A great response with some good ideas about measuring the airflow. Several
mentioned the method proposed by G3SEK and I see there were a couple of
Radcomn articles in 97 about airflow, unfortunately I disposed of my pre 98
magazines when I moved here so have not been able to read them.
I used the formula 1.76 * (Pdiss / delta T) suggested by Dirk, ON4ADZ, this
gave an approximation for airflow in cfm of 21. Comparing this with the bin
bag method suggested by several, I saw a 3.5 cu ft (approx) bin bag fill in
some 9 seconds averaged over 5 tries. I make that equal about 23.3 cfm.
Given a healthy dose of realism +/- 10% on both figures suggest this might
not be far from the truth so I'm assuming I have something just over 20cfm
of airflow through the tubes, this is with both fans operating and more than
the Eimac stated 8.6 cfm with 400 watts dissipation
I did try wiring the internal fan directly across the 120 VAC winding of the
transformer but it certainly makes a lot more noise for very little change
in airflow. The external fan, a 4.5" diameter 220 VAC bolted to the power
supply air inlet with a homemade foam rubber gasket adds less noise and
gives a much greater airflow.
I have re-soldered the coil tap with a big 200+ watt iron. Seems to be fine
so far and I have seen about 1.2Kw out on 15 and 10.
Starting to feel that amp number 1 is okay and working as it should with
enough air movement so things don't melt as they tend to do here. Attention
is turning to amplifier number 2, having robbed it for its multimeter and
one of the tubes what I have right now is essentially a chassis with most of
the parts. My thinking is that much of the hard work is done, just the
remaining 50% to get it back together as some sort of amplifier. There are
some bits missing, the aforementioned multimeter which I know I can replace,
some broken switches which I know can be sorted and strangely 3 broken
ceramic standoffs, the two big ones that are at each end of the main PA coil
and the small one near the choke, not sure about replacements for these yet.
The front panel is pretty rough but that’s only cosmetic.
The big question has to be tubes, and I know this has come up on here before
but I can't believe its not fixable. 3 more 8874's don't really seem
sensible unless somebody has a pile of good ones for $50 a piece. The
modification to use a pair of 3CX800's seems equally expensive both now and
going forward. So what to do. I have seen suggestions of using russian tubes
but do not know of any actual work done to use these on this style of amp.
In order to cover as many possibilities as possible here is what I think
could easily be done:
Remove the plate switching relay to give room for an additional transformer.
I don't feel the CW and SSB selection is a real necessity.
Re-do the rectifier and filter. I have capacitors half the size of the
originals which would either allow more space for other psu hardware or
allow more capacitors with a voltage doubler depending on tube.
Replace the tube plenum box with one for the tube or tubes of choice.
A single large triode would seem the easiest but I don't think there is
enough room for the likes of an 8877.
There has to be an alternative short of fabricating a whole new chassis so
what would seem to be the way to go?
73
Martin HS0ZED
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