[AMPS] EIMAC 8874 tubes
Richard W. Ehrhorn
w4eto@rmii.com
Tue, 28 Jul 1998 21:28:13 -0600
Hello Dean...
I doubt very much that what you experienced (in your message below) means
that all 3 tubes are "bad." It MIGHT mean that at least one tube did arc
severely from plate to grid and/or cathode, and that COULD indicate that
the particular tube is at or near end-of-life.
A loud "bang" generally means that something has exploded in the HV area.
An arc inside the vacuum envelope of a tube makes little or no audible
noise itself. But the resultant HV current spike of 50-100 amps or so while
the filter cap discharges through the arc CAN and sometimes does literally
blow up a choke in the HV line.
Maybe more likely, failure (shorting) of an HV bypass or blocking capacitor
can make quite a noise and flash as it blows apart.
I think you need to pull the plug, take off the top cover, and CAREFULLY
use a long-bladed, well-insulated screwdriver or equivalent to SHORT TO
GROUND each of the following: at least one tube plate cap; the top of the
large plate rf choke; the + HV terminal on the power supply board. All of
this with one hand in a hip pocket!
As a final personal-safety precaution, since you don't know what may have
opened up in the HV + or - circuits, I would apply a short HV clip lead
with well-insulated alligator clips on both ends as follows: first (one
hand in the hip pocket!) clip one end to the NEGATIVE end of the HV filter
capacitor stack. You may have to study the schematic to determine which
screw head on the HV filter board this is. Then similarly connect the other
end of the lead to the POSITIVE end of the filter stack. Leave this lead in
place until you are ready to make circuit resistance measurements OR to
apply power again. BUT DON'T FORGET TO REMOVE IT AND REINSTALL THE TOP
COVER before applying power, or you will damage something else.
Now you're ready to investigate the RF compartment. My bet is you'll see
something black and/or fragmented as the result of whatever caused your
BANG and FLASH. If it was one of the HV (3kV or 6kV) disk capacitors, the
cap itself probably caused the problem and I recommend replacing ALL such
disks in the RF area with new 6kV caps of the same value. They are
available from ALPHA/POWER, Inc. (970) 535-4173, FAX (970) 535-0281, e-mail
<APIcontact@aol.com> or <sehrhorn@aol.com> and (and no doubt other sources)
at very modest cost. I hope this is your problem, 'cuz repair is easy and
cheap.
In the fairly unlikely event that you can't find ANY visible evidence of
electric violence, more digging will be needed. No point in borrowing that
trouble until you've looked for the easier-to-find potential faults and at
least noted whatever else you might see inside the amp.
Good luck.
73, Dick W0ID
-----Original Message-----
From: Dean St. Hill [SMTP:dean_8p6sh@hotmail.com]
Sent: Sunday, July 26, 1998 9:36 PM
To: amps@contesting.com
Subject: [AMPS] EIMAC 8874 tubes
I own an ALPHA 374A linear amplifier (early 1980 vintage) which has seen
only moderate use throughout its life. It was not working - not even
coming on and as a project and with the help of the great guys at ALPHA,
I rebuilt the control board and it seemed to be working OK except it
wasn't giving power expected. Then suddenly POWW!!! a huge flash in the
tube compartment and fuses blown. Now I don't want to even open it up
again. Here are my questions.
1. How can I tell if the tubes are blown?? (Preferably without powering
up)
2. If they are not OK where can I get a reasonably priced pair of these
tubes??
3. Is it worthwhile looking for used tubes at all??
4. Can I use tubes from theis russian company (Sveltena-or some similar
name)
Thanks and you can send replies direct or via the reflector.
73 Dean
8P6SH
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