more high voltage supply bench racing

[imaradiostar]

For small amps intended for power amp distortion I like to have a stiff B+ supply- somehow it makes sense and I generally like the sound of it.

If I'm building a larger/higher power amp with KT88's or 6550's, do I really need that kind of plate voltage regulation? I don't intend to crank up a 100 to 200 watt bass or PA power amp up enough to create significant power tube distortion so how much reserve current do I need?

I have a PT from an old television that produces 800vct and about 8 amps of filament current. The PT secondary is about 40 DC ohms from end to end. It's a bit bigger than a Silverface twin PT I have laying around. I've spent enough time with Duncan's power supply designer and a volt meter to know it'll probably do well running a 100 watt tube amp with two 6550's or the like.

I have filament current to spare- so should I even bother to try it with four large output tubes? In power supply designer it makes about 530v at 200ma and 500v at 400ma...probably a good match for two pairs of KT77's or a pair of KT90's but I'm curious what others opinions on the subject might be.

Maybe 4 cathode biased KT88's?

I'm 90% certain it's going to end up in an amp with a pair of big output tubes and a 5k/3.5k potted output I have laying around but I thought I'd get some other opinions.

jamie

[booj]

Maybe 4 cathode biased KT88's?

my vote is yes

[Enzo]

All that heater current is what is making the thing large. It may make a high B+ voltage, but what kind of current can it provide there?

Apply mains power to it, and connect a load across the HV winding that would draw about what your intended application would draw at full power. How far does it sag?

[imaradiostar]

I guess I'm going to have to make up some sort of a rig with a bunch of 6L6's or something to load this PT and see what it'll make in real life. I'll just put a current shunt in there somewhere so I can measure what's happening. I figure loading it with tubes makes sense anyway because I'll be loading the filament windings too.

In the name of making it simple and balanced I'll cathode bias each tube then use a mosfet or another 6L6 connected to a pot to vary the screen voltage. This will even the load between the mismatched 6l6's and allow me to vary the current draw.

I suppose once it's built it can be used to test almost any transformer.

jamie

[Enzo]

You don't need a bunch of 6L6s to test the transformer, a resistor will do.

You know the voltage a winding produces, and you want to know if it will handle a current load. Ohm's Law will tell you the resistance that will draw your target current. And then calculat how much popwer that would be to dissipate. Then find a resistor reasonably close to that value. Put it across the winding and see if the voltage holds up.

Of course if you expect to have a steady stream of unknown transformers to test, then making up a variable load makes more sense.

[imaradiostar]

It's more a case of what I have around. I have a lot of 6l6's and old tube sockets and mosfets. I don't have a lot of resistors (or combinations of resistors) capable of sinking the current produced by a large power transformer.

I do have a bunch of transformers to test too...and I thought it might prove to be a useful rig.

jamie

[Enzo]

Then do it. You can use a 6L6 as a variable resistor, just watch dissipation.

[Steve Conner]

For tests like that, I mostly used series and parallel banks of light bulbs. We have 240V bulbs here, which makes things a bit more convenient.

I imagine a PT from a large TV would supply a fair amount of HT current as well as heater current. The 40 ohms end to end measurement is encouraging, and a bridge rectifier will give better winding utilization than the original one, which is good for a little extra "free power". If you know what rectifier tube or tubes it was used with, that would help too.

Somewhere on the net there's an old 60s magazine article by Norman Crowhurst or some similar guru. It explains how to ID unknown transformers and figure out their pinouts and ratings from scratch.

One trick I was taught in EE school is to measure the winding resistance when cold, then run the load you want to run for several hours until the transformer is good and hot, then quickly measure the resistance again. The temperature coefficient of resistance of copper is well known, so by this method you can take the temperature of each winding and see if any of them are running too hot.

All in all though, the "how long can you hold your hand on it" and "does it smell funny" methods always worked for me...

http://w2dtc.com/w2dtc-transformer-test-page.htm

[imaradiostar]

I thought I would post a reply from a practical test and ask for another opinion.

