PT size and reality

[WRC34]

I posted this same concern on the TW section of the forum and got dead air. Maybe because I piggybacked onto an older thread. Was I jumping someone else's train?

Anyhow, I have a Stancor P-8356 PT that I intend to use in a 4xEL84 build. It's HV secondary is 270-0-270 rated at 260mA, BUT it's size is smaller than a typical JTM45 OUTPUT transformer. I'm going to guess that this PT was originally intended for some type of service in a television? Either way, I have a small family and our dough is quite limited. Building anything at all requires extreme determination, and I don't want to waste valuable time and money building yet another amp that I'll just take apart and have to repurpose.

I built an AC30 type amp last year that used a gigantic old organ PT (the thing must weight like 18 lbs and it is about doubel the size of your average JTM45 power transformer!), yet, somehow the amp was pulling an insane amount of current with the output tubes installed. I got it working ok for a few days and then it started blowing HT fuses. I increased the fuse value a bit but then it just ate rectifier tubes and would still eventually fail. The actual ratings on the PT were unknown, but voltages on plates, screens, filaments, etc all checked out fine. I went over the amp countless times and just couldn't figure it out, so I abandoned the project and took the amp apart to use pieces to make other amps.

In my attempt to build point-to-point only I've got a sort of system for wiring up a bias circuit within a fixed bias amp a la Marshall style. It works great in every amp I've used it in. I have also had success in cathode biased amps in the lower wattage range (15-20 watts) using either two EL84s or two 6V6s. Yet, success in building the more powerful, cathode biased quad EL84/6V6 amp still eludes me.

Is there a chance that despite the enormous size of the organ PT used in my failed AC30 build it simply didn't have the current to supply the output stage, and that this Stancor P-8356 will be fine? OR am I taking a risk by building with this tiny PT? Should I just suck it up and buy a PT from a known manufacturer who makes transformers intended for this specific application?

[pdf64]

Is the Stancor P-8356 appreciably more substantial than the PTs you've used for 2 x el84/6V6 builds?
If so, then I guess it's a degree of verification that the manufacturer's ratings are good.
Consider getting a ruler and noting down the thickness, width and length of the stacks of your various transformers; sometimes memory plays tricks, particularly if things aren't side by side.

[sluckey]

It's HV secondary is 270-0-270 rated at 260mA, BUT it's size is smaller than a typical JTM45 OUTPUT transformer.

That current rating is sufficient for 4 EL84s, but I'm surprised by the physical size. If the filament winding is rated high enough to light all the tubes in an AC-30 I'd use it.

Is there a chance that despite the enormous size of the organ PT used in my failed AC30 build it simply didn't have the current to supply the output stage

There's a very good chance that's the case. That PT could be so big just because it has to light up every tube filament in the organ. And still only be required to provide modest B+ current. If the filament leads seem extra heavy to you, that may be the case. But without seeing a schematic or the organ this PT came from, it's hard to say what the B+ current rating is.

[Malcolm Irving]

Because the mains frequency in the UK is 50Hz compared to 60Hz in the USA, power transformers can be a bit smaller in the USA to do the same job. Of course, most PTs are designed for international use anyway.

[Malcolm Irving]

...
I built an AC30 type amp last year that used a gigantic old organ PT (the thing must weight like 18 lbs and it is about doubel the size of your average JTM45 power transformer!), yet, somehow the amp was pulling an insane amount of current with the output tubes installed. I got it working ok for a few days and then it started blowing HT fuses. I increased the fuse value a bit but then it just ate rectifier tubes and would still eventually fail. The actual ratings on the PT were unknown, but voltages on plates, screens, filaments, etc all checked out fine. I went over the amp countless times and just couldn't figure it out, so I abandoned the project and took the amp apart to use pieces to make other amps. ...

Problems with excessive HT current are generally due to the power tubes or their bias. As long as the PT is providing the correct voltage it doesn't really influence the HT current.

[pdf64]

Your Stancor is listed as being a valid PT for Rocket here https://tubeamparchive.com/viewtopic.php?t=24912

[ToneMerc]

Should I just suck it up and buy a PT from a known manufacturer who makes transformers intended for this specific application?

IHMO, using an unknown PT can be an exercise in false economy. In general terms an American made laydown or upright PT that will support 40-50W P-P can be found shipped lower 48 to your home for $85-95.

If one has a 3 page thread on getting a PT to work and then once it's assembled lacks confidence in it's ability, then how much did that "free" PT really cost?

TM

[rp]

The old Stancor catalogs are on the web, you can double check it. Stancor seemed to keep the same models in their catalogs decade after decade so they were time tested. I'd be more worried it is a 115V and might run high, especially the heaters, bummer when you get >6.7V.

Maybe also compare the mounting holes to something current out there like a Hammond, in case you need to swap it out you won't have to chew up a chassis too much.

[Phil_S]

AFAIK, Stancor made great transformers and you should be able to rely on the published spec. Here is the item in the 1961 catalog -- last one in section B. If this is for a personal, one-off build, I would use it.

