[AMRadio] 813 tx (EL-34)


Jim candela jcandela at prodigy.net
Sun Nov 28 19:57:51 EST 2004




    I appreciate all the comments about the EL-34. Sure a pair of Genalex
KT-77's (premium EL-34) will bring a hefty price tag, but recent manufacture
imports go for $10-15 each. These tubes with 600 on the plate, and 300
screen need about 25 volts bias. They have more gain than the 807 / 1625 in
a Viking 2, so I agree that the demand on the audio driver 6Au6 will be
reduced. Still, a Viking 1 or 2 has poor screen voltage regulation, and grid
bias regulation. The transition from AB1 to AB2 where the 807 would start to
draw grid current is complicated by the bias wiggling around, the screen
voltage diving downward, and the driver giving up all at the same time.
Keeping everything stock except a switch to a EL-34 (except for socket and
bias change) might be beneficial because the EL-34 may produce enough audio
without drawing grid current; class AB1 is all that is required.

    I bought a Gonset G50 on Ebay and instead of the usual parallel
connected 6L6 modulators, this Gonset had two good Genalex KT-77's in there.
$$$$$$ My retirement fund. This is an example where the Gonset could have
been left at the vendor's booth after the KT-77's were plucked away. These
dudes go for $700 for two:

http://www.vacuumtubes.com/6ca7.html

	You guys are talking about audio use, and I was using one of these as a Ab2
linear amp for 160-80-40-20 meters where I could get an easy 40 watts pep
with good linearity, and about 8 watts AM with headroom for > 100%+
modulation. The lengthy lead lengths, and high interelectrode capacity made
this more than just a small challenge for me; kind of like pushing a rope.
That said, it took neutralization, cathode degeneration, and low impedance
drive to get the job done. In the end it really worked well.

Here is the schematic:

http://pages.prodigy.net/jcandela/CE20AQRO/RF_Schem.jpg

You can back up a notch with your browser to see the rest of the story.

Regards,
Jim Candela
WD5JKO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Donald Chester
Sent: Sunday, November 28, 2004 2:17 PM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] 813 tx



...The EL-34 is inexpensive, readily available, has small diameter (like a
>tall 6V6), and has beefy ratings.... I was actually using a Svetlana EL-34
>that cost US $12.00.
>
>

Unfortunately, the audiophool community has got in on the action.  People
are paying top ridiculous dollar for EL-34's now.  Last year at Dayton,
someone bought an Apache transmitter at the flea market, paid the vendor's
price for it on the spot, opened it and removed the EL-34 modulator tubes
and left the rest of the transmitter at the vendor's booth.


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