Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bugman3183 on April 13, 2018, 09:58:57 am

Title: Filament transformer
Post by: Bugman3183 on April 13, 2018, 09:58:57 am

Good morning fellas.   Probably a simple question but I've never dealt with this type of situation before.   Had a little misfortune recently where my son knocked an amp off the table (chassis only) and now after some testing I'm seeing that the heaters are only running at 3.9v.  Found a reasonably priced 12.6v transformer that I'd like to use for heaters only, center tapped and rated for 4a. And wondered if this would work.   Also, not certain on how I would have to wire this up.  Apologies if this has been covered before.   Thanks.

Title: Re: Filament transformer
Post by: sluckey on April 13, 2018, 10:29:32 am

Quote

now after some testing I'm seeing that the heaters are only running at 3.9v

Tell us exactly where each meter probe is connected when you measure 3.9v.

Title: Re: Filament transformer
Post by: Bugman3183 on April 13, 2018, 11:00:16 am

From pin 2 to 7 on 6v6's and  4 to pin 9 in the preamp tubes.   What prompted me to test, is that I couldn't tell the preamp tubes were glowing at all.

Title: Re: Filament transformer
Post by: sluckey on April 13, 2018, 11:30:35 am

That 12.6v CT will work. Just tape off one of the leads. You will need to use a couple 100Ω resistors to create an artificial CT if you don't already have an artificial CT.

Title: Re: Filament transformer
Post by: Bugman3183 on April 13, 2018, 12:15:23 pm

Transformer has a center tap.  So if I'm understanding you correctly,  you're saying tape off one of the leads and connect the the other lead to either the 4 or 9 pin of preamp tubes and ground the center tap ?

Title: Re: Filament transformer
Post by: tubeswell on April 13, 2018, 06:56:27 pm

If its a push pull amp with 6 volt output tubes and 12A_7 preamp tubes, see the attachments for options

Title: Re: Filament transformer
Post by: jjasilli on April 13, 2018, 08:14:50 pm

Tubewell's way works but requires rewiring all the tube sockets.


sluckey means to use 1/2 the 12.6V CT tranny.  Tape off one of the outer leads.  Use only the CT & the other outer lead as sluckey says.

Title: Re: Filament transformer
Post by: rake on April 13, 2018, 08:45:32 pm

Why not just get a 6.3v ct transformer?
they're cheap and readily available

https://www.alliedelec.com/triad-magnetics-f-16x/70218456/
https://www.mouser.com/ProductDetail/Triad-Magnetics/F-16X?qs=b1anAsPanWzPazOh%2FLHHJg%3D%3D
http://www.newark.com/triad-magnetics/f-16x/power-transformer/dp/03F960
https://www.digikey.com/product-detail/en/triad-magnetics/F-16X/237-1923-ND/5032172

Title: Re: Filament transformer
Post by: John on April 13, 2018, 10:08:20 pm

You did make sure there's not just a broken solder joint on one of your sockets? And did you measure with all the tubes pulled?

Title: Re: Filament transformer
Post by: Bugman3183 on April 14, 2018, 12:46:44 am

I did check for a bad solder joint and reflowed them just to be sure, but did not check the Voltages unloaded.  Will do that when I get a chance.   Thanks for the links for the 6.3v transformers.   All of the ones I saw when I initially searched were much more expensive for some reason.   

Title: Re: Filament transformer
Post by: Ed_Chambley on April 14, 2018, 02:36:30 am

You need to know the total amperage needed. 12.6 at 4amps compared to 6.3 v at 3 amps. Get what your tubesmneed but if You already have  the 12.6 it is quite better iron.