Voltage doubler? Unknown PT ratings.

[Phil_S]

I've got an "unknown" PT that seems to have some promise for a guitar amp if it turns out to have enough mA capacity on the HT secondary. Preliminary testing suggests to me the PT probably has a VA rating between 225 and 300. There are four windings, two odd-ball ones and a 6.3v that is good for at least 2.2A (maybe more). The HT winding unloaded is 127-0-127.

The odd ball windings are 34-0-34 and 107-107 no CT, both unloaded. Running the 107-107 across a 1K 25W started to smoke the insulation on the leads. I'm guessing the capacity of that one is under 100mA. I didn't test the 34-0-34, but let's just say it's also 100mA for the sake of discussion and to make the math easy. So, those two together are roughly 7W + 22W, so lets round it off to 30W and double it to be conservative (you'll see, read on).

If I assume VA=225 (gotta start somewhere, this is conservative and based on the resistance of the primary winding), allow 15W for the filaments and a whopping 60W for the odd-ball windings, that leaves over 150W for the HT secondary. Let's round the output to 250V (see above 127-0-127). 150W/250V=600mA.

Is this math OK so far? I realize there is a foundation of assumptions that may not be correct.

My intention is to see if this PT will work with a doubler. I reason that I can get 500v at over 200mA on the HT secondary and if that's the case, I think I've got a winner on my hands. (LOL, building a 2.5x-er would be ideal.)

I have no experience with doublers. If I build this doubler http://www.tpub.com/neets/book7/27m.htm figure 4-49 at the bottom of the page (also attached below) for that unknown PT, any suggestions for C and R values? Is this just like any other power supply circuit? Maybe a pair of 20mf caps and a couple of 5K-10K for R1 and R2? Or do R1 and R2 need to be more like 100K? What about RL?

Thanks in advance for any advice you can offer.

--Phil

[Firestorm]

Gibson built a few amps with doublers. They used 100uF/450V caps. R1 and R2 were 220K/2W. The diodes should be the big ones -- 3 amps/1000PIV. I think RL is just the amp itself.

[Phil_S]

So far, this is good and thank you!

For testing purposes, should I assume that RL is the test load, and by deduction with Ohm's Law, I can estimate the mA capacity? My sense is that I can load it until I've got B+ @ ~250V (this depends on what it is unloaded), see what that load is, and then I have the answer for that particular application. If the PT isn't at boiling temperature, I guess it works. Does this make sense?

For example (and to simplify the math), I use a 1.5K 25W resistor and I get 250VDC, and all else isn't failing, then I can assume that I'm OK at 167mA? Or with a 1K at 250mA or 2k at 125mA? This seems too simple to be true.

[Andy Le Blanc]

when you double the volts you half the ma. I think.....
youll have to add up the draw of the circuit and do an estimate.....
you might what to look at a bridge doubler instead of the type youve posted
I think I remember reading that they are a bit more efficient....
so youll lose less ma. in the process... used to have a SG amp with 8417
as the output tubes..... had a doubler and sounded real tight

[Phil_S]

Thanks, Andy. I'm aware that the law of conservation of energy applies here. I just lack the practical experience to quickly select the right value parts for the test rig. I'm not an engineer and have no formal education or training in amp building. I wouldn't pursue this if I didn't think it was better than even odds of turning out to be a winner. The PT weighs in at 8.5lb, suggesting it's got plenty of juice in it.

If I could get 125mA on the HT secondary through the doubler, it's a keeper. I think that Firestorm did a good job of filling in the blanks.

[skyboltone]

I hate to be such an empiricist, but ignore the 34/107 windings, hook up your voltage doubler and load it with about 4k at 60 watts. (4ea 1k resistors, 15W, end to end) Anyway wire it up load it and if it drops more than 10% volts, no good.

[Phil_S]

I hate to be such an empiricist, but ignore the 34/107 windings, hook up your voltage doubler and load it with about 4k at 60 watts. (4ea 1k resistors, 15W, end to end) Anyway wire it up load it and if it drops more than 10% volts, no good.

Hi Dan. I am actually very pleased for this gimme. I can't see any plan for the two odd-ball windings, so there were going to get capped anyway. This is exactly what I needed.

The parts bin has an assortment of 5W 125...250...2k2 and 25W 1K+1K+1.5K. I can easily build the 75W 3.5K load. I am guessing that will work OK. If you think I need to push it up to 4K, I can add 2x 250 5W, so the string of them, at least in theory will be 85W rated.

