Bias voltage

[jeff]

I'm thinking of making a new amp from an old transformer I got from a tube TV set.
I also have an old Mojo 6L6 PP OT.
Unloaded the secondaries are 330-0-330.
There is no bias tap so I'll have to tap the 330 to make a bias supply.
I couldn't find a 25K pot with a set screw and don't want to use a regular pot(it could get accidentally turned) so I have to use the 10K pot and a resistor. I'd rather start with too much bias and have to swap resistors if I have to lower it.

What voltage is a good starting point for the bias. I will adjust it to get the right bias but where should I start before I put in the tubes?

[PRR]

Unloaded G2 voltage divided by Mu(g2).

That will be cold but safe.

WHY don't you just STEAL plans and values from a Marshall?

[Tone Junkie]

Ask the guys on here were they get there 25k locking pots I have two in builds and need some more myself I found them because someone here showed were they were but I dont remember.Bill

[jeff]

 Well I think a 10K will work fine but you don't have the range.

It's wired as a variable resistor so with a 15K in series I can go from 25K-15K with no resistor I can go
from 10K to 0K ect.

 That's why I wanted to know ballpark what voltage to start with. With a 10K I can't go up or down as much as a 25K. I wouldn't need to use the 15K resistor, possibly max out, then swap to a 5K, possibly max out, then swap to no resistor.

[HotBluePlates]

Well I think a 10K will work fine but you don't have the range.
...
 With a 10K I can't go up or down as much as a 25K. ...

That's not true. The apparent range of a bias pot (or any adjustable pot inside of a voltage divider string) depends on the ratio of the pot's value to the resistors around it making up the rest of the divider.

Say you have a voltage divider made of:
    Voltage Source -> 1k resistor -> 10k pot -> 1k resistor -> ground
The pot can sweep over 83% of the total voltage available. That seems nice, but you probably can't accurately set the voltage; it will seem to jump in large increments.

If the divider is
    Voltage Source -> 10M resistor -> 250k pot -> 10M resistor -> ground
The pot can sweep only 1.2% of the total voltage. You can adjust the output voltage in very fine increments of that available range, but can't get too far above or below a strating value.

Do what PRR suggested. You need to derive a bias voltage from the PT HV secondary, so copy Marshall.

As for places to find them, google "25k locking pot" and/or get creative. I see that WeberVST was out of stock on a 25k locking pot, but Surplus Sales had a range of items to pick from.

A locking pot is nice, but not necessary unless your pot shaft is in a location where it could be bumped.

[Pleximan]

What voltage is a good starting point for the bias. I will adjust it to get the right bias but where should I start before I put in the tubes?

The short answer is around -50 volts

From the info provided 330-0-330 will get you about 450-460 plate voltage with sand and lower with glass rectumfrier but we'll figure for 460 volt plate anyway.

If you bias at around 70% of tube plate dissipation for a PP A/B amp you can figure for 45mA per tube.

Assuming you are building conventional guitar power supply with choke between plates & screens then plate & screen at idle are at the same potential bar a few volts, then you can simply just look at the triode connected data for 6L6gc to determine the neg grid voltage you need.

http://www.triodeel.com/6l6gc_p9.gif

From the triode connected data you can see that @ 460 plate & screen volts the tube draws 45mA when the grid voltage is just under -50 volts

A bias supply adjust centered at -50 would be a good starting point

[jeff]

Thanks
   Jeff

[stingray_65]

I like the locking pots too.

You can order a locking nut for 1/4"shaft pots but the cheapest I've found is $11.

I have cut the shaft of the pot to about 1/8" exposed and slotted the end with a hacksaw or dremmel.

not as easy to "knock" out of whack and plenty easy to turn with a screw driver.