Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on June 26, 2011, 03:42:38 am

Title: B+ Caps
Post by: J Rindt on June 26, 2011, 03:42:38 am

http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_AB165.pdf (http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_AB165.pdf)

The last cap in a Fender AB165 is 8/450. Typical plate voltage in that area is about 220 volts. Is there any reason I cannot use a 10/350 cap, and have a trouble free power supply.?
Thank You

Title: Re: B+ Caps
Post by: kagliostro on June 26, 2011, 04:32:14 am

Quote

Typical plate voltage in that area is about 220 volts

If for in that "area" you mean near, Yes, but you are in the 320 B+ side of the anode resistor, not in the 220v side (near the plate)

I think you don't want to put a 350v rated cap in a 320v rail, isn't it ?

Kagliostro

Title: Re: B+ Caps
Post by: FYL on June 26, 2011, 08:44:02 am

A 350V cap is OK, but a 450V version will offer a good margin of safety. I'd also use at least 22µ here.

Title: Re: B+ Caps
Post by: jjasilli on June 26, 2011, 08:57:03 am

The schematics say 8uF; but most stock amps seem to have 16uF there.  I think the same is also true of some Traynor models.  People report that 8 vs 16 vs 22 makes a tonal difference.  I believe them, though I'm not aware of any scientific corroboration of the point.  Bass vs. guitar might make a difference.

Title: Re: B+ Caps
Post by: J Rindt on June 26, 2011, 10:00:15 am

10-4 to everybody.....
And yes, I totally missed the 320 Volt line.
Thanks

Title: Re: B+ Caps
Post by: Fresh_Start on June 26, 2011, 10:28:21 am

It's also worth emphasizing that the AB165 shows 425 volts at the Standby switch with all of the tubes installed.  If you powered that amp up without the power tubes installed, as you might during testing and almost certainly during initial startup of a new build, you would see more than 425 volts all along the power rail - including that last cap.  (320 VAC * 1.414 = 452 VAC unloaded)

HTH

Chip

Title: Re: B+ Caps
Post by: J Rindt on June 26, 2011, 10:45:23 am

I was just thinking about that (kind of). Like when trouble shooting a blowing fuse, and you have tubes removed to see what is going on. Thanks for brining that up. It is easy to assume the amp is always in schematic mode, bu that is frequently no the case.
Thank You