• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Can you identify this tube amplifier?

S

sardox

New Member
12ax7 / 6bq5 (7189?) stereo push-pull tube amplifier / console pull.
Seems to have a half-wave solid state rectifier (two diodes).
OPTs are labelled 1221 and 9496237.
RCA and wired speaker outputs (black/red wires).
6 pin molex on chassis, and 4-pin molex on harness.
Auxillary power outlet and remote pilot lamp.
That's about all I know...
This was an eBay find, and the price was right.
It's in great shape, everything looks as I imagine it
did when the console it came from was built.
I'd love to know what it is, and where I can find
a schematic and/or tips for converting it to a
standalone or integrated amplifier.
Any and all help is greatly appreciated!
Behind it is a Magnavox 8605 SE amp for scale.
Thanks!
Todd in Warren, Michigan

pic1.jpg pic2.jpg pic3.jpg pic4.jpg
 
Register to hide this ad
Looks exactly like this one:

http://audiokarma.org/forums/index....tube-amp-and-speakers-id.691289/#post-9237013

B+ is a voltage doubler rig.

Its believed to be a Madison-Fielding amplifier. There is a schematic I drew up in there too. Output transformers on the one sitting here at least are 2.5 ohm output. If you can post your output transformer numbers, I can confirm if they match these. I suspect M-F didn't actually make these things. Most probably it was an OEM builder that cranked them out under multiple brand names.

I didn't mark it up for stand-alone conversion, but putting a power switch in is dead simple. The switch would go between the yellow and the blue wires on the molex connector. Just confirm your wire colors are the same as the ones in this amp. One will connect to a transformer lead, the other will connect to a power cord lead.

If you need a better scan of that, PM me with an email address and I'll send it over. Its hand-drawn and my writing sucks, but its somewhat more legible in a larger size.
 
When it rains, it pours...

got one of those recently too. sounds great. I highly recommend it.

no manufacturing name on mine either....

IMG_1344.jpg
 
Gadget,
The unit is similar to the one you have, but there are differences.
To complete the power path, I need to tie together the two yellow
wires. There is no blue wire on this one. The rest of the wires on
the molex connectors seem to be redundant, probably for a preamp
unit or tuner, power/ground/6.3V/etc.
Otherwise, the schematic you posted is darn close, including the
strange capacitors between pins 1 and 7 of the 12AX7. Any idea
what those are about?
Oh, and the output transformers are numbered 1221 and 9496237.
I haven't put an ohmmeter on them yet, but I'd be interesting in seeing
if they match the ones you or Leland have on yours. I really want to
power this thing up, but I'll hold out until I can source a variac...
I plan to install a power switch, fuse, three prong grounded cord,
internal pilot lamp, and a 100K stereo volume pot to it. Might even
install a NFB in/out switch on it like I did with my 9302 Maggy.
I can't believe I found this thing on eBay for 38 bucks, shipped!
I'm completely geeked to get started with it. Thanks for all the
help/insight!
 
The 100pf cap to ground off pin 1 ? Thats an HF stability cap. Probably necessary to keep it from oscillating or something.

These transformers are numbered TO-2003, and the manufacturer string is 9263137. Hopefully yours turn out to be a more useful ratio than these are.

Not much surprise that your plug is a little different. Looks like you have less pins. Probably it was variable to some extent depending on the options. This one I believe did power a tuner. There is another jack on the side that powered the multiplex adapter. I forgot to draw that part.

I'd have bought it for 38 bucks just for the tubes.
 
Hi Sardox, welcome to AK!

My OPTs are: 1221, 9496145. They look to be the same as yours...

They look rather beefy to me....for console transformers..

Great deal on yours. It seems, sometimes it is cheaper to get these off that auction site....than driving out to look at an actual console. But only sometimes...
 
Last edited:
Gadget, there's a .05uF cap between pins 1 and 7 of both the preamp tubes.
There are four coupling capacitors that are also .05uF between the standoff connections
and the preamp sockets, that lead to the power tubes.
The caps in question are on your schematic between the AF amp and the phase splitter
portions of the 12AX7. I don't have those on my Magnavox P-P amps, so I'm wondering
their function, and if they should stay, be cut out, or replaced with wire...

