Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fiftynine on July 16, 2011, 02:06:24 am

Title: Wall voltages are not purely to blame for high B+
Post by: fiftynine on July 16, 2011, 02:06:24 am

Why do modern power transformers, even from different manufacturers give voltages that are always too high? I've just fitted a Mojotone export tap PT to a 66 Vibrolux Reverb and I'm getting nearly 355v at the rectifier off the 240v tap. OK, so there's 244v at the wall but I should see 315v and the extra 4v domestic doesn't account for the whole 45v.

It's not difficult to wind a tranny to spit out the right voltages. What's going on?

Are we expected to carry a variac around?

Title: Re: Wall voltages are not purely to blame for high B+
Post by: VMS on July 16, 2011, 04:26:00 am

If this is the PT you are talking about, then the 355V is as specified:

http://www.mojotone.com/amp-parts/transformers-blackface-power/Blackface-Vibrolux-Tremolux-Power-Transformer-Export-Taps.html (http://www.mojotone.com/amp-parts/transformers-blackface-power/Blackface-Vibrolux-Tremolux-Power-Transformer-Export-Taps.html)


Also, AA964 is the only schematic that shows 315VAC. AB568 and other schematics that have the same PT show 345VAC.

Title: Re: Wall voltages are not purely to blame for high B+
Post by: fiftynine on July 16, 2011, 10:15:56 am

Thanks, that's clear. I need the AA864 schematic but can't find it.

Title: Re: Wall voltages are not purely to blame for high B+
Post by: phsyconoodler on July 16, 2011, 02:33:49 pm

It's right here in the library of schematics.Scroll up and find it.It's there. AA864

Title: Re: Wall voltages are not purely to blame for high B+
Post by: fiftynine on July 16, 2011, 06:32:40 pm

Phsyc, I just read AA864 was plastered all over Vibrolux tube charts but no schematic was released. I can only find 964 but I'll go with that.

Title: Re: Wall voltages are not purely to blame for high B+
Post by: jeff on July 17, 2011, 08:18:59 pm

Are you saying the B+ is high or the AC at the rectifier tube is high?

 If the AC at the rect is right but the DC is higher it may be the rect tube. Not sure what you're using but if it's a 5Y3 don't use a Sovtek 5Y3. I had the same higher than expected voltage  problem when I started building and it was almost enough to make me give up.

I've heard that Sovtek 5Y3's are really another tube(5V4??) marked 5Y3, and not a true 5Y3. I tried two Sovtek's before I got bought a couple of brands of N.O.S. 5Y3's. Every N.O.S. I tried voltages are what they should be while both the Sovtek 5Y3's I tried in the same amp, the voltages are too high.

Even if it's not a 5Y3 you're using, if it's a Sovtek, I'd try a N.O.S.
If the AC is the same as on the schematic but the DC isn't that's what I'd try.

Title: Re: Wall voltages are not purely to blame for high B+
Post by: fiftynine on July 18, 2011, 06:26:10 pm

Jeff, It's solved since it was pointed out I should have 345v, not 315. I was working off an earlier schematic. I actually have 355 but expect that little extra.

Title: Re: Wall voltages are not purely to blame for high B+
Post by: fiftynine on July 27, 2011, 01:28:34 pm

Sorry, wrong thread.