Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on June 30, 2015, 11:19:40 pm
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Does anyone have experience in using different kind of PI like AB763 PI
together with cathode biased 5E3 power tubes?
Would it work?
/Leevi
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yes. graft in BF deluxe PI. discard NFB loop & insertion 47R. overall gain will be less with LTPI though.
--pete
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Maybe something like 5g9 tremolux without the trem and cathode biased.
http://ampwares.com/schematics/tremolux_5g9.pdf (http://ampwares.com/schematics/tremolux_5g9.pdf)
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Yes it will work. A blackface long tail pair phase inverter is a gain stage (the tweed Deluxe's cathodyne phase inverter isn't) so the amp will drive the power tubes harder which is a good thing.
The LTP PI also has a sweeter distortion tone than a cathodyne PI too so it should have a tighter, more modern distortion tone, especially when pushed with pedals near max volume.
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Yes it will work. A blackface long tail pair phase inverter is a gain stage (the tweed Deluxe's cathodyne phase inverter isn't) so the amp will drive the power tubes harder which is a good thing.
The LTP PI also has a sweeter distortion tone than a cathodyne PI too so it should have a tighter, more modern distortion tone, especially when pushed with pedals near max volume.
I could be wrong but think that a gain stage plus cathodyne like the 5E3 has significantly more gain than a long-tailed pair. OTOH cathodyne has unusual distortion when pushed too hard.
Chip
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You are correct Chip and the second half of your post explains why the long tail pair phase inverter is so ubiquitous in post tweed era guitar amps.
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> gain stage plus cathodyne like the 5E3 has significantly more gain than a long-tailed pair.
2X or 3X.
Separate gain-stage can develop full gain; long-tail gives half-gain to each side.
Further the separate gain-stage only has to drive the near-infinite load of cathodyne grid, and can be optimized for more gain (less load-pulling grunt). The long-tail typically has to face the power tube grids directly. If these are fix-bias then they should be around 100K, quite low for a gain-stage. (But we often cheat-up to 220K.)