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RCA PA MI-38103
RCA PA MI-38103
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Old 29th December 2017, 10:36 PM   #1
famousmockingbird is offline famousmockingbird  United States
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Default RCA PA MI-38103

I picked up this PA amplifier and it seems nicely made. Completely stock with all original RCA tubes including the metal base 7027a's and 7025's. Puts out the 35 watts (25Vpeak sine wave) cleanly with no hum. Choke input filter for the power supply. Big Transformers. There are four 1:14 nice step up transformers (RCA MI-12399) for the mic inputs that plug into octal sockets, they could be used for a nice MC SUT setup in the future I paid more than I wanted ($175) but it was in really good condition for its age and I figure all that iron, chassis, tubes and what not build something would cost me at least that. Plus the chassis and cover look super cool, I may re-purpose into a geetah amp but shhhhhh don't say them words around these parts Or I could use it as a mono amp for my workshop. Not too sure what I'll do. Maybe a vote


Does anyone have a schematic for this beast? Looks like the "phono" which I don't see any RIAA equalization goes straight to the 6AU6. The "mic" inputs go through the SUT, then to the 7025's then on to the 6AU6. After the 6UA6 it looks like the 6C7 is for the bass treble network, and the 6SL7 some sort of paraphase splitter but I will have to examine the circuit more closely if I can't find a schematic.

I was trying to figure out what my next project was going to be but I think it found me.


Thoughts? Comments?!?!
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Old 29th December 2017, 11:14 PM   #2
famousmockingbird is offline famousmockingbird  United States
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So I am still diggin around for a schematic on the ole interwebs and I found out that Jack White is using a similar amp converted for guitar use.

You can see him plugging into the phono jack at 0.11 seconds into this video. I wish I could talk to his tech to see what they did inside. YouTube

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Old 30th December 2017, 07:30 AM   #3
jazbo8 is offline jazbo8
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RCA PA MI-38103
Rumor has it that the RCA's might be just stage prop's, the real amp is a supposed to be a Vibrolux Reverb (see SNL stage setup below), then again, it's not difficult to modify them into real guitar amps - all the basic components are already in place.

Screenshot - 12_30_2017 , 9_47_05 AM.png

p.s. try searching for the SA-342, it's the same amp.
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Old 30th December 2017, 11:31 PM   #4
famousmockingbird is offline famousmockingbird  United States
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Oh shizzle I knew if I said the G word this would get moved into this section Oh well I will take it as a sign to just go ahead and convert this monstrosity into a killer guitar amp


Jazbo, I tried searching for that other model and still had no luck. My google-fu isn't up to snuff I guess. If I can't find one I will take the time to draw it out and post that way there I can get more interest and involvement from the lovely folks that lurk these waters. The closest one I could find is the MI-12155. Which I added at the bottom. I heard the rumor that these are just stage props but I don't think that's true. Jack used a Silvertone for his dry amp and a Fender for his wet reverb sound, I am thinking since I don't see the Silvertone anymore that these RCA's took their place as the 'dry' amp. Great shot of the stage btw!

I am thinking that the 'phono' input is not sensitive enough and although I haven't checked the mic stages they might be too sensitive. I may fire up the scope up tonight or tomorrow and check gain for both stages to give me some starting point.

Question: What's do you gurus think is a good sensitivity for a guitar amp? Doesn't Fender usually shoot for a 20mV input to get full output? So for a 35 watt amp that's 62db. Now I am 36yo and wasn't around back in the day to know what kind of mic's were being used with this PA amp but my hunch is saying possibly ribbon mic's, which is why they have step up transformers before the active stages. A ribbon mic, at a 100uV output the 'microphone' stage of this thing could very well be over 100db of gain Or to put it shortly way too much and my pedals would be very noisy and more feedback than Jimi Hendrix could ever want. I am thinking the "microphone" stage sans the input SUT could be what I am looking for as far as gain structure.

Side question for bonus points. I tend to usually go for very compressed low watt tube amps. My main amp (10 watts) has a relatively high power supply impedance and sags when driven hard. The push pull, cathode bias 6V6 output stage has a natural compression and is very squishy and has a really neat feel to it. It's a rock/blues/fusion amp which is mainly what I play. I have been wanting to get back into playing other styles of music that I miss like Jazz and even a little metal. I am thinking this RCA amp will be my punchy, stiff amp. You know, nice tight low end notes, articulate. I mean the choke input power supply should have good regulation and be pretty stiff. So here is my question; what cabinet and speaker combo to pair it with? My squishy 6V6 amp has a Jensen P12R alnico driver in an open back 'tweed' era cabinet. For the RCA I am thinking of a sealed back cabinet but I don't know what driver or drivers. 1x12, 2x10's, 2x12's? I hate carrying large cabinets so a single driver would be nice. I am trying to think of a guitarist's tone I am shooting for and David Gilmour kinda pops in my head. Possibly Jeff Beck territory.

