Friday, February 11, 2011

Pultec Notes

Transformer hookup...

As of August 2010, there are changes to the Hammond 270FX power transformer that make hooking it up different from what is shown in the manual.

The 270FX does not have a center tap (yellow-black) for the 5v winding, so that wire does not have to be isolated (no existe).

Also, there is now an alternate primary winding: you can choose 115v or 125v. Schematic and details here:

Due to customer suggestions we have added additional windings and a tap to our "200" series primaries. This will affect all production after August 2009.


For those customers who need the 115V input - nothing has changed - simply use the White & Gray wires and tape the Black wire (see above schematic).


For those customers who are seeing too high a secondary voltage due to higher primary line voltages (roughly 125V input ) - use the White & Black wire and tape the gray wire (see above schematic).

I used the 125v primary and it worked great. My line voltage tends to run between 118 and 122.


* Put a 1K resistor across the FBK CT RES holes
* Hook nothing up to the ratio holes
* 1" low and 1" high must be jumpered
* Do not jump load 'alternate' near the star ground
* Plane lower right corner must be jumped
* That hole in the star ground is the absolute center of the star ground , a huge 20 gauge wire to the center of this to the transformer star ground would be nice
other wise use the audio ground
* the holes above the audio ground and power ground are for soldering 18-20 gauge wire directly to the pcb . This might be best , since the ground has to once again pass through metal of a lesser conductor then copper. Both these go to the transformer star ground.
* 1k before 2k in my opinion , the 1k softens the blow to the 2k,
* It looks like the 1K resistor in the feedback circuit is missing. That could explain it. (FBK CT RES)
* INPUT LOAD ALTERNATE leave it blank
* ISOLATE leave it blank
* The HIGH LOW MUST be jumped
* The ALTERNATE spot is for the loading resistor for the input transformer.
use the 620 spot parked above right of the input transformer .
* Wattages are on the schematic. For the power supply use 2w using the schematic , anything that doesn't have a wattage indicated IS a 1/2 w resistor

Switch Hookup

Don't forget the switch hookup change


Try getting a real good twist on the power switch wires.I use a portable variable speed drill to get them even and tightly twisted

I initially had the input ground connected to a ground near the XLRs. When I moved the input ground wire over to main star ground I had a huge -5.0db improvement in noise. I have 0.6 millivolts between the two ground points! (I think the case is the culprit.)

2. A -2.0db improvement from disconnecting the power switch wire that crosses the front panel. The 20Khz noise went away, too. I got most of this back by carefully draping the AC switch wire away from other wires and assuring that all the pots and switches were grounded.

3. I reversed the "polarity" of the 6.3 volt heater supply wires from the transformer. I read about this somewhere -- It worked! A solid -1.0db difference, though I had it the right way already. Go figure. (I got a -2.0db improvement on the LA2A).

4. I discovered I could lower the noise by about 1db by moving the input ground wire away from the back panel. So I began to suspect the transformer. I know you can rotate a toroid to improve noise, so... I unbolted the Hammond and turned it 90 degrees. Hum dropped by -7.0db and the 180Hz noise was gone! It is now oriented so the laminations are parallel with the case (the bells are up and down). Mounted on back of the case. Putting the top on now does not increase noise as much.

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