[AMRadio] EL34/6CA7 temporary test replacements?


John King k5pgw at yahoo.com
Thu Dec 6 13:56:05 EST 2007


My AM transmitter uses a pair of 6CA7/EL34 as
modulators.

I have no resting or idling current showing on the
meter where ther should be 50 milliamps of idle or
resting current for the two modulator tubes.

I had a catastrophic flash in the 5V4 low voltage
rectifier tube but the primary fuse did not blow.
Whatever caused the short or overload of the 5V4 must
have cleared itself such as a momentary arc. The EL
34s/6CA7s show very weak on my tube tester. One of
them is about 50 percent and the other about 30
percent. The 5V4 low voltage supplies the screen
voltage to the two 6CA7s. The 5V4 was replaced and
there was no further arcing of the rectifier. The low
voltage supply is now working. The manual says that if
the High Voltage is working and there is no resting
current, then there is probably no screen voltage on
the 6CA7 tubes.

I don't have a pair of 6CA7/EL34 tubes, at present,
but would like to put tubes in the modulator to see if
another pair will draw resting or idle cureent.
QUESTION.....WILL ANOTHER AUDIO AMPLIFIER TUBE
SUBSTITUTE MERELY FOR A TEST? I have 6L6 tubes but
don't know if they can be substituted for a MINUTE to
see if there will be resting current. I may have some
5881s.

I am ordering 6CA7/EL34 tubes, but would like to
troubleshoot and have modulator ready when the tubes
get here.

If you can answer my question, it will be greatly
appreciated. 73, John, K5PGW



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