Leslie Model 25 Booster Amp Project

[TomCarlos]

MEF Buds....

A friend handed me a Leslie Model 25 Amp to look at. It's definitely different compared to what I have worked on before. But in the end, it should be about a Power Supply, Signal path, and Output.

I am not familiar with Leslie Speakers other than knowing things are connected via "Connectors." So this amp unit does not have an AC input per se- but that comes through the Brown Cable. So does the input signal. I see that.

Question 1 - I assume I would be safe in hard wiring an AC line to Pins 3 and 4 of the Extra Spkr socket and should also attached a ground lead to the chassis?

Question 2 - The Mallory canned Caps are specified as 30-30-30-10 (at 475 volts). I am measuring 42.3-44.5-43.7-17. I don't think that is in spec. So I might be better off buying 4 separate Caps and wiring those in manually.

Question 3 - The diodes in the Power Supply... There are 4 diodes (Yellow Green discs) rated for 400 Volts. There are 2 additional diodes (the black ones) rated at 600 volts. Being very old, I am thinking I can simply swap all of them out for some IN2007s? There are 3 on a side and all in series.

I'm gonna start with this, then move on.

Thanks, Tom

[Enzo]

Can caps in those days had tolerances like -20%/+80%. Don;t worry about the value, worry about electrical leakage. Let them be until you determine they are dried out or not.

Yes, pins 3 and 4 are where the mains came into the amp through the umbilical from the organ. You can apply 120v there.

I don't see black diodes, but do the diodes work? Then leave them alone.

This amp is a dream. So many Leslie amps have a balanced input, or take B+ from the organ, or have a speed switching circuit that uses HV DC on the balanced line as a control signal. Not unlike phantom power. This is just a power amp, a phase splitter and two power tubes.

Is this going back into an organ system? Or is it now going to stand alone as some project? Don't rewire the mains if it is going back into the organ. I have a death cord in the shop, an AC power cord with clips on the end. That allows temporary power connections to things like this. Safe? No, but then neither is a 500v circuit.

[TomCarlos]

Thanks Enzo...

This amp will go back into an old Leslie... where it belongs.

The black diodes are on the back side of the board that holds the 4 Yellow/Green diodes.

For now, I will test the diodes before putting the Mallory cap back in.

Oh, I take it no harm in grounding the chassis? I don't think anything is tied to that?

Tom

[Enzo]

Not on your bench, that is fine. But the chassis has a ground connection to the organ through the cable. If we add a mains ground at the amp, we might set up a ground loop.

[TomCarlos]

Ok.... got it.

It looks like one of the Black diodes is bad. It is an SC6. I believe it's rated at 1 amp, 600v Reverse Peak. So I'm thinking I replace both with an IN4007? Will do that tomorrow.


The other 3 caps measure out ok. But like you said, leakage is another story.


Tom

[TomCarlos]

Ok... started working on this amp.... and there are some issues.

I replaced the black diodes with IN4007.

For initial testing, I have removed the tubes.

I get DC where the diodes meet and connect to the 150 ohm, 10 watt resistor - 528 volts!! That seems very high. I realize that dc voltages can be high when the tubes are out.

If I put the tubes in, once they warm up, I can hear a loud "whining." The Killowatt meter shows 1.4 amps at idle. Something is oscillating and I see it on the output- it's like a square wave. The origin can also be seen on the plates of the 12AU7. BTW, for the Power Tubes, I have 6L6-C (HP).

If I remove the 6L6 tubes, the whining goes away but I can still see some type of square wave (SEE PHOTO) on the plates of the 12AU7 (pins 1 and 6).

This doesn't seem like a complicated amp but something is wrong. Going back to the Power Supply, I am thinking that 528 Volts is too high.

Any ideas on where to begin?

Thanks! Tom

[TomCarlos]

I am measuring approx 300mv of ripple where the diodes connect to the first 30uf cap- photo attached.

Perhaps this is what is feeding through to the circuit?

For good measure, maybe I should replace the 6 diodes and Cap can? I found a replacement - https://www.tubesandmore.com/products/C-EC30-30-30-10

Tom

[52 Bill]

A couple of things here. I don't know how you powered up the amp, but if you have connected an ac cord to the inside connections of the extra speaker jack, there will be 120 vac at pins 3 & 4 of the attached connector plug. Be sure to cover those up until you are done working on the amp.

The power supply diodes are or should all be the same types. If the two on the backside of the board are different from the 4 yellow ones, then they have been replaced at an earlier time. In the past series diode strings were used due to the limitations of the available part specs.

The 528 volt reading does seem high, but without the tubes to draw current, there will be no voltage drop. The better reading would be the voltage with the tube installed.

As for the filter cap, 300mV ac doesn't seem that far out of line to me. If your theory of the ripple feeding through the amp to cause the oscillation is to be proven, what frequency is the howling at? I would test the cap with a parallel cap before ordering a new one.

If you turn down the volume control does that change the howling?

[TomCarlos]

Thank you Bill....

Yes, I used a utility cord and soldered the AC lines on the inside of the chassis. And yes, I made sure the hot points were covered!!!!

It looks like there was a bad 12AU7 tube. I replaced it and the howling stopped. I'm not sure why that was a contributor to the problem but apparently it was. I measured all the caps and they were within tolerance. But of course, I have no idea on their leakage.

I too believe that all the diodes should be replaced- I will ask the owner before doing so. You know, some guys want that all original thing and don't replace if you don't have too. But if I do swap out the diodes, the yellow ones could also be IN4007. I don't see a need for anything bigger.

I have some single caps and might hard wire them in to see if it changes the voltage or ripple.

For now, I have output, close to 25 watts before clipping... that is just about all this amp was made to do.