Need some help on an original ST-35

[EdStiles]

Rebuilding a rather beat up original ST-35. Put in new driver boards with premium parts and am waiting for a CChong cap board.

I've also ordered (4) 400 ohm and (4) 220 mfd caps to create individual cathode bias circuits, but now notice a slight problem....

On the original ST-35, one side of the 6V AC tube heaters are tied to the bias circuit. Can anyone tell me what's the purpose and what other options I might have. One option would be to run the this heater lead to the original 100 ohm resistor and 470 mfd cap on the CChong Cap board, which is not being used. Don't think its a good idea to run it to one of the individual cathode bias circuits and not the other, think it might create an unbalanced circuit.

Any input would be appreciated.

Thanks,

Ed

[erichayes]

Hi Ed,

Dynaco decided to go cheap on the power transformer and didn't include center taps on the heater windings. Usually the CTs are connected to ground , but sometimes are connected to the output tube cathodes in self-biased amps to minimize heater to cathode potentials. Either way, the idea is to reduce hum.

What you can do is create a phantom center tap by wiring a couple of 100Ω resistors in series and connect each end to the heater winding(s). The junction of the two resistors is your phantom tap, and can be connected to ground. That will allow you to separate all the cathodes and take them to ground via their individual R/C networks. Don't forget to remove the link between the heater windings.

[EdStiles]

You stated " Don't forget to remove the link between the heater windings." Not sure what you mean. Are you referring to the jumper between Pins 3 and 4 on the tube socket that connect the heater to the bias circuit. That would appear to be the logical choice.

BTW, I feel foolish, if I had looked at the DIYTube ST-35 docs, I would have seen this solution. Now all I need to get is some 100ohm, 3 w resistors...

Thanks again,

Ed

[erichayes]

Hi Ed,

You need to get rid of the wires going from pins 3 to 4 on each socket as well as the wire connecting the cathodes from one one board's output tubes to the other board's. When you're through you should have all four cathodes isolated from their heaters, and the two heater windings isolated from each other (before wiring in the phantom CTs),

I'm working on your other question.

[Shannon Parks]

BTW, I feel foolish, if I had looked at the DIYTube ST-35 docs, I would have seen this solution. Now all I need to get is some 100ohm, 3 w resistors...

Actually Ed, you can use about any matched pair of anything: a couple of ceramic caps ala Dyna ST70, or a couple of 1k, 1/2W resistors, or some 2.7k, 1/8W carbon junkers. I think the reason I switched to 3W on that guy was due to the problem of some of the early Triode PA774 trannies shorting the HV to the heaters. A couple customers reported toasted 100 ohm resistors, so I just bumped them up even though the problem was elsewhere.