Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on February 24, 2017, 02:09:48 pm

Title: How many amps should a Power transformer be rated for?
Post by: frank57 on February 24, 2017, 02:09:48 pm

What is the rule of thumb for amps and a Power transformer?
Let's say I have 4 x 12ax7 in the preamp and 2el84 in the power section.
Everything would run at 6.3 volts.
So I would need 4x .300=1.2amps
El84 =.760 x 2= 1.52 amps
Total is 2.72 amps minimum.
Do I need a PT that can supply 50% more (4.08)? 30% more?
 

Title: Re: How many amps should a Power transformer be rated for?
Post by: sluckey on February 24, 2017, 02:29:49 pm

I'd be happy with 3 amps.

Title: Re: How many amps should a Power transformer be rated for?
Post by: NewYorker on February 24, 2017, 02:35:42 pm

I'd be happy with three amps, too - Deluxe Reverb, AC30, maybe an old Ampeg for something different.

I'm drifting off topic, aren't I?

Ed

Title: Re: How many amps should a Power transformer be rated for?
Post by: shooter on February 24, 2017, 02:38:51 pm

Remember there are 2 currents you're interested in, the Fil, which you now have a consensus, and your B+ rail, a 50ma B+ might be sketchy, even though you have 3A for filaments.

Title: Re: How many amps should a Power transformer be rated for?
Post by: pompeiisneaks on February 24, 2017, 02:54:09 pm

Quote from: shooter on February 24, 2017, 02:38:51 pm

Remember there are 2 currents you're interested in, the Fil, which you now have a consensus, and your B+ rail, a 50ma B+ might be sketchy, even though you have 3A for filaments.

Shooter's right, usually they'll have a current rating for each winding, say 1A for HT, 3A for Heaters and maybe 5A for 5V rectifier.  Make sure you're good for all of them.  You'd need to repeat the math for the same tube complement for expected plate current for them all, and then check the rectifier tube's requirements as well, (If not SS rectified)

Title: Re: How many amps should a Power transformer be rated for?
Post by: Ed_Chambley on February 24, 2017, 03:46:41 pm

http://thesubjectmatter.com/calcptcurrent.html (http://thesubjectmatter.com/calcptcurrent.html)


The link above is handy in helping determine all winding amps. I also consider response I am wanting and order what will help me achieve this. I never considered the HT ma as long as I had enough.

Title: Re: How many amps should a Power transformer be rated for?
Post by: frank57 on February 24, 2017, 03:54:51 pm

Would this page be helpful?
https://www.ampbooks.com/mobile/vacuum-tubes/12AX7/ (https://www.ampbooks.com/mobile/vacuum-tubes/12AX7/)

Title: Re: How many amps should a Power transformer be rated for?
Post by: pompeiisneaks on February 24, 2017, 05:13:13 pm

This would be better: http://drtube.com/datasheets/12ax7-rca1962.pdf (http://drtube.com/datasheets/12ax7-rca1962.pdf)
but that's specific to RCA's but newer tubes are likely to be close to the same current draw, this lists both heater current needed per tube: 0.3 amp (300mA) for Parallel operation of heaters, and for plate current, 1.2 mA at 250 V, but then you need to add those up for each tube, and on top of that, you need to repeat the lookup of a data sheet for each tube type.  so your power tubes have datasheets as well, with plate current expected at different operating ranges, and heater current.  If using a rectifier, look up that for what current it needs.  Add each type up and make sure that winding works for the expected tube complement.

~Phil