Transformer for Big Rig

[stevef]

I have the opportunity to buy a big transformer that is 220:2700 (open circuit voltage) at 750 mA CCS.

I had thought about using it for a big rig project (1500 PEP AM Rig), but I'm not familiar enough with the vast variety of tubes that I might employ to use that transformer effectively.

Since I'm guessing this could produce about 3300 - 3400 volts on a capacitor-input filter, me thinks it's way too much for the 813s and 807's I have on hand.  Just more or less brainstorming at this point to find out if I should buy the transformer or not.  Initial thoughts are 4-400, 4-500, 3-500, 100TH, 250TH.  I'm excluding consideration of big new modern expensive EIMACs to keep costs reasonable.

Thanks, Steve, KK7UV

[W9ZSL]

Suggest rather than use a standard rectifier circuit, go with a solid-state bridge and use the taps on the seconday of the plate xfmr to get the voltage you're looking for.  Just remember with the bridge, you have to multiply the voltage by 1.4 no load.  To get 1500 watts pep, input to the final, you're talking roughly 750 watts.  But do you really need all that power when output to gain in signal strength is logarithmic? I do a lot of recording and constantly have musicians comming in and saying they can double the loudness on stage if they double their amp's power. Wrong!  A 100 watt amp would have to be 1,000 watts to equal twice the volume.  Mike/Passage

[w3jn]

Pair a 4-400s in class C would do Fine Business.  3KV good for about 800 watts carrier no problemo.

[Steve - WB3HUZ]

I'd be wary of using a bridge. If the tranny you have is center-tapped, then it was designed for a full-wave rectifier. Putting a bridge on it may exceed the break down voltage and crap out the transformer. Check the specs for the hipot, breakdown or max working voltage rating of the secondary.

Also,  if 3+ kV is too high for your needs, use a choke input supply. This should get you well below 3 kV, probably closer to 2500 volts. A plate voltage of 2200-2500 is perfect for a 500 watt input (375 watt output) or 1500 Watt PEP final.

[W9ZSL]

Choke input by all means.  Never saw a broadcast schematic that uses cap input.  I stand corrected on the bridge vis-a-vis CT xfmr.  That breakdown voltage is important. Are you going to use separate supplies for the Modulator and the RF?.  Passage

[stevef]

Perhaps I need to start with understanding the PS terminology.

Using the K1JJ design as an example starting point...

http://www.amwindow.org/tech/htm/813/813ps.htm

I interpret his supply as shown is either...

TFMR:   220:2000 @ 500mA
RECT:   Full Wave Bridge
B+ :   2000 v after capacitive input filter, with 10H choke on the ground return side of the bridge.

...or....

TFMR:  220:4000 @ 250 mA
RECT:  Full Wave C.T.
B+:   2000 v after capacitive input filter, with 10H choke on the TFMR center tap.

Is the position of the choke on the K1JJ design equivalent to a choke-input filter?

With that said, I am picking up the transformer today and will report back what I have in the way of secondary taps.

[WBear2GCR]

If the cap returns AFTER the choke (on the ground side) it is a choke input filter.
IF the cap returns before the choke (on the ground side) it is a cap input filter.

The choke is on the ground side to keep the HV reqirements off the choke - so it doesn't have the chance to arc to ground mostly.

             _-_-bear

[k4kyv]

I have the opportunity to buy a big transformer that is 220:2700 (open circuit voltage) at 750 mA CCS.

Is that 2700 volts with no midtap, 2700 volts with midtap (1350-0-1350) or 2700-0-2700 volts?

Capacitor input filters can be made to work with good regulation, but it takes a transformer with extremely low leakage reactance to avoid voltage sag under load.  Most transformers designed to run choke input do not meet those specs.  Some of the top of the line slopbucket linear transformers do qualify.

MV type Tube rectifiers don't like to work into a capacitor input.  The peak current rating of the tube is usually exceeded several-fold.  High vacuum tubes work ok, but they generate additional voltage sag of their own.  On the other hand, solid state rectifiers work well with capacitor input filters.  Silicone diodes have a very high peak-to-average current capability, but they don't tolerate reverse voltage, even for a microsecond, that exceeds their PIV rating.  So the diode can handle the high current peaks caused by the input capacitor, which looks like a dead short to any a.c. pulse coming out of the rectifier.  But conversely, solid state rectifiers don't like to work into a choke input filter.  It looks like an open circuit to the rectifier, and voltage spikes and transients that exit the rectifier are fully unloaded, and can reach peaks high enough blow the rectifier diodes - much in the manner of blowing a modulation transformer by running it without any load on the secondary.

I'm not sure how the rectifier stacks that replace rectifier tubes, or the plug-in solid state versions of the 872A, 8008, 575A, 866A, etc. work.  They must have some transient protection built around the individual diodes in the series stack.

I have never had any luck with series-ing solid state diodes to create a high PIV rectifier.  Every time I have tried, they end up blowing like a string of firecrackers.  But the commercially built solid state 872A's and 866A's in my Gates BC1-T have been working FBOM for several years now.

[Steve - WB3HUZ]

The HV supplies in the schematic under discussion are choke input.

[stevef]

Thanks for the clarification, and the tips on the rectifiers.  It was explained to me that with the choke in that position, the voltage rating of the choke is not as critical.  I intend to take this design and build slow and easy with safety number one.  I have no inention of just tossing something together to see what happens.  This is a learning project as well to improve my buidling and fabrication skills, as well understand all the theory.   Life is good - no need to destroy a good thing.

Okay, so here's what I picked up today (105 lbs.  ooomph!)

Technical Materiel Corp. TF-193
Dual Primaries (115/230)
Secondary is center-tapped "2800VDC, 750 mAdc" as it reads on the label.

Why does it give the secondary rating in DC?  Is it assuming some sort of standard rectification and filter?  Both my seller and I assumed the current rating was probably CCS based on the weight of this beast, and the fact that TMC was into shortwave broadcast equipment.

Can I use this with the aforementioned choke-input design for about 2500 volts for K1JJ-style 813 plate modulated AM rig?

[w3jn]

THat sounds like the power tranny from a GPT-750.  Strapping transformer indeed!

It's good for about 750 watts plate modulated AM in the '750 which had a pair of 4-250s (or 4-400s).  It's overkill for a Tesla 360 but that ain't necessarily a bad thing!

My GPT-750 has a homebrew modulator deck using 813s so yeah, the 813s can take the increased voltage.

[Patrick WA4QJU]

That is one heavy duty plate transformer, .750amp CCS,.  It will give you well over an amp of current in amateur service.