I wired up a test rig with an old amp chassis and stuck a bunch of tubes in it- total filament current based on the textbook draw for each tube ended up being 7.15 amps.

I borrowed a bunch of 300 ohm 50 watt resistors from the lab at work and wired them up as a temporary load bank. I rectified the 800vct winding using two 1n4007's and filtered it using two 120uf caps in series with 220k resistors in place to balance the voltage between the caps.

Here's a short list of the numbers I got- voltage is DC at the load, current was measured with a 50 watt 1 ohm 1% resistor between the load and ground.

526v @ 177ma
519v @ 217ma
512v @ 286ma
495v @ 414ma
490v @ 469ma
481v @ 537ma
470v @ 630ma
459v @ 767ma (filament voltage still at 6.4 volts)
450v @ 820ma

I also wired a second pair of 1n4007's to a Fender twin reverb choke into a 47uf cap and a pair of large 4k cement resistors. While the main supply was making 526v @ 177ma the secondary (choke input) supply made:

347v @ 43ma (8k load)
346v @ 86ma (4k load)

While the main supply was making 459v @ 767ma the secondary (choke input) supply made:

323v @ 81ma (4k load)

I let it sit and bake for a while (you could cook on these resistors!) and after sitting for a long while the high voltage supply was making 536v @ 180ma into a 3k ohm load and the secondary (choke input) supply was making 354v @ 44ma into an 8k ohm load. I can only guess the voltage changes are due to line current fluctuations- I didn't bother to use the variac, maybe I should have. Line voltage here usually hovers between 120 and 122 VAC. The PT is warm but you can still leave your hand on it without any trouble.

So...from the above info how hard would you push this sucker?

thanks again!

jamie

[Steve Conner]

If you can still leave your hand on the PT after several hours, and it doesn't smell funny, then it's still good for more!

60uF is a small main filter cap, and may give you an artificially good impression. A larger filter cap will lead to a higher RMS current in the windings for the same DC output current, so the PT will heat more for the same output current.

Do you know what rectifier tube(s) and what line output tube the TV had?

[imaradiostar]

It originally had a 5u4 for most of the mains filtering and a second smaller 6 volt rectifier for a part of the circuit- looked like the audio portion was on a smaller winding with the 6 volt rectifier but it may have been the flyback diode- I don't recall. The main heavy lifting on the set was with a 6bg6- I don't recall any other large tubes. I added up the winding current from the chart on the chassis and got 8.55 amps.

I left it sit for a while longer at the aforementioned "resting" current and the PT started to get a bit warmer- still not uncomfortable to touch but not as cool as I initially had thought. I might have to try some bigger filter caps like you suggest.

Either way- I think this 11 pound PT will be pretty happy in an amp with a pair of KT90's. I've been wanting to try them. I have a potted output transformer that weighs about 7-8 pounds and has 5k and 3.5k primaries and 8 and 16 ohm secondaries. I figure I can use it as listed with 6550's or kt88's and if what I've read about kt90's is true I can mismatch them down to 2.5k:4&8 and they'll be pretty happy. Either way I figure it'll be good for 100 watts or so. I'd prefer to have slightly higher plate voltage but it'll at least make for a good experiment.

I have another (12.5 pound!) PT that I tested on this rig last night- it has multiple secondaries that can be bridge rectified and connected in series. It made 605 volts @ 200ma and 512v @ 855ma. The way I connected the bridges also gives me a 388v "screen voltage" that sags to 332v at full load. It shouldn't be tough to make 200 watts with 4 6550's using this sucker! I have another pair of very large 10-12 pound potted 4k:4-8-16 outputs that I'll use with this PT or possibly even a larger PT. I'm tempted to order up some more Antek Inc toroids and see if I can hit 150 watts/channel in a giant boat anchor tube amp! I also have a pair of 4k:8-4-2 outputs that weigh 6 or 7 pounds each- I don't know how good their bass response would be but they're bigger than a twin reverb output transformer.

Lots of options- I'm ordering some large output tubes today so feel free to make suggestions!

jamie