Is bias setting necessary on matched tubes?

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 ;(

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!)

Geoff

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?

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!

Geoff

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.
Cheers!

yes, and they will sound different at different bias settings.

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.