Setting Bias on a B52 AT100 with Sovtek 5881WXT

[stoneattic]

Hello all,

I just picked up a B52 AT100 (schematic attached) that needed a minor repair (reflow a solder joint to get the drive channel Mid and Bass pots working). I wanted to check the bias but I'm not sure what is appropriate for the Sovtek 5881WXTs that are in it. First I'd like to start off by saying it sounds great and I only wanted to check since I didn't know the history and had no idea if these are the original power tubes or replacements, and if it had ever had the bias checked or adjusted. The 12AX7s and 12AT7 appear to be replacement tubes, so I'm guessing the 5881WXTs are as well.

I've read that the 5881WXT should be treated as a 6L6 in some places and that they should be treated like 5881s in others. And to add to my confusion it seem like they may be closest to 6L6GC or 6L6WGB ,which are different from the 6L6 and each other. These are the large base 5881WXT (see pic):


B52 has a recommended way to set the bias (attached), which scares me since it looks like it turns my DMM into the fuse (see attached). I have an Amp-Head bias probe so I went that way and took the following measurements with the rectifier set to solid state:

V6 = 45.6 mA
V8 = 47.3 mA
V7 = 47.2 mA
V9 = 44.5 mA
All @ 453 VDC

Per the Weber calculator (all class AB):

a 5881 @453V should be 40.1 mA @ 70%
and a 6L6 @453V should be 29.3 mA @ 70%
and a 6L6GC @ 453V should be 46.3 mA @70%
and a 6L6WGC @ 453V should be 40.1 mA @70% (same as 5881)

So if they should be treated as 6L6GC, then I'm pretty much right on.

But if they are to be treated as 5881 or 6L6 or 6L6GC it seems like the bias current is too high.

So to be safe I adjusted it down using the pot on the PS to :

V7 = 37.0 mA
V9 = 34.7 mA
All @ 450 VDC

Which is as far as the bias pot would go. I didn't recheck V6 & V8 since my bias probe only does 2 at a time and I figured the V6 & V8 would have similar reductions.

So now I'm either too hot for 6L6 or too cold (assuming 70%) for 5881, 6L6GC or 6L6WGC.

Any ideas as to which would be the most appropriate current at 450V? Thanks!

AT-100 Schematic Diagram.pdf
BIAS SETTING FOR THE AT100.docx

[Jon Snell]

A Sovtek 5881WXT is a rugged 6L6GC.
The current setting is a matter of taste, too much current and you are wasting power, too little and it will sound like a Marshall.
I would set the maximum current as 15volts across R95 as there is no need to over heat anything. (110 mA total).

[Mick Bailey]

These are probably one of the most rugged and dependable tubes that have been produced in recent years, hence there popularity as factory-fit in many amplifiers, and as a popular rebranded tube. Don't get hung up on the 70% figure - it isn't as critical as that and this seems to be enshrined in internet wisdom as a magical target value. There's a tradeoff between bias current and reliability - cooler bias means the tubes last longer, though too cool and you get crossover distortion. I usually go for no more than 30mA per tube, sometimes down to 25mA.

[Helmholtz]

Per the Weber calculator (all class AB):

a 5881 @453V should be 40.1 mA @ 70%
and a 6L6 @453V should be 29.3 mA @ 70%
and a 6L6GC @ 453V should be 46.3 mA @70%
and a 6L6WGC @ 453V should be 40.1 mA @70% (same as 5881)

So if they should be treated as 6L6GC, then I'm pretty much right on.

But if they are to be treated as 5881 or 6L6 or 6L6GC it seems like the bias current is too high.

So to be safe I adjusted it down using the pot on the PS to :

V7 = 37.0 mA
V9 = 34.7 mA
All @ 450 VDC

Which is as far as the bias pot would go.

While it could be done, there's absolutely no reason to bias a 6L6GC hotter than an original 5881 in the same amp.
All tubes of the 6L6/ 5881 family share the same performance data, only plate dissipation and maybe voltage limits are different - but these are not performance but safe operation related.
Manufacturer recommends 30mA. I'd leave it at the values you got (35mA to 37mA).

BTW, what is the cathode voltage?

[stoneattic]

Thank you everyone for the replies.

The replies seem to point towards the B52 recommendation of ~30mA, which is where I was leaning. But, as Helmholtz suggested, just sticking with the 35-37mA I have and not trying to modify the bias circuit.

Then I stumbled on this site where he appears to have done a deep dive on the 5881/6L6 conundrum, which swung me back towards 40mA. Any thoughts, especially on the second page where he suggests that my 5881WXT are 25-26W tubes?

https://www.jayskyler.com/guitar-gea...ube-guide.html
https://www.jayskyler.com/guitar-gea...e-guide-2.html

Helmholtz, are you asking for the plate to cathode voltage?

[Helmholtz]

The replies seem to point towards the B52 recommendation of ~30mA, which is where I was leaning. But, as Helmholtz suggested, just sticking with the 35-37mA I have and not trying to modify the bias circuit.

Then I stumbled on this site where he appears to have done a deep dive on the 5881/6L6 conundrum, which swung me back towards 40mA. Any thoughts, especially on the second page where he suggests that my 5881WXT are 25-26W tubes?

You still seem to be fixated on the s.c. "70% rule". In fact there is no technical justification for such rule from tube literature (despite of all the internet rumor).
At best 70% plate dissipation could be seen as an upper limit for fixed bias PP class AB amps as it provides at least 30% headroom for plate dissipation increase with signal.

If you stay below 40mA it will be fine with 25W or 30W tubes.
Generally lower plate dissipation increases tube life.

Helmholtz, are you asking for the plate to cathode voltage?

I meant cathode voltage measured to ground.

[stoneattic]

Thanks again. I'm going to stick with the 35-37mA. I measured cathode to ground and got 6.1mVDC. This is fully powered up, off stand by, but all the pots set to zero. This is beyond my knowledge, but I was expecting more. What where you expecting?

[Helmholtz]

I measured cathode to ground and got 6.1mVDC. This is fully powered up, off stand by, but all the pots set to zero. This is beyond my knowledge, but I was expecting more. What where you expecting?

It wasn't quite clear from the schematic but it seems that the mode switch shorts the cathodes to ground at full power.
So your low voltage makes sense.