LTPI with CF stage---Fender Studio Bass Amp Output Stage

[nevetslab]

While on hold moving my Traynor YBA-1A Output Stage forward to make ends meet, I saw the thread just posted on the Fender Studio Bass Amp, which I've never seen before. When I clicked on the schematic, what caught my eye was the symmetrical LTPI driver stage, followed by a CF stage, feeding the power tubes.

This seems like what I recall member 'redelephant' describing while I was reading thru LTPI, paraphase and floating paraphase output stage circuit topologies. I'm intrigued by this, as it looks like you could drive the LTPI hard, and not have the grid current of the power tubes, when they go into clip, restrict the voltage drive of the LTPI. The CF stage would, of course. I haven't yet loaded the circuit into Multisim to see if I can get it to run. Not yet sure if that program will correctly run without a proper model of the output transformer, and I'm not up on programing component models to run in spice.

Does anyone have working knowledge of this output amplifier circuit with regards to the usual LTPI/output stage circuit, and how they compare or differ.

180W Studio Bass Amp Schematic.pdf

[Enzo]

Then also look up the SVT Classic, it also has driver stages.

[MarkusBass]

I think that they wanted to minimise distortion in the power amp. Please note that each CF stage drives 3 valves. Also the output transformer has "ultra-linear" windings (if this is the correct name). In Traynor YBA-1A you have only 2 power valves and most probably the output transformer does not have additional windings.
I simulated typical LTPI/output stage yesterday. I could also simulate Fender Studio Bass but I this I don't have model for 12AT7.

Mark

[nevetslab]

Yup....I saw the Ultra-Linear screen taps on Fender's OT. My current re-build on the Traynor YBA-1A is set up for KT-88's/6550's. I haven't had a chance to get back to making further changes to the output stage/LTPI stage. It had been suggested to me to change from the existing 12AX7 LTPI driver to maybe 12AU7 or 12AT7. Also to remove the 1k/5W screen resistors that are still present from the previous configuration when I had it built with EL-34's (now set up with a 300V Series Screen Regulator).

I was wondering if the added CF stage following the LTPI would perhaps raise the headroom of the overall power amp circuit? The present circuit topology, LTPI clips when the power tubes clip.

[nevetslab]

Ah, yes.....having just looked again at their multistage voltage amp with FB to it's cathode string/driver-PI / CF stages for the SVT-CL More stages than I have room for in the Traynor, but from the drive behavior characteristics, I'll take a look at that. I have a pair of SVT-CL's in the shop for service at the moment, so good time to take a peek at that for consideration. Thanks for the suggestion.