Question on Tube Swapping and Biasing

higher max plate voltage, and pre-burn-in.. haven’t tried them yet, will do it in next tube swap..

"Thanks for all the info! @westcoastaudiophile what is "improved" about the Telefunken KT88? Have you tried them?" 

@jaylat If you click on the link that @westcoastaudiophile provided for you and hover your pointer over the image you can read on the side of the box that it is a JJ tube that's been screened, tested and cryo'd.

Multimeters are really cheap. You simply have to be extremely careful not to get your body in the loop since the amp has to be on and they're very high voltages. Hence why they often say it should be done by professional. If you're careful as easy.

“Multimeters are really cheap” - yeah, kind of! LOL

Cheap DMMs are really cheap, and results are not accurate or predictable, due to many factors, such as RFI impact, sampling rate etc.! I have couple of Agilent/Keithley bench DMMs in my Lab, $5000+ each, and I use them for sensitive circuit tuning!

Before adjusting bias current by measuring amp’s internal resistor’s IR voltage drop, would be appropriate to test resistor(s) value itself first, which could be easily 20%+ off schematic.

@westcoastaudiophile Please! The average audiophile has to believe in something! As I stated earlier, bias is a moving target (see quote below) and is not something anyone needs to obsess about! People in the know will state quite frankly that there is no "last word" regarding bias. The most you ever need to do is maybe check it once or twice when the amp is new after a good warm up and make sure the bias isn't exceedingly high for any one tube then you can rely on the set of tubes the MFG included with the amp and maybe, maybe check it once a year. The bias at any one time can be 25% below the nominal and not have any significant effect on the sound the amp produces or any damaging effect on tube life. Check it again the next day and it may be within a few percent of the nominal spec for that amp.

"“Multimeters are really cheap” - yeah, kind of! LOL

Cheap DMMs are really cheap, and results are not accurate or predictable, due to many factors, such as RFI impact, sampling rate etc.! I have couple of Agilent/Keithley bench DMMs in my Lab, $5000+ each, and I use them for sensitive circuit tuning! Before adjusting bias current by measuring amp’s internal resistor’s IR voltage drop, would be appropriate to test resistor(s) value itself first, which could be easily 20%+ off schematic."

My comments from up thread. -

"FYI, bias is a moving target. Knock your brains out trying to get it right on the money then blink. In theory a matched quad is all you all you need to do but the primary caution is not having a single tube or tubes drawing significantly more current than any of the others. Other than that, the bias is likely to be all over the place at anyone given instant and is nothing anyone with a tube amp should ever obsess over.

Your JJs are probably still just fine and unless you have any particular indication that things are not as they should be, you should leave them alone."