Quad Bias Supply

[TIMBO]

Hi guys, With old power tubes getting hard to get in matched pairs or quads, I thought it would be great if an individual bias supplies could be done for each power tube.

Will this circuit do it. Thanks

[kagliostro]

To me it is correct





Source: http://sound.westhost.com/valves/design2.html

Franco

[TIMBO]

Thanks K, that looks like a good way to go.

[alerich]

You can also go to the Weber Speakers site and look at the schematics for many of their kits that have quad power outputs. Most of them feature bias circuits with individual adjustments for each tube. It may give you some more ideas.

https://taweber.powweb.com/store/kits.htm

[kagliostro]

The way on the schematic I posted is the same way of TIMBO's schematic

K

[sluckey]

Timbo, that circuit will work but the pots will be interactive. Everytime you make one adjustment, all adjustments will be affected. You could spend hours trying to get everything adjusted just right.
EDIT... Just took a second look. That original circuit will not work. Any pot will adjust the voltage fed to every tube exactly the same. May as well just use one pot.

Using the pots as a three terminal voltage divider like Fender does with the Blackface amps will allow you to make adjustments that do not interact. Set it and forget it. Like this...

[kagliostro]

A pair of further options

one adjust each tube, the other adjust pairs of tubes then balance (more easy to set ??)



Franco

[sluckey]

The way on the schematic I posted is the same way of TIMBO's schematic

K

They are different. Timbo's circuit will not provide separate bias adjustments for each tube. That circuit from westhost.com will but the pots will interact with each other, making adjustments difficult. Furthermore, that circuit from westhost.com is dangerous! You can set the bias to ZERO!

[kagliostro]

that circuit from westhost.com is dangerous! You can set the bias to ZERO!

Ohh, Yes, and that is a very bad thing, may be swapping the position (and value) of the pot and resistor will avoid this deadly effect

Many thanks for pointing that Steve

K

[TIMBO]

Hi guys, This is more complicated than I though.
After doing some more searching this idea has said that there is no interaction with the other pots but this was only with two tubes.
So by adding the other two tubes will there be interacting because of the coupling caps??

[sluckey]

So by adding the other two tubes will there be interacting because of the coupling caps??

No. But the fact that you can set the bias voltage to zero is dangerous.

[jjasilli]

Re -0- bias. The tube will fry instantly & dramatically.  Per the schematics of sluckey & kagliostro:  there must always be a fixed resistor under the bias adjust pot.  The value of the resistor must be enough to lift from ground a minimal bias voltage to protect the tube(s) from under-bias.  Then failure of the pot, or setting it to -0- or too low, will not harm the tube(s).

[trobbins]

I would put the most dangerous aspect as the wiper failure.  After that is secondary, such as convenience of adding extra protections for human error.

[kagliostro]

This is protected against wiper failure and human error


(R2 must be ten times the value of P1)

The diode allow C2 to be charged but avoid a quick discharge when the amp is shutdown and the filter capacitors are still charged

R2 give protection against a wiper failure

Franco

[trobbins]

The human error that is averted is a tech adjusting the wiper the wrong way without monitoring bias current and then taking a 20 minute phone call.  That wouldn't normally happen if the event was part of a bias adjustment process.  Even the time-honoured 'no meter' technique of adjusting the pot back a bit after seeing cherry red  wouldn't be fooled.

If someone started twiddling pots for no good reason, then the valves could still get damaged quite quickly depending on the margin used within the bias adjust design.

Someone could swap in different output valves, without any thought for bias adjustment - so no averting human error there.

[TIMBO]

Hi guys, Sluckeys schem works a treat.
I found a bit more info here http://www.turneraudio.com.au/2323-triode-integrated-6cm5.htm
I can assume that the added 1M resistor is to prevent 0 bias should the wiper lift.
Are the 50uf caps added to the pots required.
I have only put in a dual bias for the moment to see what results I get with the 6CM5s I have. Thanks

[lego4040]

Having almost lost a matched pair of 6v6's due to human error . I will give this a thurow reading

[trobbins]

Yes the 1Meg is for wiper safety.  Unless you know that the bias adjust pot is modern, fully enclosed, and multi-fingered, then perhaps best to err on the safety of one extra resistor per pot. 

I must be missing something but I can't see 50uF bypasses on the pots??

I would certainly recommend the effort of a separate bypass cap on the bias adjust wiper, as that is the last and most efficient location to decouple, and removes the opportunity for noise to accidently find its way to the grid if the decoupling was last done further back in the bias supply.

Patrick Turner's comments are all very hi-fi biased, and a lot of his effort to modify circuitry, or describe failings, are red herrings for a guitar amp application.  All the OT's in the old Philips PA's I have are fine for finding a speaker happy winding section (except for the small 1173 intercom amp).  The 'lack' of low and high frequency range in PA OT's is often a blessing for a guitar amp, whose circuitry would normally be forcefully rolling off frequencies at the extremes.

[TIMBO]

Thanks Tim, This has been a great learning topic, as its been a bit hard to get my head around.
There is so many BIAS supply circuit around just wanting to find one that gives some adjustment and in this case multiple supplies.
The 50uf bypass is the one you are talking about off the wiper, its on Sluckeys schem.

I did try the other circuits and the pots did interact. Thanks