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ltarini 26th September 2007 05:43 PM

EL-34 instability in EICO HF-35 amp
I recently tried some matched pairs of SED "C" EL34s
and found out that the plate current on my EICO HF-35s
would immediately run away when I started up the amps.
I know this because I put a "Bias Probe" tube socket
with leads to my Fluke in between the tube and the
actual socket, that allows me to read the current.

Testing one amp at a time, I found that as the amp
warmed up, the measured current went to about 35 mA
and paused briefly, then headed towards 50 mA, 60 mA
with increasing hum and no sign of stopping, so I shut
off the amp. I tried the SED tubes in both amps, in
different configurations, and the same thing always
happened. But when I put back in a pair of used JJ
Teslas the amps worked fine: steady current, held
their bias, no hum. I tested the Teslas and SEDs on
my Hickok 539B and they all tested the same, strong.

Then I measured all the resistances with the amp off
and the voltages with the amp on. I have the original
construction manual so I know what the specs should
be. The voltages were all within range and the
resistances were too, except that in several cases the
resistance reading was "OL" when it should have been a
finite number. I don't understand that. The meter
was working and I had good contact.

If you have any ideas, I'd love to hear them. All I
know is that the SEDs should have worked fine but
something unusual in the amp was causing instability.

For a copy of Eico HF35 schematic see:


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