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RCA MI-9377 amps are here.

Water jet cut the bottom chassis covers, and I tapped it for adding the feet. I need to figure out what I want to do with the knockout hole on the side for where the cables would go when it was in a rack. I could do some aluminum side plates, either brush them and clear coat them or I could chem film it or anodize them. Or some nice wood side panels, some nice rosewood or burls. I really haven't thought that out yet.

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Just the two speaker binders and they are wired for 8 ohms. If you want to change them I can land the wires internally to the 16 ohm terminal switch if I needed to. I added the IEC plug connector with built in fuse and illuminated switch.



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Working on the mate to this and I noticed one of the resistors is cracked off the 6L6 tube. It is on socket 3 which is the plate believe and every tube uses the same value on socket 3 looks like 47ohms. These are old carbon resistors, and I have plenty of new ones both 1/2 watt and 1 watt metal film 1% so I am thinking of using a new 1-watt resistor. I don't have a parts list for this amp, so I am unsure of what wattage resistor they have in them. One some amps in the past I used the 1/2-watt ones, but I am leaning on the 1-watt ones for this location. Anyone want to add some input to give me more confidence?



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And that larger one next to it, how would I know what wattage to use on that one, it is much larger than the small ones. I see a few of them in here, I need to determ them and check their values, but I would be unsure of what to replace them with.

Yup and here is another one same value, with a large crack going down the length.


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The big "resistor" is a capacitor 3300 picofarad (0.0033 uF) at 10%. I'm assuming the last stripe is brown, so 100 volts, unless there is a brown and a black in which case its 1000. if its just a black its zero volts which doesn't make any sense. Tied into the plate connection, I'm going with 1KV.

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the others look like they may be 1/2 watt. I'd be fine using 1 watt parts though, generally higher voltage rating on the old carbon stuff vs modern.
 
Man why do they stripe it to look like a big old resistor. Ok, good to know now more back to ordering.
 
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welcome to Sprague bumblebee caps. They were the hot ticket in the 50s. Pretty much guaranteed to be bad now though.
 
I have some .0047 1000volt orange drops, would those be ok inplace of the .0033? Seems pretty close.
 
It will roll off the highs a little more than the stock network does. If I didn't screw up the math though, changing the resistor from a 470 to a 330 will maintain the same time constant so that would let it all work the same.
 
Best I can find for .0033 is 600 volts. I would think that would be fine, as I don't think I have anything in these amps with that high of voltage.
 
At the output transformer on peaks it can get close to double the DC voltage. I wouldn't want to skimp on that one.
 
Someone prior to my ownership had installed some new caps inside the chassis to replace the two can caps, I removed them and replaced the can caps with some JJ cans, and installed some can clamps to hold them in place. I did it to the mate to this amp as well, the job done inside wasn't very tidy.

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OC3 tube showed up today set them up for a demo. Sounding pretty good for an old PA amp. Both sound pretty balanced, though the one on the right I can hear a slight hum, will have to go in there and check it out as the left channel is dead quiet. I have some TAD tubes running right now, a quad from Stuman (Tube Amp Depot) and in the right channel the b stocks. I also have some quads of old NOS Coke bottle Sylvania's for future rolling.
These are not to difficult to work on, actually a fun project.

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Any data on the frequency response? I'm curious what it is. Being a theater amp I'd expect it to be pretty decent.
 
I need to work on that aspect of learning how to repair or modify tube equipment. I don't consider anything done without using the necessary tools to measure the performance. after the work. Just because it sounds good, that might not be the final truth.

I still have some more I want to do, I haven't installed the thermistor in one of the units, and I have yet to go thru and check all the old carbon resistors other than ones that were physically damaged. Plus, only one have I cut out and installed a IEC connector for the power input. Plus work out the hum issue on one of the units.

Overall, it sounds good, but not as good as that Cary amp, or the 300B amp that I have been using lately.
 
Yeh, the 3n3 cap should be datasheet rated to 1kVac (not just dc).

There's very little gain after the input transformer, so changing to triode mode may be quite noticeable.

There is no balance adjustment related to R6, R7, R8 so perhaps check if the balance is close.
 
So question regarding changing cap values on the coupling caps, C1,and C2. I saw some talk somewhere about incresing the values or decrease the values would change the tone of an amp by change the frequencies passing thru them or not allowing hi frequencies, depending on which way you go with the cap values. Woncer what swapping C1 and C2 which is currently a .10 cap with a .22 cap?
 
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