Selecting a Power transformer for an amp build (specifically a spitfire style)

[guntherbuffalo]

Hi,

I am fairly new to all this, so I am learning bits and bobs as questions arise. There are plenty of holes in my knowledge that I am trying to fill as I go along!

Just looking for a basic overview on PT selection. As I understand it generally (I may be wrong), PTs secondaries provide a High Voltage supply to the amp circuit (is this also known as it's B+?) As well as this it supplies heaters at either 6.3v or 3.15v, and sometimes 5v for a valve rectifier. The PTs I have been looking at have a mA rating which is the current draw. The spitfire has a specific spec for it's power transformers, but I have heard of people using Fender Deluxe, 18 watt, and even Silvertone organ transformers in a spitfire style build so this suggests there is some room to play in terms of these ratings.

My questions:

-Can PTs that have a 6.3v rating for heaters be wired in a way  that they provide 3.15v like on the spitfire layout?
-How important is the mA rating, the Spitfire suggests 120mA, I am looking at a PT in the UK at 200mA will this do or is it completely out?
- I am hoping to use a SS rectifier, as opposed to the tube, which means (as I understand it) that the high voltage from the PT must be lower to compensate. I am looking at a PT that gives off 275v as opposed to the 290v suggested by the Spitfire layout. Would this suit?
- Am i right about what a B+ is?!

Any feedback appreciated!

[guntherbuffalo]

I guess what I am looking for is an explanation of the ratings on a power transformer, and how they all relate/interact within a valve amp.

[tubenit]

Looking at data sheets (see attached data cheat sheet) & the schematic, you'll get the idea that the EL84's in the Spitfire are pushed close to the max on the plates of the tubes @ around 350 volts.

So, if you used a PT that has 275-0-275 with:

5Y3GT         275 X 1.1  = 302.5v
5V4             275 X 1.2 = 330v
GZ34           275 X 1.3 = 357.5v
solid state    275 X 1.4  = 385v  exceeding the EL84 max plate voltage

IF you truly feel a need to use solid state rectification,  I'd advise a 250-0-250 PT with 120ma or more (more is fine).  That would put you at the tube's max plate voltage.

However, I prefer tube rectification and with that, I can substitute a 5Y3GT, 5V4 or GZ34 tube in the same amp for a variety of tones. And with EL84's,  I personally would prefer the volts to be 330v or less rather then have them pushed to the maximum.

with respect, Tubenit

[guntherbuffalo]

Thanks for the info. I think I will go for the SS rectifier. Does this PT seem ok to you guys?

http://livinginthepast-audioweb.co.uk/index.php?p=xfrmrvt1413

Thanks,

Paul

[eleventeen]

I guess what I am looking for is an explanation of the ratings on a power transformer, and how they all relate/interact within a valve amp.

Another way to look at it (which I prefer) is to figure the total CURRENT(s) the amp is likely to need and to be sure you meet or exceed that. Pretty simple, really, once you understand what the critical items are.

You have 3 windings, High volts, 5 volts for a tube rectifier, and 6.3 volt filaments aka heaters. The unit is milliamps for the HV and amps for the tube heaters.  

The tube rectifier is in 99% of cases either a 2 amp for a 5Y3 or 5AR4 or 5V4 or 3 amps for a 5U4. *IF* there is a 5 volt winding, typically and traditionally yellow wires with or without a center tap, it is nearly inconceivable that it would be less than 2 amps, because all 5-volt tube rectifiers use at least that much.

A tranny salvaged from a tube TV is generally far more likely to use a 5U4, but in any event, you really only need to ponder whether you require 3 amps or 2 amps and you need 3 amps *only* in the case of a 5U4. In essence, if you have yellow 5 volt wires coming out of your tranny, it's a certainty that you can use a 5Y3 or 5V4 or 5AR4 and the choice is generally one of which type you already have lying around AND which voltage drop you want, per Sluckeys' specs, above.

