A Tale of Two Transformers

bad_rudy

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It isn't though... more like a question about two transformers. I am planning a blackface vibrochamp build for my dad for christmas. I have two PT's hanging around from old radios that seem like they would be in the ballpark of ok. One is a 275-0-275, and the other is a 325-0-325. Any thoughts on which would be preferred? The latter would certainly end up running a lot hotter b+ than the schematic, but I have heard some argue that it may actually be driving an old amp with modern outlet voltage that makes people love vintage gear so much. Others have argued using lower spec PT's to get the b+ back down results in a sweeter sound. Looking for general input.
 

2L man

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I think it depents what output transformer impedance you use? For 6V6GT push pull I have used 6950 ohms when PS output rating was 260-0-260 and did like its sound more than when 8000 ohms was used with 300-0-300 and 8500 ohms with 325-0-325. Amp circuits were also different but I think stressing tubes with more current produce sweeter distortion.
 

King Fan

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Good question: What B+ to target? Tough one. Why? Because the low B+ on the original schematics (~355) wasn't anything like what the amps ran in reality (which was more like the late schematics' ~410).

My experience? My '67 (late BF) VC came to me running B+ 420. I was able to get that down to ~400 when I cleaned up the circuit, restoring original resistor values etc. But it sounded better to me when I added a rectifier-to-board 'dropping resistor' and got it down to ~ 355. YMMV. Some folks like the original hot B+ sound.

As you'll know, it also depends on the total mA current rating. A 70mA PT will put out lower B+ than a 120mA as it gets loaded down more. I take it both PTs have the right taps for 5V and 6.3V? And both have the right dimensions for any chassis cutout you plan to use?

And as @2L man says, B+ does depend some on OT impedance. I've found this B+ calculator pretty accurate, and you can enter Rload (say 8000 for 8K) to get that detail.

http://thesubjectmatter.com/calcptcurrent.html
 

72_Custom

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You can also reduce your B+ with different rectifiers and/or resistors if it's too high, but you can't make it any higher if it's too low.
 

mjcyates

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I just built a Blackface Champ using the Hammond 290AX. I used the 275-0-275 tap. That gave me a B+ at 336 vdc. My bias resistor which is supposed to be 470 ohms measures out to 460 ohms.
 




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