Amps with external bias adjustment for individual tube, BAD IDEA???

[RvChevron]

Some amps provide external bias adjustment for the whole pair of output tubes like my Aiken Invader and Heritage.

Some amps provide external bias adjustment for "each" output tube so matched pair or quartet is not necessary. For eg: Tone king, soultone and siegmund...etc.

However, an amp designer/maker told me it may not be a good idea for using unmatched tubes even if you can bias them individually.

Any thoughts!!

I have a bunch of single nos tubes and was thinking individual bias adjustment for each output tube is the best thing ever happened since sliced bread.:confused::(

[John Phillips]

It depends what you mean by 'not a good idea'. Setting unmatched tubes to the same idle current does not make them matched, so in a hi-fi application it's not a great thing to do - they will still behave as unmatched tubes under signal conditions, and so will introduce distortion.

But for a guitar amp, it's far from certain that this is a bad thing - in fact, the extra harmonic complexity caused is likely to sound better. The only real benefit of using matched tubes is to eliminate hum, and setting the idle currents individually will achieve that perfectly well.

[rooster]

If an amp is played with a LOT of distortion full out for long periods, I'd recommend matched tubes. If you play clean or on the edge of grind, sometimes a mis-matched pair will give you some really nice distortion artifacts that you can't design into an amp. They're totally unpredictable from a design standpoint, but can sound really cool.

Personally, I try to design my amps around a good matched set of power tubes with a single adjustable fixed-bias control. Matched pairs are very available these days in the new power tubes, and while they're not as good as the old stuff from the 60s and 70s, but they're getting better every year.

rooster.

[guitardr]

Hey JP and fellow members:

How does Mesa Boogie do their idea iof fixed bias and is it actually a "sound" idea?

[Mickey_C]

Matched tubes don't stay matched for very long anyway, so IMHO it's just one more way to collect money from guitarists.

External bias adjust and test jacks works out better in the long run.

[PRNDL]

>> I have a bunch of single nos tubes

The optimal way would be to match the tubes in your amp using a bias probe.
Some people write the current on the boxes to make it easier to find pairs the next time.

[rockon1]

Hey JP and fellow members:

How does Mesa Boogie do their idea iof fixed bias and is it actually a "sound" idea?


They tell you to by tubes number rated by them, from them, with the same ratings as the ones it came with keeping the tubes in the proper range. Thier tubes are expensive too. Bob

[John Phillips]

They tell you to by tubes number rated by them, from them, with the same ratings as the ones it came with keeping the tubes in the proper range. Thier tubes are expensive too. Bob
Expensive relabeled junk, actually... :(

They are certainly tested and graded, but you can't turn a crap tube into a good one by selecting the ones that pass the spec.

I like Mesa amps, but I wish they'd use better tubes - not even NOS, just not the worst Chinese and Russian junk.

Mesa believe that by setting the bias permanently they are avoiding potential problems caused by users or techs biasing the amp incorrectly, and up to a point they're right. It does rely on using tubes that are fairly close to the correct spec for the tubes too, but in fact most good quality tubes are - I've never had a problem with any Mesa amp being out of the right bias range when using old-production or better-quality new tubes - and it also relies on biasing more conservatively (cooler) than is currently fashionable, so the bias point is in the middle of the good range, rather than at the hot end of it... but IMO this sounds better anyway (contrary to what's usually thought). It's certainly true that even within the right range, there is a difference in tone, and if you really want to be more exact, you will need to fit an adjuster - but it certainly isn't true that you can only use Mesa-branded tubes in a Mesa amp.

At the very least, most tube resellers will be able to select a set which matches one of Mesa's grades, or you can just stick any tubes you like in and check the bias is OK... which is what I do :). I've never bought a new set of Mesa tubes, ever.

[fuzzbomb]

Expensive relabeled junk, actually... :(

and it also relies on biasing more conservatively (cooler) than is currently fashionable, so the bias point is in the middle of the good range, rather than at the hot end of it... but IMO this sounds better anyway (contrary to what's usually thought).

Hi John, so given that Mesas are biased cooler...could you select tubes graded to sort of compensate for that? I guess it'd be tubes...graded on the lower end of the range, so that the bias would be a bit hotter for them?

I bought Mesa tubes once, from GC even, before I discovered 1) this place and 2) some good tube vendors and learned a *lot* more about tubes.

[John Phillips]

Hi John, so given that Mesas are biased cooler...could you select tubes graded to sort of compensate for that? I guess it'd be tubes...graded on the lower end of the range, so that the bias would be a bit hotter for them?Yes, but the other way round - you want tubes with higher current readings, so they run hotter at a given bias setting.

[fuzzbomb]

Yes, but the other way round - you want tubes with higher current readings, so they run hotter at a given bias setting.

Ahhh. Okay. I'll have to remember that next time I re-tube my Dual Rec. Thanks! I'm considering an Electrical Engineering minor, maybe then this stuff will start to make more sense. =P

[mooreamps]

It's probably a good idea for a push-pull amp to balance the static dc bias going through both power tubes, as it tends to balance the dc bias flowing through the primary coils of the output transformer. However, a better point for measuring plate current, gee, I do it off the plate circuit through a 100 ohm resistor. Others may disagree...

-g