The tube filaments, generally 6.3 volts, you just add up the number of little tubes time .3 amps each. To that, you add the heater power for your output tubes. For 6L6, it's 900 mils, 900 ma, or .9 amp. Two of them, 1.8 amps. A Super reverb with 2 qty 6L6 (1.8 A) plus 6 little tubes @ .3 A each wants 4 amps, minimum. For your EL84's, .76 A or 760 mils. Oh yeah...and a pilot light! 150 mils. (not a big deal)

The true issue is how much high voltage (yes, "B+") current it can supply and what's needed is an assessment of what your output tubes will need. We think of 6V6 tubes needing 35 mils, tops, so your tranny needs to able to supply 70 mils, minimum, plus your preamp tubes, and for safety we'd probably like to see 100 ma. Your EL84's can use up to 50 ma, so we'd like to see maybe 120 ma. 6L6's can use up to about 75 ma, though they seldom do, but point being, we want to see somewhere near 175 ma for a dual-6L6-fired amp.

The tranny you pictured can be used placing the 240 HV windings in parallel (60 mils + 60 mils = fine) into a type of rectifier configuration called a full wave (solid state) bridge, and you'd have about 1.4 * 240 = 336 volts, which would be just about right. You won't use the 5 volt winding and won't have a tube rectifier.

Hope this clarifies.

[guntherbuffalo]

Brilliant. Thanks eleventeen, that clarifies a lot of things. Are there any specs in terms of full wave bridge rectifiers I should be looking for that would be suitable for this PT/circuit?

I shall be getting most of my parts from these guys, and here is a selection of rectifiers I have found:

https://www.tube-town.net/ttstore/advanced_search_result.php?keywords=bridge+rectifier&x=0&y=0


Thanks again for your help, it has answered a lot of questions and made a lot of things clearer!

[guntherbuffalo]

I am guessing this one is best:

https://www.tube-town.net/ttstore/product_info.php/info/p5393_Bridge-Rectifier-B500C1500-W-W-.html

as it has a max input voltage of 500v, so could handle the 330 or so from the PT.

[tubenit]

Two HT secondaries can be connected in series or parallel for 0 – 240 – 260VAC 120mA, or 240 – 0 – 240 and 260 – 0 – 260 full-wave, 60mA

I've never seen one advertised that way?  IF this means that the 240-0-240 is only rated 60ma, then I don't think that will work?  I hope you'll get some clarification from the seller.

With respect, Tubenit

[sluckey]

Does this PT seem ok to you guys?

I would not buy that transformer. The tubes in the Spitfire require 6.3v @ 2.12A and that transformer does 6.3 @ ONLY 2A. Now if I already had one of those transformers, I'd use it for my personal amp but not for something I might want to sell.

[eleventeen]

Tubenit, a preliminary read of the spec sheet shows that EACH secondary can handle 60 ma, however, I take your entirely valid point, that tranny *does* look a little smallish for that in my experience. Clarification with the mfr/supplier is indeed called for.

Agree with Sluckey that heater wise, this is just a tad on the skimpy side. However we are talking about a 5% "overload" (not much; .12 amp * 6.3 volts = .756 watts and since we are not using the 5 volt * 2 amp = 10 watts it's probably OK.) 

Gunther, in terms of diodes, one could use a packaged bridge as you show, but I would not be all that happy with a 500 volt one which is apparently entirely within specs. I would prefer to make one out of four individual 1N4007 diodes (or...faster recovery ones) which are rated 1 amp, 1000 volt. Or, use a 1000 volt bridge...which might be tough to source. 1N4007 are very cheap. No doubt there is ai different part number over there. One thing we like to do is to use parts well within their specs, not pushing the envelope. Never hurts. Alternatively, a 1000 volt bridge might cost 1.3 times as much but will potentially save you lots of grief by not blowing up.

[PRR]

> either 6.3v or 3.15v

No. All our standard tubes heat with 6.3V. (*)

However we often like a Center Tap on that.

With CT the heater power swings 3.15V on both sides, oppositely, semi-cancelling the hum.

So you do find PTs with _two_ 3.15V windings, which you will connect together to makes 6.3V, split both sides of ground.

