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Is it kosher to parallel PT?

4/20/2000 2:14 PM
Don Symes
Is it kosher to parallel PT?
Putzing with ideas for a 2x6V6 + 2xEL84 thing and would like to be able to run 'em all at once.  
Planning on a B+ a bit under 300V, but think I may need more than 60mA with all tubes running.  
Hammond doesn't make a more ampacious PT at an appropriate voltage, so practical options are requested.
4/20/2000 2:24 PM
Don Symes

Ohhhh. I was looking at the 300-series universal input PT. The 120-only line is broader.
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4/20/2000 3:14 PM
Unfortunately, no; at least not in general.  
For machine wound coils on a single core, *maybe* if the manufacturer recommends it. For separate transformers, I wouldn't unless I did some testing and perhaps some mixing circuits.  
Here's why - the primary and secondary coils may vary by a few turns if they're not wound identically. This makes for a slight difference in voltage on the secondary, and the circulating currents between the two secondaries will be equal to the difference in voltage divided only by the secondary wiring resistances. The currents can be very large, and even if benign subtract from the power capability of the transformers as they're powering each other.  
That being said it *may* be OK for the special case of  
- high resistance secondaries; tube power transformers may qualify, you have to measure the resistances; this will limit the circulating currents  
- addition of the outputs after rectifiers; the higher output will take all the load until it sags to the level of the lower output. Then they'll both conduct, but the higher output voltage will still be held at where it starts to sag, so it's working harder. At least you don't get circulating currents.  
To find out what you have, you measure the DC resistances of the primaries and secondaries of both transformers, refer the primary resistance to the secondary by the turns ratio, and then test the actual voltage differences. You power both primaries in parallel, and connect ONE secondary wire from each transformer together. You then measure the AC voltage between the other two secondary wires. One phasing will cause the two to add, so you'll get a very large voltage. The other phasing is what you want so reverse either one primary or one secondary. This will produce a very small, perhaps zero voltage. If the voltage you read divided by the sum of the secondary-referred resistances is a small enough current (maybe 5% of the rating) you may be able to go ahead and parallel them. If it's too large, don't.  
email with questions if this isn't enough.
4/20/2000 3:56 PM
Don Symes

Hmm. So the answer becomes either get one wound the way I want (by whom?), or do the Zener thing.
4/20/2000 4:03 PM
andy fuchs

Let not forget the other problem: The placement of the trannys (physically) can calso cause hums, and voltage cancellations and weird interactions you may not enjoy either.  
4/20/2000 4:59 PM
Ray Ivers

Would you consider a choke-input supply? If so, the Hammond 273X and 273BX would supply around 300 volts at 110 and 175 mA respectively (I would go with the BX, myself). The 193D, 193H, and 159S chokes seem OK to use (the 159S is unshielded, and will throw a hum field the size of a cantaloupe). Both PT's are set up for a tube rectifier (both can use GZ34/5AR4's, the BX can also use a 5U4 and a bunch of others), and tube rectifiers LOVE choke-input power supplies. If you're going to be running in Class A, I realize conventional wisdom says that a choke-input supply is overkill, but imagine this: a tube guitar amp completely free of ghost notes! If you're looking for the 'sag' sound, though, this is definitely NOT the way to get it.  
Just an idea: using an ultralinear output transformer (Hammond, again?) rip off the Boogie Simul-Class idea, but in a different way - connect an EL84 to the ultralinear tap and a 6V6 to the plate lead, and the reverse on the other side of the center tap. You won't affect DC balance/LF transformer saturation, but you will get the sound of both tube types at both loading conditions. I can't guarantee it will sound good - I've never tried it - but it WILL sound different, and isn't risky at all, really (the UL-tap-connected tubes will be loaded more heavily and will wear out somewhat faster, but should contribute a little more to the tone, theoretically).  
Ray Ivers  
R.A.G.E. Electronics
4/20/2000 7:53 PM
Don Symes

I was planning on AB1 mode and something a lot like the Blue Angel's 'Progressive Linkage (tm)'.  
You're suggesting:  
Plate - UL - CT - UL - Plate  
6V6 - EL - B+ - 6V6 - EL  
as opposed to the more obvious arrangement:  
6v6 - EL - B+ - EL - 6V6,  
I just may have to try both setups.  
I've also been assuming 'choke input' referred to a PI filter, which boosts my B+ by a good 60V. That correction alone does very good things for my heartburn.  
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