QUESTION: Biasing a '65 Reissue Deluxe Reverb

[Mango Moon]

O.K., I've got a quick question regarding the bias adjustment on a DRRI (probably a 'newbee' quiz for most techs out there), but when there's just one visible bias pot, what's the point of measuring the actual bias on both power tubes?

In other words, as best I can tell from looking inside the DRRI, there is only one bias pot (immediately behind the power tubes and main transformer), yet there are two 6V6 power tubes. So if I attach a Weber Bias-Rite adapter (with multimeter) to either power tube and I adjust the bias to (say) 24-28 mA as recommended using the lone bias pot (which apparently sets the current for both tubes?), why would I need to bother checking the bias on the 2nd (remaining) tube?

Multiple video and written sources of guidance on this subject suggest doing both tubes, but if the single setting changes it for both tubes simultaneously, what's the point? Am I missing something somewhere?

Thanks All!

[52 Bill]

...why would I need to bother checking the bias on the 2nd (remaining) tube?

Measuring the second tube will let you know how well matched the two output tubes are. If they are perfectly matched, the bias current draw will match on both tubes at the same bias level setting.

Don't expect the two tubes to be matched that well. You may have to set to a mid-point where one is a little too hot and the other a little too cold.

[Mango Moon]

Bill,

Thanks so much for the feedback and I apologize for the delay in responding - I dropped my internet connectivity about 2-3 hours ago.

Anyway, the two replacement power tubes that I recently purchased (JJ 6V6-S) came as a matched pair, reportedly biased at 26 mA each, so I'll take it from what you've said here, that I can confirm this by setting the bias at 26-28 mA on the 1st tube, and when checking the 2nd tube, no additional adjustment should be necessary (i.e., I should read 26 mA +/- a tad).

In other words, there is no reason to check both tubes when replacing them as a matched pair, but one can confirm that the matched pair is indeed "matched" by setting the bias on one tube and confirming compliance of the 2nd tube (+/- ~ mA or so) using the same process. I get it!

Thanks for the education my friend. Long live the internet!!!

[Enzo]

To me the point is to not assume anything. The tubes may be matched, or at least were when they left the factory. But that doesn't guarantee they both work in your sockets. By setting it on one tube, you have a reasonable expectation that the other tube will fall in line, but by verifying that secind tube you not only find out that is indeed the case, you also find out that that other tube is working in this amp.

SHop advice: never think up reasons NOT to check something.

[Mango Moon]

Yup, in the words of Roger Waters . . . "it makes perfect dollars and cents"!

Thanx Enzo!