(*) There _are_ plenty of non-6V tubes. The popular 12AX7 can be wired either 6V or 12V. Before 6v caught on (in the 1930s!), 4V and 2.5V were popular. 1V and 2V tubes for battery radio. Series-string radios and TVs had to scale the heater voltage by the size of the tube, so the various bottles had 3V, 9V, 25V, 35V, even 50V heaters. There's a 2-tube stereo phono design with two 60V tubes across the 120V wall-power (ouch!). There's people who love oddball tubes (they are less popular and sometimes very cheap), so you will see designs with 11V and 13V tubes.

[guntherbuffalo]

I will not be selling it, it would just be a gift for my brother.

How about this one:

https://www.tube-town.net/ttstore/product_info.php/info/p527_Toroidal-50VA.html

Sec 1: 250 V @ 0,12 A

Sec 2: 6,3 V @ 3 A

[guntherbuffalo]

And while we're on the subject, been looking at this OT:

https://www.tube-town.net/ttstore/product_info.php/info/p5905_Fender-Style-OT-25-W---8-Ohm-125A1A.html

[sluckey]

Both of those transformers look good.

[guntherbuffalo]

Next question is to do with the choke...

I have heard of a lot of people using a big resistor instead of a choke here. I think I know of someone using a 1Kohm 3 watt resistor for the Spitifire. Would this suit?

Is it as simple as just connecting the 2 ends of the resistor into the 2 turrets that the choke would be wired to? Or is there more to it than that?

Thanks

[guntherbuffalo]

Also - Just checking that OT would be fine with a 16 ohm speaker.

[eleventeen]

Next question is to do with the choke...

I have heard of a lot of people using a big resistor instead of a choke here. I think I know of someone using a 1Kohm 3 watt resistor for the Spitifire. Would this suit?

If you are not using a choke...then you are not using a choke. If you are not, and it is by no means compulsory, then the sole function of a resistor in the place of a choke is to reduce overall B+. End of story...well, 95% of it. It *does* provide a bit of inrush-current-prevention to your filter caps, but if your caps are over rated (and I hope you've been convinced that this is a wise move) then should you achieve a 350 volt B+ and that is your "target" for your B+, then you want to use 450 volt power supply caps (my opinion) and thus, there is no real need to "snub" inrush power to the caps. It could be argued that a choke could reduce diode-switching noise by a small amount. I have not tested that thesis but it would not surprise. *If* you are using a choke, it is traditional to power the output tube plates from your B+ BEFORE the choke and power the screen grids from a node AFTER the choke. There's usually less than 10 volts difference produced there...a little bit less on the screens than the plates.

Where people are replacing a choke with a resistor, it usually occurs when they are replacing an old speaker: the field coil of a field-coil (eg; electromagnet vs permanent-magnet) speaker not only provides the magnetic field in which the voice coil works, it acts as a choke. Neat in some ways, clunky in others. We are not necessarily happy about running B+ all the way out to the speaker, even though zillions of old radios/TVs did it. But it is a teeny bit different in the open back of an amp where we are likely to be throwing guitar cords (500 times) and beer bottles and the like....versus the back of a TV set that is "never" accessed.

*IF* you are doing such a thing, do not use a 1K resistor, but a 200-300 ohm one in place of the choke. And yes, it's simply a direct swap-in for the old choke. And...3 watts could be a little bit skimpy. Assuming your EL84's use 35 mils each and your preamp tubes use 5, total 75 mils, then the power through such a resistor would be (via ohm's law) given by: I [squared] * R .  So: .075 * .075 * 1000 = 5.63 watts, for your 1K resistor. Thus 3 watts is not enough, you'd cook it. But, .075 * .075 * 200 = is 1.13 watts, so a 3 watter is OK, if and only if your R is down in the 200 ohm range. Nobody would criticize for using a 5-watt resistor here, a 3 watt one *will* get hot.

Is it as simple as just connecting the 2 ends of the resistor into the 2 turrets that the choke would be wired to? Or is there more to it than that?

Yup.

Again, if you are happy with your B+ level as it finally comes out of the rectifier....under load of course...then there's no particular purpose for this resistor.

