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Old 5th December 2017, 01:13 PM   #1111
lehmanhill is offline lehmanhill  United States
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Kevin,

Thank you for your explanation. Part of my education from this project is the variability of tubes. I will get the plate - cathode voltage in the correct range, not sweat my measurement problems, and try different tubes.

Thanks again.

Jac
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Old 16th December 2017, 08:37 PM   #1112
drpro is offline drpro  United States
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ImPasse Preamplifier
A question for those that have used one of Jack's board's. I am finally getting to the stuffing stage of assembly. There are two cap position, C106 and C107 directly above the 6SN7 socket. They are not on the BOM. What value if any goes here?
Thanks
David
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Old 16th December 2017, 08:59 PM   #1113
mrdave45 is online now mrdave45  United Kingdom
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I think they're 0.01uf 600v ceramic disc, probably x rated (although I don't think they have to be, probably just because 600v caps are often like that) they're decoupling on the elevated heater supply, have a look here:
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Old 16th December 2017, 09:56 PM   #1114
drpro is offline drpro  United States
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Thanks Dave. Got it now, page 14.
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Old 17th December 2017, 02:30 AM   #1115
drpro is offline drpro  United States
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Another couple of questions from folks using Jack's boards. Referencing Jack's board, where does one connect the 6.3vac heaters? I think I would connect at the two side by side holes at the top of the 6DJ8 between R11 and R25. Just want to confirm.

Since the Allied transformer in the original build is no longer available I went with a Hammond xformer (272X). It does not have a center tap 6.3vac. Just 6.3vac on two wires. Will using this cause me difficulties? or ........

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David
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Old 17th December 2017, 08:00 AM   #1116
mrdave45 is online now mrdave45  United Kingdom
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You won't be able elevate the heaters without the centre tap. This will most likely result in more noise and/or hum. I'm not sure whether the design requires the heaters elevated to avoid damaging the tubes. Some designs you do need this other not. If you get hold of a 2x3.15 tx (wire like a dual rail supply and stick the elevated supply on the 2 wires you connected together, this is now your centre tap) or 6.3v ct tx of suitable current you could use that instead just for the heaters.
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Old 17th December 2017, 08:28 AM   #1117
jan.didden is offline jan.didden  Europe
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Why can't you elevate the heater without center tap? Just connect one of the heater sides to the elevation voltage.

Jan
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Old 17th December 2017, 08:29 AM   #1118
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by drpro View Post
Another couple of questions from folks using Jack's boards. Referencing Jack's board, where does one connect the 6.3vac heaters? I think I would connect at the two side by side holes at the top of the 6DJ8 between R11 and R25. Just want to confirm.
David
Can't you just do a continuity check with the DMM between those points and the tube socket heater pins? Rather than trust a reply that may or may not be correct?

Jan
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Old 17th December 2017, 12:37 PM   #1119
mrdave45 is online now mrdave45  United Kingdom
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I think the heater supply wants to be balanced for emi suppression and connecting one side would send this off kilter. Check this page out
The Valve Wizard
It looks like you can create an artificial centre tap.
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Old 17th December 2017, 12:51 PM   #1120
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by mrdave45 View Post
I think the heater supply wants to be balanced for emi suppression and connecting one side would send this off kilter. Check this page out
The Valve Wizard
It looks like you can create an artificial centre tap.
That depends on a lot of things, for example the heater inside the tube may or may not be symmetrical against the cathode.

Anyway, there is no reason a non-CT heater winding cannot be elevated and that was the issue.

And, as you say, you can always create an artificial CT with a pair of low value resistors.

Jan
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