Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Planobilly on November 25, 2016, 08:51:11 am

Title: Other than cost why use a voltage doubler with a center tap transformer?
Post by: Planobilly on November 25, 2016, 08:51:11 am

Hi,
Is there any advantage to using a voltage doubler style reficiter circuit other than cost of the transformer?

I have a transformer that puts out 186VAC center tap to each side of the winding. It is rated to about 750 ma. I need around 480V to 500V loaded plate voltage. I assume 186V X 1.4 = 260.4 X 2 =  520.8V unloaded. I am guessing the voltage drop to be less than 40V. This circuit is powering four 6l6gc output tubes and seven 12AX7 preamp tubes in a 100 watt guitar amp design.

Thanks,

Billy

Title: Re: Other than cost why use a voltage doubler with a center tap transformer?
Post by: sluckey on November 25, 2016, 09:10:48 am

I would just use that PT with a bridge rectifier.

Title: Re: Other than cost why use a voltage doubler with a center tap transformer?
Post by: PRR on November 25, 2016, 04:55:59 pm

> 186VAC center tap to each side

So end-to-end it is 372V?

As said, 372VAC in plain FWB comes out near 520V DC. Which is a fine value for ~~2KCT OT and 100W goal.

Ignore the PT CT unless you need a hefty 260V voltage tap. (Some of the very big pentodes at higher plate voltages are happy with lower screens, see 100W ratings on 6550 and 8417.)

Oh, the PT CT can also be a solid bias for stacked 350V caps to cover your >500V B+.

Title: Re: Other than cost why use a voltage doubler with a center tap transformer?
Post by: Planobilly on November 25, 2016, 05:54:03 pm

Thanks for the feedback guys. I am sure  the PT will work. I just have no experience with voltage doublers.

Billy

Title: Re: Other than cost why use a voltage doubler with a center tap transformer?
Post by: sluckey on November 25, 2016, 06:10:15 pm

Quote from: Planobilly on November 25, 2016, 05:54:03 pm

Thanks for the feedback guys. I am sure  the PT will work. I just have no experience with voltage doublers.

Billy

Don't use a doubler. Use a full wave bridge.

Title: Re: Other than cost why use a voltage doubler with a center tap transformer?
Post by: PRR on November 25, 2016, 08:55:40 pm

I don't know where you got that PT(??), but I'm 99% sure it was used with a simple bridge, NOT a doubler.

> I just have no experience with voltage doublers.

So get a small 12VAC CT transformer, diodes and caps, experiment. If you are very clever you could arrive at 36V, which is not very fatal. Better to learn at low voltage. Not like when I re-PT-ed an Ampeg and accidentally created a doubler (oops!), saw my "400V" B+ headed for the 800V range.

Title: Re: Other than cost why use a voltage doubler with a center tap transformer?
Post by: Planobilly on November 26, 2016, 10:15:31 pm

 The transformer in question is a new Mercury Magnetics APS FTW135-P. The output transformer is a Mercury Magnetics ToneClone O100JM-VZ.

I just went to a bridge.

Here is the schematic of where the PT came from.

http://schematicheaven.net/fenderamps/twin_reverb_sf_135_schem.pdf (http://schematicheaven.net/fenderamps/twin_reverb_sf_135_schem.pdf)

Billy

Title: Re: Other than cost why use a voltage doubler with a center tap transformer?
Post by: PRR on November 28, 2016, 08:15:41 pm

> where the PT came from.

That may be Fender's worst-drawn plan ever.

Let's untangle it.