Newbie needs advice choosing and biasing new tubes

Thanks Newbee. As you say, I can well imagine that biasing the tubes isn't anything too challenging. It's just that I've never done it before! A technician does my guitar amp for me amd my headphone amp is autobiasing (apparently).

Your post is very clear and the advice much appreciated. Just one thing I wanted to query....you say to turn the bias pots counter clockwise. I assume the intention here is to lower the voltage..makes perfect sense and counter clockwise is usually what achieves that result. But, in the instructions that come with the amp, it states that turning the pot counter clockwise increases the bias and it is clockwise that reduces the bias.

Do you think maybe the instructions are incorrect? The amp is chinese and the manual contains it's fair share of poor phrasing/grammar/spelling, so I wouldn't be surprised if they go this bit the wrong way around:-)

Thanks
David

Congrats on your new purchase and venture into tubeland.

I prefer the Winged-C KT-88 Svets, and would stick with them. The two types (6550 and KT-88) are usually interchangable (you want to check with the manufacturer first before spending the dough). That said, I'd go with the KT-88's myself. 6550's in my different amps and
experience have seemed less dynamic.

One matched pair would go into V1 and V2, the other pair
should be similar in transconductance (gain) and would go into V3 and V4.

The biasing should be exactly as the manufacturer stated in the manual, for sure no higher, but could be set slightly lower if desired or needed. Don't worry about being exact, close is usually good enough. Set the bias
lower upon turn on (about .5vdc in your case), look at it again in 5 minutes to make sure it is not too high, and again after about 20 minutes of tame warm up. You'll want to bias it again the next time or so until you feel comfortable with it remaining pretty constant. After that, maybe check it again in 2 weeks, after that maybe only every month or two. You'll develop a level of comfort and familiarity with the process and be able to make your own judgements.

Many tubes fail early in their life, if they make it past the first few days, you're probably good to go. Also, be aware the bias will go up as the amp warms up, hence the set a bit low comment earlier. This bias voltage should not be varied regardless of manufacturer of the tubes. The tubes are designed with operating parameters that should keep them similar. Likewise, the amplifier is
designed with the specified bias in mind for the tubes to operate safely and with a given linearity.

Also, don't set the bias with music playing and be sure to have your speakers connected for a load for the amplifier.

Some amps can be turned on with no tubes in, some burst into flames. I'd avoid attempting anything without all
the tubes in place.

Hope this makes sense. Good luck and above all have fun. Sorry you can't listen immediately, but it's kind of like Christmas- almost as much fun to look forward to.

RFG

David, Re your question about the Chinese explaination. I think that they are 'technically'correct' in thier verbiage and we understand the process backwards, i.e. you reduce bias by increasing voltage not increasing voltage increases bias - we just assume so because we are watching a meter which is measuring voltage (not the actual bias). I'm not a techie - this is just my understanding. But, FWIW, if you think you should start with your bias pot anywere but the maximum counter-clockwise position don't do it without specific clarification. I've never seen a bias pot work 'backward' and I've seen quite a few. :-)

I have seen bias pots work backwards on a half dozen occasions. If you have no idea where it will be (as in with a new set of tubes), start in the middle and upon turn on, quickly(!) make sure the bias won't run up too high. Then set again as outlined in our previous posts.

David, although some amps can stomach having tubes unplugged when switched on, I suggest you have all your tubes plugged when switched on.