DIY 1624 from Hammond M3 Parts

[martin manning]

The difference is cathode current includes screen current, which is something like 5% of the total. If you use the resistance and voltage drop across each side of the OT primary you will get plate current by itself.

[dawsonaudio]

So one side of the OT measures 250 ohms and the other 285 ohms. With the amp turned on, the center tap of the output measures 365vdc and the plates measure 355vdc. There is a 10vdc difference. I divide this difference by the resistance of each side and the total plate current of both tubes is 75ma. Did I do that correctly?

[martin manning]

Yes, but even better you also know that one is at 35mA and 11.7W, and the other is at 40mA and 13.3W.

[Smokebreak]

Nate, I revised my original layout, to hopefully make the tremolo wiring a little clearer. Note the square on the back of the speed pot is the spst switch. Make sure that jack is isolated! i just used a cliff but you can use shoulder washers with a switchcraft metal type. The pic should be big enough to zoom in and see, as well. Glad you got it goin!

edit: disregard the 1M resistor across the pot. I only had a 1M switched pot so that was a hack to get the speed w/i the range I wanted.

[dawsonaudio]

Thank you for the help. With the difference in amp draw, is that considered a matched set of tubes? I did notice that the one drawing more current was the tube that was a little red in the middle...red plating? I'll play with the values of cathode resistor to hopefully bring that down. The tubes came from Antique Radio Supply in AZ. Outside labels on both boxes say 24/2.6 and were ordered as a matched pair.

And thank you for the revised schematic. I'll go through it tomorrow and get it working and post some finished images of my build.

Nate

[Smokebreak]

Thank you for the help. With the difference in amp draw, is that considered a matched set of tubes? I did notice that the one drawing more current was the tube that was a little red in the middle...red plating? I'll play with the values of cathode resistor to hopefully bring that down. The tubes came from Antique Radio Supply in AZ. Outside labels on both boxes say 24/2.6 and were ordered as a matched pair.

And thank you for the revised schematic. I'll go through it tomorrow and get it working and post some finished images of my build.

Nate

5mA difference here is good enough for me. Much more than that I might be peeved, only because of the money spent for matching. Sonically, some prefer the harmonics produced by mismatched tubes, to an extent. You can also separate the cathodes, put a resistor on each cathode, and match them closer , and see what you like.
As a rule of thumb, set the upper current draw limit (12W or whatever you choose) by the tube pulling the most current

[dawsonaudio]

Would it be possible to add a 30ohm resistor to the 250ohm side of the transformer to balance them out? If so, how large of a resistor in wattage? I've got a 30ohm/10 watt here from the M3. That difference in resistance off the transformer primaries seem to be having a direct effect on the different plate current readings. Is that correct?

[martin manning]

It's the difference in the tubes that is responsible for the majority of the difference in plate current. With separate cathode resistors you could balance them, but it's not worth worrying about IMO, and they might change after a few hours of use.

[dawsonaudio]

So I tried several different cathode resistors and the best was a 350ohm. My readings were 250 and 285 ohm resistance at the primaries with an 8 volt difference on one side and 9 on the other. This gives me about 32 and 31.5 ma plate current per side. That is about 11.5 watts, correct?

Should I be concerned with the higher plate voltage being at 380vdc?