Choosing proper PT

[fevzay]

How far above the tube recommendations are you guys going when spec'ing at PT? For an Vox AC30 (single top boost channel amp), I calculate the current needs to be roughly 4.5A for the 6.3V secondary. I'd get a 3-4A 5V secondary as well even though a GZ34 doesn't need that much.

Would you aim for 6A on the 6.3V?

How about the HT? How many mA would you recommend for an AC30? Looking at 290-0-290 or 300-0-300 PT's. I see the 290NX Hammond showing 264mA on the HT, but those transformers have gone up in price quite substantially lately!

Looking at other Hammond transformers...would you go for the 272HX or the 272JX?

thanks in advance!

[martin manning]

The vintage current specs are history; can’t change that! Here’s a lower cost alternative (@ $132): http://www.classictone.net/40-18050.pdf

[fevzay]

Thanks Martin! Yeah, I've seen people having good luck with MC.

Playing devils advocate though....what am I gaining by getting an "AC30" PT instead of just spec'ing out a 290-0-290, 6.3v @ 6-8A, and 5v @ 3-4A?

[pompeiisneaks]

In my understanding, if you have them build a custom PT for you, it's going to cost a LOT more than choosing one that's already right for the amp. That's why you'd want to pick those vs others.

Basically they know the AC30 is a very commonly built amp, and can sell them so they make up a bunch of them. OTOH if they're making one custom, it's a much larger time sink/cost for them, so they pass that on to you. To get a better price break, though, you could order them in larger quantities, 10/20/100 etc. and they'll split the 'setup' costs over the batch and therefore make it a lot cheaper for you per unit.

Same way most any manufacturing process works.

~Phil

[fevzay]

In my understanding, if you have them build a custom PT for you, it's going to cost a LOT more than choosing one that's already right for the amp. That's why you'd want to pick those vs others.

Basically they know the AC30 is a very commonly built amp, and can sell them so they make up a bunch of them. OTOH if they're making one custom, it's a much larger time sink/cost for them, so they pass that on to you. To get a better price break, though, you could order them in larger quantities, 10/20/100 etc. and they'll split the 'setup' costs over the batch and therefore make it a lot cheaper for you per unit.

Same way most any manufacturing process works.

~Phil

Hey Phil. I could have phrased my post better. Instead of saying "spec'ing" out, I should have said to use another off the shelf model. It seems that the opposite is happening in this case (paying for the name, not the specs).

For instance, the Hammond 272HX and 272JX are WAY less expensive than the 290NX (AC30 Vintage PT). Is there any advantage to the 290NX when the other PT's spec out close to the 290NX? My source has the 272JX roughly $70 cheaper than the 290NX (AC30).

[pompeiisneaks]

Oh gotcha, that gets into a more emotional discussion.

There are many that feel very strongly that iron that is done right costs more, and those manufacturers that make replacements for a specific amp are attempting to clone the exact way it was made to create some of that 'mojo'. I don't know enough to speak intelligently about it, as i've never side by side compared a 300$ PT and a 100$ one.

I can say, that so long as the PT has enough current handling for all three requirements and you've done that math, then yes, it should be fine.

You seem to have done the math but basically you look at each tube type in the amp, and see it's power requirements for the normal operation (milliamps range for 12AX7 and power tubes) and the heater requirements

sum those all up, and make sure the PT has that much or a bit mroe to spare so it runs cool. Then do the same for the 5V circuit for your tube rectifier and bam, you're done.

I think you already did that, as I said, so if your current requirements fit that PT and it's at the right voltage, you're in business.

The only way to know if the more expensive PT is better would be to build two identical amps, one with each PT and listen to them in the same cab for differences

~Phil

[fevzay]

Here's the math on these two. 290NX is $70 MORE than the 272JX.

Hammond 272JX
115/125VAC, 60 hz (would choose the 125V)
300-0-300v CT @ 287mA
5v CT @ 4A
6.3v CT @ 8A

Hammond 290NX
Univ. Primary
290-0-290v CT @ 262mA
5v CT @ 3.4A
6.3v CT @ 6.8A

AC30 (4- EL84, 3 12AX7)
6.3v = .9A x 4 + .3A x 3 = 4.5A needed
5v = ~3A common
HT = ~50mA x 4 EL84 + 2.4mA x 3 12AX7 = ~207mA needed

Seems like the 272JX would get things done easily. Even the 272HX, maybe.

I'm a little fuzzy on the plate current for EL84's. Haven't built an EL84 amp yet.

[martin manning]

Hammond 290NX has an unusual multi-tap primary. That and other things that make it a drop-in replacement may be driving up the cost.

[sluckey]

You're overlooking an important spec for PTs... Form factor, ie, footprint. Just because a standard off the shelf transformer has the same electrical properties does not mean it will physically fit the original chassis. For example, the AC-30 chassis is actually two chassis joined at 90°. And the PT ( as well as the OT) bolt to both chassis. This requires a certain physical size and form factor. That 272JX won't fit a standard AC-30 chassis.

No big deal if you're gonna build your own chassis and are not following the original layout. But it's a big deal if you want a 'drop-in' replacement PT for your old AC-30. So, a drop-in replacement must meet the original electrical specs as well as fit the original footprint. You will have to pay extra because it is more specialized and costs more to build.

Edit... What Martin says too. There's a voltage select switch on the original AC-30. That 272JX does not have the primary taps to fit that requirement.

[fevzay]

Great points, guys!

I'll be building out a project chassis, so I don't need an original fit. Definitely prefer having an X pattern as well. Trying to buy vertical mount PT's when possible for easy installation (less chassis work).

I was a little confused about the 115/125 VAC primary, but the 125 should be fine given I have a higher wall voltage (don't we all these days?).

I think I've talked myself into the 272JX (or 272HX). They should get the job done well.