The only concerns I'd expect are related to the differential between the 117V original spec in relation to present day line voltage. I'll guess at 122v vs. 117v, a boost of 4%. For HT secondary, I would mostly ignor the 4% factor. This should produce filament voltage at around 6.6V, which isn't a problem. Since this beast is rated 6.3V @ 8.8A, you might find it running a little higher. If it is more than 6.9V, you can insert a wirewound 0.5Ω oe 0.25Ω resistor (you'll have to calculate based on actual load) between the PT and the first tube, and that should drop enough voltage without having to resort to superhuman tricks.

The last time I used an old transformer that gave me filament voltage that was too high, I had anticipated the problem. Since, in an ordinary build, I install transformers, sockets, and filament harness first, I simply completed that part of the build, put tubes in it, and dealt with it at that stage.

[WRC34]

Thanks for the replies everyone and sorry for the typos in the OP - apparently I was exhausted from working

The P-8356 I have is NOS and has a clear sticker on it with all the ratings. I just have a hard time believing that at its size it could manage to do what it says it can. Oh and I did not get it for free. I can't remember how much it cost me but it was a 'good deal'

rp, I like your suggestion of minding the mounting holes. In fact since it is so small I could always just leave enough open space around it to accommodate a larger/appropriate PT, if so the old holes would be covered up by the replacement, so no worries there.

Phil S - great idea regarding a small value resistor in series with the 6.3V winding before the first tube. I know a lot of builders out there use or re-use older PTs that are designed for 115 or 117 mains voltage and I have read numerous threads about filament voltage being too high. I'm surprised I've never heard this solution suggested by anyone before. I know you said wire wound but how high a rating would you use? Is 5 watts big enough to be safe?

In regards to the failed AC30 w/the massive PT, yes the 6.3V filament wires were exceptionally thick. Something dawned on me today while reading over these replies too - there are other windings on the PT and I think one is rated for 12 or 15V. Maybe some of these older organs only ran the output tubes on the 6.3V filament winding and ran the 12V preamp tubes on a separate winding? That would certainly explain a) the overload symptoms with the output tubes installed, and b) the presence of the higher voltage winding in the first place.

I'm going to go for it. I won't get to the build until April/May since I'm about to leave home for a 5 week tour. Plenty of time to think about the build, buy parts and draw up schematic/layout while I'm gone!

[Phil_S]

What resistor for dropping filament voltage? Here's how to figure it, but first, if it's under 6.9V, I'm not sure if I'd do anything because it could be challenging to achieve such a small drop.

Let's assume 4x EL84 and 3x 12AX7. (You will adjust to what you actually use.) These seven tubes require 6.3V @ 3.94A. Let's add something for the pilot lamp (#47 bulb needs .15A.) Round this off to 4A, which is close enough to make the estimate.

Let's say you socket the tubes and find the filaments are running at 7.0V. (I like round numbers for examples.) We need to drop 0.7V @ 4A = 2.8W. This resistor is going to get hot. Ordinarily, it derate at 2x, which is 5.6W. Given the abuse, I'd go with a 10W over a 5W if you can make it fit. If you find that 2x is under 5W, go with 5W.

What value resistor? Ohm's law is our friend. 0.7V/4A = .175Ω. I'd go with 0.25Ω, which you should be able to find. It is better to go higher than lower. If you have difficulty tuning the voltage, you can always try multiple resistors in parallel, in which case 5W rated should be more than adequate.

When you do the testing, do yourself a favor an use a terminal block which will allow quick swaps and accommodate 2 or 3 resistors if you need to go that route. I've posted a picture of the kind of block I like to use.

You may find you're dealing with 7.5V, in which case, the math says you'll need 0.3Ω. I imagine this is the worst case scenario. Using 0.25Ω (or maybe it is 0.26Ω that is the standard value) is close enough to give you an adequate voltage drop. Also, remember that these resistors are 5% +/- at best and with such small values, you just need to get close to get a satisfactory result. I say anything between 6.0v and 6.6v is an excellent result.

[rp]

You can get those <1ohm resistors as adorable little chassis mounts, that guess what - need adorable #2 nuts & bolts! Now you have a heads up.

[Phil_S]

Mouser carries regular sandblock 1% axial lead resistors in several values under 1&#937;. The resistors cost less than $1. Shipping is another matter. You might think about resorting to eBay.

[WRC34]

Outstanding, thanks for the formulas and the target voltage range Phil S!


I've attached some photos for scale and further inspection. The P-8356 is on the left, an old Stancor A-3802 OT in the center (which is huge) and a Triad choke on the right which is about the same size as an average 18 watt OT. The P-8356 dimensions are:

Height - 3 3/4"
Width - 3 1/8"
Depth - 3 1/2"
Core width - 1 1/2"

[drew]

You can also use diodes to drop the heater voltage. http://www.valvewizard.co.uk/heater.html