Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ampnewbie on June 17, 2021, 08:46:09 am

Title: PT for S6420 Thunderbolt?
Post by: ampnewbie on June 17, 2021, 08:46:09 am
Hi all, I'm looking for help sourcing a PT for a S6420 build (schematic here: https://el34world.com/charts/Schematics/files/Supro/Supro_6420ss_tbolt.pdf). It needs to be SS-rectified (I don't have room for another tube), but I can't seem to find a non-center-tapped tranny that meets these specs. Anyone know of one that would work?

Thanks.
Title: Re: PT for S6420 Thunderbolt?
Post by: sluckey on June 17, 2021, 09:04:07 am
The Hammond 276X will work just fine, but just replace the FWB rectifier with a conventional two diode rectifier and connect the center tap to ground.

     https://www.hammfg.com/files/parts/pdf/276X.pdf

     ***CAUTION! DO NOT USE THE 276X WITH A FWB!***
Title: Re: PT for S6420 Thunderbolt?
Post by: dbishopbliss on June 17, 2021, 09:33:31 am
Quote from: sluckey on June 17, 2021, 09:04:07 am
     ***CAUTION! DO NOT USE THE 276X WITH A FWB!***
I wish I had a small fraction of your knowledge... what do you see in the 276X specs that tells someone to not use a FWB?
Title: Re: PT for S6420 Thunderbolt?
Post by: sluckey on June 17, 2021, 10:30:18 am
The 276X puts out 640VAC with center tap. A FWB will produce a B+ of 640 x 1.414 = 905VDC! That's a bit high for 6L6s.   :icon_biggrin:

But, using the 276X with a conventional full wave rectifier will produce 320 x 1.414 = 452VDC. And that 325VAC PT with FWB shown on his schematic will produce 325 x 1.414 = 460VDC. Of course all these B+ numbers are with no load. Once a load is connected the voltages will go down some.
Title: Re: PT for S6420 Thunderbolt?
Post by: PRR on June 17, 2021, 01:43:11 pm
Quote from: sluckey on June 17, 2021, 09:04:07 am
....replace the FWB rectifier with a conventional two diode rectifier and connect the center tap to ground.....

If the FWB is healthy and firmly nailed-in, you can probably get away with cutting the "-" leg.
Title: Re: PT for S6420 Thunderbolt?
Post by: tubeswell on June 18, 2021, 11:14:16 am
Quote from: sluckey on June 17, 2021, 10:30:18 am
that 325VAC PT with FWB shown on his schematic will produce 325 x 1.414 = 460VDC. … Once a load is connected the voltages will go down some.

Amen to that. I’ve built a couple of t-bolts using 325-0-325 high tension, and gotten a B+ of 400VDC with a shared 220R (10W) cathode resistor on the 6L6s.
Title: Re: PT for S6420 Thunderbolt?
Post by: SoundmasterG on July 01, 2021, 04:54:53 pm
One of the earlier schematics listed a replacement from Heyboer at my suggestion. I'm not sure why Sean took it off later versions of the schematic. Here is the info however. These were cloned from an original transformer. This is the non-tube rectified version.


Replacement Power Transformer:
Heyboer HTS-4664
Primary: 120VAC
Secondary 1: 325-0, 200mA
Secondary 2: 3.25-0-3.25, 4A


Hope that helps,


Greg