RCA mono tube amp

[Mr.JD]

Hi guys

My name is Jon, I've been casually playing guitar for years and years while about a year ago i came to love tube amp sound. I read a great deal of literature about different manufacturers back then, and got just enough info to a)be dangerous and b) know not to pass up on a cool original amp from 50 years ago when I find one.

To my knowledge, and from what I can find, this is an RCA MI-12236 mono tube microphone amp.


[img:800:600]http://i14.photobucket.com/albums/a309/ ... aca608.jpg[/img]

[img:800:600]http://i14.photobucket.com/albums/a309/ ... 63521d.jpg[/img]

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[img:800:600]http://i14.photobucket.com/albums/a309/ ... 51986a.jpg[/img]

It appears to be very similar to the amp in this auction
http://www.ebay.ca/itm/VINTAGE-RCA-MI-1 ... 0645172633

Though mine has clearly been altered with a 1/4th audio input in the front and a much bigger power transformer. The guy I bought it from told me that his father was in a band back in the 50's and 60's, and this was part of his gear. He also showed me an original (and big) silvertone head and amp, and a whole box full of spare tubes (will be buying it all soon). Who knows what else i've not seen.

Anyway

I'd like to test this amp and see if it is functional before re-capping it... but i'm not going to plug it in without hooking at least resistors to the speaker outputs.

My question is this: how do i hook up speakers to this older tech? All there are are single outputs on the back marked 0, 4, 7.5, and 15. I've got a good 4 ohm cabinet... but? any input would help.

Jon[/img]

[ToneMerc]

My question is this: how do i hook up speakers to this older tech? All there are are single outputs on the back marked 0, 4, 7.5, and 15. I've got a good 4 ohm cabinet... but? any input would help.

Jon[/img]

0 = ground and then you have 4, 7.5 and 15 ohm taps. Just configure an spade lug to a housed rca jack.

TM

[Cantplay]

Power it up slowly on variac, without tubes and check for heater voltage and AC to rectifier.

Then plug in the rectifier and check dc at empty tube sockets. If all that is cool fire it up full tubes.

I would also change those anode cap wires and add grommets where they rub the holes in the chassis.

John

[Baxtercat]

Wow, you scored two of 'em for a hifi rig!
[Oh, it's a mirror.]

[xtian]

Is there an output transformer hidden inside?

[Mr.JD]

Is there an output transformer hidden inside?

Yes there is. I'll try to post pictures of the guts tonight. Sadly I don't have a variac

[Leo_Gnardo]

Sadly I don't have a variac

Then you'll want to construct the TAG Light Bulb Limiter - see sticky above. Before applying any power 1) make sure your fuse is appropriate (in this case a 1A fuse is enough) and B) examine the power cable & replace it if the insulation is cracking & conductor showing through.

After making up your Light Bulb Limiter, use the smallest lamp you can find to start.

Monitor your amp's voltages as you swap in larger lamps.

In an amp this old you can pretty well expect to need new filter caps.

Also beware if there are any selenium rectifiers - you definitely don't want to let the smoke out of these.

One 6L6 and a 5Y3 - looks Champ-like right there. Good start.

Please don't "just plug it in and hope for the best." Not everybody's lucky.

[martin manning]

If you are hoping to reform the large capacitors, the vacuum rectifier will prevent you from doing it directly with a Variac. I suggest you build a bulb limiter (see sticky; good for future use), and temporarily put a couple of silicon diodes and a large resistor (100k 3W, say) between the PT and the reservoir capacitor. The diodes and the resistor could be scabbed onto the rectifier socket. Use the bulb limiter with a 7W bulb to start and see what happens. Work up to a 100W bulb, and measure the voltage across the 100k resistor to see how much DC current draw there is. Leave it on for a while and see if the voltage across the 100k drops over time. You can leave the smaller metal tubes in, but pull the 6L6 so you don't damage the output transformer. I can draw a diagram if you want to give this a shot.

[Firestorm]

Look at the can caps from inside the amp. If any are pushing gunk up through the relief vent, you might not want to power up even with the limiter.

[Mr.JD]

yea, i'm clearly in over my head. Thanks for the help though guys. I'm glad people like you are out there that know their stuff.

[LeftyStrat]

Update your profile to include a location. Perhaps we have someone near to you that could help out.

[Mr.JD]

I appreciate the offer, but I am moving from Alabama to NC in about a month, so I am just going to move this amp on to the next person. I don't have time to work on it nor the facilities at present... and I'd hate for something to happen to it during the move. I just have rescued the thing from a cabinet after 50 years, lol.

Thanks guys

Jon

[khawk]

Mr. JD,
I inherited on of these amps a few years ago and have been trying to find info on it with out much luck. I saw you post and wondered if you had had any luck getting yours up and running. Any info you might have would be of help. I have a schematic and spec sheet which I would be willing to share with you if needed.
K

[Leo_Gnardo]

Mr. JD,
I saw you post and wondered if you had had any luck getting yours up and running.

From Mr.JD's last post it looks like he's given up on trying to do anything except sell/give it to someone who might do something with it. May be best to start a new thread and we'll try to help you out.

Same advice from other posters holds for you and your gem. Please don't just plug it in and hope it works. That's the fast way to a smoke generator not an amp. Filter caps deteriorate after years & decades of storage. Find a variac or use light bulb limiter and slowly try to reawaken your amp. With the proper size fuse in place of course. You could even put in a smaller value fuse for safety's sake on its initial warmup after long term storage.