vintage RCA amplifiers MI-12172-A (pics)

[PRR]

> they all have different gains. #1: -13 dB #2: -17 dB #2: -29 dB What sort of variation would they have considered healthy back in the day?

Oh, gosh. +/-3dB might have been allowed, but anything much over +/-1dB means something aint right. You used the same tranny and tube? Then check all the DC voltages. Not sure what is right, since you don't have even two the same. Also power-down and wait overnight for caps to bleed, check all the resistors.

I assume the gain control was set the same.....

> would I go after similar parts (carbon resistors?) or simply get the cheapest modern equivalent?

These are frankly low-end amps and RCA did not obsess about their "resistor quality".

I use carbon film (cheapest) for about anything. Nothing wrong with them. Some guitarists do favor carbon composition (what you probably have), but they are obsolete so you have to hunt down old stock. Some audiophiles favor exotic materials, with some justification. Wire-wound (mostly obsolete) would be contemporary with these amps, though too costly for general use (a classy mixer might have wire-wound for the first plate resistor, maybe the feedback network). Metal film is too-new, not-cheap, but good stuff. And then there are the exotics, which make sense for like a Jensen-tradition 990 mike amp but seems like lily-gilding for these amps.

See if Radio Shack has your value (it will be carbon-film) so you can fix the busted resistor. Figure out your gain variations before you worry about resistor types. Whatever affects gain that much has to be affecting the "sound". And don't apologize for carbon-film unless you are charging more than $1000 for the box. Everything you hear on records has been through scads of carbon-film resistors; a few more can't and doesn't hurt.

 

[Mailliw]

Hi PRR,

I gutted the low-gain amplifier and discovered it had a 180K resistor on the output, hidden under that insulation/shrink tubing.

Turns out 4 of the 6 amplifiers have it. Of the 3 I recapped for testing, 1 of them had the resistor - the low gain amplifier.

Since I took it apart before realizing, I can't test to see if this caused all that gain loss. The pot seems good though.

I don't understand why those resistors are there, don't they just steal a good portion of the output voltage? My only guess is that at some point, they had the 3-position switch (included in that pot assembly) hooked up as a rotary-switch type gain control.

Thanks

 

[PRR]

> discovered it had a 180K resistor on the output, hidden under that insulation/shrink tubing.

Like this?

I was wondering what they did for a mix-network. I was hoping it was in the harness or a missing module. It is a little odd to find it inside the preamp module, because that eliminates the possibility of sub-mixes, effects loops, etc. (However a basic mixer like this never dreams of anything so fancy.)

It is "possible" to mix channels by shorting all the plates together, but that tends to increase distortion. Also the 0.1uFd output cap suggests >100K total circuit resistance, and the 12AY7 plate is only ~20K. Strapping several such plates directly would give bass-cut around 100Hz, OK for speech but PA preamps usually did better and let the master module or power amp apply any bass-cut needed to keep the speakers from slapping.

Cut it out or short it with a wire. For individual channel duty, you don't want it, and indeed it "steals a good portion" when feeding modern ~10K inputs.

 

[Mailliw]

Hey PRR,

I wired up an amplifier with new caps and resistors... doesn't work off the bat (very little gain), so I'm going to have to troubleshoot it. I have a few questions:

I got all 1/4W resistors except for the 51K B+ resistor for which I got 1/2W because its carbon comp counterpart was bigger than all the rest. I sort of guessed at that, I hope thats fine. Resistors are metal film 1%.

I ordered a few WIMA polypropylene caps just as a test to see if I can tell the difference between the old paper/oil, the polyesters I originally ordered, and these propylenes.

The WIMA propylenes are rectangular with plastic cases and have very short leads, I guess they are more suited for pc mounting? Live and learn...

For wire, I've been using this radioshack stuff, I started with 18 gauge but sometimes the holes aren't big enough, so I use the next smallest I have, which is 22 guage (left over from my cmoy amp).

I know they used something thicker than 22 in the original amps, is 22 too small? The resistor leads look pretty close to 22, so I'm reassured bv that.

