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Old 16th August 2012, 04:33 PM   #31
ChrisA is offline ChrisA  United States
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Join Date: Jan 2008
Originally Posted by puginfo View Post
i use DC filament supply for both the input - AX7 and invertor - AU7 stages but AC for the output stage - 6550. so do i need to ground or reference the DC filament supply cct? its floating right now as it has its own line transfo and the whole thing does not tie to any part of the main HT supply cct.

Yes floating is not good. It can chrage up to any un-known voltage. 'd at least use a high value resistor to ground. But if being extra carful why bias it it up? It only takes one more resistor
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Old 17th August 2012, 08:04 AM   #32
puginfo is offline puginfo  Malaysia
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think i will try a 20k resistor on the -ve leg of this 12.5vdc heater supply to the amp star ground and see what happens. did not think of biasing up to the HT line (via voltage divider of course) because this is still 12vdc biasing on another dc HT voltage of a totally different transfo.
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Old 19th August 2012, 12:24 AM   #33
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Thanks, all. Just been peering into my amps for the first time in years.

When I made them, some years ago, I agonised for some time over heater grounding. Small valve heater supply is current source, and heaters are in series. Eventually I decided to ground the middle of the chain, probably on the basis that any fluctuation in current would cause least common mode voltage variation. Actually current is as constant as I've ever seen a constant thing be.

When I get round to it, I'll ground the negative end of the chain, so the valves with greatest gain are least positive, and the elevated drivers will see 20-odd volts. I guess the ultimate would be to connect the centre of the chain to an elevated ground, but I don't care enough.

The amps have been happy for years as they are. Heaters are left at half power when not in use, so they've been on for years. There is no apparent difference in the condition of the valves with negative heaters.

I've been a bit lazy about bias I notice...
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Old 21st August 2012, 04:06 AM   #34
jt1 is offline jt1  United States
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While I am still on a pretty steep learing curve about all this it occurs to me that some confusion may come from the way this technique is
explained, as I recall I read that referencing a DC voltage allowed the AC componet to "Float" on top of the DC stablizing the leakage current from heater to cathode...or something close to that. I do know it helped to reduce hum in my 12au7 preamp.
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Old 21st August 2012, 04:15 AM   #35
AJT is offline AJT  Philippines
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yes, that's the thing....
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
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