GZ34 in 5f4 build

[Bruce / Mission Amps]

so i assume theres no physical and or machanical difference between a basic 1/2w vol/tone control pot and a trim pot. now, what determines the values of the components, resistors, pot, bypass cap of a bias circuit?

NO a pot is a pot if the ratings are the same.
You don't need a ten turn anything with this.. a regular old 50 cent 1/2 watt linear trim pot is fine.

Actually a 1/4 watt trim pot ought to be OK, but I'd look for a 1/2 watter.

Look at it like this....
A given PT has a 50v bias tap.
50vac x 1.414 (reverse rectification and filtering) = about -71vdc
With a traditional 1/2 wave rectifier, that is the max -DC voltage your bias supply could make if it was not grounded out.
However even with the supply built up correctly, that -71 volts goes somewhere.
With a good bias filter cap, that "somewhere" eventually has to go to ground through all your bias control resistors... why? Because the grids of the power tubes (UNDER NORMAL OPERATION) won't be feeding bias voltage anywhere except as a static charge on the grids so the resistance and the filter cap in the bias supply are the only path to ground.
A good 47uF to 100uF @ 100v cap ought to last for many many years.

Lets say you have a 1/2 watt, 25K trim pot sitting on top of a 1/2 watt carbon film, 10K resistor.

That is a total of 35K

Ohm's law for current: E/R=I
-71v/35000ohms = 2.03ma

Ohm's law for wattage: P=I^2*R
.00203a^2 * 25K (trimpot) = .103 watts
Just a bit more then a 1/10 of a watt is used in the trim pot.

Now look at the 10K resistor.
It also has to be able to handle the 2.03ma of current in the bias supply, so you can solve for it's wattage ratting the same way.
.00203a^2*10K (fixes limiting resistor) = .0412 watts
Less then 1/20 of a watt in the 1/2w setting resistor.

So lit looks like the wattage of the resistors and resistance in the whole bias supply (not including the actual R of the PT winding) is less then 1/4 watt.
So I'd use 1/2 watt resistors and or trim pots to be ultra safe.

You need say -45v?
The trim pot now offers a variable and lower -DC resistance path to ground for the -71v, lowering the total amount of -DC bias voltage.
The 10K resistor limits how low you can go with the total DC grounded resistance so there will always be some kind of -VDC on the power tube's grids, ...least you launch them into outer space with high current by having none.
Regardless, what ever -DC voltage is left over is always applied as a static charge to the grids of your power tubes for bias voltage.
10K might not be much voltage but for the few moments in time that it happens the tubes will just get super hot and not blow up.
It "might" allow them to draw enough current that the mains fuse could sag an pop.

If not too preachy and wording... I hope that was helpful..

**********************
Of course there are much more sophisticated ways to build up a bias supply but this is the basis for all of them, unless they are using lots more parts like transistors and precision current sensing resistors in the power tubes for tracking power tube current and auto adjusting bias voltage...etc

[scole]

awsome bruce, thank you very much. it looks like weber has locking bias pots, i assume they are good quality. i cant seem to find any other source.

[Bruce / Mission Amps]

awsome bruce, thank you very much. it looks like weber has locking bias pots, i assume they are good quality. i cant seem to find any other source.

I don't know... if it is a standard built pot with a slotted shaft, then the bushing nut will be on the outside of the chassis and so will the slotted shaft.
If you are using 1% 1 ohm, current sensing resistors on the cathodes of the power tubes and have leads going to some small 1/16", or so, dia PIN sockets, mounted to the outside of the chassis for easy access, then you do everything without taking the chassis out.
If you make up an L bracket, you can mount the POT sideways inside the chassis so you can get at the slotted shaft from the inside as you are measuring the tube sockets from the inside.
Some variables to think about.

[scole]

what i meant by source was where to buy the pots, anyway, i like the idea of 1 ohm resistors on the cathodes with leads to a couple 1/16 jacks for easy bias checking.

[Paul P]

what i meant by source was where to buy the pots, anyway, i like the idea
of 1 ohm resistors on the cathodes with leads to a couple 1/16 jacks for easy
bias checking.

I have this but I'm not so sure anymore that it's all that useful. The thing
is, you need the value of the power tube plate voltage to calculate the
dissipation at idle, but changing the bias will cause plate voltage to change
and you won't know what the new value is. And I don't think having the
plate voltage available to be measured from the outside is a good idea,
security-wise. The biasing tools that plug into the powertube socket can get
at the plate voltage.

Paul P

[scole]

hey paul, i do have a bias meter, pretty handy. still need a source for the bias pots though.

[tubeswell]

If you are using 1% 1 ohm, current sensing resistors on the cathodes of the power tubes and have leads going to some small 1/16", or so, dia PIN sockets, mounted to the outside of the chassis for easy access, then you do everything without taking the chassis out.
If you make up an L bracket, you can mount the POT sideways inside the chassis so you can get at the slotted shaft from the inside as you are measuring the tube sockets from the inside.
Some variables to think about.

That's more or less how I am doing my 5G9. - albeit without the permanent PIN sockets. I figure I'll just use insulated 'gator clips from the cathode side of the 1R to ground. Trickiest part herebouts is finding 1%1R resistors.

Heaps of trimpots tho' - Maybe I could stick a few in the post for scole :-)

[scole]

what kind/brand of trim pots do you use? do you get them from mouser, digikey?

[Bruce / Mission Amps]

That's more or less how I am doing my 5G9. - albeit without the permanent PIN sockets. I figure I'll just use insulated 'gator clips from the cathode side of the 1R to ground. Trickiest part herebouts is finding 1%1R resistors.

Heaps of trimpots tho' - Maybe I could stick a few in the post for scole :-)

Then use 1% 10 ohm resistors and slide your decimal point over one click when converting from mv to ma.... 250mv would be .25v or, .025ma with a 10 ohm resistor.

[tubeswell]

what kind/brand of trim pots do you use? do you get them from mouser, digikey?

'Nother (Ozzie) place that has branches here in NZ (one just down the road)

http://www.jaycar.com.au/productResults.asp?FORM=CAT

http://www.jaycar.com.au/productResults.asp?FORM=CAT

http://www.jaycar.com.au/productResults.asp?FORM=CAT

email me if you can't get them, I'll stick some in an envelope (and maybe one day you can flick me some 1% 1R 5W resistors ;-)

[tubeswell]

Then use 1% 10 ohm resistors and slide your decimal point over one click when converting from mv to ma.... 250mv would be .25v or, .025ma with a 10 ohm resistor.

But of course!

[scole]

anyway, with the 5u4 rectifier i have 443 volts on the plates of the jj 6l6gc power tubes and 28ma idle current. i put a messa gz34 in and got 461 volts and 34ma. the amp is a little louder at 28ma but its fine with the gz34. i think ill just let it be.