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Hi folks...I am shooting in the dark here, I have posted a similar request to a couple of other forums, but unfortunately all I hear is crickets. Must be a real stumper. So I came across this site during one of my forays into the dark recesses of Google's servers and I thought I would give it another shot.
I was recently given an old Hammond Organ Amp chassis without tubes that I intend to turn into a guitar amp with my son. The chassis itself looks very similar to an A100 AO-39 Power amp, same size and for the most part layout, however the circuit is completely different and is set up for octal output tubes that are cathode biased, instead of the 6BQ5's. I drew a schematic which I have posted here:
and a photo of the circuit side of the box here:
I have gone through every schematic that I can find on the net and I haven't really even come close. The best I can do is think that it could possibly be from maybe a Leslie tone cabinet, but even any of those are not the same. I have several questions that I really need to try and determine before I move forward, so it's kind of a shot in the dark that some of you may have seen one of these before. The date codes on the Transformers are 1960 and the pot is 1962, so obviously early 60's.
1) Is there a way to determine the PT and OT specifications from the old Hammond part numbers? Those p/n's are on the schematic above. I don't think so, but its worth trying.
2) Is there a way to determine the tube lineup? I assume its either 6V6 or 6L6 on the power and probably 12AX7/12AU7 on the PI. Also, any idea about the rectifier? I assume there its either a 5AR4 or 5U6 (does this really matter?). I have no way of knowing what the OT specs look like, so this is probably not gonna happen either.
3) Also, look at the power filtering on the schematic - it seems like a lot of filtering with the two pairs 100uF/300V Cap + 100k/2W tied together on either side of the 2K10W dropping resistor. Was this purely to get the capacitors up to 600V's at 50uF? This makes me wonder why so much filtering is required unless the plates are looking at some pretty high voltages which would imply maybe more power needed, leading to maybe the 6L6 scenario. This would give me an idea about how the PT can handle higher voltage requirements...I don't want to over juice the PT. What about figuring out the filament current output capabilities of the PT? Trial and error?
Along the lines of that, I marked both the schematic and the photo with letters that correspond to each other so it is easier to make out what is going on with the H+ power circuit side.
The circuit itself looks eerily similar to a Fender Tweed circuit with the cathode bias circuit, but with a NFB loop.
Key point here: I have calculated the OT turns ratio N=64, which gives me an impedance ratio of ~4000:1. With that, I can assume 6L6's are most likely out, and it is probably looking for 6V6's feeding a 2ohm speaker load. I don't have any speakers or cabs set up for a 2 ohm load...I guess 4x12's (at 8ohm ea.) all in parallel will get me there, but what I don't want to happen is to hook it up and have it be too low of a load, resulting in a fried OT.
Question for the experts: Were there many amps made back in the early 60's that were using 2ohm loads in push-pull with 6v6 or 6L6 tubes? That just seems awfully low and right on the cusp of being a short circuit.
Another consideration is I don't want to trash this thing by converting to a guitar amp if it is as rare as it seems and may actually be useful back inside an organ for someone that needs it. But if it isn't valuable, I think it will be a good project for my son to turn it into a tweed clone, he likes helping me work on amps and this would be something for him to work on over the summer.
So, if you can offer any thoughts - anything at all - I (and he) would be really grateful. I am just coming up with a big goose egg trying to find any information on this particular amp. If all else fails, I can start with the trial and error plan, but I prefer to know what I am dealing with if at all possible.
Thanks in advance!
Craig C
I was recently given an old Hammond Organ Amp chassis without tubes that I intend to turn into a guitar amp with my son. The chassis itself looks very similar to an A100 AO-39 Power amp, same size and for the most part layout, however the circuit is completely different and is set up for octal output tubes that are cathode biased, instead of the 6BQ5's. I drew a schematic which I have posted here:
and a photo of the circuit side of the box here:
I have gone through every schematic that I can find on the net and I haven't really even come close. The best I can do is think that it could possibly be from maybe a Leslie tone cabinet, but even any of those are not the same. I have several questions that I really need to try and determine before I move forward, so it's kind of a shot in the dark that some of you may have seen one of these before. The date codes on the Transformers are 1960 and the pot is 1962, so obviously early 60's.
1) Is there a way to determine the PT and OT specifications from the old Hammond part numbers? Those p/n's are on the schematic above. I don't think so, but its worth trying.
2) Is there a way to determine the tube lineup? I assume its either 6V6 or 6L6 on the power and probably 12AX7/12AU7 on the PI. Also, any idea about the rectifier? I assume there its either a 5AR4 or 5U6 (does this really matter?). I have no way of knowing what the OT specs look like, so this is probably not gonna happen either.
3) Also, look at the power filtering on the schematic - it seems like a lot of filtering with the two pairs 100uF/300V Cap + 100k/2W tied together on either side of the 2K10W dropping resistor. Was this purely to get the capacitors up to 600V's at 50uF? This makes me wonder why so much filtering is required unless the plates are looking at some pretty high voltages which would imply maybe more power needed, leading to maybe the 6L6 scenario. This would give me an idea about how the PT can handle higher voltage requirements...I don't want to over juice the PT. What about figuring out the filament current output capabilities of the PT? Trial and error?
Along the lines of that, I marked both the schematic and the photo with letters that correspond to each other so it is easier to make out what is going on with the H+ power circuit side.
The circuit itself looks eerily similar to a Fender Tweed circuit with the cathode bias circuit, but with a NFB loop.
Key point here: I have calculated the OT turns ratio N=64, which gives me an impedance ratio of ~4000:1. With that, I can assume 6L6's are most likely out, and it is probably looking for 6V6's feeding a 2ohm speaker load. I don't have any speakers or cabs set up for a 2 ohm load...I guess 4x12's (at 8ohm ea.) all in parallel will get me there, but what I don't want to happen is to hook it up and have it be too low of a load, resulting in a fried OT.
Question for the experts: Were there many amps made back in the early 60's that were using 2ohm loads in push-pull with 6v6 or 6L6 tubes? That just seems awfully low and right on the cusp of being a short circuit.
Another consideration is I don't want to trash this thing by converting to a guitar amp if it is as rare as it seems and may actually be useful back inside an organ for someone that needs it. But if it isn't valuable, I think it will be a good project for my son to turn it into a tweed clone, he likes helping me work on amps and this would be something for him to work on over the summer.
So, if you can offer any thoughts - anything at all - I (and he) would be really grateful. I am just coming up with a big goose egg trying to find any information on this particular amp. If all else fails, I can start with the trial and error plan, but I prefer to know what I am dealing with if at all possible.
Thanks in advance!
Craig C
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