Power Transformer Questions

[Structo]

Hi,
As some of you know I am going to build a 100w nonHRM ODS amp.
I'm trying to figure out what power transformer to use.

I've been interested in trying the Magnetic Component brand and their Fender 100w Twin PT, OT and choke.
The specs are:
420v @ 450ma, 60v bias tap, and 6.5v @ 5.5A

I am thinking that the 420v is the under load voltage. It seems most of you guys are running higher plate voltages on your amps.
I will be using 6L6GC tubes by the way.

While other companies may state 325v @ 300ma. which is what my D'lite PT is. So that would be 650v before load.
It's a Heyboer and I end up with around 430-440v on the plates.
How do you figure out how much the load is going to drag the plate voltage down.
I mean, 650v down to 430v is quite a drop!

Seems I remember somebody saying you want at least 450v on the plates for ideal operation.
So any help on this perplexing thing would be appreciated.

[ChrisM]

650V is CT so its actually 325V per side.

325 * sqrt(2) = 459.55V

It will drop to around 440V with load. 440V is what your aiming for. Look at #124 notes.

I like the Hammond Twin PT myself.

[Structo]

THanks Chris, I actually know with the CT that you add the two halves.

Can you explain the square root of 2 formula?

I know the one with the 1.4, is that where you are getting that since 1.4 is the root?

I've tried figuring the tube load but haven't had much luck figuring it out.

I see that Merlin quotes a drop of 10%-15% but wouldn't that depend on the number of tubes and the type of tube.
That is the drop across the windings when current is drawn by components.

I've used that on PT's where they give the peak voltage.
You multiply it by 1.4 to get the actual voltage before load.
So a 300-0-300 tranny multiplied by 1.4 is 420v.

For instance, my D'lite PT is 325-0-325.
That should be 455v, but in practice I only get about 435v on the plates.
And that is only a two holer.

I'll check out the Hammond.

[talbany]

Tom
The voltage drop is generally 15-20v with a decent transformer 100w included..You never know for sure till you fire it up..

I use the 1.41 formula too and it's pretty reliable.. You can also download Duncan's Power supply calculator if you get into voltage doublers different rectifier configurations and filter cap values/ bleeders..Something to play around with..


http://www.duncanamps.com/psud2/index.html

Good Luck

Tony

[Structo]

The Hammond 290FX Twin PT only has 320v on the HT @ 450ma.
So that would be 448v without a load.
It does have a 60v bias tap and 5.5A on the heater winding.
$74.16

Here is a 100w OT for a Tone Master? Never heard of it.
http://www.tubesandmore.com/scripts/fox ... =P-TF26478

2350 ohm primary
4, 8, 16 ohm secondary which I want for versatility.
6.25 lbs.
$76.95

Compared to the Hammond Twin OT
2K primary
4 ohm secondary only, crap
5.53 lbs.
$70.61
http://www.tubesandmore.com/scripts/fox ... m=P-T1750W

AES has Hammonds 10% off right now too.

[M Fowler]

The Tone Master sounded pretty darn good I played through on not long ago.

[johan]

Have you check with Heyboer? They will wind it to your specs. Or maybe they have something on the shelf already

[mad_dog]

Mojo (heyboer) has a high power tweed twin PT that I was thinking of using so that you can use the 5v rectifier taps for the relays. I'd be interested to hear others thoughts on this one

[Structo]

I always forget about Mojo.
Heyboer huh?
I have been told that most of their trannys are made by Magnetic Components anyway.

They certainly are much more expensive, by about 40%.

Their blackface Twin PT seems to have higher voltage in the specs at 340v@450ma.
But it is $106!

The high power tweed twin PT is $128 because you have to buy the export one and only puts out 300v.
They don't even list the current supply in their specs.

[Merlinb]

How do you figure out how much the load is going to drag the plate voltage down?

You can't figure it out exactly unless the transformer manufacturer tells you what the transformer 'regulation' is, or if he tells you what the primary and secondary coil resistances are.

Incidentally, this will be covered in some detail in my next book! Guess you asked too soon

[Tonegeek]

Look at the MC Marshall PT if you want more voltage. I use the 50 watt ones and I got ~ 455-460V on the plates using their Marshall OT w/El-34s and ~475V using 6L6 with the Merc bassman OT. Plus they don't take up any room inside the chassis.

[M Fowler]

Tom I am using MC transformers Fender Twin reverb replacement in two of my ODS. Not that it matters but I can't see any reason not to and the price is good.

Mojo has dealer pricing and drop ship is only $5 so that is an option as well for a much lower price on their trannies. In Mojo's catalog it states they use three different companies and no I am not in a position to get the catalog and tell you the names, I can tonight.

[sergio]

I ever using the 125P34AX by TAD I think they buy from mojo! and I've always been good company!

http://www.tubeampdoctor.com/product_in ... cts_id=111


I would like try a PT from Magnetic Component but they answered that have only 110V type. This is strange!!

[mad_dog]

I always forget about Mojo.
Heyboer huh?
I have been told that most of their trannys are made by Magnetic Components anyway.

They certainly are much more expensive, by about 40%.

Their blackface Twin PT seems to have higher voltage in the specs at 340v@450ma.
But it is $106!

The high power tweed twin PT is $128 because you have to buy the export one and only puts out 300v.
They don't even list the current supply in their specs.

Last I heard all of mojos trannies were heyboer.

I actually need export PT so probably only looked at that. The specs for the Tweed twin Export are actually 350v @ 250ma (I'm guessing that is 250ma each side so 500ma total???)

MC don't actually do a Twin with export taps that I need unfortunatley otherwise I would love to try one, with all the good reports I've heard and the great value price point.

[David Root]

Tom there is a quick-check method for estimating what PT CT voltage you need for a given DC B+.

For example, you want 440 VDC. Divide by 0.95 for up to 3, maybe four 12A*7s, or 0.90 for more than that. Dividing 440 by 0.95 = 463. Multiply by 1.4, = 648. Divide by 2 = 324. So you need a 325-0-325 PT.

Note that this gives the CT voltage with no signal load. You should be OK with 300 to 310 CT if those numbers are rated at full stated load. But an earlier post is right too, there are other parameters involved.

0.95 I would use also with two power tubes. With four, use 0.90. That would change the above calc. to 342 unloaded, about 320 loaded.

However I have found that a useful calculation.

With an M-C Twin PT I get B+ ~438V with three 12AX7s and four 6L6GCs biased to 35 mA cathode current in my #124.

I would go with the M-C Twin at $64. How can you lose?