Bias reading question

[bucketbot]

As a new Mark V owner & an incorrigible fiddler :wink: I decided to check out what the actual Bias readings were on my head.
I measured it with my Tube amp doctor Bias probe which permits measuring all 4 tubes at once & this is what I got:
Values are going from R to L viewed from chassis rear after the Rectifier tube.
Measurements taken with all 3 channels set to 90W mode. Clean channel engaged. All Masters & Output turned down to zero.

V10 26.2 ma Outer
V8 33.6 ma Inner
V9 58.1 ma Inner
V11 35.2 ma Outer

Quite a variance!! Should I be worried? The amp is new & only a month old.
Powertubes are 6L6 with Grey bands on bases with writing saying 10AC YEL.

One thing I noticed was if I swapped the tubes around I always got a similarly high reading for v9?
Also similar readings for the other 3 slots regardless of which of the 4 tubes I put in each position?
This type of amp is all new to me so is this normal?
All this Simulclass stuff is a bit magican territory for me so maybe this is quite OK as I am normally used to measuring the Bias on my Carvin Heads were I expect all 4 tubes to be around the same value.

Anyone allay my fears or tell me that I have a problem so I can get it sorted while the amp is still new? :?:

 

[domct203]

FWIW- that V9 position is running at about 91% of it's max specs (at idle) if there is 470V on the plate.

That's alot of difference between tubes of the same color group. I know the center sockets run higher due to their (sort of) Class A specs but I dont think they should be that hot, or that different within the pair. If I remember correctly the pairs in my MKV were within a few mA's.

Personally, I'd be super paranoid about that V9 tube red-plating on me.

Dom

 

[domct203]

I found an old thread of mine where I took some bias readings with some different tubes. You can see that the pairs are well matched using both Mesa YEL & GRN codes as well as Mesa grade =C='s.

http://forum.grailtone.com/viewtopic.php?f=33&t=46948

Dom

 

[bucketbot]

Thanks Dom,
Guess that's just what I expected to hear but not what I wanted to hear.

I have a very well matched set of JJ 6L6GC's in my Carvin X100B so may be worth briefly putting them in & re-checking the bias & see if I get similar variances to ensure I don't have duff tubes before I contact the dealer who sold me the head :?:

 

[domct203]

I have a very well matched set of JJ 6L6GC's in my Carvin X100B so may be worth briefly putting them in & re-checking the bias & see if I get similar variances to ensure I don't have duff tubes before I contact the dealer who sold me the head :?:

I was going to suggest that, guess great minds think alike. If you don't want to risk a tube failure in the MKV due to unknown cathode current draw of the JJ's you could also put the Mesa tubes in the Carvin and check their matching.

Dom

 

[bucketbot]

I have a very well matched set of JJ 6L6GC's in my Carvin X100B so may be worth briefly putting them in & re-checking the bias & see if I get similar variances to ensure I don't have duff tubes before I contact the dealer who sold me the head :?:

I was going to suggest that, guess great minds think alike. If you don't want to risk a tube failure in the MKV due to unknown cathode current draw of the JJ's you could also put the Mesa tubes in the Carvin and check their matching.

Dom

Good Plan, :wink:
In fact I may try that first as I know 100% for sure my Carvin is fine.

 

[domct203]

Good luck. Be sure to post your findings.

Dom

 

[screamingdaisy]

Seems odd that both the V9 and V11 are out of spec with their matching pair. Probably something simple like a bad bias resistor or something.

 

[domct203]

It's odd that it is one half of each operating class pair that is off. I would think that the inside (class A) sockets would have their own bias voltage, as well as the outside (class AB) pair. But then again, 10W is parallel single ended, which combines one half of each pair so there could be something related there too. Without a schematic it's all speculation anyway.

Dom

 

[bucketbot]

Good luck. Be sure to post your findings.

Dom

Will check it out tomorrow & post. :wink:

 

[bucketbot]

Good luck. Be sure to post your findings.

Dom

Will check it out tomorrow & post. :wink:

OK, I checked it all out today & here are my findings! But to summarise: Good news! Panic over!

I thought before I took the tubes out of the X100B to try the Mesa tubes in it I would just fit my TAD Bias Master & re-check the Bias readings as it’s about 6 months since I last did it.
This was actually the 2nd time I have used the TAD gear (First time being when I got the results that prompted this post) as up to then I always used a single GT Bias probe & my multi-meter.

