Schematic

[den1976]

  Anyone out there have a schematic for this amp mine has nothing on it and I can't find one on line anywhere. It is an RCA MI38104A. Picked it up for cheap and wanting to use it for PA and run my guitar thru the Phono section it has all original tubes RCA brands 59-38 date codes with a 6AU6 and 7025 in the preamp section a 6SL7GT not sure what this is used for, two outputs 7027A and a 5U4GB rectifier.

[stratele52]

Good luck to find it . Very little chance pictures of the amp could help but we never know.

Or do the schematic by yourself, a sketch,  many hours of pleasure. I done that very often.

[PRR]

Is it on the bottom?

[stratele52]

Is it on the bottom?

Bottom of the cab ? Maybe no cab ?

[den1976]

  Well I have read all the post and no it isn't on the bottom of the cover plate although it looks as if something had been there in the distant past color change on the plate around the edges.I did manage to find some old RCA hifi schematics that are close to the same but I am unsure about the values on the resistors. I did bake the unit at 175 for an hour upside down in my stove with the cover removed powered it up over a two day period with no output tubes in with a variac. Then ran it at 89 to 90 volts with the power tubes in and hooked up to a 12"  1967 speaker. Found the caps were pretty much shot so replaced them all. It seems to work rather well no complaints and no hum or popping did have a bit of pot noise that I cleaned up and tested them again and all is quite good. I would still like to see a full schematic if anyone ever runs across one please email me denniso@coastwidelabs.com Thanks for the feedback!

[stratele52]

IMO
This is a amp with lot of mods or it is an handmade amp from scratch amp chassis. For sure. This is nothing to do with an original old RCA hifi. 

Where it come from ?

The previous owner must used some schematic to do that.

[HotBluePlates]

It seems you've been all over looking for this schematic. Gearslutz, AudioKarma, etc.

I hate to say it, but you'll probably have to draw it out yourself. Or find a collection of old RCA schematics in some manual online.

While there appear to be replaced caps in your unit, it doesn't look completely mangled. But there's a chance that an original schematic won't match what's in yours if the previous owner got happy with the soldering iron.

[den1976]

 :icon_biggrin:I think I made it clear but the reason why it has the mods done to it is I cut out the old caps tested them and found them to be way off so I replaced them before I powered it up with the tubes and a speaker. Everything was returned to the proper place with the exception of the cans I left them on the top of the unit and found room underneath to mount them there. I also put a three wire power cord in it and grounded it to the chassis on a transformer lug under the large blue caps. The circuts have not been changed they are still as they were when I got it. The cans were 40mfd and 80mfd in one and three ten's in another. I was hoping to find someone who had a schematic so I could replace the resistors also if needed. I may not ever find one but it seems there are a few of these still out there. Thanks!

[stratele52]

Ok den1976 , now I understand more why it look so modified . New caps take so much place in the picture.

Good luck to find any schematic ; No model name , nothing. Like HotBluePlates, you have to draw by yourself . I do often myself.

[den1976]

  It is an RCA MI 38104A PA Amplifier has two mic inputs and one phono. Did some looking into the cost for the tubes on line and found they are pretty pricy about 6 bills for nos my guess is this was a well built unit and not used much not much wear on any part of it even at the input points for the mics. The knobs and paint on the exterior all look like new paid right at 1 bill including the shipping got about 2 into it right now and it works great. I have seen a couple more had been sold via ebay a couple of years ago and was hoping that someone would find one with the schematic. I am not a electronic whiz by any means just like to tinker and I know enough not to get myself hurt with high voltages. Have always been one to tear things apart and try to make them work again. My math wasn't a strong point so I never took any formal training just learned as I went along. I can read a schematic but have no idea about the technical points of this stuff. That dosen't mean I wouldn't like to learn and really appreciate all the help along the way. Thanks!

[stratele52]

Sorry de1976, you write the model on your first post  ; )


I find this , just pictures close tou your's;

www.radiomuseum.org/r/rca_pa_amplifier_mi_38103_a.html

[den1976]

Yes that is close mine only has two mic inputs and so one less tube than that but i belive it would have all the same circuts I would think. Thanks!

[HotBluePlates]

... Did some looking into the cost for the tubes on line and found they are pretty pricy about 6 bills for nos ...

Only the 7027's would be pricy, but that's only because they're hard to find. In essence, they are a 6L6GC with a different pin-out. You could rewire the socket to accept 6L6GC's.

All other tubes are cheap to dirt-cheap (you almost can't give away 6AU6's except to a radio guy, and no one wants a 6C4 which is the same as a half-12AU7). You also can use run of the mill 12AX7's instead of generally pricier NOS 7025's.

[den1976]

Well if you look on line the 7027A are 225.00 a pair the 5u4gb is 100.00 the 6sl7gt is 55.00 the 7025 is 220.00 the low one is th 6au6 at 25.00 this was for RCA brand tubes which match what are in the unit. I wasn't saying that I couldn't find something else that would work or a different brand just what was meant to be in there and is now what I have. All have date codes of 1959 the 7027a's are the metal base grey/silver ones all have the RCA radiotron ELECTRON TUBE and RCA logo on them.

[Tom_Hull]

ebay ..

check out tubes with guys that have 100%.rating
your find a good source for tubes

[HotBluePlates]

Well if you look on line the 7027A are 225.00 a pair the 5u4gb is 100.00 the 6sl7gt is 55.00 the 7025 is 220.00 the low one is th 6au6 at 25.00 ...

Ripoff prices. And don't get hung up on having RCA labels on everything; the label is no guarantee that RCA actually made the tubes.

Antique Electronic Supply will sell you NOS 7027's for $63; they don't guarantee which brand you'll get, though you can call them and request a particular brand.

Also, you can take my word or not, but by the time 7027's came out, they were essentially 6L6GC guts in a different pinout with very similar ratings (by the late-60's, there was probably no difference in what was in the two bottles).

AES sells an NOS 7025 for $44, but ask others here... you'll almost certainly not notice a performance difference between the 7025 and 12AX7 unless you use a very early 12AX7. Probably any old-production 12AX7A will give similar performance.

AES has military 6AU6's for about $4. As someone who has nearly 1,000 old production tubes, I can tell you that even this price is higher than what you could sell a 6AU6 for... like 12AU7's, no one seems to want them. Now if I could figure out how to convince audiophile folks to buy from me and pay the ridiculous prices some dealers charge for 12AU7's (and apparently the 6AU6 you found), I'd not have to work at least the rest of 2013, maybe 2014.

AES 5U4GB for about $19.

AES 6SL7GT for $24.

Note: If AES doesn't specify a brand, it will be old-production U.S. or European tubes from the 70's or earlier.

Regardless, if you're trying to buy a complete set of 1959 tubes from a particular brand, the sky's the limit on the price. That's because you're a motivated buyer with few options for competition or alternative vendors, and the seller will take advantage of that. I'd humbly recommend you seek other sources or expand your selection criteria to reasonable alternatives.

Maybe a poor example, but I once bought 20 flat-plate Telefunken ECC83's for $20... in a hardware store. The store owner was a ham enthusiast, and didn't gouge people on prices, especially if the item sold wasn't a vintage radio.

[DummyLoad]

is this it?

--DL

edit: errr... never mind! should have read the whole thread. ugh!