Odd bias behavior - Mazda 6V6's

[Bluewail]

I tried to bias up a pair of Mazda 6v6's in my amp last night using a Bias Probe. The reading kept fluctuating up and down and when it would settle down, I would try to tweak the bias into the correct range. When I did this would set off another series of fluctuations getting me no closer to the target range. I did this for about a 1/2 hour and never did get them set-up right. What causes this? Are the tubes shot? I just got them from an ostensibly reliable supplier so I can probably send them back. Oddly enough they sounded pretty damn good, but I wonder for how long?

 

[aeolian]

How much fluctuation? Maybe the bias pot is dirty. Is this in your Carr? Not sure how the bias voltage is derived. Have to look at the bias on a scope as see what kind of noise is on it.

 

[Bluewail]

Hey Steve. Yep, its the Carr. I doubt whether it's the bias pot as when I try other tubes (NOS Marconi, NOS Brimar, new JJ's, some old used GE's) it sets up fine with no fluctuation other than a little drift as the amp warms up. If it does drift, I dial them right in and it stays where I set it. So it does seem to be isolated to the Mazda's. Also there is no other noise associated with that fluctuation that I can detect. Weird, huh?

 

[Bluewail]

I'll try swapping them and check the pins as well. Got to wait until later as I'm headed out of town. I'll post back with the results next week. Thanks all.

 

[Blue Strat]

Mazda 6V6s are famous for wild drift until they're burned in. I've sold nearly 1000 of these suckers, trust me, I know.

They seem to settle down after about 6 hours or so. Until then, it will be almost impossible to bias them properly.

This behavior is sometimes exhibited in burned in tubes in which case it's time to toss them.

 

[TheAmpNerd]

Originally posted by Blue Strat
Mazda 6V6s are famous for wild drift until they're burned in. I've sold nearly 1000 of these suckers, trust me, I know.

They seem to settle down after about 6 hours or so. Until then, it will be almost impossible to bias them properly.

This behavior is sometimes exhibited in burned in tubes in which case it's time to toss them.

Interesting.

That is pretty long to be that unstable. I wonder how
their emissive serfaces are coated differently than
other tubes?

How do you like the Mazdas compared to the JJs compared to the black plate RCAs?

 

[Blue Strat]

Originally posted by TheAmpNerd
Interesting.

That is pretty long to be that unstable. I wonder how
their emissive serfaces are coated differently than
other tubes?

How do you like the Mazdas compared to the JJs compared to the black plate RCAs?

Yes, they're definitelly unique. Most other tubes will drift over the first 5 to 20 hours but not nearly as eratically as the Mazdas.

They're great sounding tubes. I don't do comparisons often. Processing 9,000+ tube orders has taught me that no single opinion matters.

 

[drbob1]

There's a tube comparison on the net somewhere (Tubeworld? KCA?) that included Mazda, RCA, Visseau and some newer tubes. I think the Mazda scored well, just below the RCA overall, but perhaps a bit ahead in distorted performance.

 

[fullerplast]

Originally posted by drbob1
There's a tube comparison on the net somewhere (Tubeworld? KCA?) that included Mazda, RCA, Visseau and some newer tubes. I think the Mazda scored well, just below the RCA overall, but perhaps a bit ahead in distorted performance.

Our own forum member Mook has done a 6V6 comparison test. It's a bit dated, but does include the tubes you are asking about.

Mook's 6V6 Taste Test

 

[Blue Strat]

Originally posted by Old Tele man
BlueWail--did you try letting those Mazda 6V6s "burn-in" at an idle for awhile (8-20 hrs?) to see if they would "settle down" as Blue Strat suggested? I'm interested to see if they did...

What OTM said. The "good ones" eventually do settle down. I can't remember seeing any that didn't. Some did fail for other issues though.

 

[Bluewail]

Just got back in today. I'll give the burn in a try. They do sound good on initial listening with slightly more headroom then the Brimars or Marconi's. I'll post again once the burn in (or up).

 

[Blue Strat]

Mazdas are known for less headroom and nastier breakup (not that this is a bad thing....) than either the Brimars or Marconis.

 

[Bluewail]

Well I let the Mazda's cook for a day on my Super Champ and sure enough when I stuck them back in my Carr, they bias right up with none of that jumping around they had done earlier. Thanks for the tip Blue Strat. BTW your comment on the headroom had me go back and fool with my 6V6 stash again. My initial impressions of the headroom on the Mazda's was based on swapping out tubes in a Univalve and simply listen to how loud they were at the same clean setting relative to one another. Other than a JJ, the Mazda was louder at the same setting than either the Marconi or Brimar - but duh, I know that's not headroom. When I went back and pushed them into break-up, the JJ stayed cleanest (didn't sound to good but did have more headroom), the Brimar was second, then the Marconi and breaking up the earliest was (drum roll please) the Mazda. Right on the money Mike! Thanks for all the feddback guys.

 

[aeolian]

So maybe I should get some Brimars for my DR.

 

[Blue Strat]

Originally posted by aeolian
So maybe I should get some Brimars for my DR.

Those and the Marconi 6V6Gs are favorites with my DR customers.