BIAS Voltage problem!!!

[grtamp]

Hi , My bias voltage with open circuit is -55V , when connect the wire to Pot(10Kpot + 27K) , the voltage go down to -20V.

what's the problem , PT fault ?

thanks

[tubedogsmith]

Walk us through your bias circuit from the trannie to the output tube grids so we can help you.

[DonMoose]

With no load, what AC voltage do you get on the bias filter?
Then with load?

That load is _barely_ 1mA, so my suspicions are:
1) filter cap backwards (bias is negative, so the (+) terminal goes to GND)
2) bad solder joint(s) in the series path (winding, diode)
3) short or unexpectedly high load in pot+resistor - is wiper connected to anything? - what are voltages across pot and across resistor? - in other words, might that be a 270R instead of a 27K0?
4) bad winding - what is DC resistance of bias winding?

Hope this helps!

[grtamp]

The 100uF 100V cap was broken, replaced and now the voltage is better but low -46V.
I replaced the 27K res. with 51K and now I have -52V.

[phsyconoodler]

What circuit is this?Sounds like a Fender.You likely won't need -52v.

[grtamp]

But -46V seems low to me for 4x6L6.

[phsyconoodler]

It depends on the plate voltage.What amp is it and what is your plate voltage?Not unloaded,but with tubes in.What PT do you have and is it tube rectified or diodes?
For example,if you have 450v on the plates then you will likely only need about -40 to -45v.It really depends on the tubes and the parts you have.If you have -52v that's fine,as long as you can get it in a lower range too.What's the lowest (most towards positive) that you have now?Turn the 10k bias pot and find out.If you have a good range from -40v to -52v then leave it alone.But don't set the bias by voltage,use the current.Average is about 35 to 40ma at 450v.

[FUCHSAUDIO]

Most raw bias supplies in a Fender style should be 60-80 V. If the tap is low, there are other ways to make bias (high voltage winding etc), but rule out perhaps if you made an error somewhere.

[phsyconoodler]

No Fender amp that I know of can supply 80v from the bias supply tap.The most I've seen is 60v on later model Silverface amps.
The average is 60v and many even show in the schematic a 50v bias tap.
It depends on the transformer.

[FUCHSAUDIO]

Perhaps "most" isn't the best choice of words. Yes, they can run around 50-60 (many Sf and BF's have 50-V rated bias caps curiously). Some (better word choice) will do higher (Rivera Concert, 75, a few others). It depends on the load the bias pot network provides too:

The bias tap is usually around 45 to 55, so at 1.3 times the AC value it's maybe 50-60 or more unloaded. Add in the resistor in series with the rectifier diode and then the bias pot network and it goes down somewhat.

Here's a Vibrolux: http://www.mojotone.com/core/media/medi ... a&_xt=.pdf

Shows a 50 V bias tap, which means about 65 raw, unloaded.....

But what do I know ?

[phsyconoodler]

Ok,so we know what we need,the answer to the OP is get the amp up and running,check the bias and change the range resistor as necessary to get the correct current.

[grtamp]

My bias voltage range go from -43V to -52V but with 51K tail res. Probably low tap. it's a Mercury PT.

Phsyconoodler.... my amp is on work and it's not again finished! I haven't given anode voltage , I'm just checking the bias voltage ( my stand-by is off)

[phsyconoodler]

That will be fine to get the amp up and running.Then check the plate voltage.
Measure from the OT center tap to pin 3 in ohms and record that reading. Then fire up the amp at the maximum bias setting and measure the voltage from the OT center tap to pin 3 again.
e.g. say you get 1v and 112 ohms. 112/1 = 112.Now divide that by 2 to get your per-tube ma current. 112/2=56ma per tube.
These numbers are just out of the air.Your actual readings will be different.
But that will give you an idea where your bias voltage needs to be with the tubes you have now.Remember,more range in the bias adjustment means you can use many brands of 6L6's without changing the range resistor all the time when swapping tube brands.

[grtamp]

changed 3,3K to 1,2K in bias circuit , with 10Kpot/51K tail res.

I put the power tubes: pin 3= 450V VBias= -55V 36/37mA

[phsyconoodler]

There you go.Done.