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 Post subject: Searching for info on RCA MI-12158 70 Watt PA Amp
PostPosted: Mar Thu 08, 2018 4:40 pm 
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Joined: Apr Mon 24, 2006 3:01 pm
Posts: 641
Location: Lombard, PA
I just came across one of these and tired to find info on the web but found almost nothing. Radiomuseum doesn't have it either. It's a nice amp with FOUR large transformers. Likely one is the power, one a choke and one an output. I'm trying to figure out the fourth one. They are all big and all slightly different. This amp has four mic ins and a phono in.

I'd love a schematic. I also read that there was a field mod that replaced the two 6146 output tubes with a pair of 6550s. Does anyone have any ideas on that?


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 Post subject: Re: Searching for info on RCA MI-12158 70 Watt PA Amp
PostPosted: Mar Thu 08, 2018 6:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4128
Location: Boston, MA USA
The fourth transformer may be an interstage transformer to drive the two output tubes. That would be normal for an amplifier running Class AB2 as many PA amplifiers did. Not exactly high fidelity, but plenty loud.

-David


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 Post subject: Re: Searching for info on RCA MI-12158 70 Watt PA Amp
PostPosted: Mar Thu 08, 2018 10:36 pm 
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Joined: Apr Mon 24, 2006 3:01 pm
Posts: 641
Location: Lombard, PA
Still looking for a schematic, but I've studied it a bit and the extra transformer is a separate filament transformer. All four transformers are large and heavy. The unit puts out 79 watts, so the output transformer is the biggest.

Also I read that there is some controversy as to whether these should be converted to 6550s. Like a lot of things related to tubes, it's hard to tell facts from beliefs.


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 Post subject: Re: Searching for info on RCA MI-12158 70 Watt PA Amp
PostPosted: Mar Fri 09, 2018 4:02 am 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 2757
Location: Vancouver Island, Canada
I think you might be stressing your filament transformer if you put in 6550's.
The 6550 heater draws 1.6 amps while the 6146 only draws 1.25 amps.

Not a huge difference, and not saying it can't be done, but you need to check the specs of the transformer. You might find numbers on it that can be researched.

I don't know why that 6146 tube does not appear in the GE characteristic manual. Also check if your tubes have an A, W or B suffix, as those were improved versions, with higher power output. In fact if your amp claims to be 70 Watt, it may be running the newer tubes. The plain 6146 had a max ouput of 25 Watts, while for the 6146B, the output is bumped up to 35 Watts, about what you have there.

In that case, you won't gain by running 6550's in the amp, except that the tubes might be more readily available But probably not cheaper due to popular demand. Anyway, those are my thoughts on the subject, just from browsing the specs.

BTW, here is a blog you might find interesting:
http://www.qth.net/pipermail/boatanchor ... 31942.html

Further info here:
http://faculty.frostburg.edu/phys/latta ... 6146b.html

Quote .. "the 6146B is not always a drop in replacement for the 6146/6146A. In some cases the slight difference between the interelectrode capacitances in the 6146/6146A and 6146B causes problems with neutralization. Some transmitters originally designed for the 6146/6146A do not have sufficient range on their neutralizing capacitors to be properly neutralized when using the 6146B. These transmitters must have their neutralizing circuits modified to obtain proper neutralization.

There is also a 6146W, which is a military version of the 6146/6146A or 6146B. The trouble is that some 6146Ws are 6146/6146As, and some are 6146Bs! If you are going to use a 6146W in place of a 6146B, be sure the 6146W your are using is rated for service as a 6146B. If you are going to use a 6146W to replace a 6146/6146A, then be aware that it might be a 6146B and you might have the neutralization problem mentioned in the previous paragraph."

None of the above applies in your case, as you are not transmitting, just powering speakers. Just some background on the tubes and how they differ. It seems the 6146W would be a good choice price-wise and a rugged tube to boot.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


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 Post subject: Re: Searching for info on RCA MI-12158 70 Watt PA Amp
PostPosted: Mar Fri 09, 2018 11:16 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 8103
Location: Beaver Falls, PA. USA
The field mod was for the MI-12182 amplifier. RCA also made a bigger amp for drive-in theatre use, which had four 6146 tubes in parallel push-pull; there was a 6550 conversion for that one as well. I'll have to dig through my RCA drawings to see if I have anything on the MI-12158.

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Tim KA3JRT


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 Post subject: Re: Searching for info on RCA MI-12158 70 Watt PA Amp
PostPosted: Mar Sun 11, 2018 3:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4986
Location: Rochester NY USA
From what I understand, the MI12158 is the same as the MI12182 with added preamp stages. Here is a link to the MI12182 schematic that was posted at AudioKarma: http://www.audiokarma.org/forums/index. ... df.457978/


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 Post subject: Re: Searching for info on RCA MI-12158 70 Watt PA Amp
PostPosted: Mar Tue 20, 2018 2:04 pm 
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Joined: Apr Mon 24, 2006 3:01 pm
Posts: 641
Location: Lombard, PA
Thanks everyone! Just to close the loop, tom is right that the schematic for the amp part is online. I just did a search for MI12182 and then clicked on "images." It's there is high def.


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