I'm kind of unsure on how to test the OPTs. I get .6 Ohms on the speaker side of each
of them, and on the primary side, i get around 220 Ohms between the red wires, and
454 Ohms between the red wire on the one EL84 socket and the black wire on the other EL84 socket.
Between the red wire on the HV side and the black wire, I get 233 Ohms. How does one
determine the ratio?

Leland, they do have beefy OPTs, a lot larger than the ones on my 9302, for sure!
 
close.jpg
I just snapped a tighter photo of one of the 12AX7 sockets so you can see what I'm
referring to. The cap is connected to pins 1 and 7 of the socket and the leads are
shielded with black tube. The other two caps are the coupling caps. All are
Standard brand .05uF and 400V.
 
Oh, that one. Thats the audio coupling cap from the voltage amp stage into the phase inverter. Without it, you'd have no sound. This uses a concertina, cathodyne, or split-load inverter depending which name you like. One output tube drives from the plate, one drives from the cathode. The Magnavox uses a paraphase inverter. On that, both output tubes drive from a plate, but there is a connection from one output tube's grid back to the driver tube via a resistor divider network to get the inverted phase.


working out the impedance requires some voltage testing and a little math

http://www.radioremembered.org/outimp.htm

Nutshell version, you apply voltage to the input and measure voltage on the output. Divide input volts by output to get the ratio. Once you have the ratio, take that number, square it, and multiply by the speaker load connected. For a 6bq5, you usually want to end up somewhere between 7000 and 10,000 ohms, with the 7500-8500 ohm range usually being the happy spot.
 
It looks a lot like the amp i removed from what I believe was a Faircrest or crestfield (not sure but close) console. It had dual tuners for am/fm and would mix them for stereo, also had mpx i believe. Was pretty cheesy built so I pulled the amp. I will get a pic tomorrow.
 
would be interesting to see what brands these were sold under. I'd really like to know who made the thing in the first place. Its not badly built. I half suspect a Wells-Gardner or a Belmont or someone like that.
 
Tonecrest. That was it. here is one but the amp pics are missing.

http://audiokarma.org/forums/index.php?threads/new-to-me-tonecrest-console-questions.374539/

NOT MY PICS, but this is the amp. So thats where the "Ultra tone" quad came from.

35k1z5d.jpg
2hckew4.jpg
2r3fh8j.jpg
9gwqk2.jpg
11ahj84.jpg
11jyu6a.jpg
2iqdd88.jpg
 
Interesting that its ever so slightly different. The one here has 4 input jacks that are wired in pairs. No idea why, but the setup is L,R,L,R with the two lefts and two rights just tied straight together. This one also has no external 120v socket on the side, but it has an external power feed to the multiplex adapter.

Looks good though. Anyone know what store sold "Tonecrest" brand stuff ? JC Penny maybe?
 
The ones on the side look like outputs, but on the one here I have outputs on the side and 4 inputs on the end, plus there are wires coming out for speaker outputs. Its got more connections on it than you can shake a stick at.
 
vrusso123 said:
What, no atta boy?
Click to expand...

LoL....

Mine had the UltraTone labeled tubes in it also. They are all Mullards....:)

I've put them aside...for a rainy day. What Brand of tubes did yours have Sardox?

Yes, I agree...those RCA jacks on the side are extra speaker outputs.

IMG_1352.JPG
 
Last edited:
well, seems reasonably solid that everyone else's version of this amp has 12ax7's. The one here showed up with Bugle Boy 12AU7's. Doesn't seem like that makes sense though, so I'm thinking somone changed them. It also had Amperex 7189 tubes, and the voltages and whatnot simply don't justify those. I'm thinking someone re-tubed it somewhere along the way.

Did anyone else get the multiplex adapter with it? I have the one from this and it also has a 12au7 and I think a 6C4, some small single-triode tube. I haven't looked at it beyond that, but most MPX units seem to use 12ax7 or 12at7 tubes. Just curious what others might have.
 
You must log in or register to reply here.
Back
Top Bottom