Replacement Output Transformer for Fender Studio Bass...

a fine line between stupid and clever

Postby dcgillespie » Sun Mar 08, 2009 8:28 pm

Chris --

I know my response is late -- but based on your original question, I wanted to give you some input about the Hammond transformer you questioned using, that I don't think was addressed in the other responses. I would also like to suggest in the kindest terms, that some of the information was simply wrong.

The 1650T transformer with screen taps would be a marginal transformer choice for the super twin -- and it would require more circuit modification than indicated to make it perform even at a marginal level.

Many people in the guitar amp world mistakenly refer to the Fender tapped screen circuit as an Ultralinear design. It is not, and the taps were not intended, nor are they capable of producing Ultralinear operation. Typical Ultralinear operation for 6L6 class tubes requires a 43% tap (based on turns ratio), which is about what the Hammond transformer provides at 40%. True Ultralinear operation results in much lower distortion, and much lower power output -- about 35 watts is the absolute maximum RMS power output that a pair of 6L6s can give in an Ultralinear design. Therefore, when using the tapped version of the 1650T in the Super Twin, the amp would not produce much more than about 110 watts continuous RMS output. For this transformer to work to it's best ability, the screen taps would need to be taped off, and then have a standard choke type filter system installed for the screen B+ take off. Under these conditions (strict pentode operation), the amplifier would produce about 165 watts continuous RMS output.

As for the actual design of the Fender tapped screen circuit, the purpose of these taps was a cost saving measure on Fender's part, in that they eliminated the need for the choke to provide adequate filtering for the output stage. That was it, and nothing more. This is not an opinion, but a fact that came out of a conversation I had with the circuit's designer. As such, the taps are appropriately placed at only 12.5% of the winding -- enough to provide a small amount of B+ filtering, but hardly enough to reduce power output and cause Ultralinear operation.

Finally, contrary to what was claimed, the Super Twin is quite capable of putting out 200 watts continuous RMS -- if the P-P impedance is correct to allow for it. In this case, the original Super Twin transformer has a P-P impedance of 1500 ohms, which is proper for six 6L6s in Fender's tapped screen design. This is why the Hammond transformer, as a 1900 ohm unit, will not allow full power output to be produced even without screen tapped operation.

When the impedance mismatch, inappropriate tap positioning, and power rating of the transformer are all considered, you can see why I maintain that this transformer would be a marginal choice at best.

Your final decision to operate the unit as is was best. Using the existing transformer into a 4 ohm load will reduce power output somewhat, and is a little harder on the tubes, but it is clearly the lesser of all evils.

David G.
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Postby jar240 » Sun Mar 08, 2009 9:33 pm

Hey dcgillespie, your timing is perfect! I gigged the amp this Friday for the first time, using a 1x15 8-ohm and a 4x10" 8-ohm cab. There were three acts using my rig, including an upright, a Fender Jazz and my Franken P, and it kept humming all night, with good output, although the whole chassis was HOT! I was running about 8 on the pre and about 7.5 on the master. It was plenty loud for any gigs I'll ever have.

I'm going to keep it as-is but will install a small fan in the back panel for longevity. Shlepping those cabs and that head instead of my usual, compact Euphonic Audio rig was a bit more work, but the sound was excellent!

Thanks again,
Chris
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