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PostPosted: 12 Feb 2013, 23:23 
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Joined: 08 Jan 2012, 10:09
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I was looking at tube specs and see that the heater to cathode voltage is 200 volts for the el34, if the heaters are not grounded how would you measure this voltage.
also in a SRPP circuit can the upper cathode of say a 6sl7 at 100 volt heater to cathode limit go higher then this?

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PostPosted: 13 Feb 2013, 01:27 
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Floating heaters are limited to the insulation breakdown rating on the 6.3V winding of the transformer. They are noisier and you can bypass the noise for AC with a capacitor of appropriate voltage (if you have a 200V cathode and float the heater, use at least a 250V noise bypass cap).

SRPP is best done with two tubes; one for the top section, one for the bottom and fed with two different 6.3V supplies and lift them (I've posted elsewhere here on how to lift a heater supply).

If the Vh-k of the tube isn't exceeded, you can do it with one supply. Some tubes are made for cascode or SRPP operation, like the 6DJ8/ECC88 and one section has a Vh-k of 50V, while the other has 150V. The 50V section should be used as the "bottom" tube.

Sometimes you can't help but use a single supply where you're exceeding the Vh-k, like a single tube SRPP or mu-followr. As long as you don't exceed DC+AC, 2 x the Vh-k rating, you can float the heater supply and use the noise bypass cap.

Cheers!

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