[AMRadio] 6CA7/EL34 replacements


Jim Candela jcandela at prodigy.net
Thu Dec 6 20:38:55 EST 2007


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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