I also use a big 140/100 Watt Weller for a lot of my soldering, its so much quicker! But in the back of my head I'm a little worried its too much for some of these components?

Thanks

 

[PRR]

> is 22 too small?

No.

Your resistor sizes sound fine.

> suited for pc mounting?

That's how everybody does it these days. So you get some wire-scrap and extend the leads. For this job, any wire stiff enough to support the cap mechanically is plenty-big electrically.

> I also use a big 140/100 Watt Weller

Oh, gosh. Swatting flies with a sledgehammer.

Yeah, these amps can be handled a bit brutally, though with the big nose on a Weller down in that box you can fry parts other than the one you are soldering. I'd really prefer to use a 35W iron. (Not those dinky 12W things they sell these days for microcircuits.) My 37W and 45W will do work like this as fast as a Weller 140W (faster counting Weller warm-up), with a lot less wrist-strain. The 45W needs practice to get in and out before things char; 35W is a safer size.

 

[Mailliw]

Hi PRR,

Now I have 3 working variants:

^ I put in some radioshack pots and it freed up a lot of space, nice and tidy.

#1: Has WIMA polypropylene caps in place of the old paper-oil .1uFs

#2: Has metal film resistors in place of the carbon comps and polysesters in place of the paper-oil

#3: Untouched (except for the electrolytic replacements)

I did another gain test with a 1 kHz tone (used a neat test tone generator computer program) and,

#1: - 23 dB
#2: - 24 dB
#3: - 27 dB

The two "more worked on" amplifiers are only off by 1 dB, thats a lot better!

I ordered some xlr male connectors for the output of the amplifiers, but when I hooked them upto my mixer's mic xlr input, I was getting a lot less gain than previously with the 1/4" going into the line input. I looked up the input impedances for the line and mic inputs on my soundcraft mixer and the line (1/4") is 11k and the mic (xlr) is 2.4k, so that explains it, I went back to 1/4" connectors on the output of the amplifiers.

Another thing thats bothering me is how the waveform is very unsymmetrical in adobe audition:

(Thats singing)

Does that fall under "tube imprecision"?

As always, thanks for your time!

 

[NewYorkDave]

The human voice (especially the male voice) has an asymmetrical waveform. Do a search on "phase rotator" to learn how broadcasters have tried to tackle that issue. How does the preamp do with a sine wave?

 

[ToobieSnack]

hey i saw this on ebay and thought it might be of some help ...
maybe this guy can offer some info ...
i have found ebay to be a great source for info as well as a good lil vintage shopping place .. :grin:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7336361129&rd=1&sspagename=STRK%3AMEWA%3AIT&rd=1

anyway hope this helps

later
ts

 

[Mailliw]

Thanks toobie!

Thats hilarious, 1000 bones...

I also see that power supply has the GND and B- connected right there, just like me

 

[Mailliw]

Back at it...

I tried changing the components PRR suggested for greater output, but the gain went down a little!

I didn't have any 2uF caps, so I series'd two 4.7uFs left over from my G9 project, that gave me ~2.4uF.

Is it possible to connect two of these together for more gain? If I bypass the input transformer on the second module and go into the next tube?

If I could get 15 more dB I'd be laughing. They'd be great for recording vocals and guitar, they sound really nice.

Thanks

 

[NewYorkDave]

Without reviewing the whole thread... Did you remove that series resistor on the output, and did you try bypassing the second stage cathode? Try them in that order, if you haven't already.

 

[PRR]

> the gain went down a little!

It is unclear if you want more gain or more maximum output. Or both.

> I didn't have any 2uF caps, so I series'd two 4.7uFs

The 4.7s would have been fine. 2uFd is just the minumum here.

> If I could get 15 more dB I'd be laughing.

Cut this wire and see how you like it. (Be prepared to put it back in case you don't laugh.)

I'm having trouble opening page 2 of this thread, which has the key schematics.

 

[PRR]

And as NYD says: any series mixing resistor should be gone, and you can put 22uFd-100uFd 16V on the output stage cathode (though that only gives ~5dB more gain, and increases THD a bit).

 

[ciminosound]

What's up with page 2 of this thread? Still not openning? :?