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Old 3rd July 2003, 03:17 AM   #1
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Default Frankenhouse

My latest semi-project.. modding the ol' Frankenhouse. (Name is a contraction of Frankenstiened Westinghouse, referring to shoehorning in a Philco PT and filter cap, which boosted B+ about 150V...)

Started off as uh.. I forget what it used to be. 12AX7 something I guess. 6V6 PP cathode biased output. 2nd latest mod put in a 12AT7 diff. amp/phase splitter and fixed bias (-26V) for the 6V6s, but no global NFB.

Current mod:

Click the image to open in full size.

Comments?

Tim

P.S. I brought this up in the class B thread because I figured it was pretty close to that.. guess I really need more like -50V available for bias eh? (Current range is -18 to -36V.)
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Old 3rd July 2003, 07:47 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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How about a dual triode like 12AU7 as phase splitter? That half a tube could still be useful...

And how about tone controls? No-one does tone controls any more
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Old 3rd July 2003, 09:19 AM   #3
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If I used a 12AU7 I'd need like 10V input! Even with a 12AX7 I wouldn't have enough gain. That's what led me to a pentode front end.
Plus I don't feel like cutting another hole in the chassis.

Tone controls are on my preamp, which is right beside me in the desk drawer -
*taunts other members by turning up the radio without moving away from the computer*

Tim
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Old 6th July 2003, 08:12 PM   #4
Joel is offline Joel  United States
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Can I suggest a .22uF coupling cap with the 100k grid resistors following? 0.1 seems a tad low.
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Old 6th July 2003, 09:40 PM   #5
jam is offline jam  United States
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Tim,

Can you self bias the output tubes and is there any advantage to doing that?

Regards,
Jam
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Old 6th July 2003, 11:57 PM   #6
Joel is offline Joel  United States
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The only advantage is that you don't need to make adjustments to the bias value - if that can be seen as an advantge.
The disadvantage is reduced power, added heat, and no adjustment room.
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Old 7th July 2003, 04:17 AM   #7
jam is offline jam  United States
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Joel,

Thanks for the reply.

Regards,
Jam
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Old 7th July 2003, 10:29 AM   #8
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Quote:
Originally posted by Joel
Can I suggest a .22uF coupling cap with the 100k grid resistors following?
No.

j/k. Don't think I have any .. well, the block shaped things ya pull from computer PSUs maybe.. oh well, doesn't really matter to me. It's an old commercial OPT, can't be too great! Even so it sounds pretty good, nice bass.

Jam: The tubes cannot be self-biased because then it would have to be class A. At 400V, this would melt the tubes! And the 6V6 is hearty as red-plate goes...

Tim
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