[DogWizard]

Hopefully one of you fine folks can help with this.

I've been using an AV123-era SP3 as part of an office system for years now and the original Chinese 5881 power tubes finally gave up the ghost (could no longer keep them biased properly). I ordered a matched quad of the new Tung Sol 5881 from Tube Depot and they arrived today but I can't find any info on the recommended bias outside of the official data sheet.

The SP3 recommends a bias of 1.15V, however the Tung Sol data sheet states the following (the only reference to "bias"):

"Maximum Grid Resistance (Fixed Bias) 0.1 MEGOHM"
"Maximum Grid Resistance (Self Bias) 0.1 MEGOHM"

which appears to be a different "bias" measurement/parameter than I am accustomed to seeing (non-voltage based).

I am admittedly new to power tube swaps and while I have done some rolling with this amp, it's never been the power tubes so the proper bias voltage was never a major issue.

Does anyone have any recommendations on the proper bias voltage in this application? I'd rather not screw this up and I haven't been able to find anything useful using "search". I sent an email to the Tube Depot but haven't heard anything back yet and I would like to get this baby back up and running sooner rather than later.

Am I overthinking this? Can I just stick with the default amplifier bias of 1.15V?

Any assistance or suggestions would be much appreciated.

Thanks!

 

[HAL]

The SP3 has a resistor in the cathode circuit that you measure the bias voltage across.  It converts the plate current to a measureable voltage to set the bias.

Just adjust the trim potentiometer to 1.15VDC on the meter and you should be good to go.

[JakeJ]

Hi DogWizard,

Bias in a tube power amp is the value the operator/owner manual tells you to set it at.  Tubes themselves do not come with a bias setting from their manufacturer.  Tubes are designed with specific operating parameters for a specific application.  Equipment mfr designs their circuit based on those operating parameters and the bias setting is generally similar for a tube type even if you have two different units.  If you are so inclined you could check out the tube data sheet for your 5881 and see what the designer must provide in his circuit.

Here is an excellent resource:
http://tdsl.duncanamps.com/tubesearch.php

And a link to the 5881:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5881.pdf

If it's all gobbledygook to you then I apologize for overwhelming you with info.  Hopefully my explanation makes sense.

[rlee8394]

Your quote of the data sheet info is just stating what the value of the grid resistor should be based on how the amp is biased, fixed or self. It has nothing to do with setting the bias current. Follow HAL's advice and adjust to 1.15 volts.

Ron

[JakeJ]

No.  You miss my point.  No matter.  The amp mfr specs the bias not the tube mfr. although the tube's operating parameters limit the range of that bias setting.

Oh, and HAL's recommendation is in the original post.  It's the mfr's setting.

[DogWizard]

Thanks for the replies guys - looks like my mistake was in thinking that different tubes would require (or benefit) from a slightly different bias setting. I have seen multiple references to people adjusting bias higher or lower to accommodate a different tube and I assumed that it would be needed in this case.

Do different tubes not have a "preferred" bias voltage, regardless of the amplifiers recommended default? While I understand that the amplifier will have a specified target value for the intended tube type, it seems odd that it would be universal and apply to all tubes of that type, regardless of style or manufacturer.

Sorry for my ignorance on this, just trying to understand.

Thanks again!

[HAL]

The 1.15VDC bias setting applies to the original 5881's the amp was designed for.

Other output tubes require different bias to work.  The design was done for 5881's, so that family of tubes maybe similar.

The original specs you posted are for the grid stopper resistor used with the 5881's.  That does not apply to the plate bias current spec.   The bias voltage measurement is a way to set the plate current for the tube to make sure they are not running to hot for long life.