View Full Version : help identifying Conn organ power amp
06-08-2007, 08:16 PM
hey everyone, first post here. I found a conn organ demolished on the side of the road (i think it fell out of someone's truck) anyhow i salvaged the power amp from it. I am in the process of re-capping it but i need a schematic for it. I can't figure out what model organ it is from to look for a schematic. Reason being it has this weird fan thing that looks like a variable capacitor that controls the volume and another white ceramic square inside that has a screw- you tighten it and in increases gain. It's several mica and metal discs. I'd like to figure out how to wire this stuff all up. Right now i substituted a pot for the variable cap and it works ok but i'd like to have it as original as possible. </P>
Here is a picture- 2x6L6 cathode biased, 12au7 and 12at7. It has an 0A3 voltage regulator and 5U4G rec tube. Below is a picture, i'm guessing it's about 20-30 watts since it runs pretty clean even with (2) 12ax7s.</P>
any help identifying or directions to a schematic for this is greatly appreciated it! It sounds really nice.</P>
06-08-2007, 09:08 PM
It would be a help if you could describe the rest of the organ, such as may have survived. </P>
Did it have banks of 12au7 tubes above the amp, or were there transistor circuit boards ?</P>
Was there a single 12" speaker, or several -perhapsincluding a 5-6" in a small housing down low surrounding awide black pedal.</P>
Could you tell if the keyboards were 37-44 notes staggard, or 61 notes parallel.</P>
Was the front below the keyboards covered with grill cloth from side to side. or was there a single round speaker hole.</P>
Was there a brand name still visible between the keyboards that said CONN, or did it say CONSONATTA.</P>
06-09-2007, 12:39 AM
That amp is almost surely out of one of the very earliest Conn organs, built in the 50's. Possibly one of the 700 or 800 series, or maybe even older, perhaps one of the "Connsonata" models or model 1A (I think that was the designation??). The variable cap for a volume control marks it as really ancient.</P>
There may be a number stamped on the chassis somewhere in reddish-brown or black ink. That would be the Conn assembly number and would tell you exactly what it is.</P>
Anyway, you might contact Organ Service Corp in Marengo, INdiana. They have the service manuals and might even be able to positively ID it over the phone.</P>
It looks very clean. Must have been well cared for.</P>
06-09-2007, 04:54 PM
I found the amp in about 2002 or so. There wasn't much left of it, like someone had come and picked up what they could and left this and a couple other small pieces. Thankfully they left this part! Trust me, if all the other tubes had been there i would have nabbed them!</p>
This isn't a picture of my amp, I found this one for sale on Ebay so i nabbed the pic for future reference. Mine isn't as sparkling clean as this but it is in pretty darn good condition. The seller told me it was from the 700 series so i bought a manual for one but the amp isn't in there. A couple of them are close but they use 6X4 and no variable cap. So it's basically missing the only section i need. </p>
This thing sounds awesome on guitar. There is one more punch out for another preamp tube, so i am making a switchable gain stage with it. There's an antique audio place by me that probably has the right tube socket, a little brown and BRITTLE phenolic one. I have a parts amp from ebay that i bought for $15 and i tried to drill out the rivets for both of those sockets but they shattered. I plan on putting a switchable gain stage in there to get some more fuzz out of it.
As for the number- you can see on the above pic that there is a label on the front showing the tube layout. There is a number printed on it that says 56665 (blank line)-7. The 7 is hand stamped. On the edge there is a black stamp, 95009. There is also a black round stamp on the top about the size of a nickel but it is smeared, probably from the day they stamped it. It's almost illegible, there is another one on the inside that i can barely make out "inspector" so i don't think it has any actual amp information.</p>
I will look that shop up and give them a call. Thanks for the help guys!
06-09-2007, 04:57 PM
Oh i forgot to ask.... how does that variable cap work? And why would they use that vs a pot? I have a pot hooked in now and it works fine. Also what's the function of that small square mica thingie?
06-09-2007, 10:27 PM
The variable cap works as a volume control by simply being in series between the input and the pre-amp circuit. When the VC is turned one way so that the plates mesh only slightly, very little signal is passed. As the control is turned so that the plates mesh more and more, the capacitance increases hugely and therefore more signal is passed. Technically, in an ordinary circuit, a VC like this would pass high frequencies much more than lows, and the lows would become more prominent as the capacitance increased. However, in this circuit the capacitance is so small thatthe actual resonant frequency is far above the audible range, so the roll-off of the lows is not noticeable. If youfind the schematic, you may even see a resistor shunted across the VC, which would have served to pass somesignal even when the VC is in the open position. I actually don't remember that, but if I were designing it I'd probably put one in for that purpose.</P>
They used this device rather wisely, as pots are known to deteriorate badly in organ expression shoes. The constant movement (when many players keep a foot on the expression pedal) quickly wears the pot. This VC has nothing to wear out except the air between the plates! So it was designed to last forever, quite a nice thought on the part of the builders. Hammond used the same thing on some of the classic tonewheel models.</P>
The little mica thingie is probablysome kind of a variable capacitor, perhaps used to trim the signal in some way. I don't have the schematic at hand, but I have seen such things in older amps and other stuff.</P>
You might be able to draw out that schematic if you take the time. It isn'tvery complicated. Just draw in your tubes, put in the socket pins, start with one tube and show all resistors, caps, etc. It is probably very similar to the amp you see in the 700 manual with the exception of the input stage and that VC. You could start by drawing out just the input stage and go from there.</P>
06-10-2007, 02:11 AM
As a completely amateur/novice technician I was just wondering ... why would you need to replace any caps in this power supply other than electrolytics or those old paper caps (if it has any)? I thought the other types remain failry stable over the years. With the few organs I was involved in restoring I only replaced the electrolytics. Thanks for any advice.</P>
06-13-2007, 07:30 PM
awesome john, thanks so much! Yep, there is a resistor shunted across the VC. From your description i can figure out how to hook it back up, basically in parallel w/ the resistor. It's an interesting control, i'll have to see if i can employ it in something else i build. </p>
That small mica thing is indeed meant for a trim control, it has a tiny set screw in the face of it. I think i'll adjust it to taste and "set it and forget it".</p>
Thanks a ton, this has really helped a lot!</p>
As for the caps, in my amp i think they are all electrolytic. There's one plastic coupling cap that looks like it is from the 60s or so but it's still way over the 10 year rule of thumb for replacement. Film ceramic, and dipped mica caps are the only kind that don't dry out over time but high heat can damage them. Amps that the tubes hang under the chassis are especially prone to this, like old ampeg amps. Improper ventilation or amps left on for long periods of time can create enough heat to do so. So far i have replaced the filter caps and the motorboating has decreased significantly, but i still need to do the cathode bypass and coupling caps as well. That should take care of the rest of it.
01-14-2010, 07:06 PM
I just removed an identical amplifier from a model 514 spinit Conn Organ. *The variable capacitor volume control is defective. *I need one to repair this amp for the church organist who uses this as a practice organ.</P>
Anyone have a spare variable capacitor?</P>
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