The AM Forum

Hi.  I acquired two identical Hammond 278CX 800vct 535ma transformers for my 813 amp project that I would like to connect in parallel (both primaries and secondaries).  I am using a voltage doubler, so I want the extra overhead. The amp will be on a dedicated 125VAC 20 amp outlet.  What should I do with the center taps?  Should I connect the center taps and ground or just tape and leave aside?  Alternatively, I could connect the center taps and use for reduced output while tuning (hot switching with a 10a 250VDC SPDT switch).

The amp is a x2 813 grounded grid that I am building.  Using a voltage doubler, I am hoping to obtain around 2.2KV on the plates and running the two 535ma transformers in parallel would give me plenty of room.  I had planned to connect the primaries and secondaries in parallel.

I would suggest putting the 120 volt primaries in parallel, as you planned anyway, and then placing the secondaries in series. You could then use a full wave bridge and not need to do a voltage doubler.

Either way you go, you'll need to make sure that the transformers are in phase. If you don't they could get destroyed when you apply power.

There's plenty of ways to ensure you have the connections right. I've placed the 120 volt primaries in parallel, but fed them with 6.3 VAC from another transformer. The reduced voltage makes sure nothing blows up. Then connect the secondaries in series. You should get double the voltage output. You'll need to do the math to figure out what that should be with only 6.3 VAC on the primary.

If you don't get what you expect, reverse the connections on the secondaries.

My 2 cents...

Jon

what is the specification on the center tapped secondary ... many older hv xfmrs require that the ct remain close to ground due to small amounts of insulation .... a hipot test will give the answer if the manufc'tr does not

Hammond power transformers are rated for use with the full secondary  and a full wave bridge. I would think that using the secondaries of two transformers in series might cause a breakdown of the insulation.

Pat
N4LTA

I've used voltage doublers, but I don't favor them. Given the choice, I go with a FWB when possible. One way to do that, having two transformers, is to build a FWB and filter on each transformer then "stack" (connect in series) the DC voltages. This method eliminates any phasing issues and uses the same string of capacitors that a doubler would use... just a few more inexpensive diodes.

The Hammond transformers you're using have a Hi-Pot test of 2,000V RMS, so I think you're in the ballpark... but close.

All unused secondary connections should be well insulated from ground.


Don

Also with a doubler, if you have to use it, give each transformer its own set of diodes so that the 'parallelled' secondaries will be isolated from each other just as with the bridge. Hammond winds to the exact turn number, but any small imbalance can still waste juice in a loop. hehe also give each one its own primary fuse.

If the thing is set up as shown, each cap is charged each half-cycle as opposed to one transformer into a doubler.

A bit off the subject but I have used salvaged microwave transformers successfully in pairs for HV power supplies. I have a supply that handles my BC-191 and BC653 transmitters made from a pair with choke input and it has been bulletproof for 5 years now.

The issue with using these babies is that one of the HV output leads is actually tied to the frame of the transformer. They used a half wave shunt rectifier system in the old monster microwaves. So it is downright dangerous to use a bridge on them, even if you manage to disconnect the winding from the ground. The grounded winding is simply not designed to be at a high potential. The other thing I had to do was knock out the chunk of metal jammed into the winding that acts as a lossy element which tends to even out the load in the microwave.

So you use the grounded lead on each of the transformers as your grounded center tap and conventional full wave 2 diode, and all is well. Oh and make sure you use fairly identical transformers! The economy hookup is a no-no, again because it raises the windings above ground. But a grounded centertap with a bridge is fine for a HV bipolar supply.

E-mail sent.

73DG

In my younger days I worked for a transformer manufacturer, a small outfit. To save time, their "Chief Engineer" would have us stack EI laminations three at a time instead of interleaving each layer. He also carried a fountain pen which he would use to scrawl illegible notes on the work orders. A fountain pen... in the 1980's, not the 1880's. He was a real dandy.

As I recall, he also wore a bow tie.  ::)

I know this is off topic but I'm curious if the profile picture for N4JOY is that of a Lettine 130 HF transmitter. I have one and it was my first plate modulated AM rig. I have never found a schematic or any info on it. It's a 6L6 oscillator, pair of 807's in the PA, a pair in the modulator and a 6N7 speech amp, tansformer coupled. Power was from a pair of 866's. In my youth I butchered it pretty badly for use on 200 meters (Pirate Radio) and I'd like to get it back to original. 

The profile pic is one of my Lettine 240s and not the rare 130s.  I have yet to find any documentation (other than an ad) on the 130.  I would love to get my hands on a schematic too!