Several years back, I gutted a Gulbransen Organ, and my sister's ex-fiance was able to shorten the cabinet to make a very interesting housing for the built in single rotor Leslie speaker, and I kept the amplifier lying around under the assumption that I would one day get around to finding a schematic for it, and also to learning how to interpret said schematic. I was able to find the schematic in question on page 23a of this service manual:
http://www.jimmyandsharonwilliams.com/gulbransen/gulbransen_664DEJVKF.pdf
After a couple days of brushing up on how tube amps work, I'm fairly confident in rigging up a power supply and output, but I'm concerned that pins 6,7, and 8 may require a load for proper function of the amplifier. The pin guide on page 22 says that these are for manual keying(6 and 7) and Omega snubbing(for 8, and I have no idea what that means). Is it okay to ignore these pins and just use this strictly as an amplifier with no modification aside from connectors, or should I wire up some kind of dummy load?
I apologize if this is out of place for this forum, and if needed I can go bother the folks at The Organ Forum instead
http://www.jimmyandsharonwilliams.com/gulbransen/gulbransen_664DEJVKF.pdf
After a couple days of brushing up on how tube amps work, I'm fairly confident in rigging up a power supply and output, but I'm concerned that pins 6,7, and 8 may require a load for proper function of the amplifier. The pin guide on page 22 says that these are for manual keying(6 and 7) and Omega snubbing(for 8, and I have no idea what that means). Is it okay to ignore these pins and just use this strictly as an amplifier with no modification aside from connectors, or should I wire up some kind of dummy load?
I apologize if this is out of place for this forum, and if needed I can go bother the folks at The Organ Forum instead
Hello Morgan,
Assuming that you want to use this amplifier as a general purpose audio amp, the quick answer to your question is yes you can basically ignore these connections 6,7,& 8. But there are several caveats to this answer.
These connections go to a dual low voltage supply used to supply power for voicing the organ through external relay switches. By leaving this unloaded the total power transformer load will be slightly less. This will allow it to operate a little cooler. Consequently the amplifier B+ and filament voltage to raise up a little. But I think this will be minimal and tolerable. (more on this below)
Assuming figure 22 (actually page 31 of the pdf) is the schematic of the actual amp you have (verses 22A), I think SR1 & SR2 are selenium rectifier full wave stacks. These have limited current capacity but appear to be updated in 22A.
If this were my amplifier and I wanted to use it as a general purpose amp, this is what I would do. I would remove, or disconnect, the selenium rectifiers leaving the two brown and center brown/white leads unconnected and taped up. You no longer need this source.
Because I don't know just what the load on the supply was, you could ultimately connect a resistor across this winding (brown to brown) to simulate the original load. This will depend on just how much higher the filament voltages is compared to what the tubes want. Today's AC line voltage is higher then when this amp was designed. But the B+ level can withstand a little higher level. This is something you will have to experiment with if you deem necessary. You'll have to estimate current drain and use simple ohms law. (R=E÷I)
There is a small capacitor (.05/600v) to ground on the power transformer primary, This is to help filter noise on the AC line. But it is dangerous because it can short putting the mains AC on the metal chassis. Because of this we call it a death capacitor. Remove it!
Depending on how old this amp is and how long it's been sitting, the main B+ filter capacitors may need replacing. In any event you should not power up the amp without first forming, or at least checking, the existing filter capacitors. Forming requires a slow power up using a variac. Since you have a tube rectifier, temporarily substituting a pair of SS diodes makes this process a lot easier. Using a multimeter, start with 50 volts on the B+ rail and run for a time. Then incrementally increase the voltage by 50 volts until you can reach full input. You are looking for no heat from the three capacitors. (1 is a dual can type) I just use my hand to feel the capacitor temperature. How long this process takes depends on the capacitor's condition.
You should also check (or replace) the 50ufd/50v cathode capacitor for the 6BQ5 output tubes.
There is an "expression" filter between the first and second stage that I would remove or disable since it will color the amplifier's sound. Jump the 270K resistor and disconnect the .04/50v capacitor just below it.
Of course the AC power switch (pins 11 & 12) needs connection, Also the B+ ground connection (pins 1 & 2) at the speaker plug.
Assuming that you want to use this amplifier as a general purpose audio amp, the quick answer to your question is yes you can basically ignore these connections 6,7,& 8. But there are several caveats to this answer.
These connections go to a dual low voltage supply used to supply power for voicing the organ through external relay switches. By leaving this unloaded the total power transformer load will be slightly less. This will allow it to operate a little cooler. Consequently the amplifier B+ and filament voltage to raise up a little. But I think this will be minimal and tolerable. (more on this below)
Assuming figure 22 (actually page 31 of the pdf) is the schematic of the actual amp you have (verses 22A), I think SR1 & SR2 are selenium rectifier full wave stacks. These have limited current capacity but appear to be updated in 22A.
If this were my amplifier and I wanted to use it as a general purpose amp, this is what I would do. I would remove, or disconnect, the selenium rectifiers leaving the two brown and center brown/white leads unconnected and taped up. You no longer need this source.
Because I don't know just what the load on the supply was, you could ultimately connect a resistor across this winding (brown to brown) to simulate the original load. This will depend on just how much higher the filament voltages is compared to what the tubes want. Today's AC line voltage is higher then when this amp was designed. But the B+ level can withstand a little higher level. This is something you will have to experiment with if you deem necessary. You'll have to estimate current drain and use simple ohms law. (R=E÷I)
There is a small capacitor (.05/600v) to ground on the power transformer primary, This is to help filter noise on the AC line. But it is dangerous because it can short putting the mains AC on the metal chassis. Because of this we call it a death capacitor.
