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Confusion about a 5Y3GT rectifier

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I'm looking at the data sheet for my rectifier tube and something is confusing me. It says it has a filament voltage of 5V but it says that it is direct heated and it only uses 4 pins. I thought this meant that you hooked up the high voltage AC to two pins and you got DC on the other two. Where does the filament voltage enter into it?
That's correct. Pin 8 will go to your first filter capacitor, choke, fuse, or whatever your power supply requires. On a directly heated rectifier 5Y3, 5U4, etc the B+ can be taken from either pin 2 or pin 8, however if you were to use an indirectly heated rectifier such as a 5AR4/GZ34 the heater connections are still pins 2 & 8 but the cathode is connected to pin 8. So wire your output to pin 8 and you be able to use all of the common rectifiers. Using pin 2 on a 5AR4/GZ34 as the output will still work but usually not done.

An additional point, directly heated rectifiers, like the 5Y3 and 5U4, yield the best possible hum performance by taking the B+ from a center tap on the 5 VAC winding. However, types with cathode sleeves, like the 5AR4, yield the best hum performance by taking the B+ from pin 8, where the sleeve is connected.
Thanks for the answers - pretty much what I imagined! :eek:/
I'm building a 300B amp and do have one monster c-core Tx with 5v and 10A of 6.3v outputs but it has 250v and 450v HT too, whch is a pain as I'd rather keep the filament and HT supplies completely separate.
I was lucky enough to get a pair of monoblocks with Partidge c-core output Txs for £260 a few years back but their build quality was pretty slap-dash so I'm starting from scratch.
Filament is a constant current draw whereas HT varies with the music programme. You get a better sound with twin mains transformers.

I've had the luck to listen to and live with a lot of seriously expensive gear, there are amazingly delicate amps with single mains Txs, the Kondo Kageki (apparently it was labelled wrongly but Kondo Sans amps where all prototypes - it was a PSE 2A3 amp) was the only amp I've had thats reproduced orchestra correctly - I saw the Russian Ballet one Christmas, walked home and put the CD of the same orchestra on and it was just exactly the same sound. Remarkable!
5Y3 2.0 Amp filament. 5AR4 1.9 Amp filament. 5U4 3.0 Amp filament. All 3 types have different voltage drops from plate to filament when the load current is the same. All 3 types have different voltage ratings. All 3 types have different current ratings. The 5AR4 warms up the slowest (B+ delay). I think all 3 types have different maximum input capacitor ratings.

Other the above, and and any size differences, they are all the same.
5U4 is 450v 225mA 32uF output cap
5R4 is 750v 250mA 3uF output cap

5R4 sounds thinner but is better with 211 / 845 and lots of secondary capacitence after the current limiting resistor / choke. Inside the Ongaku is a section which is just lots of capacitors, I think it was 10x 100uF or 220uF 550v wired in pairs. I could be wrong on the wiring though, the HT is 1,250v on that amp, must be threes. Long time since i looked, might have to dig out the photos.
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