Voltage doubler? Unknown PT ratings.

[Phil_S]

Hmmm....I may be over thinking this....what do I do about the ground for this test circuit. It's all screwed to a wood board. Run it back to a tranny bolt? Ignore it?

[Voodoo_Man]

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.

I used the attached schematic for mine. No balancing resistors. I'll be adding those after reading NEETS. My PT has a CT also. Thought I would play around with a half power switch.

[skyboltone]

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.

Makes no difference Phil. Just cap off the CT.

[Phil_S]

Dan,
I understand about capping the CT. My question wasn't clear.

In the schematic, there is a wire that runs up the center. It starts with one leg of the PT, and runs between the two caps and the two balancing resistors. Should it be grounded? If so, how?

--Phil

[skyboltone]

Dan,
I understand about capping the CT. My question wasn't clear.

In the schematic, there is a wire that runs up the center. It starts with one leg of the PT, and runs between the two caps and the two balancing resistors. Should it be grounded? If so, how?

--Phil

No, no no. It won't voltage double then right? As the AC peaks + on the ungrounded set of diodes it loads the top cap. When the it peaks + on the bottome lead of the transformer it loads the bottom cap. The ground in this arraingement is only at the doubling caps. In your scheme, ground point B.

Here's another way to view it.
[IMG:929:622]http://i28.tinypic.com/sfgm4w.jpg[/img]

Dan

[Phil_S]

...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.

I built the doubler. I used a couple of recycled Mallory can caps that are rated at 40u, but meter at ~60u. I had to use two cans because of the ground arrangement. The balancing resistors are 220K 2W.

Loads are build with three 25W sandblock resistors. Load 1 is 5.89K and load 2 is 3.5K. Recall, that the unloaded HT secondary puts out 127-127VAC.

Through the doubler, load 1 = 644VDC and load 2 = 623VDC. I am measuring voltage across the two ends of RL, points A and B on the schematic. I get 109mA @ 644VDC and 178mA @ 623VDC Is this right?

I did not allow this to run for very long, perhaps a minute or two at most with each load. In doing a test, I was disinclined to allow it to cook until I could digest the results.

Ss this calculation right? 127-127 winding is 254VAC. 254VAC * 1.4 ~=360VDC. 360 * 90% = 324VDC (allowing for the load) as the expected output. On that basis and the test data, I think perhaps the reasonable use of this PT is no more than 110mA at the doubled voltage?

All in all, not a bad outcome, don't you think? Would you agree I could run a pair of 6AQ5 or 6V6 plus three 12AX7? Let's ignore design spec for plate voltage on the power tubes; I'm aware and I've gone over it before.

--Phil

[skyboltone]

Why double if you're going to use EL-84? I know that folks overvoltage tubes but that much? Only headache I think. Why not just bridge rectify the 254 volts and just ground the negative end of the bridge? THat gives you 355VDC. Right nice for 6BQ5 I think. Cap off the center tap. You can either self bias or take your bias off the Ht before the rectifier. The only thing I can think of to do with 600+ volts on that tranny is a single ended EL-34 or 6550/KT-88.

[Phil_S]

Dan,
I don't know how I missed this, but I must have my head up my you know what. I keep thinking about using a 250-0-250 or there-abouts, with a tube rectifier. What you say, it is the right approach. Thanks so much for setting me straight.

AFA mA, loosing the doubler, I'm north of 200. I can't imagine needing that much. Nice iron for cheap!

BTW, I can take fixed bias supply from the 34-0-34. At least it could be used for something!

Anyhow, many thanks for your guidance.

--Phil

[flood]

while we're on the topic of unknown PTs (WARNING: NOOB ALARM!) what would be the safest way to determine the current put out by the secondary of an unknown PT?

i have this tranny i salvaged from a radio, and would like to do a 2 x 12AX7, 1xEL34 build sometime. dunno if the pT will stand up to it - the radio it was in had a handful of tubes, with a 6BM8 output.

if it doesn't work out, i will settle for building the derailment.

[flood]

while we're on the topic of unknown PTs (WARNING: NOOB ALARM!) what would be the safest way to determine the current put out by the secondary of an unknown PT?

i have this tranny i salvaged from a radio, and would like to do a 2 x 12AX7, 1xEL34 build sometime. dunno if the pT will stand up to it - the radio it was in had a handful of tubes, with a 6BM8 output.

if it doesn't work out, i will settle for building the derailment.

[Phil_S]

Flood: If you know the equipment it was salvaged from, you know that at least it will support that tube compliment. For example, if there were a pair of 6V6 and a pair or three of 12AX7, you add up the current requirements. That set is about 100mA depending on the particular circuit, etc., and that is a very round number. From there you can always push the envelope by one preamp tube, as they don't use much, but it would be unwise to go to 4 output tubes, which really suck current. So, yours isn't exactly "unknown", it's unpublished, and there is a big difference.

The one I'm dealing with is NOS, NIB. The seller had no clue as to the intended equipment. So I must test it. The best course of action is to test it for the load you expect to apply. If you want to know the max, it requires you to let the smoke out, and what good is that? I'm saying that you should be satisfied to know it is appropriate for your intended use.

