Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on March 20, 2023, 09:41:01 am

Title: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: jeff on March 20, 2023, 09:41:01 am

Hello,
I'm in the process of replacing a Crate V18's blown switch mode power supply with an Epi Valve jr. PT.
The Crate heats the power tubes and PI with 6V DC... But heats the two preamp tubes with +12.9V(series heater) and -12.9V(bipolar output full wave bridge rectifier with c.t. transformer to get + and - voltages)
I'm planning to use the Epi's 12V-0-12V taps but worried about the current draw.


Can we assume/deduce current ratings by looking at the fuses?


The 12V-0-12V taps have inline fuses rated: T1.6AL@250V
Each 12AX7 draws .150 plus the draw of the TL072 chips .0153333333
Total draw .330


Seem good?
Thanks

Title: Re: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: tubeswell on March 20, 2023, 10:12:09 am

Quote from: jeff on March 20, 2023, 09:41:01 am

Can we assume/deduce current ratings by looking at the fuses?

Yes

Title: Re: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: acheld on March 20, 2023, 10:17:47 am

And current draw is correct.

Title: Re: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: jeff on March 20, 2023, 10:18:38 am

Quote from: tubeswell on March 20, 2023, 10:12:09 am

Quote from: jeff on March 20, 2023, 09:41:01 am

Can we assume/deduce current ratings by looking at the fuses?

Yes

Thanks.
One more question:
Can we assume/deduce we can draw .330A from a PT with 1.6A fuses safely?

Title: Re: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: sluckey on March 20, 2023, 10:50:50 am

Quote from: jeff on March 20, 2023, 09:41:01 am

I'm planning to use the Epi's 12V-0-12V taps but worried about the current draw.

Are you certain the VJ PT has a 12-0-12 filament winding? All the schematics I find show a simple two wire 6V winding.

Title: Re: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: jeff on March 20, 2023, 06:42:11 pm

PT's marked
12V-0-12V   6.3v-0    260V-0
Blu Blk Blu  Org-Org  Red Red


The blues have inline T1.6AL 250V fuses
Orange has T4AL fuse


I'm worried the amp uses half wave rectifier to get 6.3V on PI and EL84s(2) lotsa filtering. Diode, 1000uF(X2)51ohm to ground, zener, 1000uF cap.

It uses the +17(through 22ohm resistor + 2200uFcap) for 12.6V on one preamp tube(series) and -17v for the other.


Hoping to full wave bipolar rectify the 12v-0-12v to get the +17/-17 to run the heaters and tl072(x2) chips.

Title: Re: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: acheld on March 20, 2023, 09:20:41 pm

 :icon_biggrin:   They musta got a great deal on the PT they spec'd.   

But, if it works, it works and  :worthy1:

Title: Re: 12V-0-12V Transformer w/ T1.6AL 250V inline fuses.
Post by: jeff on March 21, 2023, 05:50:45 am

Yeah,
Think they used the same PT on Jr. Sr. Etc.
12V taps weren't used on Junior(heat shrunk ends, n.c. twist tied) so maybe that's why it doesn't appear on schematic. ???
I may have an early, early, revision. ???