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PostPosted: Fri Apr 28, 2017 11:50 am 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
Anyone experienced in bias adjusting an old M II A (1979)?

I measured my output tubes to around 20 mA which i find kind of low. I used a Bias cable. Actually it was this one https://www.eurotubes.com/store/pc/viewPrd.asp?idproduct=149&idcategory=6.

What might be the correct values?


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PostPosted: Mon May 01, 2017 6:04 pm 
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Mark III

Joined: Thu Jun 02, 2011 4:00 pm
Posts: 264
Location: France
alagami wrote:
Anyone experienced in bias adjusting an old M II A (1979)?

I measured my output tubes to around 20 mA which i find kind of low. I used a Bias cable. Actually it was this one https://www.eurotubes.com/store/pc/viewPrd.asp?idproduct=149&idcategory=6.

What might be the correct values?


On my MKIIA, under mains voltage at circa 226VAC/50Hz and voltage selector switched on 240VAC, amp at idle, no signal, 100W position.

secondary transformer HV = 2X314VAC + 53VAC tap.
A+ = 424VDC
B+ = 422VDC
C+ = 411VDC
D+ = 371VDC
6L6 plate = 423VDC
6L6 G2 = 422VDC
6L6 grid bias = -48.5VDC

Of course, these values are a bit lower than expected because of the 240V selector voltage for a 225-230V mains voltage, but it is a personal choice of a smoother working point.

Sorry I do not remember the cathode current (I noted it somewhere...) but I would not be surprised by circa 35mA par tube. I'll re-check it...

A+!

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MKIIA from March 1980 (S/N° 4915) - dixit Daniel "igfraso" (thanks !)
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PostPosted: Fri May 05, 2017 7:24 am 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
Posts: 1457
Location: NYC
That "20ma" you posted means nothing without knowing the plate voltage and calculating the dissipation wattage.Mesa does bias very cold.I like mine to be about 70% of total plate dissipation,which is typical for 6l6GC in class AB.So if plate volts are at,lets say,425vdc I adjust to about 49ma,which would put the idle dissipation at about 21 watts.Also be aware that to adjust the bias you need to install an adjustable bias pot first.You have to be careful when raising the idle watts in these hi gain amps,if you go too high the lead drive distortion can get harsh and ratty sounding.


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