Remove it! Depending on how old this amp is and how long it's been sitting, the main B+ filter capacitors may need replacing. In any event you should not power up the amp without first forming, or at least checking, the existing filter capacitors. Forming requires a slow power up using a variac. Since you have a tube rectifier, temporarily substituting a pair of SS diodes makes this process a lot easier. Using a multimeter, start with 50 volts on the B+ rail and run for a time. Then incrementally increase the voltage by 50 volts until you can reach full input. You are looking for no heat from the three capacitors. (1 is a dual can type) I just use my hand to feel the capacitor temperature. How long this process takes depends on the capacitor's condition.
You should also check (or replace) the 50ufd/50v cathode capacitor for the 6BQ5 output tubes.
There is an "expression" filter between the first and second stage that I would remove or disable since it will color the amplifier's sound. Jump the 270K resistor and disconnect the .04/50v capacitor just below it.
Of course the AC power switch (pins 11 & 12) needs connection, Also the B+ ground connection (pins 1 & 2) at the speaker plug.
HollowState,
Thanks for taking the time to look over the schematic and make an informative response, I appreciate it very much.
I'm only planing to use the amp with the accompanying Franken-Leslie that the organ was made into, for use with the Hammond Organs I own and occasionally guitars. I'd like to be able to drive harder than originally intended, but I suppose that might be an issue with the slightly higher voltage from the organ keying loads not being present? I admittedly am not qualified to approach the subject, both by my limited understanding of the maths behind the circuitry, and in my lack of appropriate equipment to do as you've suggested. I'm now sure that I'm in over my head, and will be taking this along with a Lesle 147 amp I've got to the local tube amp guy.
I'm going to do my damnedest to get on track to understanding all of this, as I'd love to have this as a hobby, and now I now where to go when I'm ready. Thanks for the heads-up on that death cap by the way, it reminded me that the old Leslie amps usually had those as well.
Thanks for taking the time to look over the schematic and make an informative response, I appreciate it very much.
I'm only planing to use the amp with the accompanying Franken-Leslie that the organ was made into, for use with the Hammond Organs I own and occasionally guitars. I'd like to be able to drive harder than originally intended, but I suppose that might be an issue with the slightly higher voltage from the organ keying loads not being present? I admittedly am not qualified to approach the subject, both by my limited understanding of the maths behind the circuitry, and in my lack of appropriate equipment to do as you've suggested. I'm now sure that I'm in over my head, and will be taking this along with a Lesle 147 amp I've got to the local tube amp guy.
I'm going to do my damnedest to get on track to understanding all of this, as I'd love to have this as a hobby, and now I now where to go when I'm ready. Thanks for the heads-up on that death cap by the way, it reminded me that the old Leslie amps usually had those as well.
The valve amp can be pushed in to normal 'guitar' overdrive performance doing a few typical modifications, such as:
- remove feedback 100k link.
- add capacitor bypass to V1A and common V2-3 cathodes.
- remove 470pF shunt on v1B anode (as Hollowstate indicates)
- add grid stoppers to all valves
- add screen stoppers to V2-3
The volume control is a shortcoming in its present position. There are options to move it to before or after the phase inverter (which would definitely mean that the feedback loop needs to be removed from its present position).
By looking at post threads on each of those changes you will gain some insight on why they are normally used in a guitar amp, and what values to nominally choose. Those changes may cause a problem with re-inserting feedback if you wanted for plain organ use, but maybe you might prefer the modified amp with an organ and without feedback.
Ciao, Tim
- remove feedback 100k link.
- add capacitor bypass to V1A and common V2-3 cathodes.
- remove 470pF shunt on v1B anode (as Hollowstate indicates)
- add grid stoppers to all valves
- add screen stoppers to V2-3
The volume control is a shortcoming in its present position. There are options to move it to before or after the phase inverter (which would definitely mean that the feedback loop needs to be removed from its present position).
By looking at post threads on each of those changes you will gain some insight on why they are normally used in a guitar amp, and what values to nominally choose. Those changes may cause a problem with re-inserting feedback if you wanted for plain organ use, but maybe you might prefer the modified amp with an organ and without feedback.
Ciao, Tim
Several years back, I gutted a Gulbransen Organ, and my sister's ex-fiance was able to shorten the cabinet to make a very interesting housing for the built in single rotor Leslie speaker, and I kept the amplifier lying around under the assumption that I would one day get around to finding a schematic for it, and also to learning how to interpret said schematic. I was able to find the schematic in question on page 23a of this service manual:
http://www.jimmyandsharonwilliams.com/gulbransen/gulbransen_664DEJVKF.pdf
After a couple days of brushing up on how tube amps work, I'm fairly confident in rigging up a power supply and output, but I'm concerned that pins 6,7, and 8 may require a load for proper function of the amplifier. The pin guide on page 22 says that these are for manual keying(6 and 7) and Omega snubbing(for 8, and I have no idea what that means). Is it okay to ignore these pins and just use this strictly as an amplifier with no modification aside from connectors, or should I wire up some kind of dummy load?
I apologize if this is out of place for this forum, and if needed I can go bother the folks at The Organ Forum instead
Hello,
I know this is an old thread, but I was wondering if you have that schematic for the ABS-22 available.
I have one of these and have not been able to find a schematic.
Thanks,
Joe Y
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