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Old 31st December 2017, 12:49 AM   #5
JMFahey is offline JMFahey  Argentina
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1) the phono input is hard to drive, it was designed for a Crystal pickup cartridge which could put out up to 1V , 500mV for sure.
Even a ceramic pickup would put out 100 to 200mV.

2) For guitar use the Mic inputs but without the low impedance transformers, pull them out and jumper whatever´s necessary to go from input jack straight to first tube grid, sensitivity will be perfect.

3) I also think Jack White´s amps are props, he goes all for stage presence and attitude.
And IF they are used (which is possible), they will have anything but the original circuit inside.
Just look at the new controls labelling.

Old Silvertones *were* Guitar amplifiers from Day 1 , these are PA amps, not the same.

Of course you can plug a guitar and play as is, but sound will be somewhat crude Lo Fi.
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Old 31st December 2017, 12:51 AM   #6
Enzo is offline Enzo  United States
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Your phono input probably has no RIAA because it was meant for a crystal pickup. Notice how it has LESS amplification than the mic inputs rather than more?
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Old 31st December 2017, 02:46 AM   #7
famousmockingbird is offline famousmockingbird  United States
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Ahhhh yes crystal pickup that makes sense there is no eq.

So everyone seems to agree that the phono won't have enough gain and the mic w/ transformers has too much so bypass the transformer and it should be good to go.

It wouldn't surprise me that these are just stage props for Jack White but as noted there are custom faceplates with labeled controls which is why I think they are being used. But yes I am sure his are modded.

With a some modifications I don't see why it wouldn't be a decent guitar amp. It won't be driven into clipping, it will run clean, and if I want distortion I will use some TS9's in front.

I bet George (tubelab) has plugged a geetah into one of these things at some point.

I wonder if this cabinet will pair nicely with the head?

Jet City JCA12S 1x12 Guitar Cabinet

The price doesn't seem too bad. Anybody have any experience with these cabs?
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Old 31st December 2017, 07:04 AM   #8
jazbo8 is offline jazbo8
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Sorry, I got the model numbers mixed up... it should be SA-354!
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Old 31st December 2017, 04:20 PM   #9
PRR is offline PRR  United States
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Ribbon mikes were not used for common commercial PA. This will be dynamic mikes.

You can't "figure" the gain on some of these products. Many have WAY MORE gain than could ever be used. They can't be run with input above 5 and Master down near 2. It appears there was a "gain spec war". PA salesmen and buyers don't get gain any better than guitarists. "More is better". You have to have "more gain!" than the competition when buyers go only by spec-sheets. Adding a pentode is cheap specsmanship.

A hasty pre-coffee runthrough suggests all-up input sensitivity is 0.2mV at the grids, an astonishing 20 MICRO volts through a mike transformer. Absurd.

Convert V3 to Triode. Run Master way down or hang about 10K on the Bridging jack. That gets near 20mV at the V1V2 grids, a good starting point. The bandwidth will be fine. The James-type tone control is useful for guitar.

Detail- SA-354 biases the input grids with a large cap and an over-size resistor, cathode grounded. This gives maximum gain and a debiasing action on overload which tends to clip short speech transients non-rudely. Many PA amps did this. 1953 g-amps did, but by 1957 most everybody had gone over to cathode bias. For all-around best sound, I think I'd do cathode bias: omit the grid cap and make 2Meg 1Meg. Since you have several inputs, you can do them all different.

If you have the Mike/Mag switch, input 4 will take mag phono carts (with jack conversion). The Phono jack and knob are for ceramic, but could take your iPod etc.

Power stage self-bias. Choke-input power. When you get crazy, go fix-bias and solid-state cap input. You will get near 700VDC and can extract near twice power. Put a 8r load on the 4r tap to keep current demand reasonable. I do not know how long 7027 will stand 60+ Watts; 6550/KT88 would be safer. Even so, Vg2 is awful high. Considering China will ship you a complete 100W class D amp for less than the price of two power bottles, this is probably not a good plan.
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Old 1st January 2018, 06:41 PM   #10
famousmockingbird is offline famousmockingbird  United States
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Jazbo, you are a life saver!!! It's not a terribly complicated circuit but I was really not looking forward to drawing it out so thank a million!!!

PRR, wow so 20uV that's crazy talk and way too much gain for sure. I too was thinking of changing the grid leak bias on the 7025's to typical cathode bias. As for power, I really don't think I need much more power than 35 watts, especially with certain highly sensitive speakers. I might pair this amp with a cannabis rex so 35 watts will be very loud.

I haven't dug into it yet, still in the brainstorming process, especially with all the great idea pouring in. I also don't have a cab to play though yet but I guess I could just plug it into my combo amp speaker. I would like to get the cab I will be using and then start tweaking so I don't have to do things twice.
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