Tube biasing: how long do you wait for a reading?

[azzzy]

Greetings. This is my first attempt at biasing an amp (Bogner Uberschall to be precise). I bought a Weber Bias Rite, followed the instructions to a T, and then discovered that the readings change significantly after a time. I let the amp on for about 20 minutes and the readings seemed to continue to climb - on some tubes they went up 10 ma over the initial reading. So, my question is how long do you wait before taking a reading after you turn the amp on? As in switch it off of "Standby" into the "Play" position?

 

[azzzy]

So? Anybody?

 

[mniel8195]

It may not be the tubes but voltage fluctuation in the house. This may be to much but i turn it on let it sit for 30 minutes. Take first readying and adjust wait another 30 minutes. continue if necessary .

 

[azzzy]

It may not be the tubes but voltage fluctuation in the house. This may be to much but i turn it on let it sit for 30 minutes. Take first readying and adjust wait another 30 minutes. continue if necessary .

Thank you for replying. Well, if it were the voltage fluctuation in the house you would expect it to go up AND down, no? In my case I tried 5 or 6 different quads of old stock Mullard tubes (most of them XF1's), and the readings kept going up every time. It's just that the first 3 times I didn't wait long enough to see how significant the increase was. But later I left a quad sitting a little longer and the readings kept climbing! How high to you let it climb before adjusting it down?

 

[Loudness250]

You have a bias rite. So you are making sure the plate voltage isn't rising too right? 10 ma isn't that much but if you already know where you want to bias it, then turn it back down to where you want, and then check like a half hour later, if you need to turn it down again, do that and then just play it and check again later.

 

[azzzy]

You have a bias rite. So you are making sure the plate voltage isn't rising too right? 10 ma isn't that much but if you already know where you want to bias it, then turn it back down to where you want, and then check like a half hour later, if you need to turn it down again, do that and then just play it and check again later.

The plate voltage doesn't change much - hovering around 470-480 (depending on the tube). I wasn't yet adjusting the bias - I was looking for the tubes that measure close enough to be in a matched quad (hence trying multiple sets of tubes in a row). 10 ma may not be that much but I don't know how high it would have climbed if I didn't turn off the amp.

P.S. My Uber blew the HT fuse - so no tube testing until I get the replacement. For a while I thought it was the Bias Rite that crapped out. Luckily it's just the fuse in the amp.

 

[LP Freak]

I use the Bias Rite and everything seems to max out within 5 minutes or so on all my amps.

 

[mniel8195]

You may want to send your Bias Rite in to be serviced. Have you contacted Weber?

 

[PatF]

The heaters should be ready in 1 minute. If you put in a new set and the bias is way off it could damage them in seconds after throwing the standby. So you would make sure its close right away. Then leave it on and make sure it stays in range.

 

[Alex_S]

I have the same thing on some of my amps (change after a while of playing). I wait for 5-10 min usually as the values seem to stable then. But I would wait longer if I see it's not long enough. As the amps seem to get hotter, I don't want them to have a to high bias and prefer to start playing with a to cold/low setting instead of damaging tubes later on. Esp with amps where I have NOS tubes in. Funnily enough my Bogner Shiva seems to be the most extreme regarding the topic. Probably something special with their circuit.

 

[rustymetal951]

sorry if this is wrong, but guess I'm not fully vetted to post new topic yet. Need help with a randall thrasher 120 head.
bought used from GC (excellent condition) outside pair by fuse blown, mismatched 6l6 ptube.. bias fluctuation in good pair, new tube set, and by fuse? there are some tonality issues, so was thinking full tube kit. very first tube head,and not enjoying the step up. thoughts?

 

[rustymetal951]

ht fuse (stupid auto correct)
one fuse was blown and one tube out of matched quartet is diferent. biasing is al over the place.. suppose to be set at 72mV for ruby 6L6..
does anyone here suggest a new tube set for pre and power.. might solve issues?

 

[azzzy]

ht fuse (stupid auto correct)
one fuse was blown and one tube out of matched quartet is diferent. biasing is al over the place.. suppose to be set at 72mV for ruby 6L6..
does anyone here suggest a new tube set for pre and power.. might solve issues?

If the tubes are too mismatched the amp will not sound good. They need to be matched as close as possible - I like to keep them within 5% of one another. Unless you have individual trim pots for biasing each tube (which is unlikely).

As for the HT fuse, one of the power tubes might be bad. If it shorts it will blow the HT fuse. If you don't know which tube is bad (may not be the one that's different, unless you mean it's a different type of tube), just get a new matched quartet. If it's 6L6 I would suggest spending some money and getting NOS RCA black plate 6L6GC's. They can be had at $50 a pop. $200 for a quartet may seem like a lot but they will last for a loooong time - some of my vintage Fenders and Ampegs still have original tubes in them. That's over 40 years!