Newbie needs advice choosing and biasing new tubes

Hi, I received my first tube amp today - a chinese made Yaqin MC-100B. Unfortunately one of the 4 KT88 tubes that came with it is damaged (glass smashed), despite the fact that everything was well packaged. Anyway, what I intend to do is seek out a replacement set of tubes tomorrow as I'm really itching to get my new 'toy' powered up and working. I'd appreciate some guidance on the following.

I've heard a lot about the Svetlana 'Winged C' KT88's and these are available at my local stockist. However, they also have the 6550C version - are these two types of tube really interchangeable? Which would you go for?

The tubes are sold in matched pairs. How would I install these? One matched pair in V1 and V2 and the other in V3 and V4?

The amp has testing points for each tube and an adjustment slot. The manufacturer states that the "working point setup of this unit shall be 0.55v-0.60v." Does that range apply regardless of which brand of tube is installed?

Any do's and don'ts that I should be aware of when setting the bias? I have a 'cheapish' digital multimeter. the instructions talk about setting this to DC1V and then adjusting the volume to minimum.

Is it dangerous to turn the amp on without all the tubes inserted? If it's not then I'll put all except the broken one in this evening so that I can practice measuring the voltage of the 3 good tubes.

Sorry for the torrent of pretty basic questions and thanks in advance to anyone who takes the time to respond.


Sonics - The KT88's will have a bit of mid bass bloom (sounds good!) a bit forward in the mid-range, and fairly extended highs. In comparison the 6550 is more linear from the bass through the mid-range and a bit soft in the highs. I use both of these tubes in amps.

Amps or speakers which are inherrently warm can sound a bit overblown with KT88's and the 6550's would help produce a more neutral tone. Conversely a KT88 would warm up a 'cooler' amp or set of speakers. Kind of a matching to taste issue. I'd recommend that you start with the tube type selected by the manufacturer, i.e. KT88's. SED's are a good rugged tube and worth buying. After you have spent a long time with this tube you can proceed with experimenting with other power tubes and small tubes to tune your system to taste. Those little tubes can make a huge difference, don't overlook them. Its really a combo of power tubes and small tubes that will get you were you may really want to be.

V1 & V2 go together as do V3 & V4. Matched tubes is good but since you have individual bias pots if you have to replace a single tube (again) you can just get a new one of the same brand with a similar bias point.

The bias range should be close with each tube type especially if its established for KT88's. IME 6550's are a bit more rugged and can take a higher bias point. For example I will bias my 6550s between 45 and 50 ma in my amps whereas with KT88 its more around 40 ma.

The biasing procedure. After installing the tubes, and before you turn on the amp, turn the bias pots counter clockwise to a zero setting. Turn on your amp with no sources operating and let the amp warm up for a few minutes. The set the bias as recommended, say 55 ma and let the amp run for a few hours, then go back and adjust bias again (it will probably have fallen off a few ma). I usually check it again in a few days and then I check it every couple of months thereafter.

Re - your 'cheap' multimeter. Probably good enuf, so long as it provides for reading DC in ma. Some use digital. I like analog. Mine cost about $15 to $20 at my local electronics store.

I don't know if operating you amp without all of the tubes in place would actually damage the amp. But, I would err on the cautious side and not recommend it. Besides biasing the new tubes really isn't, as you will discover, a big deal.

Have fun with your new toy. Oh, by the way, most tube amps like to be connected with speakers when you turn them on.
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.


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 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:-)

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.