Choke Selection Question-Dumble Reverb

[Charlie Wilson]

Hello, I have a device that I am restoring and it was missing the power transformer and the choke. I sorted out the power transformer but I could use some help on selecting a choke. I have this in the Dumble section because it is a Dumble device but unfortunately I cannot share much more than that right now. Attached is my hand drawn power supply.
CW

[bepone]

can be any choke but use 3H 150mA it is more or less standard for Fender/type amps

[martin manning]

Hello, I have a device that I am restoring and it was missing the power transformer and the choke. I sorted out the power transformer but I could use some help on selecting a choke.

4H is a pretty common inductance. If you know how much current the power transformer can supply, you could select a choke to match that, or better yet determine the current required and use that figure. Can you identify the chassis mounting holes for the choke? That might give you a clue. My guess is this is a relatively low-current device, and a 50 mA Deluxe Reverb choke will be sufficient.

[Charlie Wilson]

Thanks guys. I can say that the device is a three tube stand alone reverb that Dumble did in the early eighties. It uses two 12ax7s and one 5751 as the reverb driver tube. The cut out for the power transformer is the venerable Triad R-2C, so 15mA DC. The hole spacing for the choke is 1 3/4", pretty small choke. The reverb driver transformer is a Triad S-13x which has a rating of 10mA DC. From the looks of the filter caps, this thing appeared to work at least for awhile. The power transformer, choke, and power cord where removed, so maybe the power transformer finally died. I had Heyboer wind a beefed up R-2C for me with 40mA DC secondaries and 1A filaments that fits the cut out. I think a 4H 50mA choke would be good and the Triad C-4X (4H 50mA 360 ohms) might be a good choice. It has a 2 inch hole spacing, so I will have to drill an additional hole but there are so many unused holes in this enclosure that I think I can get the owner to agree to it.
CW

[martin manning]

I can say that the device is a three tube stand alone reverb...
It uses two 12ax7s and one 5751 as the reverb driver tube...
the power transformer is the venerable Triad R-2C, so 15mA DC
The hole spacing for the choke is 1 3/4", pretty small choke
The reverb driver transformer is a Triad S-13x which has a rating of 10mA DC...

From the looks of the filter caps, this thing appeared to work at least for awhile. The power transformer, choke, and power cord were removed, so maybe the power transformer finally died. I had Heyboer wind a beefed up R-2C for me with 40mA DC secondaries and 1A filaments that fits the cut out.

Almost certainly the R-2C couldn't handle the current required by the doubler with 6 mA load, and I'd bet it didn't last very long. What do you have for DC resistance on the primary and secondary of your 40 mA PT? You may still be cutting it close.

[bepone]

very small chokes are probably rated 50mA, normal sized 100mA, chokes for 100W are 150mA, some empirical medium values

[Charlie Wilson]

Martin, the primary is 60 ohms and the secondary is 235.3 ohms. I wanted a transformer that would fit the existing cut out and I needed enough filament wind for 3 preamp tubes, so the 40 mA was as much as we could get for secondaries. Bill at Heyboer said he also beefed up the primary a bit. For a 6mA load, how much more current draw does the voltage doubler circuit put on the transformer?
CW

[martin manning]

... how much more current draw does the voltage doubler circuit put on the transformer?

I'd say at least four times the DC current, but the waveform is spiky and will peak at a much higher value depending upon the DC resistance and reservoir capacitors. For this case I'm getting 65 mA peak and 25mA RMS, so you have sufficient DCR to keep it under control. Here's what it looks like with 140VAC input and 6 mA DC output. I(V1), blue trace, is the current draw from the transformer.

[Charlie Wilson]

Thanks Martin.
CW

[Charlie Wilson]

Although not a choke question, another interesting thing about this device is the little Triad transformer used as the reverb driver. It is a S-13X which has a primary of 25k and a secondary of 3.2 ohms driven by a 5751 tube. So a mismatch with the 8 ohm input of the reverb pan. Not sure if he was trying to soften the reverb or the pan is tolerant of an impedance mismatch.
CW

[martin manning]

It is a S-13X which has a primary of 25k and a secondary of 3.2 ohms driven by a 5751 tube. So a mismatch with the 8 ohm input of the reverb pan.

Maybe to present a 2.5x higher primary impedance for the driver tube?

[didit]

Hammond 154G?

Fits and has the basic correct specs.

..

[Charlie Wilson]

Hey look at that, thanks. The only thing that I wonder about is the 300VDC rating.
CW

[didit]

Don’t put too much into that particular spec ..

[Charlie Wilson]

... how much more current draw does the voltage doubler circuit put on the transformer?

I'd say at least four times the DC current, but the waveform is spiky and will peak at a much higher value depending upon the DC resistance and reservoir capacitors. For this case I'm getting 65 mA peak and 25mA RMS, so you have sufficient DCR to keep it under control. Here's what it looks like with 140VAC input and 6 mA DC output. I(V1), blue trace, is the current draw from the transformer.

Hey Marin, I was wondering if the 6 mA DC current we used was the probable current draw idle or while in use? I ask this because I called Heyboer to confirm what I thought they told me last time and I was wrong. The current rating for my transformer is 30 mA DC not 40. It was the most I could get and still fit enough wire for the 1A filaments and some thicker wire for the primary. What do you think?
CW