SOLVED How do you adjust bias on a valve/tube amp?

Hey guys, I just today took the plunge and ordered a new Yaqin 10L MC-10T tube amplifier based on the many favorable reviews of this amp online. One thing that has been mentioned over and again is that from the factory the bias hasn't been adjust correctly for the tubes the amp ships with.

My understanding is that this is adjusting the flow of current so that electrons move freely through the tubes, and that if this isn't set right it changes the sound of the amp and can drastically shorten the valve/tube life span.

Is there anyone here who can explain how to bias the tubes and if any equipment is required to do so?

Very much looking forward to recieving this. I'll be hooking them up to some vintage Bowers & Winkins floor standing speakers. Should should great with my turntable

Amp link - http://www.ebay.com.au/itm/310816534422?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Ugh this site stresses me out sometimes. ROUND THREE...

As for biasing, you just need to find the plate voltages and the tube specifications. They SHOULD have something in the manual. For tools, you just need a screw driver and voltmeter that displays miliamperes. You can buy testers for it too.

Heres what can happen ->
1) Nothing will happen, tubes are fine
2) bias is "too hot" and the tubes burn hotter -> shorter life on the tubes
3) bias is "too cold" and the tubes run colder -> longer life, but much lower quality sound

Since you're buying a new amp, I would recommend you don't mess around with the bias. Find a tube amp tech around you (search for guitar stuff), probably cost 50 bucks ish. For the price of tubes, thats pretty reasonable rate to make sure everything is copasetic. You're also playing with severe voltages, and run the risk of hurting yourself. I would say leave well enough alone personally.


Finally, make DAMN SURE your speakers resistance match the 8 or 4 ohm outputs on the amp. If you have it wired too low, you can and will reduce the life of your amp (potentially drastically shorter).

Good luck

Yeah, my speakers are 8 ohms so that's ok. I guess I'll get someone to do it for me, though it would have liked to learn myself so I could do some tube rolling

mjw21a said:

Yeah, my speakers are 8 ohms so that's ok. I guess I'll get someone to do it for me, though it would have liked to learn myself so I could do some tube rolling

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Im a very "DIY" guy, and I've been playing and building guitars for about 15 years now. I've replaced capacitors, I've done all kind of wiring and builds... to this day I still pay someone to bias my amps. I have biased a few amps, but its just so much of a PITA and nitpicky, I'd rather just pay someone 50 bucks and be done with it.

Its like adjusting timing on my car. I'd rather rebuild the engine than set the timing. Leave it to the people with the specialized equipment that actually do that for a living.

Your user manual should mention how to bias that particular amplifier; as each amp has its own methods and settings required. Give it a go if you're feeling frisky, just passing on my experiences. Granted, Im a little... biased ( sorry haha) based on working with guitar tube amps that have a dozen tubes in them.

I might give it a crack. My understanding is you don't have to touch the pre amp tubes don't need adjusting, which leaves just 4 tubes to bias....

How "vintage" are your speakers ? They may benefit from re-capping. Elna , Sonicap , and Mundorf all make extremely high quality audio capacitors.
You should have a very nice sounding combination there. I had the pleasure of hearing four MFA 250 watt tube amps feeding a pair of B & W 801 Matrix speakers at a friend's house and it was a musical experience to rival my favorite concerts.

mjw21a said:

I might give it a crack. My understanding is you don't have to touch the pre amp tubes don't need adjusting, which leaves just 4 tubes to bias....

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Nope, just the power tubes, those would be your el34 tube sections. Its not "hard", but it is very easy to screw up. Just be careful and take your time

All you really need is a multimeter and screwdriver...Bias is a -voltage put into the grid circuit to turn off the tube when audio is not present AFAIK.. You might get a better idea what to do at the following link http://www.hans-egebo.dk/Tutorial/biasing.htm It shows several methods and the simplest is to insert a resistor into the cathode circuit which will leave the circuit "self biasing", but I don't know how efficient it is. Worth a looksee though.
Hope this helps,
Doc

Edit: forgot to say that usually there is a bias control trim pot in most modern amp circuits.

mjw21a said:

Yeah, my speakers are 8 ohms so that's ok. I guess I'll get someone to do it for me, though it would have liked to learn myself so I could do some tube rolling

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The thing to keep in mind assuming it's the same as guitar amps is the power tubes should be matched sets which gets expensive switching out but also the pre amp tubes are what make the biggest difference in sound, not that the power tubes don't have some effect but not as much as the pre amp, which depending on your power tubes can be substantially cheaper as well.

Yes, I've 4x matched Amperex Bugle Boys coming for the pre stage. I'm not sure if they'll actually work though as the amp currently has 4x 12AT7's. I'm hoping its compatable with the Amperex tubes which are 12AX7's.

Alaric, the B&W speakers I'll be using are 1970's manufacture speakers. The first of the speakers with polyproylene injected cones. B&W were pioneers in that tech. They should sound great.

Awesome, thanks for the link Doc, I'll have a read of that before proceeding

12at7's and 12ax7's are interchangeable, 12ax7's are just higher gain

Excellent. I've a good stock of 12ax7 tubes to try. The Amperex Bugle Boys should be the best of them though

Wouldn't hurt to try mixing them up as well, can do wonders in guitar amps

Hmm, most of my tubes are current production tubes.... I tried Mullard reissues in my phono pre except the Chinese tubes sounded better. on the other hand they sound amazing in my ART microphone pre.

I’ve heard good things of the Tung Sol reissues. I didn't like them in my mic pre though they might be good in my stereo amp....

Yep tube are a funny thing and sound is all subjective to the listener. Sometimes the cheap ones sound better some times not. I have one amp with a NOS RCA 12at7 and an older Realistic el84 and the other with a NOS Jan Phillips 12at7 and 4 JJ 12ax7s and JJ el 84's. I hated the sound from the Chinese tubes in the second but I know of some people who ended up putting them back instead of the new tubes. The other came with solvteks which weren't too bad.

It's helpful to research the properties each brand is known for to get the sound you want

I found the sound of the Chinese Shugang tubes has really changed after about 30 hours burn in and became much richer and more musical sounding. These NOS Amperex tubes are meant to be spectacular in tube amps though so I'm very keen on listening to them. I may change the tubes in my phono pre once I have the tube amp. Every component change modifies the sound after all