View Full Version : First leslie, model 145....but have a amp with dual preamp tubes.?
08-28-2011, 03:47 PM
My son is a teenage keyboardist, and yesterday we came home with his first Leslie cabinet, a 145, dated coded from June 1967, bought with his summer job earnings.
We cleaned it up a lot, and I had the amp out to get the years of dust and grime off of it.
I had looked at the pictures of other 145s ( or 147s) and noticed that they all have (1) 12au7 preamp tube. but this one has the knockout removed and a second 12au7 there, with a short bottom shield.Its still labeled a 147.....in fact there is a phono jack where the fuse holder normally is and the fuse is moved up on the label where the tube chart is. We already have put a line out on Sonny's M3....but I'm not sure how to utilize the "guitar Jack", since it can't power the leslie.....
Can anyone tell me about this modification ? Was it done to make it a 122 amp? ( I notice they have 2 preamp tubes....)
Perhaps its a common mod, but I can't find any internet references to it.....
Whats up with that? Any Ideas?
08-28-2011, 03:58 PM
Welcome to the forum. Unless you post some pictures we have no idea of what modifications have been made as that 'vision' is only held by the person doing the mod and therefore cannot give any safe, sound advice. We can visualize the outside but it's the wiring underside that's most important. Guessing is not a good idea on these.
08-28-2011, 05:01 PM
The 3 terminal phono jack is wired with the tip to pin (2? or 8, not sure which way to count.....)) of the "additional" 12au7, and the other (sleeve, wired to both remaining jack solder points) goes to a 4 position fiber terminal strip ( 2nd from bottom) just near the front side of the 2 electrolytic capacitors, where the is also a termination of a turquoise wire from the choke. ( sorry for the color choice, but there are green wires here and blue wires, but this is neither of those....)
That extra 12au7 socket is very busy, with 5 resistors and 2 capacitors wired to it.....
08-28-2011, 06:40 PM
That's better but it's still hard to tell. Back out a little with your camera and show an overall picture from the edge to the transformers inside edge. With the added small transformer on top I get the idea it may have been converted to a model 251 with the added input for another keyboard, etc, except I don't see any output tubes for that type. It may just be some sort of additional input for a special effect. The safest thing you can do is convert it back to a stock 147 amp unless you totally understand what the mod is, which we don't so far. This is still just a guess at this point and hard without having it in my hands. Show where the wires from the 6 pin connector go, especially pin 5. It's a little dark around the phone jack. Blow all that dirt and dust from the wiring.
08-29-2011, 06:06 PM
I've heard this work before we loaded it up to bring it home ( with a Porta-B) so I won't be taking it back to stock anytime soon.....it's delayed gratification enough to not have a hookup for it to play with either the M3 ( which has a line out installed, or Sonny's other keyboard ) while we hunt for a reasonable combo preamp pedal......
There seem to be a couple of things here- the 9 pin 12au7 # 2 installed in the knockout spot......now, I'm assuming its a 12au7 tube, but the markings are all rubbed off......
- A set of wire terminals screwed into the side , hosting a pair of yellow electrolytic caps between them ( Aerovox , (1) 8 microfarad, 450 vdc (top one in pics,)
and (1) 12 uf 450 vdc on the bottom. there are several resistors (1) back (1 M) , (1) front 3.3 K, (1) between back and front , 220K.
-The 3rd small transformer you mentioned above ( 2 yellow wires from it, (1) to multi-section can, and the other to bridged both 6550 pin # 4.
Here's my best shot at the backside connections to the 6 pin amphenol socket :
1. Black wire to load resistor switch, Burgandy wire to 10K volume pot, then on to 9 pin 12au7 #1, bridged to pin #5, then to ground cluster.
2.Violet to relay , 1 end of the large green Cornell 1 MFD cap
3.Blue ( to lower fast/slow plugs)
4. (2) orange wires , 1 to relay, 1 to fuse terminal now on topside....
5. Red to relay, other end of green Cornell 1 MFD cap
6. Green to 10K volume pot
I can also list connections to the 12au7 #2 if that's helpful to anybody......
08-29-2011, 09:11 PM
It appears that the 6 pin amphenol has not been changed so it should work with the regular 147 hookup (6W). That was my main concern, that it had not been converted to 122 (6H) type or 251 type. You don't need a combo preamp for the M3 unless you are using another keyboard also. The standard hookup would use a 26-1 box or it can be wired directly by skipping the box and just using another 6 pin connector with wires to the various locations if you don't need the internal organ speaker. A simple hookup can be made with the organ speaker either on or off and just switch for the Leslie motor control.
It simply appears the added things are some kind of additional input maybe to raise the input level for other type instruments otherwise simply the jack directly to the 1st tube will work. Seems whoever did it did good work. We may never know exactly unless you can find who did the work. It's unique! I've bought a few oddballs myself. You give good detailed descriptions.
08-29-2011, 11:03 PM
Admiral isn't that a "output" xformer that looks added to the top?
09-09-2011, 12:14 PM
I still haven't solved this puzzle, as the leslie is still being re-habbed while I wait for the hardware to build out a 26-1 connector box, but I have looked closer at the second "preamp" tube and see that it is a Realistic (only partial lettering left, but it starts "Real") 6U8A. I also see upon googling that it is a substitution for a 7199, which I'm familiar with as a preamp tube in a Dynaco ST-70 stereo amp. Anyway, I'm kinda hopeful that this is a built in preamp for the 1/4 " phono jack, and will support a second keyboard in the setup.
09-10-2011, 11:31 PM
I also see upon googling that it is a substitution for a 7199, which I'm familiar with as a preamp tube in a Dynaco ST-70 stereo amp. Anyway, I'm kinda hopeful that this is a built in preamp for the 1/4 " phono jack, and will support a second keyboard in the setup.
A line level preamp was my guess too. Is one of the grids on the 7199 tube connected, in any way, to the 1/4 jack? (pin 9, 7, 6 or 3)
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