2. amps-preamps
  3. 1180551872

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 1 response by divad68

divad68 OP
6 posts
 
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
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