Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Captain chunkulus on November 12, 2018, 10:48:47 am

Title: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: Captain chunkulus on November 12, 2018, 10:48:47 am

Hey, I'm trying to find a power transformer that will do high voltage for a three tube high gain preamp that also has a secondary tap for the heaters.  I know I can do voltage doubling as an option as well. Trying to find something that could fit in a 1 rack space chassis. Any of you guys know of anything that might work? Thanks for any help.

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: shooter on November 12, 2018, 11:51:36 am

quick search since your parameters are pretty vague;

 https://www.edcorusa.com/xpwr011
800vdc, plenty of amps n TWO filament windings 6.3 and 5

might not fit in you're box  :icon_biggrin:

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: kagliostro on November 12, 2018, 11:56:21 am

Toroidal transformer 230V -> 270V, 14V

Primary 230V, secondary 270V @0,08A and 14V @2A.

Diameter: 78mm
Height: 37mm

https://www.musikding.de/Toroidal-transformer-230V-270V-14V (https://www.musikding.de/Toroidal-transformer-230V-270V-14V)

or

Toroidal 47 VA

Pri: 0-220-230-240 V
Sec 1:  250 V / 0,1 A, 
Sec 2: 6,3 V / 1,5 A
Sec 3: 12,6 V / 1 A
Size: d = 80 mm, h = 35 mm

https://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-47VA::5391.html (https://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-47VA::5391.html)

Franco

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: 2deaf on November 12, 2018, 02:55:30 pm

I don't know what country you are in, but if you can get Signal Transformer products and you are willing to use two transformers, LP-230-25 and LP-12-450 will fit into a 1U space.  I've used them when they were called Flat Pack by MagneTek and they worked fine.

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: DummyLoad on November 12, 2018, 10:39:16 pm

https://toroid.com/Portals/0/205.5076.pdf


1.4" high x 3.2" dia. should fit. use the washers.


--pete

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: nandrewjackson on November 17, 2018, 03:35:35 pm

Try the power transformer for the standalone fender reverb unit. I've used it for several preamps, I've used it for 4-12ax7's at a time. It may require a 2 space rack unit though.

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: Captain chunkulus on November 26, 2018, 08:03:41 am

The fender reverb transformer looks like what I'm looking for. Thanks for that.

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: Captain chunkulus on December 06, 2018, 08:40:08 pm

So I bought one of those standing alone fender reverb trannies. It says 306v dc. I'm assuming that's after rectification? I'm using a full wave rectfier for my build.

Quote from: nandrewjackson on November 17, 2018, 03:35:35 pm

Try the power transformer for the standalone fender reverb unit. I've used it for several preamps, I've used it for 4-12ax7's at a time. It may require a 2 space rack unit though.

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: PRR on December 06, 2018, 11:27:46 pm

The Hammond item has more traditional specs. 237 V 50mA AC, which is indeed liable to peak-up then sag to something over 300V 27mA DC. Plus 6V 1A AC for heat.
https://www.hammfg.com/files/parts/pdf/290WX.pdf

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: Captain chunkulus on December 07, 2018, 07:51:30 am

I hear ya, I was just wondering if this other transformers specs are indeed 306vdc after rectification or if it's 306v ac. The data sheet says 306v dc which I'm assuming means after it gets rectified.

Title: Re: Suitable transformer for 3 12ax7 high gain guitar preamp build.
Post by: sluckey on December 07, 2018, 08:47:52 am

The data sheet is very specific... 306vdc at 27mA dc with a FWB. Says nothing about AC voltage for that winding, but you would need approx 216VAC to produce 306Vdc with a bridge.