FULLY TESTED AND GUARANTEED

FULLY TRACKED DELIVERY SERVICE

SECURE ONLINE SHOPPING

Cathode Bias – What Exactly is it?

Do you own a valve guitar amplifier and wondered what ‘cathode bias’ means?

Have you heard of ‘biasing’ but perhaps are a little confused about what bias means and in particular what cathode bias means?

Don’t worry – you’re not alone! Biasing is a highly confusing subject for the owners of valve guitar amplifiers.

The two sorts of bias are cathode bias and fixed bias. Both of those terms are highly confusing, but we’re stuck with them. This article aims to address this very confusing topic.

Let’s begin. You’re going to like this first revelation. ‘Fixed Bias’ actually means ‘Adjustable Bias’! Work that one out!

So a ‘fixed bias amplifier’ is one on which you can adjust the bias to get the ‘tickover’ current (also called the bias current) through the output valves to be the right level (usually around 30-50mA). It’s called ‘fixed bias’ because once you have adjusted it then it stays the same (i.e. ‘fixed’) until adjusted again. You’ll probably agree that ‘fixed bias’ is not the clearest term, but we’re stuck with it.

Key Point: if you have a ‘fixed bias amp’ you can adjust the bias voltage (usually with a pot, or by swapping out a fixed resistor). You can bias a fixed bias amp or get a tech to do it for you. In fixed bias amps, when the output tubes are changed (always for a matched set of course), you readjust the bias until it is correct. (The correct bias current depends on the plate voltage and valve type. As I mentioned earlier it’s typically around 30-50mA).

Cathode Biased – Does that mean I don’t need to bias my amp?

In contrast to fixed bias we also have cathode bias.

A better name for this would be ‘variable bias’ but it’s not called that. Just because it’s ‘variable’, don’t run away with the notion that you can vary it – you can’t. User-adjustable bias is the province of fixed bias amps (feeling confused yet??)

So in a cathode biased amp, the bias voltage varies up and down (that’s the ‘variable’ bit) automatically to keep the correct bias.

It’s kind of like a thermostat. Too much bias current and it’s turned down automatically. So too little bias current and it’s turned up automatically. In contrast, in fixed bias amplifiers, once set, the bias current does NOT move up and down automatically. It stays FIXED at whatever you set it at. And that’s where the ‘fixed’ bit of ‘fixed bias’ comes in.

You may be interested to know that all preamp valves are cathode biased which is why you never have to bias them. If your amplifier features cathode-biased output tubes also, then you don’t need to bias that amp – ever. In fact, you can’t, there’s no way to do it.

Some amps have cathode-biased output tubes (EL84) and unlike fixed bias amps, these amps don’t need biasing. In fact, they can’t be biased as I just mentioned.

Which Amps Are Cathode Biased and don’t need re-biasing when you change the power valves?

For reasons I won’t bore you with, it’s only low-power amps that have cathode-biased output tubes. These are almost always EL84 valves. Anything over 30W is highly likely to be fixed bias (i.e. you need to adjust the bias when putting in new tubes).

Here is a list of some of the amplifiers that are cathode biased and therefore do not need biasing:

  • Ashdown: Peacemaker 40.
  • Blackstar: Artist 10 AE, Studio 10.
  • Bugera: BC30, V5, V22 and 1990n Infinium.
  • Carr: Viceroy, Super Bee.
  • Epiphone: BC30, Valve Junior.
  • Fender: Deluxe, Bassbreaker 15, Bassbreaker 30R, Vaporizer, Champ, Champ 600, Vibro Champ XD and Excelsior.
  • Friedman: Little Sister, PT20 (note: all Friedman EL84 amps are cathode biased).
  • Genz Benz: Black Pearl.
  • Hayden: Lil Mofo, Mofo 30.
  • Hughes and Kettner: Tubemeister 36.
  • Laney: Lionheart, L20H, LC30, LC30-II, VC30.
  • Marshall: Class 5, DSL20, DSL1, DSL5, SL5.
  • Matchless: Lightning.
  • Orange: Tiny Terror, Dual Terror, Dark Terror, Jim Root Terror, TH30, Rocker 30, AD5, AD15, AD30 and OR15.
  • Suhr: Badger 30.
  • Supro 1642RT Titan.
  • Tone King: Imperial (MKI, MKII and 20th Anniversary), Sky King and Gremlin.
  • Traynor: DH15 (Dark Horse 15).
  • VHT: 12/20 and Special 8/8.
  • Vox: AC10C1, AC15 (all variations), AC30, AC30C2, AC30C2-TV, AC30C2X, AC30C2H, AC30TB Top Boost, AC4C1 and AC4TV.

SPECIAL NOTE ON MESA AMPS: Mesa Boogie amps are fixed bias. However, we can supply correctly balanced and biased valves for all Mesa amps which mean you will not need to bias the amp. These valves are a direct swap for the Mesa tubes supplied with the amp. Simply tell us it is for a Mesa amp (in the notes field when you order) and we will do the rest.

90% of guitar amplifiers are fixed bias. Fixed bias on the output tubes means they are capable of being adjusted to raise or lower the bias current. This is usually done with a pot or swapping out a fixed resistor. I have done other blogs on biasing so I won’t go into the details here.

So here’s what you should now know:

Fixed bias = capable of adjustment (!) but it stays the same (fixed) once adjusted (until next adjustment of course).

Cathode bias means non-adjustable and it varies up and down automatically with conditions (so it’s ‘not fixed’). Neither you nor a tech can adjust the bias on these amps. They don’t need biasing when changing tubes as they automatically ‘settle’ to the correct bias current.

For replacement valves please visit https://www.ampvalves.co.uk/choose-your-amp/

Related Products

Sorry, we couldn't find any products that match this article.

Scroll to Top