Now the other consideration is if the power supply puts out too many volts. In that case, placing the choke AFTER the diodes but BEFORE the first cap is known as a "choke input" filter....and that will probably reduce your eventual B+ by (guess) 35-60 volts. Just remember that if you need it. Better than a heat-generating resistor.

But...if you're NOT using a choke, then if your B+ is too high, you whack it with a series resistor. At 75 mils, every 100 ohms will eat 7.5 volts. 500 ohms would eat about 37.5 volts. Wattage? .075 * .075 * 500 = 2.81 watts.

[guntherbuffalo]

Awesome thanks for that. I think I will have to read that a couple more times to take it all in! I think I will use a resistor instead of a choke, as it will be lighter, easier to install, and cheaper than a choke. So I should look for a resistor of around 200-300 ohm, at around 5 watts, and just slot it in where the choke is meant to fit.

Just so you know, this is the layout I have been looking at in reference to my questions, and is the one I will most likely use.

http://www.ceriatone.com/images/layoutPic/matchlessLayout/SpitFireCeriatone.jpg

[eleventeen]

Most folks would simply mount the R we are talking about hanging off the filter-cap board, like the other ones do. There would be no choke, and you would just connect "A" to "B" together via said resistor. Now of course we become concerned that the leads are too flimsy to support it. It's always something. Know that any such resistor should have some air around it, meaning, it has to stand up on its leads a tad. It should *not* be mounted tight to the board. It will get warm.

It may thus be a bit smarter to construct your 200 ohm resistor out of 3 qty 600 ohm 2 watt resistors in parallel or 2 qty 400 ohm 3-watt R's. You'll note that most 5 watt resistors are of the rectangular "sand-cast" form factor. Like the ones Doug sells, the Xicons. They are heavyish. So maybe we don't want to have it "up in the air" on spindly leads. OTOH, 2 watt resistors are little guys and easily support themselves stood up off the board.

Just one of those arcane considerations passed on by the tribal elders.

[eleventeen]

One special consideration in the case of toroidal transformers is the mounting technique. With toroids, you have a sort of "use" [yoose] conflict: You need to mount it quite securely to whatever it is mounted to, but you cannot puncture the insulation, obviously. Toroids were sometimes mounted with a "U" bolts, but such bolts indeed acted like a half turn about the core of the transformer and a voltage differential can be produced from one end to the other! In some cases this produced weird eddy currents in the chassis, worse if the bolt got loose, worse if the chassis was steel and it corroded. This problem has in large part gone away with the more modern method of mounting toroids with the shaped washer in the middle, but you still to be sure you have a nice clean area of the chassis against which it will mount.

[HotBluePlates]

... I think I will use a resistor instead of a choke, as it will be lighter, easier to install, and cheaper than a choke. ...

I would advise against using a resistor, and instead use a choke.

Look back up at the schematic Tubenit included in his post. For the Spitfire, Mark Sampson copied the approach of Vox amps (as he did in basically all of his amps). After the tube rectifier, you will see power supply node A, then the choke, then power supply node B. At this point, Fender would have used a resistor between nodes B & C (in the Spitfire we see a 22kΩ here), and another between nodes C & D.

But the Spitfire, like Vox, has a parallel rather than series arrangement of the power supply dropping resistors. So there are two 22kΩ resistors from node B, each feeding their own filter cap.

Why did Matchless (and Vox) do this? Because they already started with a lowish supply voltage (Vox's was closer to 320vdc), and they needed to maintain the supply voltage as high as possible for successive filter nodes. Said another way, they didn't start with a high enough B+ voltage to all it to be thrown away in the power supply.

That's why Vox and Matchless used a choke in the first place. And they further used parallel filter nodes, even though this results in poorer filtering with the same parts, because a series arrangement would have give successively lower supply voltages.

If you're buying from TubeTown, any of their chokes look like they should be large enough to do the job. If you wanted to skip the choke due to budget, the Hammond 154G looks like the least expensive, and at 9H @ 40mA, appears to be ample for filtering the screen and preamp current.