ‘60s.  They had shelves of transistors
from GE and RCA, coils from Millen, and on and on.  There were even a
couple of unsold and ‘new’ Lafayette phone patches on the shelves.  I do
remember passing on some of the sand-state replacements for 5U4
rectifiers.  *sigh*  I miss that place.  When the shop was closing, the
owner lamented how people just don’t build things in their garages the way
that they used to.  I miss that toystore regularly.



*>>20A's are a kick to use barefoot.  Or you can drive an amplifier*



Barefoot’s the plan, at least for a little while.  Even if the time were to
magically appear to work on the 813 project soon, the 20A would need a
preamp to drive that beast.  I’m thinking a pair of 1625’s or maybe even a
6146.





*>>The 9 Mc. crystal oscillator runs just a bit high in frequency, maybe 1
1/2 to 2 Kc.  This *

*>>makes a **difference when you calculate crystals needed to operate
voice. *



Can’t resist asking, but would there be any value to VXO’ing the crystal
with 3 turns or so through an FT-243 and a trimmer cap to put the frequency
right at 9 MHz?



*>>There are lots of 20A mods on the internet, and you could spend endless
hours *

*>>changing this and adding that, but even just bone stock the 20A is a
blast to operate.  *


Except for the power supply changes that should help prolong transformer
and tube life, I think I'll leave mine in.  For what difference it might
make, I will put in a resistor to hold down filament voltages with modern
AC sourcing.  Sure, I have Variacs, but any help with inrush ought to
benefit, right?


There is another question that I have which concerns the manual insistence
on RCA 6AG7 tubes.  Was there a connection between CE and RCA, or is there
an actual difference?


73

Stan

KD4BTH