I have salvaged from an old Hammond Organ, an amplifier. I would like to convert it to a guitar amp. It is an AO-68-1a. It has something else that plugs into the chassis that has a 12ax7 and an ef86 tube, this 2 tube setup has its own mini chassis with a male plug that plugs into to its partner. The mini chassis has a printed mark on it that identifies it as a AO-64-1. Does anybody have any info on this set up? Thanks in advance!
I have one of these setups. What info would you like to know? I haven't gotten around to working on it yet but have done some advance work looking at the schematic. You are correct in how it goes together, the AO-64 plugs into the AO-68. The EF86 is the first gain stage and the 12AX7 is the second. It is structured as a cathoode follower and drives the signal through the expression pedal. This then goes to the main chassis and the 6FQ7 is the signal recovery for the expression pedal, so the 6FQ7 is the input for the main AO-68
If you can find a service manual, it was from the Hammond K-100 series organ. I found a copy on ebay and have tried to scan the schematic but doesn't work very well because of the center fold of the manual prevents it from fitting under the top and I'm not willing to cut it out of the manual for a better scan. I want to keep it in one piece. I'll try later for a better scan and upload the schematic then.
If you can find a service manual, it was from the Hammond K-100 series organ. I found a copy on ebay and have tried to scan the schematic but doesn't work very well because of the center fold of the manual prevents it from fitting under the top and I'm not willing to cut it out of the manual for a better scan. I want to keep it in one piece. I'll try later for a better scan and upload the schematic then.
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I am working on the ao-64/68 combination at the moment as well. Thanks a lot for the schematic scans!!
I measured the speakers of the K-100 to be 16ohm on the two 12" ones and 8ohm on the small speaker wich is connected to the 20u capacitor. My thinking was that the three parallel speakers are resulting in a load of 4ohm at the output transformers secondary. I read somewhere that the 7591 push-pull comfiguration is usually designed to work with a 6600ohm load. So I am thinking that the OT has a winding ratio of 40. I am a beginner though and would like to hear your opinions, so that I do not blow the OT when wiring my poweramp.
Thanks for any thoughts!
I measured the speakers of the K-100 to be 16ohm on the two 12" ones and 8ohm on the small speaker wich is connected to the 20u capacitor. My thinking was that the three parallel speakers are resulting in a load of 4ohm at the output transformers secondary. I read somewhere that the 7591 push-pull comfiguration is usually designed to work with a 6600ohm load. So I am thinking that the OT has a winding ratio of 40. I am a beginner though and would like to hear your opinions, so that I do not blow the OT when wiring my poweramp.
Thanks for any thoughts!
pnadora,
As long as you don't modify the original design much (at first) and have a load connected to the output while it is powered up, the output iron will survive. Far more output irons are blown by running them with no load (or way too high impedance load) connected to them. In the schematic posted before, you can see on the secondary (speaker) side, the headphone jack has a 30 ohm resistor across the winding when the headphones are active. I would not go above 30 ohms, ever, when the unit is making output.
Loading a tube amp up with too little ohms can damage things also, but in practice this takes a decent amount of punishment to accomplish. When the iron core saturates or the output tubes run out of muscle, even a novice operator can notice the change in fidelity.
As long as you don't modify the original design much (at first) and have a load connected to the output while it is powered up, the output iron will survive. Far more output irons are blown by running them with no load (or way too high impedance load) connected to them. In the schematic posted before, you can see on the secondary (speaker) side, the headphone jack has a 30 ohm resistor across the winding when the headphones are active. I would not go above 30 ohms, ever, when the unit is making output.
Loading a tube amp up with too little ohms can damage things also, but in practice this takes a decent amount of punishment to accomplish. When the iron core saturates or the output tubes run out of muscle, even a novice operator can notice the change in fidelity.
Speaking of running things without a load, the 6FQ7 reverb driver is run as a low power SE amp with T702 as the output tranny and the reverb tank as the secondary load. If you don't have the tank hooked up, disconnect the 275v supply or some other means to either add a load or to disable the tube. You can't just pull it because the other section of the tube is the expression pedal recovery stage. Having the tank not hooked up is the same as not having a speaker hooked up.
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Hey, I just went down and put the output tranny on my variac and came up with 33.5:1 turns ratio. Since the impedance ratio is the square of the turns ratio, that comes up as 1122:1. So it looks like to get to the 6.6k pri. impedance you correctly stated for 7591s, the secondary load is 6 ohms. So I would think anything from 4 to 8 ohms will be fine since Hammond usually overbuilt these great old amps. Hope this helps.
I apologize for digging out this rather ancient thread, but I scrapped a Hammond K-100 organ last weekend, just to get my hands on it's amplififer 
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If boobtube was right in determining the OT's turns ratio, and I do not have any doubt that he might not, and because the load is two speakers of 16 ohms each in parallel (the tweeter doesn't count), the pair of 7591's looks into a 9 kohms load, which matches the class AB1 suggestion in the last coloumn of any datasheet.
Btw, I'm quite amazed of how big this 25 watts OT is.
Best regards!
.If boobtube was right in determining the OT's turns ratio, and I do not have any doubt that he might not, and because the load is two speakers of 16 ohms each in parallel (the tweeter doesn't count), the pair of 7591's looks into a 9 kohms load, which matches the class AB1 suggestion in the last coloumn of any datasheet.
Btw, I'm quite amazed of how big this 25 watts OT is
.Best regards!
Hi, the transformers are really massive. I think that might be a reason that the amp sounds so good. I transformed it quite a bit, basically completely rebuilding it to turn it into a guitar amplifier. It sounds amazing (if i may say so) and combines elemtents of many of my favourite amps (EF86 input, cathode follower to drive a Fender tonestack, Fender reverb circuit, etc.). Below you can find the circuit diagram Good luck with your build.
Good luck with your build.
...Below you can find the circuit diagram
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