THE BIG ONES . . . RCA MI-9359 resurrection begins . . .

markus

markus

Master Procrastinator
I've been sitting on these for a loooooong time . . . they've scared me from the day I brought them home :yes: there's a reason those tubes are behind a cage . . . I figured it was time to leave the kiddie pool and see how the deep water is . . .

a quick rundown of the spec's, that I know of:

4 809's in Class B / Push Pull Parallel for output tubes
2 2A3's to drive those 4 Transmitting tubes
1 6SL7 as (i think) a phase splitter and voltage amp
3 5R4's in the power supply, putting out WELL over 600 VDC . . .

designed to run a complete drive-in theater, the rated frequency response is 50-5000 Hz, at 125 watts RMS. When I got them, they were missing most of the tubes. Over the years, I snagged a pair of Valve Art 2A3's, 4 5R4's, and a few 809's (already had a couple 6SL7's kicking around . . . )

I still need to track down 2 more cheap 2A3's (new production of course) and 3 more 809's (ideally I'd have 8 'matching' NOS 809's, as I see no way to adjust bias on the 809's . . .) two of 809's I DO have separated from their top caps, so for testing, I soldered the plate wire coming out of the top of the 809's to the plate cap socket.

I brought the first up on my variac, and although there is a substantial low level buzz (sounds like a ground issue, not a power supply issue), it is making music through my test speaker. Clearly, it'll require a recap - power supply uses all BIG oil caps, and the signal caps are all 1000V which I don't trust AT ALL :D

The plan is to get the remaining tubes I need over the winter, recap both amps, and bypass the input transformer, which I believe is the circuit component limiting the frequency response and see how well they sing. With iron this serious, I'm very hopeful :thmbsp:

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Mwahhahahahha!

Gotta love

1) Tubes with top caps.
2) Tubes protected by cages.
 
Leave the oil caps alone. As long as they're not leaking they are most likely good. I wish the best of luck with your restoration.
 
While a different RCA design, this page might give you some ideas for taming these beasts into excellent hi-fi amps: http://www.montagar.com/~patj/mi12188a.htm RCA liked to drive their power tubes to the edge of melting, I guess it was a good way to sell more tubes if nothing else.

2A3s as drivers?!?
 
The oil caps will likely out last all of us put together. If they show no signs of leaking (and I bet they don't), then it is virtually guaranteed that they are still as good as the day they were installed.

Dave
 
DAMN! Those look like part of the Stack we have on the sub for internal comm's. The Stack is 2 amps, 2 sets of switching gear (overlapping voice comm's, collision, General and Diving alarms) is a bit over 6 ft tall. And LOUD!?!?!?! DAMN STRAIGHT! Scares the hell outta the kids and mom's when someone hits the General or Collision alarms. Ever seen a 3 year old try to climb back to where they were born and Mom is wearing slacks? Hit the diving Alarm (motor driven klaxon), and watch the fun. You can hear it outside on the pier with the Hatch's shut.

It definately will heat an auditorium. Our's on the sub is stuffed into a corner in the control room. No need for space heaters in that compartment. The one thing we found when we started restoration was one of the transformers had started to leak (oil filled) due to rust on the outside. Had to replace it. 2 days to get to it, 1 day to replace and 2 days to reinstall. Saturday's only so it was a month and a 1/2. WHAT A PITA. But it runs fine. And it makes 60 and 70 year old guys(our Volunteer Crew) feel 20 again. Which is fun when you're 56 and one of the younger guys in the Volunteer crew, someone hits the General Alarm and everyone starts running(limping, hobbling or crawling as fast as they can) to Battle stations without thinking about it (it's 2nd nature). But the next morning, we're are all groaning about sore muscles, and aches and pains. Is it worth it?? DAMN RIGHT IT IS. The wives won't agree tho. Nothing gets done at home for a week afterwards. LOL!!!

Larry
 
NICE amps Markus! I'm just itching to find a pair of RCA MI-12182-A theater amps (the ones with 6146 output tubes) and go to town on them!
 
dcgillespie said:
The oil caps will likely out last all of us put together. If they show no signs of leaking (and I bet they don't), then it is virtually guaranteed that they are still as good as the day they were installed.

Dave
Click to expand...

Dave is 100% correct with regard to the oil caps.

Markus, one other thing to check is those old wires. Make sure there's no breaks or cracks in the insulation that could arc over.
 
And replace all those wax paper caps. They are guarenteed to be bad (they absorb moisture out of the air, and then mix with the acid in the paper, making the cap electrically leaky, which is bad news in tube amps.
 
wa2ise said:
And replace all those wax paper caps. They are guarenteed to be bad (they absorb moisture out of the air, and then mix with the acid in the paper, making the cap electrically leaky, which is bad news in tube amps.
Click to expand...

Correct again!
 
ab0ez said:
Dave is 100% correct with regard to the oil caps.

Markus, one other thing to check is those old wires. Make sure there's no breaks or cracks in the insulation that could arc over.
Click to expand...

wires are in (surprisingly) good shape . . . still nice and pliable. I've already replaced the .1 / 600V caps on one of the amps, but will need to send away for more caps for the rest of the circuit . . . there is only four signal caps in each amp - still trying to trace out the circuit to see what's going on. both amps have hum to them . . doesn't sound like power supply issues though. I might try running them with the voltage amp chassis to see if that improves things.

one of the neat features is the switch and guage assembly on the front of the ams - while running, you can switch the meter to check strength on the 6sn7, 2a3's and both pairs of 809's :yes:

even with the hum, running through one of my test speakers, I can tell these amps have BALLS . . . I would LOVE to hear these running a pair of horn loaded Altec, or better yet, RCA 15 inchers, with some multicell horns up top . . .

big potential here me thinks:thmbsp:
 
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