[Central-Electronics] 20A 6AG7 voltages

[Jim Candela]

Jon,

   Many will debate the point of modify or keep stock. I too have seen >400v in 
a stock 20A in standby. In fact the filter caps go over the rated 450v during 
the turn on when the 5U4 is quickly heated, and the other tubes take a lot 
longer to conduct. If you just lower the B+, you might also find the filament 
voltage around 6.9 volts. So here is what I'd do if I wanted a near stock 20A on 
the air:

1.) Pull the 5U4 and solid state the HV power supply (use two 1n4007's on each 
leg)

2.) Change the pi C-L-C power supply filter a choke input filter L-C

3.) Add an additional R-C filter, maybe 270 ohms (5-10w) and 40 uf to feed the 
audio and low level RF stages.*

4.) Use the 5U4 5vac filament winding to 'buck' the AC line voltage such that we 
drop the incoming AC by 5 volts.

*The additional RC is necessary since the AC ripple is higher with just the 
choke input filter.

Now the 6AG7's will be around 300v, and the low level stuff about 275v. The 
filament voltage might be around 6.5 volts. This all assumes a line voltage 
around 120v. the 6AG7's will no longer cook the paint off them, and the output 
power on 160-80m will be closer to the spec 15 watts PEP.

Just one mans opinion. :-)

To those that want the rig 100% stock, I say use a variac, use a fan, and use 
500v rated filter capacitors..

Jim
WD5JKO


----- Original Message ----
From: k6jek <k6jek at comcast.net>
To: central-electronics at mailman.qth.net
Sent: Mon, March 28, 2011 11:33:19 AM
Subject: [Central-Electronics] 20A 6AG7 voltages

Folks,

The plate and screen voltages on my 6AG7's are too darned high, at least I think 
so.  Do you guys see 300 V or something greater?  The bias seems correct, -10V.  
I have the AC cranked down to 110 and still see 375 VDC on the plates and 
screens.  Yikes, I say.  Yikes.

BTW.  Dave, K6IR, solved the mystery of the screen resistors which none of the 
rest of you guys seem to have.  They were in the early, early, early 20A's.  He 
came up with the original wiring diagram and there they were.  There were no 
parasitic chokes on the plates of the 6AG7's. The grid resistors were 100 ohms, 
not 15, and the screens were fed with 180 ohm resistors both fried in my unit. 
And, of course, the oscillator is 1/2 of a 12AU7a, not a 6U8.  All this was 
changed after just three months of production.

One more thing.  Dave has the early wiring diagram.  I don't suppose any of you 
actually have the early schematic, first version, 1954, not the revised version 
on BAMA.  I can figure things out from the wiring diagram but a schematic would 
make it easier.

Thank,

Jon

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