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Old 19th February 2018, 03:24 PM   #1
Ritchie is offline Ritchie  Canada
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Default tubelab se jumper question

The tubelab se schematic calls for a jumper from pad3 to pad4 when using 300b.This seems to short the C2 filter cap in the 5842 filament supply taking it out of the circuit. Is this correct or am I missing something?
Assuming the C2 is required for lower noise from front end?

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Old 21st February 2018, 10:10 PM   #2
Tubelab_com is offline Tubelab_com  United States
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C2 does indeed get shorted out and is not needed if only 5 volt tubes are being used.

The jumpers and extra cap is a clever way to convert the rectifier seen by the filament regulator from a Full Wave Center Tapped setup when using 2.5 volt tubes (4 volts DC) to a Full Wave Bridge for 5 volt tubes (6.7 volts DC). This is required for operation with 2A3 tubes which will draw 5 amps.

The 5842 always sees a FWB rectifier and gets 6.3 volts DC regardless of what the filament regulator sees. C2 is a filter cap for the 5842 heater voltage when 2.5 volt tubes (45, 2A3) are used.
Tubelab, I blow stuff up so that you don't have to.
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Old 22nd February 2018, 03:30 PM   #3
Ritchie is offline Ritchie  Canada
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Okay, thanks George.
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Old 1st February 2020, 06:44 PM   #4
1001meier is offline 1001meier  Switzerland
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Default 2.5V filaments ar humming!

Hi George & Ritchie
looking for a solution for my problem I came across this thread. Maybe it contains the answer to my problem but I can't see it:
I have been using my tubelab SE with 300B tubes for a few years, lovely amp and absolutely no hum. I recently put in some old 45 tubes after lowerin B+ to 320V and setting the jumpers as indicated in the schematic (PAD1-4 closed, PAD5-6 open). I got the amp playing music, but with a lot of hum! Any idea what could be the cause and the how to solve it? (would another cap in parallel with C2 help?)

I also saw that the polarity of C2 is reversed in the schematic on the tubelab site, but on the silkscreen the markings are correct, right?
Thanks for the help
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