Using Isolation Transformer and Voltage Doubler for B+

[avwalker]

I've been reading about this on some audiophile forums. Using a Triad N-68X Isolation Transformer (data sheet) (http://system.netsuite.com/core/media/media.nl?id=2638&c=ACCT126831&h=b50d30e579072bdce35e&_xt=.pdf) for $11 with a Voltage Doubling solid state rectifier to produce around 300V B+. The filaments can be powered by a separate transformer for about 6 bucks. What I'm confused about is what tubes this thing could handle. I've read that using a voltage doubler in this manner reduces the current availability by upwards of 80%.

Say we're using two EL-84's at 300V. The max current draw for the tubes is max plate dissipation (12w) divided by the plate voltage....or 40ma X 2 tubes for 80ma of current draw. Now if this transformer has a .435a current capacity, even if we reduce that to 20% capacity using a doubler we still have 87ma available.....and I don't plan on running the tubes at max value.

The single most expensive of most amps I've built is the power iron, and if an effective power supply can be built for only 20 bucks, it's a considerable cost savings. I just haven't seen anyone using something like this for guitar amp projects that most people are working on these days. I think univox might have done something like this back in the day.

Any thoughts?

[phsyconoodler]

no,it won't work.You have to spend the money and get a PT and OT that will work properly.Two EL84's will draw so much current that the PT will quickly burn up.Voltage doubling is only to double the voltage,it won't double the current handling of a PT.
Lot of amps use voltage doublers but they all can handle the current.The manufacturer saves money on a lower voltage unit,but it still has to be able to handle the current

[avwalker]

So what tubes could this thing handle do you think? Any options for using lower powered tubes like a couple 6AQ5? a SE 6V6?

[Trout]

So what tubes could this thing handle do you think? Any options for using lower powered tubes like a couple 6AQ5? a SE 6V6?


Why not just get one of these?

Allied Branded Hammond (https://www.alliedelec.com/Search/ProductDetail.aspx?SKU=2270081&MPN=6K56VG&R=2270081&SEARCH=2270081&DESC=6K56VG)

TRANSFORMER, PRI:110-120V, 50/60HZ, 540VCT @120DCMA, 5V @3A, 6.3V @3.5A, LEADS

That would cover loads of low power projects and is hardly a bank buster.

[avwalker]

being unemployed.....everything is a bank buster at this point. trying to make things as cheap as possible.

[BillyJoeJimBob]

Any thoughts?

No thoughts terribly useful... but encouragement to go for it and see what happens. Sounds like you've confirmed it's at least plausible and it sounds like you might be at the point of "one test is worth a thousand expert opinions". So I think it would be cool if you try it and pick some tubes that fit what you actually get out of the thing - and be sure to post your results! Although, I appreciate that that might not be what you're trying for and maybe you don't like a good experiment as much as I do. In such case, my thoughts are that the "bird in hand" is usually better on the pocketbook in the long run.

You could try a scale model, but it would be tough to do that cheaper than what you have lined up already.

Good luck and don't forget to post your results if you do it!

[phsyconoodler]

How about a pair of 6AQ5 tubes?Or what about a nice single ended amp?
Or an MA-1,with a single 50L6 power tube for about 3 watts?
All doable,and all sound good.

[ontariomaximus]

no,it won't work.You have to spend the money and get a PT and OT that will work properly.Two EL84's will draw so much current that the PT will quickly burn up.Voltage doubling is only to double the voltage,it won't double the current handling of a PT.
Lot of amps use voltage doublers but they all can handle the current.The manufacturer saves money on a lower voltage unit,but it still has to be able to handle the current

It'll work. The tranny is 50va and .435 amps. The OP's original estimation is correct and you would get around 300VDC. I would go with a Musical Power Supplies PT275 II though.

[Trout]

It'll work. The tranny is 50va and .435 amps. The OP's original estimation is correct and you would get around 300VDC. I would go with a Musical Power Supplies PT275 II though.


I was going to recommend that as well, but MPS is currently out of stock (http://musicalpowersupplies.com/6.html)on that model.

[ntasher]

You do realize that for a doubler to work you will need two high capacity/high voltage electrolitics per doubling stage? take a close look at the schematics of a doubler and you will see... plus the diodes must be over rated. The current spikes out of the tranny are 3-5 times higher than those of a standard PT arrangement. This will produce noise that will be hard to get rid of.
So you look at 3 electrolitics+diodes+6.3V tranny - kind of puts you back at square 1, doesn't it?

[donnyjaguar]

There is one benefit to voltage doublers in that the PIV of the diodes doesn't need to be as high as it would otherwise be. This was a design consideration in the 1950's and why you see then on some vintage (and very well made I might add) gear of that period in time.

I use them when I have a power transformer that has the VA but not the voltage I require. You can make a 300Volt supply right off the line cord, but these designs are inherently dangerous if you don't know how to tame them.

[Trout]

There is one benefit to voltage doublers in that the PIV of the diodes doesn't need to be as high as it would otherwise be. This was a design consideration in the 1950's and why you see then on some vintage (and very well made I might add) gear of that period in time.

I use them when I have a power transformer that has the VA but not the voltage I require. You can make a 300Volt supply right off the line cord, but these designs are inherently dangerous if you don't know how to tame them.


With all due respect, And I Mean VERY High respect,

IMO, you should not even make mention of anything line based other than warnings of eminent danger and death. Just as sure as bears shi****g in the woods some guy will fry himself.

There is only 1 way to truly tame line input, A proper transformer.
This is a public service announcement from the coalition of humans against funerals.

[avwalker]

I've also heard of people using the 230 Volt primary and wiring that as the secondary to step the voltage up. The person said it wouldn't make 230 volts....more like 200VAC.....but rectified that would be around 250V for the B+....great for a couple 6AQ5's

[PRNDL]

You might want to consider these

http://www.tubesandmore.com
P-T262E6 - $20 Primary 115 VAC; Secondary: 120 V, 87 mA; Filament: 6.3 V, 1.2 A
P-T31 - $14 Power: 8 watts; Primary: 5K ohm single-ended; Secondary: 8 ohm

Weber WRVBPT $25
Primary: 120VAC; Secondary: 260V/100mA, 6.3V/3A

Allied 6K56VG. $41.33
PRI:110-120V, 50/60HZ, 540VCT @120DCMA, 5V @3A, 6.3V @3.5A
http://www.alliedelec.com

http://www.musicalpowersupplies.com/
PT190V $33 190-0-190V CT @ 220mADC, 6.3@3A, 5V@2A
OT15PP $25 15W to 22W PP EL84 or 6V6 8000/7000 Ohm inputs with 4,8 & 16 Ohm Outputs

The main differences are how much power (current) they deliver, and whether it has a center tap.
You also save on shipping if you can order both from the same place.

[donnyjaguar]

Good point, Trout! I mentioned this because it is in designs of equipment I've serviced. I also have a large collection of 5 & 7 tube table radios and this is standard practice in those, but entirely for cost-saving reasons. In the case of these you have only the plastic or bakelite knobs protecting you from the potentially, and often, live chassis. A musician plugged into this type of design doesn't have this level of protection. Remarkably I've seen one in for repair with a plug that wasn't polarized. You plug it in one way and you get a jolt, the other way you (hopefully) don't. If your rehearsal space has bad wiring, see funeral above. Yes, this was another cost-reduction strategy of yesteryear.