Who's done a buck transformer?

[briane]

Hey all, I need a little info on how to install a buck transformer.

Its in a SVT style build, 710vdc on the rectifier, need to drop it a bit. Just about everything is heating up too much or running red hot. Its down to bucking or mosfets to get past the heat buildup.

I'm using the kevin o'conner idea, with the 4 diodes and cap (see his SVT/tut3), though my output is at 160v...Which is not right, especially given that that the diodes should outward rectify at -160v. I'll admit using a smaller cap (22uf) across the diodes, though, as did not have the correct one in the junk box, not sure if that would cause this.

I'm using a pair of hammond 166n6's for the bucking transformers.

The way this should work is (if I can diagram - all in parallel)

dc in -> (<diode) BT (<diode) -> dc out at (Vin-160v)
-> (<diode) BT (<diode) ->
-> +220UF/200V ->

//< shows diode direction, BT is bucking transformer 120v taps, 6v taps connected to 6v/4a supply of other xformer

Can anyone point me at the right way to do this?

Thanks,
brian

[briane]

Heres a couple of pics of the beast. Dont worry about the 'smell' under those diode to the left, just a burned resistor of couple of months ago. Actually those diodes are the ones refered to in the posts diagram, as their the circuit.

[jaysg]

I've done it. You need to keep in mind that all the current through the amp, also goes through the buck power supply. Mine was a Condor 48V 5A supply.

[M Fowler]

OK I am not familiar with this Bucking bronc transformer thing, why in the hell would you need a transformer like that with what appears to be a 50-70 watt amp. My mig welder has a smaller transformer.

Mark

[Structo]

[dano-rator]

I can't see it in the picture, but I'm sure there is a forklift lurking in the background.

There is some info here on bucking, toward the bottom of the page:
https://tubeamparchive.com/viewtopic.ph ... light=buck

[Structo]

I don't understand why you would use that PT?

Why not downsize it so you can at least lift the damn thing?

[briane]

in regard to the xformer....This is a 300 watt 6x6550 build (as I said SVT)....Thats why...You either need a massive piece of iron, or a couple of xformers to get the plate current you need. If I did it again, I would put 2 formers together, and avoid the step down. All told theres 5 formers in that build, and one could argue for one more.

This is not a reference to the brand buck, rather to 'bucking the plate voltage down'. Sometimes called a step down xformer.

Even though conner says its all right, 710VDCV is too high for 6550's, and I have found that this design sounds best around 500 VDC....So I need to buck the voltage down.

I tried a 100 uf cap across the diodes last night (replaced 22 uf), worked and sounded good (22uf did not), but after2-3 seconds heard a cracking/popping noise (assume from the cap). got scared and shut it down. No idea what the sound was...I suspect its possible the 220 uf may be the ticket.

yep, its a beast at 60+ lbs. No forklift, but my muscles have grown.

thanks for any help you can offer.

[M Fowler]

I sure don't remember my old SVT being that excessive in the transformer section but I am sure my chassis was a lot bigger than the one your using unless the photo is playing tricks on my old eyes.

So you now have 4 diodes bolted to the chassis or heat sinked for voltage reduction.

I now know what your doing with the transfomer coupling but never heard of it being called bucking.

Could be an interesting build once you figure out what you need.
The SVT is a complicated circuit for my brain.

Mark

[breakfastbuddy]

well you can use kt 90 or you can use a buck transformer from center point at pt to ground negative , your 160 vac works good i think

[briane]

I sure don't remember my old SVT being that excessive in the transformer section but I am sure my chassis was a lot bigger than the one your using unless the photo is playing tricks on my old eyes.

YES. I have a SVT II sitting right here, rack mount, very nice, all tube, (likely pre-80's, I dont know...sounds great), and this is just a hair bigger overall, but much larger in the tranformers. Its what I had to do, and I have had the iron sitting around for a while looking forward to this...and the pre, well thats my own design, but basically is the marshall 1986 style with cathode follower pi, LNFB, and master vol. They actually are very comparable in bass tone,both sound awesome, though of course the SVT II has that nice EQ.

Not sure if those pics are coming through though....

So you now have 4 diodes bolted to the chassis or heat sinked for voltage reduction.

nope. this is KOC svt power section style off a hammong 717 for bucking in tut3. Rectifier into 1n4007 (band to rect.) to a hammond 166n6 (6v, 4a heater transformers - 2 of them, 1 for heater, second for bucking down the voltage), on both primaries of the buck tranformer. then each of the 2 legs through 1n4007 (band to primary) to output dc, with a 220uf connecting the in dc to out dc. Though so far I have been only able to squeeze 100uf in there, and that may be the problem.

Have not gotten to the zeners yet, but the'y would likely be too hot for what I want...The 500 ohm/150 watt resistor I was using for the initial drop resistor just got way too hot, way too quick. that thing is running at near an amp on the plates, and 10A+ through the heaters. Its like stacking 24 6v6 tubes on top of each other in some ways, just using bigger tubes.

[Andy Le Blanc]

Holy Cow......

Why not go with KT90? There are a few radio tubes that would be more suited.
May be even Svetlana 3CX300A1. It seems like it might be better to see if the operating
conditions and loading of that design would suit a different valve, rather than
modifying (limiting) the design to suit one tube or tube family.

[skyboltone]

I agree in a way with Andy on this one. I've fought the overvoltage battle in a variety of ways and none of them are ideal. The buck transformer program at least maintains the sag built into most power supplies. The mosfet or darlington cure with diodes regulates. I don't think amps sound good regulated.

A buck transformer is hooked up by using a separate lower voltage transformer wired "out of polarity" with the primary winding of the main power transformer.

First you calculate how much line current you want to lose. If you've got 710 DC you most likely have a 525-0-525 secondary voltage transformer. What you'ld like is about 375-0-375. So we need to buck 150Volts. The turns ratio of your 525 V tranny is about 1:8.75. So 375X2=750VAC V (the desired voltage) divided by the turns ratio of 8.75 means you need about 86 volts on the primary of your main transformer. So 120 line minus 86 needed means we buck 34 volts. The 34 volt transformer only need be rated at about .25 current of the full load. Ratio/proportion.

So we wire the primary of the buck transformer in series with the secondary, and hook line voltage to the primary leads, and we'll get a voltage between the outside leg of the primary and the free leg of the secondary. If we've hooked it up with additive polarity (boost) we'll get 154 VAC. If we hook it up with subtractive polarity (buck) we'll get about 86 volts. We use this resultant as the line into our main power transformer. The filament voltages will be all screwed up.

If you want to buck the secondary HT of the main transformer you'll need a 150 VAC buck transformer with the primary and secondary hooked together as explained before but with both the primary and secondary insulation rated for high voltage. It's screwy as hell to do it that way so we're back to start here.

Buy the right transformer to begin with. Call Phil at Heyboer and get the right one wound. He'll charge about $100 for one with that KVA rating but it will be perfect for the use and you'll be happier. I guarantee it.