2. amps-preamps
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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.

David
divad68

Showing 2 responses by rf_gumby

rf_gumby
54 posts
 
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.
rf_gumby
54 posts
 
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

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