[JMS] 90881 Power amp rebuild

Wed, 4 Feb 2004 18:37:01 GMT

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. 

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". 

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. 

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

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