some questions about tube amp build

[abbey road d enfer]

I just wanted separate adjustable bias...

Why not? It allows finer balance of current in the OT. However, it is not a guarantee that the output halves will be more precisely matched. In conclusion, it does not free you of using matched pairs.

the PTX i used is mojo 762ex (export) fender replacement. the bias winding states 53v on the diagram i found.

Is it 53Vac or 53Vdc after rectification/smoothing? If it is 53Vac, it means the rectified voltage will be about 70V.

Abbey says the published design will fail, has excess pot dissipation and suggests a route for a corrected single bias (maybe easier to work out with known ptx above) but not how to dual this.

You just need to duplicate the 10k series resistor, the 50k pot and whatever padding resistor you may want to put in series with the pot.

so, lets assume i'm getting exactly 53v on the bias.

This is not likely, because the xfmr specs says 53Vac, so you will get more.

from what i can find on EL34 biasing the recommended is -38v, as weber suggests 'starting' at about -55v can i also assume a more negative voltage/more resistance to ground, will be less current draw?

Correct; the more negative the grids, the less current is drawn in the anode.

how do i determine the current here?

You have to read the graphs in the spec sheet. There's always a graph that shows Ia vs. Vg, generally with different anode voltages.

If the current (which from what i understand but is not well defined) will be about 20mA from bias to tube (each tube in push pull?), then this is 40mA to the tube grid. how does this stress the pot?

No; the bias voltage goes to the grids but there is no current flowing (or so little it is negligible).

acting as a voltage divider the current would be going to the tube only and not through the remaining trim value, which will just determine a reference point for voltage? 

Even if there was current flowing to the grids, Kirschoff's law would still make current flow to the rest of the circuit.
With 400V B+, the bias voltage to cut-off the EL34 (i.e. the bias voltage for zero anode current) is about 55V. It is good practice to allow the possibility to put the tubes in cut-off, that means you must make sure the bias voltage can reach 55V. The recommended bias of 38V should be with the pot more or less at mid-position. The data sheet shows there is no need to make the bias going below 20V. You may want to do the maths, or experiment.
Please note however, that guitar amps tend to run at lower quiescent current, so the actual bias voltage would be a tad higher.
There may be a discrepancy in the Mojo specs. The M762EX xfmr is supposed to be adequate for the AB763 circuit, but on the AB763, the bias smoothing cap is rated at only 50V. With a 53V winding, this cap will be seriously stressed, although there's a 470R drop resistor in the ac branch that drops about 10-12V.

 

[kepeb]

excellent.

the most important part was the first bit, If this wont allow me to use random old tubes together then my initial reasoning to put this in there... was silly.

i will put a single bias together with the 10k and 10k series drop before the cap so i can check the actual rectified voltage and adjust it after that.

I have a 150v cap here so should be ok. thank you very much for taking the time to clarify for me.

I think i'm getting there  :-[

 

[abbey road d enfer]

i will put a single bias together with the 10k and 10k series drop before the cap so i can check the actual rectified voltage and adjust it after that.

10k before the cap may induce a very large drop. In the AB763, the 470r induces about 10 v drop.
The reason is that the current charges the cap during a tiny fraction of the time (about 1/20th) so the current  peak is quite high. As a consequence the I.R drop is much higher than if the 10k resistor was after the cap.
Anyway, it 's not a big problem, you can turn the circuit on and AUT.

 

[CJ]

there are actually less parts when using cathode bias, one resistor, and one cap. that's it.

with the fixed bias, you have a diode, 2 caps, 2 or 3 resistors, plus a pot, a knob, and more wire than you need.

weber is well aware of the pwr issue with the bias pot,

if you read his great books on amplifiers, you will hear him spec out a cermet pot, ceramic-metal, for the bias pot.

i used to have an amp that had a switch for cathode bias and fixed bias,
only used to run cathode bias as it sounds better.

especially if you are using solid state rectifiers.

change a solid state rect/fixed bias amp into a tube rect and cathode bias and you will hear a world of difference.

plus, if your fixed bias circuit somehow gets screwed up, the tubes go into meltdown mode.

went thru the 6l6/12ax7 phase, now into the trainwreck/el84/ef86 phase,
tube rect/cathode bias.

no stomp box needed.
plug straight in for great tone,

use more than one speaker type in the same cab if you want even better tone.

maybe a jensen and a celestion bass speaker side by side,

or maybe a jbl d150 alongside a vox bulldog alnico,