Valve biasing

[Covenant]

If you don't bias a valve when you install it what is likely to happen?

[Macca]

[worrasf]

If you don't bias a valve when you install it what is likely to happen?

There is a risk it could run "to hot" (or "to cold") which in addition to affecting the SQ also may shorten valve life if running to hot. Macca's doomsday scenario while certainly possible is unlikely unless the bias is way out). With power valves its good practice to check the bias when swapping but not so crucial for phono/line stage valves - especially if previously biased correctly. Each amp will have it's own bias range. When I discussed this with Glenn Croft he recommended I check the bias in my S4s monthly I have to say I did this religiously for a while but then stopped as I never found any drift. Probably a good idea if you are putting in new tubes as there is some drift for the first few weeks IME. Also, I swapped the ECC83's and BH7's repeatedly until I got the "right" combination and the bias of the power tubes (GE 6550A's) was never affected by this.

Steve

[Covenant]

Thanks chaps-it's an ECC88 I will be replacing in a headphone amplifier. I have bought a Mullard, don't want to knacker anything so I had better learn how to set the bias on my headphone amp.
Basically I am a lazy sod!

[worrasf]

Thanks chaps-it's an ECC88

Indeed - if an NOS Mullard you don't want your dollar going phut

As an aside, more as an experiment I recently bought a pair of HAMA Cryo ECC88's from Watford valves for my SOHA II headphone amp (I've got a pair of NOS 6N1P-VI's on the way as well) as I fancied switching from 12v - 6v tubes) - anyway, despite my reservations the HAMA cryo's sound superb - just a thought in case you ever wanted to compare something different with your Mullards - but you probably won't as they are so darn good

Steve

[Covenant]

TBH Steve, I am more than happy with the JJ Electronics tube that's in my headphone amp now. It's just curiosity and reading so many good reviews about Mullard's that has made me order one.

[worrasf]

It's just curiosity and reading so many good reviews about Mullard's that has made me order one.

Beware Jerry - that way lies madness

I speak from experience -"every day in every way I'm getting better and better"



Steve

[DaveK]

Thanks chaps-it's an ECC88 I will be replacing in a headphone amplifier. I have bought a Mullard, don't want to knacker anything so I had better learn how to set the bias on my headphone amp.
Basically I am a lazy sod!

Jerry,
HA10? - if so I think you have a treat awaiting - I put a balanced ECC88 Techtronics (?) from China in mine a while ago and the improvement was very noticeable.
Another numptie question: -
Does adjusting the bias acheive the same thing as putting in a valve with matched triodes (i.e. a valve with matched triodes does not require balancing because it is already balanced?)
Said it was a numptie question, didn't I? I know nowt.
Dave.

[worrasf]

Does adjusting the bias acheive the same thing as putting in a valve with matched triodes (i.e. a valve with matched triodes does not require balancing because it is already balanced?)
Dave.

Nope - different. Biasing sets the cathode current for the valve and so determines the "voltage" it sees. Biasing a valve with matched triodes balances the entire valve regardless of the 2 triode status. Even if a triode is internally matched if the bias current/voltage is incorrect it will not perform as intended in that circuit and could potentially blow if run to hot.
Biasing has 2 objectives
1) to set the optimal current/voltage for the valve to perform to spec and 2) if you have a pair to balance the cathode current ("voltage") for each so that they perform as a balanced pair. If you have a quad then you balance bias for the 2 pairs.

Steve

[Ali Tait]

It also depends on the circuit the valve is used in. Many circuits use auto biasing, which means you just plug the new valve in and forget about it. If it's a fixed bias circuit, then bias needs to adjusted manually using the pots and a multimeter.