I was horrified to see that on one tube socket in the X100B I was getting readings in excess of 100ma when I had previously biased the amp to around 45 & all 4 tubes were within 2 ma of that. :shock: :shock:
Well sh1t...that just can’t be right I thought :roll: so I removed the TAD bias probes & re-fitted my trusty old GT Bias probe & multimeter & checked each tube in the X100B one at a time & ended up with the readings I expected with all 4 tubes at around 44ma.

I then did the same with the Mark V & got the following readings:

V10 26.7 ma Outer
V8 35.0 ma Inner
V9 36.3 ma Inner
V11 25.3 ma Outer

No problem!! Panic over! :mrgreen:

Clearly a case of duff measuring equipment! :evil:
E Mail to TAD manufacturers imminent! :evil:

Good thing I never attempted to actually bias any of my amps using the **** thing!!! :wink:

 

[domct203]

Glad the amp is OK! Those new bias reading are very close to the ones I got in the thread I linked.

Good luck now with the TAD probe issue.

Dom

 

[bucketbot]

Glad the amp is OK! Those new bias reading are very close to the ones I got in the thread I linked.

Good luck now with the TAD probe issue.

Dom

Cheers Dom....to be honest I'm temped to just bin it & revert back to the old method which has never let me down! :wink:

However I only bought the **** thing in April so should still be warrantied.

 

[bucketbot]

Tube related Q Here:

Always a good idea to have a few spare Pre's knocking about just in case so I plan to get 2 or 3.
I know any brand will do but as the amp is new for the sake of the warranty I will stick with Mesa Branded ones initially, so is there much to be gained by using the SPAX7 variety rather than just the stock 12AX7's?

 

[screamingdaisy]

Tube related Q Here:

Always a good idea to have a few spare Pre's knocking about just in case so I plan to get 2 or 3.
I know any brand will do but as the amp is new for the sake of the warranty I will stick with Mesa Branded ones initially, so is there much to be gained by using the SPAX7 variety rather than just the stock 12AX7's?

Yes and No. The SPAX7 is a regular Mesa 12AX7 that's been selected based on low noise performance. It's recommended for sensitive positions; such as the V1 position, as any noise generated at that stage is further amplified by every gain stage that follows it. After that you're throwing money away for no reason.

Note that SPAX7s are chosen for low noise performance and not superior sound performance. They sound pretty much the same as any other Mesa 12AX7.

 

[domct203]

screamingdaisy is spot on. I would just pickup a couple of standard Mesa 12AX7's.

You may also want to get a spare pair of powertubes as well.

Dom

 

[bucketbot]

screamingdaisy is spot on. I would just pickup a couple of standard Mesa 12AX7's.

You may also want to get a spare pair of powertubes as well.

Dom

Thanks Guys.
Yeah Dom, I was thinking that. :wink:

Re the Pre's I may well get 1 of the special Pre's & stick that into V1 now & keep the other as a spare & get another Standard 12AX7 for good measure too.

 

[bucketbot]

Glad the amp is OK! Those new bias reading are very close to the ones I got in the thread I linked.

Good luck now with the TAD probe issue.

Dom

Cheers Dom....to be honest I'm temped to just bin it & revert back to the old method which has never let me down! :wink:

However I only bought the **** thing in April so should still be warrantied.

Thomann accepted the return & will replace it for me.
Result! :mrgreen:

 

[domct203]

Glad the amp is OK! Those new bias reading are very close to the ones I got in the thread I linked.

Good luck now with the TAD probe issue.

Dom

Cheers Dom....to be honest I'm temped to just bin it & revert back to the old method which has never let me down! :wink:

However I only bought the **** thing in April so should still be warrantied.

Thomann accepted the return & will replace it for me.
Result! :mrgreen:

Excellent! Good customer service can be hard to find sometimes now a days, glad it turned out well.

Dom

 

[bucketbot]

Having lived with the amp for a while I'm now starting to get to grips with it.
One thing i've noticed is there is a huge difference in the way the amp behaves once you switch all 3 channels to 90 Watts.

Really opens everything up & what's more when switched to Variac mode it still sounds amazing at relatively low voulume even at that power level. :mrgreen:

Guess that's the Simul Class Magic coming into effect! :wink: