Resistor biasing

nut

Wammer
Wammer
Oct 26, 2014
2,342
256
118
Eton wick, berkshire
AKA
Phil
HiFi Trade?
  1. No
I was looking at an amp for sale.
An el34 valve amp.

It has resistor biasing.

Is this the norm for non self biasing amps?

Seems a faff to replace resistors every time the power tubes are changed.

Is is a complex affair to convert to self biasing?

Does it only apply to power/output tubes?

Thanks
 

toprepairman

Wammer
Wammer
Dec 6, 2010
1,190
573
158
Surrey
AKA
Henry
I'm guessing here but the implication is that it is resistor cathode bias, which is the same as self bias so the resistors will not require changing.
 

Jazid

Wammer
Wammer Plus
Oct 13, 2009
4,861
1,556
193
London
AKA
James
HiFi Trade?
  1. No
And if you do have to adjust the bias manually then there will be trimpots installed somewhere to allow this, so no resistor swapping.
 

Disarmamant

Certified foo free
Wammer
Mar 30, 2013
660
154
68
Brackley, Northants
AKA
Martin
HiFi Trade?
  1. No
If you are referring to the KEL34 amplifier I posted about, it is self biasing or cathode biased. The output valves have a resistor between the cathodes and ground. The grids of the output valves are tied to ground via high value resistors.

The current flowing in the cathode of the output valves develops a voltage across the cathode resistor which puts the cathode of the output valve at a higher potential than the the grid of the output valve, which is at ground potential. Thus the grid is more negative than the cathode.

The characteristics of the output valves determines the current flowing in the cathodes and thus the cathode to grid voltage, so the circuit is self biasing. There is no need to change the values of the cathode resistors to adjust the bias.
 
Last edited:

Lurch

Piss-taker & Gobshite Par Excelle'nce
Wammer Plus
Sep 17, 2016
5,994
7,419
183
HiFi Trade?
  1. No
Thanks for the above, it appears that my eyes are universal translators, so that I can read Klingon and other alien languages. Pity this skill didn't include comprehension 😄
 
  • Haha
Reactions: Ady

Disarmamant

Certified foo free
Wammer
Mar 30, 2013
660
154
68
Brackley, Northants
AKA
Martin
HiFi Trade?
  1. No
Thanks for the above, it appears that my eyes are universal translators, so that I can read Klingon and other alien languages. Pity this skill didn't include comprehension 😄
In that case maybe you just need to find a copy of the Hitch hikers guide to the galaxy. 😊
 

nut

Wammer
Wammer
Oct 26, 2014
2,342
256
118
Eton wick, berkshire
AKA
Phil
HiFi Trade?
  1. No
It was another valve amp i was contemplationg.
Not the kel34.

The seller said to bias, resistors had to be changed.
 

rabski

Everything in moderation
Staff member
Dec 2, 2006
32,873
1
26,119
173
Kettering
AKA
Richard
HiFi Trade?
  1. No
I suggest the seller has probably misunderstood.

No fixed resistors ever need to be changed for biasing purposes, because it's impossible to know what the 'different' value would need to be. At least, not without knowing all the characteristics of each valve and either using a sim program or doing some serious calculations.

The 'resistors need to be changed' may mean that it isn't fixed/auto bias, and that there are variable resistors that need to be adjusted to set the bias.
 
  • Like
Reactions: mac72

nut

Wammer
Wammer
Oct 26, 2014
2,342
256
118
Eton wick, berkshire
AKA
Phil
HiFi Trade?
  1. No
I suggest the seller has probably misunderstood.

No fixed resistors ever need to be changed for biasing purposes, because it's impossible to know what the 'different' value would need to be. At least, not without knowing all the characteristics of each valve and either using a sim program or doing some serious calculations.

The 'resistors need to be changed' may mean that it isn't fixed/auto bias, and that there are variable resistors that need to be adjusted to set the bias.
Confusing.
I didn't realize there were such things as variable resistors.
I have only heard of bias pots.
 

Jazid

Wammer
Wammer Plus
Oct 13, 2009
4,861
1,556
193
London
AKA
James
HiFi Trade?
  1. No
Confusing.
I didn't realize there were such things as variable resistors.
I have only heard of bias pots.
In this instance they are the same thing. Bias pots, trimpots, volume, balance and bass and treble control pots are all examples of variable resistor.
 
  • Like
Reactions: rabski and mac72

rabski

Everything in moderation
Staff member
Dec 2, 2006
32,873
1
26,119
173
Kettering
AKA
Richard
HiFi Trade?
  1. No
Yup. A variable resistor is just the umbrella term that I use because I'm old. Technically, anything with three terminals ought to be called a potentiometer, but let's not confuse things even further :ROFLMAO:
 

pmcuk

Wammer
Wammer
Sep 12, 2015
7,317
1
4,879
148
Kensington, London
HiFi Trade?
  1. No
It was another valve amp i was contemplationg.
Not the kel34.

The seller said to bias, resistors had to be changed.

Maybe the meaning was that if you used different types of valves you'd change the value of the cathode resistor, which could be true.
 

Latest posts

Forum statistics

Threads
113,444
Messages
2,451,263
Members
70,783
Latest member
reg66

Latest Articles

Wammers Online