Dobro amp 6107

[maxkracht]

I bet that would look the part more than a new hammond thing. Should work fine, assuming it doesn't make the amp hum and doesn't get hot.

[maxkracht]

Can always add some more resistance after the choke to cut down hum if needed.

[martin manning]

I have a Baldwin 512-30 K choke with the correct spacing for mounting almost identical size.

Perfect!

[angelodp]

Max, yah new choke in no shorts and seems good. Next try some tubes. I need to shore up the fuses and make that all safe. Or... can I get rid of those 300ma fuses and just go with the 2A main fuse. I am hopeful that this amp may come back to life. Appreciate your help.

A

[angelodp]

Ok Martin, just happen to have a few in the box.

A

[angelodp]

So given the way this owner will use the amp, I don't think that including the HT fuses seems necessary, but I could be wrong. I would keep the backup diodes, but looking for a good design to have that rock solid. There are no NC tabs to work with and putting them in series with the PT wires leaves them hanging. I suppose a terminal strip, small one, in the right spot might be the ticket. I could put one small hole in the front panel and hang the strip just above the socket, then make the connections, unless there is a better solution.

I am a bit nervous with regard to adding the tubes, but it all seems to be stable so safe to proceed. Old amps are nerve wracking.

[angelodp]

Could not take the suspense, with the fuses, diodes and all I put in the tubes one at a time, no shorts.... but no sound until the variac passed 85VAC then all good and loud. Shut it down and will neaten it all up and make it solid. I did rebuild the grid cap connector, which is where the trouble started. I think it will be good to get the 5Z3 tube in there with the protection diodes, get the cab finished up and bye bye. This one had me in a sweat.

Max big thanks for your guidance on this project. Martin, SteveM, Sluckey always appreciate your input.

[maxkracht]

If there isn't a convenient place for fuses, it's probably more harm than good. Ht fuses are not absolutely necessary, especially with backup diodes in place, but they are a nice feature. Many people are too intimidated to change regular fuses, so I would not expect anyone to consider something internal but if they blow, something bad happened. A normal person couldn't deal with that either.

You can attach diodes directly to the HT wires, then heatshrink on top for a bit of insulation and mechanical stability.

If I were working on this, I would reinforce all of the filter caps with a dab of 100% silicone. Just a little squeeze under anything heavy will stop things from shifting and breaking a lead or tube socket pin.

[angelodp]

Good points thanks. So this replacement choke is not at the same resistance as the previous, which was about 470ohms, this new one is closer to 150ohms. I do hear more hum now. You mentioned adding some R to tame that. Would that be in the order of 1k 5W just after the choke? I have a friend who has the 83v rectifier available so I might just go with that. Could be that my little fuse arrangement is acting like an antenna right now. I need to pull that out now that it's running again. I do see dim light on the limiter as I bring the voltage up near 115vac. I am hesitant to bring this amp up to 120.

[maxkracht]

You mentioned adding some R to tame that. Would that be in the order of 1k 5W just after the choke?

Not sure If it would trust the resistance of your old choke if it is faulty. I'm sure 1k 5w would be perfectly fine to tame things. If you are shooting for a specific voltage, you can play around until you get the right value, but It shouldn't be too critical. I would also be suspect of the bad choke damaging a filter cap. That might be your hum problem.

Could be that my little fuse arrangement is acting like an antenna right now.

Fuses shouldn't make any difference for noise, anything getting picked up should go right to ground at the first filter cap.

I do see dim light on the limiter as I bring the voltage up near 115vac. I am hesitant to bring this amp up to 120.

Dim bulb is normal, some people even call it a dim bulb tester. Try it with another amp of around the same wattage and you should get about the same amount of glow. Not much difference between 115 and 120v

[angelodp]

Rectifier connections with Diodes? I might put in nylon screws? Or use silicon to safety....

[maxkracht]

Little close, but probably fine? Nylock should add a bit of isolation from ground. Could cover with nailpolish. Works kinda like solder mask and easier to remove than silicone if you need to resolder. Also adds another layer of threadlocking to the nylock.

If the distance gives you that sinking feeling, just redo the board with eyelets closer together. Annoying, but better than having it in the back of your head.

[angelodp]

Better, safer.

[angelodp]

A few questions. I notice that the amp eliminates the hum altogether when the volume is rolled up towards

7 - wide open? The amp has the dual pots, is this showing some imbalance in this part of the circuit or is it done to the Power tubes?

I am running the amp on the solid state rectifier setup right now. Is their any value in putting in a Tube/SS switch for the rectifier, or leave it stock with the 5Z3. I realize the sonic differences, I am mindful of not adding stress to the circuit.

[maxkracht]

I notice that the amp eliminates the hum altogether when the volume is rolled up towards 7

I would guess a ground loop, but perhaps someone else has other ideas. Make sure you didn't move any ground points from where they were originally and everything is tight. Possible that the amp always hummed a bit.

I am running the amp on the solid state rectifier setup right now. Is their any value in putting in a Tube/SS switch for the rectifier

That's mostly personal preference. Make sure the ss rectifier isn't raising voltages too much for caps/tubes. If it is, you could put in a series resistor to emulate a tube rectifier.

I could be misunderstanding the picture, but I think you want that radial electrolytic on the other side of your 1k 5w resistor. Might not make too much of a difference, but if you probably want choke>resistor>cap.