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Old 7th January 2006, 07:49 PM   #1
5u4 is offline 5u4  United States
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Default Loftin-White

Hi, I'm new to this group. I'm working on a new project (FI Primer Loftin-White 2A3) & could use some advise. I ordered a custom power transformer & it came back with no 5V rectifier winding, so I'm considering the possibility of using SS diodes. Not really sure how bias for the 2A3 is arrived at. Guess 99V comes from the SRPP after it warms up placing the 2A3 at zero bias during turn on since 2A3 warms up before 6SL7? But what about the 390K load resistor to voltage divider. It seems there would be about 99V at voltage divider as soon as 2A3 warms up? Is there still about 99V through the 390K at 2A3 grid since there's little or no load? Can someone advise if SS is a viable option or not? What about using a Thermometrics Inrush Current Limiter (CL-70, CL-80) in power transformer primary to let all voltages come up slow reducing currert surges? Finally a standby switch would work for sure as long as no mistakes are made in turn on sequence. I'm really not open to using a relay, timer, etc. except as a last resort. It seems a tube rectifier solves all these problems. My schematic doesn't seem to want attach, so here's a link....... <http://tinpan.fortunecity.com/saints...mer/2a3-se.gif>Thanks , Jerry.
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Old 7th January 2006, 08:23 PM   #2
Eli Duttman is offline Eli Duttman  United States
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SS rectification is definitely viable. Use 1200 PIV Cree Schottky diodes from Parts Connexion. Watch the B+ rail voltage carefully. It will be higher than you would get with vacuum diodes.

There are honest differences of opinion about the best place to put NTC inrush current limiters. I prefer them on the rectifier winding, as it allows some time for the filaments to heat before the B+ rises. A pair of CL150s wired to the ends of the rectifier winding will get the job done.
Eli D.
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Old 7th January 2006, 08:27 PM   #3
Fuling is offline Fuling  Sweden
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Make sure that the tubes starts conducting in the right order, or you might get into problems when it comes to direct coupled amps.
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Old 7th January 2006, 10:46 PM   #4
TugaTweaker is offline TugaTweaker  Portugal
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And , please 5u4, would you be kind to tell us how it sounds ?

Good 2006 to DiyAudio community .

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Old 8th January 2006, 08:27 PM   #5
5u4 is offline 5u4  United States
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Thanks for the info about the zero recovery 1200v diodes, I have some Harris fast recovery diodes, but never heard of zero recovery. I will try them for sure. They're expensive, but not any more than a half way decent tube rectifier. I will post photos when I get her done, probably in a few months. OPT's are Transcendar, P-xfmr & choke are E-P.

Had another idea about how bias may work in this amp. Tell if I'm wrong. Since the 2A3 filament is at 144v, zero volts on the grid would place the 2A3 in cutoff. When driver warms up, 99v is applied giving -45v bias? Any DC experts out there?
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