Is bias setting necessary on matched tubes?

Hi Guys,

Just bought my first tube amp and it was delivered with broken/defective tubes. I just bought a matched quad of Genalex KT88's to replace them and was wondering if biasing is a must, or can I just "plug and play" them as they say.

I'm pretty much a newbie with tubes so any advice is really helpful.


Yes, the bias will need to be reset. Make sure the bias pot is turned all the way down before turning on the amp. Let the tubes warm up for 10 minutes and then bring the bias up slowly to the recommended settings. With new tubes, check the bias again after a couple of hours playing time and again in a weeks time. Enjoy.
yes, and they will sound different at different bias settings.
Unless your amp is of the "audobias" type (primaluna is one brand) then you will have to readjust the bias. Matched tubes just means that they are very similair in terms of operating characteristics. Important! If your amp has only one bias adjustment for each channel then it's recommended that you ONLY buy matched tube sets and replace them all when one goes bad. RAM tubes website has a good primer on matched tubes.
Only if you have a fixed bias amp, if you have a cathode bias amp then the tubes will be self-biasing and no bias controls or check points will be present on the amp.

If the amp has bias controls and there are individual controls for each tube, matched pairs of tubes, while desireable, are not necessary. If the amp has one bias pot per pair of tubes then each pair must be matched to prevent current hogging by one of the tubes, leading to premature failure and asymmetry of the waveform.
OK, it looks like my amp the Mingda 368-b90 has manual bias adjustment, one for each tube. I've got a multimeter, but I'm about as new to tube bias adjusting as it gets. Is it easy to do? Where can I get info on how? I don't have any info on the specs save that I'm putting in a set of KT88's. (I think it came with 6550s) Should I just bring it to a guitar shop? Am I being a sissy for not wanting to stick my hands where I know not what? I really don't want to smoke a new set of Genalex Gold Lions or myself for that matter!

Thanks for your input!

OK I talked with a repair tech and he said it would be $125 to bias the amp. In addition he's going to go over the amp to see that it's all working correctly. I think it's a fair price for that service, am I wrong?
Geoff, we all have different comfort levels as far as dealing with our gear goes, but if you own a tube amp that requires biasing, it is best to learn the simple task. As components, and tubes, age, and one's mains voltage varies, it is best to check bias, at least monthly, and adjust it as needed.

It is usually as easy as putting the red probe from your multi-meter into one hole, and the black into another, or touching it to the chassis.

If you are electing to have your tech check the amp and bias it for you, why not just have him give you a ten second lesson in how the biasing is done? Bring your own multi-meter so that he can show you which range to set it on, if it is not auto-ranging. You will be surprised at how easy, and safe, it is, and you will take pride in keeping your gear optimally adjusted for best sound and tube life.

Marty Kohn
Portland, OR
Hi Marty,

Thanks, and I'm with you on that! The tech said that he would show me the procedure for biasing when I picked the amp up. The real reason for it costing $125 is that he's going to go over the amp to make sure there's no other problems. Right now I've got one fried tube (I'm assuming that it was damaged in shipping) but I want to make sure that the amp isn't compromised in a way that would fry another.

Soon to be a master bias'er (that sounds really wrong!)

On the MingDah the voltage from earth to pin 8 is to be set between 32 and 35 milli volts adjustment it is easy with the correct screwdriver but if the amp is started from cold set the bias at the low side and wait till all warms up properly and recheck until it stays where you adjust. I smoked my MingDah last year (well someone else did it for me while I was a way sailing) one of the tubes got smashed while it was running and the intestines didn't like having this done to them. I found an service request box on their web site and left them a message and within ten minutes I had been contacted by one of their service techs who explained what was needed and passed on the schematic for the amp and the instructions on how to adjust the bias etc very good service I thought.

PS no one owned up to dropping anything on to the amp and the cats carried the blame for the damage ;(