I have a 76pa with the trio of 8874 and have considered the same swap. The
practical problem with the GI7b is that at 2200V that we have typically in
the Alpha, you really end up driving the tube pretty hard to get power out
of it. Their sweet spot is around 2700V. The grids in the tubes are rated
at 7W.
I have a pair of GI7b with 2900V on the plate and it will run a comfortable
900w for the pair; an easy KW on the low bands - while staying within the
350w plate dis limit. That is with about 50w drive - resulting in well
under 100 mA grid for the pair (IIRC).
Considering the low voltage, I have wondered if one of the Chinese 8877
would be a good pick. The 2200 B+ would be a nice fit for that tube and
it's got decent gain and nice Pd. Needs a grid protection circuit though.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Glen Zook via Amps
Sent: Wednesday, April 23, 2014 4:21 PM
To: Chuck Curran ; amps@contesting.com
Subject: Re: [Amps] Alpha 78 with 8874 Triodes
The GI-7b / GI-7bT has a plate dissipation of 350-watts per tube versus
400-watts for the 8874. The GI-7b and GI-7bT are pretty easy to make a
socket, at least for HF. A couple of years ago I built a 160-meter only
linear amplifier using a pair of GI-7bT and the socket that I made for them
is pictured in the article at
http://k9sth.com/uploads/160-meter_amplifier_article-1.pdf
So far, my Tempo-2001 linear amplifier, which uses a pair of the 8874 tubes,
is still working fine. But, if the tubes did go soft, I would seriously
consider replacing the 8874 tubes with GI-7bT.
Glen, K9STH
Website: http://k9sth.com
On Wednesday, April 23, 2014 2:48 PM, Chuck Curran <ccurran@wi.rr.com>
wrote:
Hello the Group:
I am working on an Alpha 78 with three 8874 triodes, that are still in good
shape. This particular amp was one that was built in late 1980, and the
interior is in like new condition. However, it was one of those that was
built with the "SCRATCH PROOF" paint. The catch is the paint broke down on
all of the Alpha amps that used it and is sticky like a coating of honey was
applied to all surfaces.
I was able to obtain a brand new front panel and I stripped and repainted
the sheet metal cover. The cover turned out beautiful, just like the new
front panel is, using a color coordinated paint my wife picked out, I am
really bad at selecting complimentary colors!
So here is the question. If an owner would want to run the amps for a long
time, you have to consider tube availability. The 8874/3CX400A7 is listed
at $895 each at RF Parts, so I was wonder about re-tubing?
I searched the archives and found a discussion/argument from February of
2002. There a GI-7B was being discussed, but no results were ever posted.
Now, 12 years later, have any on the list come across a tube that can be
considered an available replacement with new sockets, for the three 8874's
currently in place?
I would greatly appreciate any advice available, Thank You
Chuck W9KR
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