Bias current/Voltage

[Vin Diezel]

I measure 37mV in one pair and 32mV in the other. Is that a problem or is that still within the tolerance (35mV is what Peter suggests)?
There is only 1 trim pot for all 4 tubes.

 

[Riffingapex]

What amp? On my d moll it's 70mA for a pair (35mA each tube)

 

[Vin Diezel]

Hi!

Lil Fokker. I guess the dMoll has 2 trim pots?

 

[Crunchtime]

It may be possible to mix tubes around to get better matching. That is the nice thing about my Weber Bias Rite. You know the current of each individual tube and can assign pairs according to that.

For example:

tube 1 = 32
tube 2 = 37
tube 3 = 32
tube 4 = 37

So, if you match 1 with 4and match 2 with 3.

You will get an average of 34.5

 

[Vin Diezel]

That's the way I did it. There is one resistor for each tube to measure.

 

[Scuba.Duba]

What amp? On my d moll it's 70mA for a pair (35mA each tube)

Yeah, Papa never speaks to use gringos in bias measurements of "mV", only "mA" because it would be stooped.

**********

You either can use the .6 factor (60%) or the .7 factor (70%). 70% will yield a better clean sound while 60% will yield a more tight and percussive sound.

The EL34 power tube has 25 watt of maximum plate dissipation:

25W x .7 = 17.5W

17.5W / 480DCV = 0.036A = 36mA for each individual EL34 power tube. To be safe against low main voltage set a pair to 70mA.

**********

 

[Vin Diezel]

The Fokker has resitors built in so you can measure the voltage drop at the resistor which indicates the current. Therefore it is unnecessary to measure the current directly. That is the opposite of stupid, Einstein.

And you did not address my question: Are my tubes too far apart or are those values still acceptable?

 

[CEG85]

Its fine.
Setting your bias generally depends on your plate voltage.
People are getting PV readings anywhere from 450v to 500V on Diezels.

70% dissipation @ 500pv = 35ma
70% dissipation @ 450pv = 38ma

Sometimes I run tubes slightly cooler when changing them out (between 60/65%)

You can always check the plate voltage in your amp if your curious.
http://www.ebay.com/itm/Bias-Tool-P...hash=item360bb870d2:m:mf-K0XlJ0M8AxHyR9-mO5zw

BTW where are you buying these tubes from?
Id ask the seller to make sure they are within 1ma of each other if your buying a matched quad set.
I'm assuming your using EL34's?

 

[Vin Diezel]

The problem is that 2 tubes of a new quad set died and I had to put in 2 older ones.
JJ KT77s I'm using.

 

[Peter Diezel]

I´m not picky, a tolerance in between 5mA / 5mV is great.
To measure the right amount of current it would be
good to use the shunt method on the output transformer.

Cheers,
Peter

 

[Scuba.Duba]

I measure 37mV in one pair and 32mV in the other. Is that a problem or is that still within the tolerance (35mV is what Peter suggests)?
There is only 1 trim pot for all 4 tubes.

This is the method Peter sent to me many years ago on this subject.

Tube Type Dissipation in Watts

For:
6V6, 14 Watts
6L6GC, 25 Watts
EL34, 25 Watts
E34L, 25 Watts
KT77, 25 Watts
6CA7, 25 Watts
KT66, 25 Watts
6550, 35 Watts
KT88, 35 Watts

Calculation for Herbert (480 DCV)

You either can use the .6 factor (60%) or the .7 factor (70%). 70% will yield a better clean sound while 60% will yield a more tight and percussive sound.

The EL34 power tube yields 25 Watts of maximum plate dissipation:

25W x .7 = 17.5W

17.5W / 480DCV = 0.036A = 36mA for each individual EL34 power tube. To be safe against low main voltage set a pair to 70mA.

 

[Vin Diezel]

I´m not picky, a tolerance in between 5mA / 5mV is great.
To measure the right amount of current it would be
good to use the shunt method on the output transformer.

Cheers,
Peter

Thanx, Peter!