p-90's and zero bias operation

[Andy Le Blanc]

I've been playin with some really old fashioned amp designs.... paraphase invertors and the variations of which can be readily found..... and zero bias operation of a pre amp stage..... the inspiration are all those cheesey amps that many older musicians seem to pine for.... some how it seems that the amps I have constructed useing a zero bias pre. favor any guitar with P-90, soap bars.... any make ..... they sound the balls..... but fender style single coils and gibson style humbuckers ( series wired or parallel) do not.... it makes for a quirky expectation of what an amp is suposed to do.... I have heard from more than one old fart that in the good old days a musician had to match the amp to the guitar... this seems to lend credibility.... zero bias uses grid leak thru a cap and high value resistor on the grid to create bias and wires the cathode to ground..... the plate resistor sets the gain ..... IE: the signal from the instrument first encounters a .02 cap then the grid (on the tube socket) and a 10 to 15 meg. resistor to ground..... why would it favor P-90's....

[muchxs]

I cloned my '62 Supro for a New Year's Eve project.

A supercomputer it ain't! At first glance it looks like there's almost nothing inside it, further investigation reveals the main circuitry to be half a dozen 1/2 watt resistors and a couple ceramic caps on an 8 lug terminal strip. There's a cap can, a couple more resistors and a cap at the input and that's about it.

Most of the Supros are grid leak bias. Long after Fender gave up on grid leak as obsolete Supro continued to use it.

I worked out three different layouts for it. The stock layout is pretty sloppy with much more wire than necessary. A tag board tidies things up but looks funny, there's all that board with almost nothing on it. True point to point is simple and direct. Pics are useless, there's nothing inside it.

[Andy Le Blanc]

Thats part of the attraction..... the simplicity.... the reason ,I guess, why its not favored is that theres a inherent distortion of the negative side of a sine wave being reproduced from the get go..... but that doesn't explain why it favors one type of pickup.... the amp is fantastic.... but only for P-90's...

[muchxs]

that doesn't explain why it favors one type of pickup

I suppose it's a matter of juggling component values just like any other amp. There has to be a way to make it work for Fender single coils, most of Fender's amps prior to 1954 were grid leak bias on the preamps.

Maybe that's a clue: Try a 5C3 Deluxe or a 5D3 Pro circuit. Use a 6SC7 tube in the preamp. Change that 10-15 meg resistor, Fender typically used 5 meg. Supro used 6.8 meg. If your preamp is wired for 12AX7s switch to a 12AY7.

[Johnhenry]

I've been useing 2 6SL7's for a few build's and other than a few tube 's being a little microphonic the tone is very good with my P-90's or the Strat, i got 2 more build's started with the 6SL7's,
The Supro, Magnatone, Valco, Silvertone, Gretch amp's can sound so good when Cranked,
Johnhenry

[Andy Le Blanc]

the current project is nearly identical to a 5C3..... and has 12AY7..... the 12AX7 has a more limited bandwidth but sounds phatter ...but the 12AY7 has the detail and sweetness... the next on the bench will be close to a 5D3 with 6SL7 thru out..... they'r great after warmup.... itll be good to A\B a paraphase with a self balanceing.... the grid resistor tightens the bass as it increases in value......and starts to roll of the highs going the other way.... so it may be that the right compromise is yet to be found.... but it sounds so good with a P-90 the way it is ... thats the problem