Hammond L101 Smoking Power Amp Capacitor

[ayelex]

Hey everyone, new to these forums.
I was recapping my L100 and cleaning off all the wood that has collected a thick dust layer over the years. When I put everything back together it seemed to have not made any sound, then one of the multi sectional capacitors (50uf + 50uf 450v, C318 or C319) started to smoke and I turned the organ off immediately. I replaced the entire power amp with a used one I got for very cheap (at least cheaper than buying new caps) and recapped it. When I flipped the switch I was finally able to get sound out of it and was pleased. Though after messing with the keys for 5 seconds or so, the same capacitor (C318 or C319) started to smoke again and I turned the organ off immediately. THENNN, I took C316 + C317 out of my old power amp (that seemed to be in decent condition) to replace the newly damaged one, but the same outcome happened as just before.

My question is what could be the cause of this problem? Are there tubes needing to be replaced? I can see that these caps are connected to the 5U4 tube and wonder if this is the problem. I would rather not go replacing every tube in the organ before asking if there could be a small issue. Any ideas? Any input is greatly appriciated.
Also my whole organ has original Hammond brand tubes, except one 12AU7 I replaced in the percussion preamp.

Thanks
AN

[Adam Caldow]

Did you use the same 5U4 on both amps? If so I'd change that out first before you replace any more capacitors. Those are rather failure prone. Also, make sure the big power resistors underneath didn't burn up with those caps. Obviously you'll need to replace the damaged sections of the can capacitors now too.

[kziss]

Did you use the same 5U4 on both amps? If so I'd change that out first before you replace any more capacitors. Those are rather failure prone. Also, make sure the big power resistors underneath didn't burn up with those caps. Obviously you'll need to replace the damaged sections of the can capacitors now too.

Hi Ayelex and Adam.
A possible cause of this problem is a faulty 5U4 rectifier tube which has internally shorted out and thus is sending the unrectified AC voltage on to the power supply filter capacitors.

A simple and cheap safeguard to protect against such damage when a rectifier tube fails like this is to wire up two diodes in series with the two secondary winding high voltage wires of the power transformer which go to the pins 4 and 6 of the 5U4 tube.

The anode sides of the two diodes go to the two secondary winding wires of the power transformer, and the cathode sides of the two diodes go to the pins 4 and 6 of the 5U4 wire.

Last week I partially rebuilt a friend's AO67 amplifier from his M100 organ with the Tonewheel General Hospital M100 refresh kit, and in order to better protect the new power supply filter capacitors from a 5U4 tube becoming faulty, I wired up two 6A10 diodes which are 1000 volt / 6 amp rated diodes between the two high voltage secondary winding wires of the power transformer and the pins 4 and 6 of the 5U4 rectifier tube.

The 6A10 diodes are heavy duty diodes and you can use less heavy duty diodes, but I do prefer to use more rugged quality components, and the 6A10 diodes only cost a few dollars.

With this modification, the two diodes convert (rectify) the high voltage AC coming from the two secondary winding wires of the power transformer into a DC voltage so therefore if the 5U4 tube malfunctions by internally shorting out, then the two diodes will prevent the unrectified AC voltage from going to the power supply filter capacitors so therefore if the 5U4 does become faulty and has an internal short, the amplifier will continue to work, but with a somewhat higher DC voltage being sent to the power supply filter capacitors.

Because the diodes produce an inconsequential 0.6 volt voltage drop when they are wired in series between secondary winding wires and the 5U4 tube, this means that with a properly functioning 5U4 ( or any other rectifier tube), the two diodes will have no effect whatsoever on the operation or the tonality of the amplifier or to the "sag" effect of the 5U4 when the amplifier is driven hard.

Here is an article by R.G Keen which explains about this simple modification in more detail. I also recommend that you read R.G Keen's other articles in the "Immortal Amplifier Mod" series which can be clicked on underneath this article.

https://www.premierguitar.com/articl...mplifier-mod-1

All the best.
Kon.

[ayelex]

Did you use the same 5U4 on both amps? If so I'd change that out first before you replace any more capacitors. Those are rather failure prone. Also, make sure the big power resistors underneath didn't burn up with those caps. Obviously you'll need to replace the damaged sections of the can capacitors now too.

Thanks for the reply! I did in fact use the same 5U4 on both amps. I had a hunch when looking over the schematic, but I am not an expert in the field. When I took a general look over each amp I did not notice any burned up resistors, but I will take a closer look at them. I figured I needed to just replace all the tubes The organ was sitting for a while.

[ayelex]

Hi Ayelex and Adam.
A possible cause of this problem is a faulty 5U4 rectifier tube which has internally shorted out and thus is sending the unrectified AC voltage on to the power supply filter capacitors.

A simple and cheap safeguard to protect against such damage when a rectifier tube fails like this is to wire up two diodes in series with the two secondary winding high voltage wires of the power transformer which go to the pins 4 and 6 of the 5U4 tube.

The anode sides of the two diodes go to the two secondary winding wires of the power transformer, and the cathode sides of the two diodes go to the pins 4 and 6 of the 5U4 wire.