Read the thread carefully. The procedure consists of both trial and error, as well as educated guessing. The first hint is to look up the VA rating of the transformer based on the ohms you read across the primary winding. This is an overall limit. This is the lookup table that I received from what I consider a reliable source:

For a 120 volt AC supply the VA rating and primary resistance is as
folows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply. Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings.

In my case, the primary falls between the 225 and 300VA ratings, and sure enough, it proved to be about right.

On the HT secondary, one very general way, is based on a copper loss of say 4%. The HT winding rating is probably the most important. Find the secondary voltage, e.g. 300Vac. Take 4%, giving 12V. Measure the winding resistance; one half if 300-0-300V (i.e. a 300V winding), if bridge then the whole winding. The current will now be that which causes a 12V drop across the winding d.c. resistance. Thus dividing 300V by the resistance would give a ball-park figure for current (Ohms Law).

The copper loss method seemed wrong to me on this one. It weighs 8.5 lbs. and has a bunch of odd-ball secondaries. The HT is 127-0-127, and assuming use with a FW bridge, 254 * 4% ~= 100mA. This just seems way low. I'm thinking it is twice that. So, you need to think things through.

Heaters more difficult; same method, but low voltage winding resistance is usually impossible to measure accurately. The heater current could be expected to be in line for the output stage that would require the previously calculated anode current.

Generally speaking, though, allowing 15W or so as a starting point for the heaters is a reasonable guess (6.3v * 2.5A).

One degree further entails loading of the transformer. I did this with three wirewound 25W resistors, giving me a 75W rated load. If you do this with, say three 5W resistors, you will smoke them in less than one minute. Such a load across the ht winding for >1 hour should cause the transformer to get only slightly warm, as you still have no heater load, unless you simulate that too. Very generally heater and HT load can be assumed to have similar power figures.

This is why I put a load on the PT. When you know the load, you can easily use Ohm's Law (V=I*R) to determine the mA for that particular load. It is best to work your way down, from maybe 6-10K to around 4K -- the right values will change with your measurement of unloaded voltage, the physical size/weight, the estimated VA rating, and your measurment of voltage on the unloaded secondary.

Under load, a 5% drop in voltage is reasonable. A drop of 10% probably exceeds capacity. You can get close, but you will never find the exact number. Again, that is why you should determine whether it is suitable for your intended use, rather than to determine the true rating.

[Phil_S]

while we're on the topic of unknown PTs (WARNING: NOOB ALARM!) what would be the safest way to determine the current put out by the secondary of an unknown PT?

i have this tranny i salvaged from a radio, and would like to do a 2 x 12AX7, 1xEL34 build sometime. dunno if the pT will stand up to it - the radio it was in had a handful of tubes, with a 6BM8 output.

How many of what tube were in the radio? We can look up the typical current draw and tell you how to get that info. You add them up. Sorry, the other post is long and a bit technical. Not noob stuff.

--Phil

[flood]

Phil,

thanks a LOT for your patience in writing out that long post! i admit it has gone a bit over my head, but i'm astounded as to how much i learn on these forums.

i reallydon't remember the tube complement that well - there were about 5 tubes in the radio, o0ne of them was certainly an ECH81, ECL82 power tube, i think an EF86 and a couple more. i junked whatever was broken and one of the tubes that was useless to me and salvaged the variable capacitor, tranny and a couple of the tubes that remained.

i will go through my stuff, maybe i can get the complement together. but on the whole, it *mightÜ just work out for something like the projet G5...

and thanks for the heads up on the U=i.R *slaps head* ...and i dare to call myself an engineer? will check the DC resistance of the 6.3 winding sometime soon.

[Phil_S]

R for the 6.3v won't tell you much. Five tubes is a decent number. I would assume the heater winding has at least 2A. The exception would be if there was a 6.3v rectifier, in which case, its rating is about 1A more. Jus tdon't look to put in a high mA Octal tube like KT66 or EL34. I doubt it will be able to support the current draw or produce adequate B+ supply to wake up that sort of tube. You should be fine with 6V6 or EL84.

Put your meter on the primary and find out how many ohms it is and use the chart. Be aware, that unless you've got a Fluke, your margin for error for low ohms is fairly high. You can test your meter on a low R resistor and see what it reports on the display.

That VA rating pretty much sets the bar. Let's assume you've got only the HT and 6.3V windings. If the VA from the chart is 120, subtract 15 for the filament winding (or for each one if there are two. like a 6.3 and a 5.0). That leaves 105VA for the HT. If the HT puts out 200VAC unloaded, it will give you 1.4x through a full wave bridge, or 425VDC. If your capacity is 105VA (or watts), then you've got 105/425; almost 250mA. You can safely use 65% of that number or 160mA. So take that as your upper limit. An old radio PT won't be this robust.

So, just sub the correct numbers and I think you'll have a better than even chance of doing OK with it.

[jamdigs]

Phil, I must be missing something here! How did you arrive at 425vac in your last post? If you have 200vac times 1.4 you wind up with 280volts.