[JMS] 90881 Power amp rebuild

Fri, 6 Feb 2004 15:07:03 -0700

Cal,

Sounds like you are doing a very good job.  The only suggestion I can =
make is to put the exciter and amplifier into an enclosure that can be =
shielded so that TVI-causing harmonics can be contained inside.  I don't =
run my 90800 above 40 meters because it bothers off-the-air television =
reception in my own home on the higher frequency bands.  I have a Globe =
King 275 with a pair of V70D's in push-pull that is tolerable on 20 =
meters if the neutralization is set just right, but it wipes the TV set =
pretty well on 15 and very well on 10.

Enjoy!

Jim, W8KGI
  ----- Original Message -----=20
  From: [email protected]<mailto:[email protected]>=20
  To: =
[email protected]<mailto:James_Millen_Society@mailman.=
qth.net>=20
  Sent: Wednesday, February 04, 2004 11:37 AM
  Subject: [JMS] 90881 Power amp rebuild

  All:
  Last week, I obtained James Millen Co. RF Amplifier 90881, serial #52 =
in So. Cal. Item was in restorable but slightly surface rusted =
condition. After doing much thinking, I decided to rebuild her from the =
ground up. So far, here is what I did:

  1) All components stripped off the chassis. Everything from the front =
panel tags to the tube sockets, and all hardware were taken off. The =
chassis is now stripped down to "parade rest" and awaiting powdercoating =
at a coating shop. Unfortunately, they won't be able to do texture on =
the front and smooth black at the rest of the chassis so it will be all =
textured black coated. Also, whoever "TESTED BY S.H.B."won't be there =
anymore. That stamp of approval by the James Millen Co. is going to go. =
Maybe I can have a stamp made up for that, hmmm?

  2) Many of the metal components developed a "green verdigris" coat to =
them, presumably from having this unit stationed in some dank storage =
area. This included the neut caps, some of the jack bar hardware, and =
the main tuning cap. With the exception of the latter, I dipped all the =
affected hardware in a vinegar solution. That green stuff just turned to =
slime and fell off. Now I have fairly clean and shiny components ready =
to go.

  3) I stripped all the soldered components off of the amp. Antenna and =
power strips have been cleaned and desoldered. I'm going to follow up =
with stripping the tube sockets.=20

  4) Front panel i.d. tags have been stripped off and cleaned. The main =
i.d. tag where the plate and grid meters reside had been cleaned and a =
light coat of neutral shoe polish has been applied to restore a dull =
luster to the facia. Looks good.

  5) New Svetlana 812A's will be ordered in the near future.

  6) I'm going to relocate the plate current measurement of the 812A's =
from the centertap of the 6.3VAC filament transformer rather than on the =
original configuration where the plate B+ goes through the meter! I know =
I'll be reading both grid and plate current at that point but I think I =
can get fairly reasonable plate current readings from the cathode of the =
tubes since grid current will be fairly insignificant compared to the =
plate current. However, if anyone disagrees with my assertion, please =
correct me while I'm in the "design stage".=20

  Today, I'm going to strip out the 812A tube sockets and clean up both =
grid and plate meters. Once the chassis comes back, I'm going to put all =
components back into the chassis. Since I'll probably never own a KW-1 =
or some large broadcast transmitter, this will probably be the closest =
thing to owning such a monster. I plan on putting a 90800 transmitter, =
the 90831 modulator, the 90881 amplifier, and homebuilt power supply in =
a compact 28" Hammond enclosed rack cabinet. It will be short and =
compact but will have anywhere from 250-500 watts out from this little =
powerhouse trasmitter.=20

  Guys, let me know your thoughts on this. Cal, N6KYR.=20

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