I realize, the object is not to go too low on the load.

Many thanks.
--Phil

[skyboltone]

500/3500=143ma x 2 = 285ma on the windings. What are you going to build again? 500/6000 would put about 167ma on the winding. 83.5 watts. 83.5 x .5 for AB1 = 40 something watts out. Maybe no though. A pair of EL-34s need about 190ma at 500 or so to make 40 watts. That's about 95 watts in divided into 40 out gives 42% efficiency. So, we need about 380ma at 250? So see what happens at 285ma and go from there?

Dan

[Phil_S]

I didn't say what I'd build. In part, it might depend on what I've got, but probably nothing that beefy. I'd like add a clean-ish/Fender-ish type amp. I'm a big fan of the 6AQ5. I've got about 50 of them. I've also got some 6V6 laying around.

I can't remember where, someone posted a schematic (and piles of documentation) for what he called the Hound Dog Deluxe, a 5E3 variant, and the clips sounds very nice. We're looking at a ~20W amp; lets call it 22W to be conservative. The preliminary findings suggest this will be well within the capacity of the PT.

I"m testing just to be sure, or maybe, let's call in pre-design work, as I have no experience with doublers. Nevertheless, at 8.5 lbs, I'm thinking there's gotta be plenty of smoke in there. As you know, the object is not to let the smoke out.

If nothing else, I'll get some satisfaction just for solving the "problem".

When I get it built out, I'll post what I find out.

[Voodoo_Man]

I built that circuit a few weeks ago. With 135VAC input I get 385VDC 0ut.
A lot more than I expected . Using the same PT with a bridge I got 186VDC out.

God Bless,
John

[skyboltone]

I didn't say what I'd build. In part, it might depend on what I've got, but probably nothing that beefy. I'd like add a clean-ish/Fender-ish type amp. I'm a big fan of the 6AQ5. I've got about 50 of them. I've also got some 6V6 laying around.

I can't remember where, someone posted a schematic (and piles of documentation) for what he called the Hound Dog Deluxe, a 5E3 variant, and the clips sounds very nice. We're looking at a ~20W amp; lets call it 22W to be conservative. The preliminary findings suggest this will be well within the capacity of the PT.

I"m testing just to be sure, or maybe, let's call in pre-design work, as I have no experience with doublers. Nevertheless, at 8.5 lbs, I'm thinking there's gotta be plenty of smoke in there. As you know, the object is not to let the smoke out.

If nothing else, I'll get some satisfaction just for solving the "problem".

When I get it built out, I'll post what I find out.

I'd say you got plenty for a pair of EL-84s just by the weight. It's a wierd combo though eh? All those goofy windings.

[Phil_S]

I'd say you got plenty for a pair of EL-84s just by the weight. It's a wierd combo though eh? All those goofy windings.

Yes, a weird combo. That's a good reason to test. I'm unsure about the filament capacity. A 3 ohm load dropped it from 6.6vac to 6.35vac. So, it's good for something in the low 2's. Don't know if it will go to the middle 2's, even though we expect it would by weight. I figure 2 power + 3 pre.

[Phil_S]

I built that circuit a few weeks ago. With 135VAC input I get 385VDC 0ut.
A lot more than I expected . Using the same PT with a bridge I got 186VDC out.

God Bless,
John

Actually, the way I do the math, it's spot on. 135 * sqrt(2) = 190. 2*190 =380. That's close enough to 385. If your 135 was really 136, that would do it. Please clarify, that was input of 135-135 or 270, or 135 total?

[Voodoo_Man]

Actually, the way I do the math, it's spot on. 135 * sqrt(2) = 190. 2*190 =380. That's close enough to 385. If your 135 was really 136, that would do it. Please clarify, that was input of 135-135 or 270, or 135 total?

I was a little vague . The PT measures 135V total. It's known current rating is .5 amps @ 120V. It has a 117 primary and my power runs a little high. Haven't loaded it down but I'm sure it will work for my purposes.

God Bless,
John

[Phil_S]

Yikes! That means when I run mine thru the doubler, it should put out 360-360 without a load and with load, about 90% +/- and that would be around 320-320 or 325-325. We'll see soon enough. I don't have the resistors I need for the doubler.

I am suspicious because this winding has a CT, suggesting that the intended us was not with a doubler. Another day or two...I'll post findings.