Last week I partially rebuilt a friend's AO67 amplifier from his M100 organ with the Tonewheel General Hospital M100 refresh kit, and in order to better protect the new power supply filter capacitors from a 5U4 tube becoming faulty, I wired up two 6A10 diodes which are 1000 volt / 6 amp rated diodes between the two high voltage secondary winding wires of the power transformer and the pins 4 and 6 of the 5U4 rectifier tube.

The 6A10 diodes are heavy duty diodes and you can use less heavy duty diodes, but I do prefer to use more rugged quality components, and the 6A10 diodes only cost a few dollars.

With this modification, the two diodes convert (rectify) the high voltage AC coming from the two secondary winding wires of the power transformer into a DC voltage so therefore if the 5U4 tube malfunctions by internally shorting out, then the two diodes will prevent the unrectified AC voltage from going to the power supply filter capacitors so therefore if the 5U4 does become faulty and has an internal short, the amplifier will continue to work, but with a somewhat higher DC voltage being sent to the power supply filter capacitors.

Because the diodes produce an inconsequential 0.6 volt voltage drop when they are wired in series between secondary winding wires and the 5U4 tube, this means that with a properly functioning 5U4 ( or any other rectifier tube), the two diodes will have no effect whatsoever on the operation or the tonality of the amplifier or to the "sag" effect of the 5U4 when the amplifier is driven hard.

Here is an article by R.G Keen which explains about this simple modification in more detail. I also recommend that you read R.G Keen's other articles in the "Immortal Amplifier Mod" series which can be clicked on underneath this article.

https://www.premierguitar.com/articl...mplifier-mod-1

All the best.
Kon.

That's worth a try, I might do this and replace the tubes so I can have that safety net. Thanks for the informative reply! I will defiantly check it out.

[Guest]

Hi Ayelex and Adam.
A possible cause of this problem is a faulty 5U4 rectifier tube which has internally shorted out and thus is sending the unrectified AC voltage on to the power supply filter capacitors.

A simple and cheap safeguard to protect against such damage when a rectifier tube fails like this is to wire up two diodes in series with the two secondary winding high voltage wires of the power transformer which go to the pins 4 and 6 of the 5U4 tube.

The anode sides of the two diodes go to the two secondary winding wires of the power transformer, and the cathode sides of the two diodes go to the pins 4 and 6 of the 5U4 wire.

Last week I partially rebuilt a friend's AO67 amplifier from his M100 organ with the Tonewheel General Hospital M100 refresh kit, and in order to better protect the new power supply filter capacitors from a 5U4 tube becoming faulty, I wired up two 6A10 diodes which are 1000 volt / 6 amp rated diodes between the two high voltage secondary winding wires of the power transformer and the pins 4 and 6 of the 5U4 rectifier tube.

The 6A10 diodes are heavy duty diodes and you can use less heavy duty diodes, but I do prefer to use more rugged quality components, and the 6A10 diodes only cost a few dollars.

With this modification, the two diodes convert (rectify) the high voltage AC coming from the two secondary winding wires of the power transformer into a DC voltage so therefore if the 5U4 tube malfunctions by internally shorting out, then the two diodes will prevent the unrectified AC voltage from going to the power supply filter capacitors so therefore if the 5U4 does become faulty and has an internal short, the amplifier will continue to work, but with a somewhat higher DC voltage being sent to the power supply filter capacitors.

Because the diodes produce an inconsequential 0.6 volt voltage drop when they are wired in series between secondary winding wires and the 5U4 tube, this means that with a properly functioning 5U4 ( or any other rectifier tube), the two diodes will have no effect whatsoever on the operation or the tonality of the amplifier or to the "sag" effect of the 5U4 when the amplifier is driven hard.

Here is an article by R.G Keen which explains about this simple modification in more detail. I also recommend that you read R.G Keen's other articles in the "Immortal Amplifier Mod" series which can be clicked on underneath this article.

https://www.premierguitar.com/articl...mplifier-mod-1

All the best.
Kon.

Hi Kon that's great advice. I look after two A100s in studios in London and will add this modification to their power amps when I go there next week. Apart from an M102 I owned and restored years ago, Ive never owned a Hammond with a power amp.
Best
Dave

[kziss]

Hi Kon that's great advice. I look after two A100s in studios in London and will add this modification to their power amps when I go there next week. Apart from an M102 I owned and restored years ago, Ive never owned a Hammond with a power amp.
Best
Dave

Hi Dave.
Adding diodes in series between with the two secondary output wires of the power transformer and the rectifier tube is a suitable and very worthwhile safeguard modification in any amplifier which uses a tube rectifier including the AO10 and the AO28 preamps and the pre 1960's era Leslie amplifiers and the Hammond tone cabinets etc.
All the best.
Kon.

[Guest]

Hi Dave.
Adding diodes in series between with the two secondary output wires of the power transformer and the rectifier tube is a suitable and very worthwhile safeguard modification in any amplifier which uses a tube rectifier including the AO10 and the AO28 preamps and the pre 1960's era Leslie amplifiers and the Hammond tone cabinets etc.
All the best.
Kon.

Hi Kon
i should do the AO43 that I use as a combo preamp in that case as well.