[AMRadio] 6CA7/EL34 replacements


Jim Candela jcandela at prodigy.net
Thu Dec 6 20:55:29 EST 2007


resend...

John King Said:
 Thu, 06 Dec 2007 13:08:48 -0800

Please disregard my earlier post requesting
information about substituting other numbered tubes
for the 6CA7 for testing purposes.

I got the transmitter working with the "old" 6CA7/EL34
tubes. Now to order some!! Has anyone had much luck
with the cheaper import 6CA7 tubes such  those sold by
Antique Radio Supply under the brand names of JJ
Electronics and Valve Art ???

Thanks for reading. 73, John, K5PGW
 ------------------------------------------------------------------------

Reply by Jim Candela, WD5JKO:

John, I have one of my QRO Central electronics 20A projects where I
 tried a EL-34 for RF Ab1 linear use for 160-20m. I tried the Svetlana,
 Sovtek, and an old vintage KT-77. None of them did as well as a Teslovak
 E34LS (this is not a typo, the tube number changed). This tube is
 carried by the Penta Laboratory folks. This tube is more robust with a higher
 Pd rating than the plane Jane 6CA7 / EL-34 (30w instead of 25w Pd).

You will find these hi-fi tubes a little finicky because the high Gm
 means high gain, and variations in grid 1 bias and/or screen voltage
 really move the idling current a bunch. Having provision to vary the static
 G1 voltage is really a must, and regulating the both G1 and G2
 voltages are both very desirable. 

In my main QRO 20a I use a pair of 7591's (get 60w PEP out) and I had
 to regulate the G1 bias and G2 screen to maintain a stable DC operating
 point for these tubes. The 7591 is kind of a baby 6550 and both are
 very high Gm Hi-Fi tubes. Tubes like the 6L6/5881 have lower gain, and
 therefore require considerably more peak grid swing.

Here is the link to Penta Labs:

http://www.pentalaboratories.com/Audiophile.aspx?dept_id=1516

My vote is to pay a little more and buy from Penta Labs,

Regards,
Jim
JKO







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