If I put new power tubes in my 2x10 HD150, do I need to have the amp re-biased?
I just bought a mint one that has Sovtek 5881 wxt's in there...these are good workhorse tubes but want something that sounds better like JJ 6L6gc's or Tung Sol 6L6 STR's....the amp sounds very good, but kinda sterile tonewise......
Thanks!
150's are in need of bias
150's are in need of bias check and set when replacing tubes.
Are you sure? I thought the
Are you sure? I thought the MM combo's that did not have the 12ax7 were plug and play?
Trust him...
Hi there,
you can certainly trust Mike on this one (and in general). Only the RD-50 series (#1650RD chassis) are non-adjustable fixed bias. All 2275/2475/2100/2165 series amps are adjustable fixed bias. Your amp is chassis 2275-150.
Cheers,
Lars Verholt
Thanks, this bias hard to do?
Thanks, this bias hard to do? Need a matched set of 6l6gc's?
Safety First
Hi Woody, Congrats on the 150 you just got. The bias adjust is pretty easy to do but be very careful of getting shocked by the 700VDC+ Hi Voltage for the tubes. If you're not comfortable with that, you may want to consider taking it to a qualified technician for adjustment. There's a procedure in the Downloads section that covers bias adjustment. Take a look and see what you think. As far as getting a matched set of 6L6's, many folks think it's important to do that. For the very early MM amps, tube matching makes a lot of sense, but for the later models (like your 210-150) it's not nearly as important. This is because in later models, the tubes ride on top of two power transistors that do the job of controlling tube current flow whereas the older models sourced the tube current through simple resistors. In your case, the important thing is that the tubes are in good physical working condition. I believe some of the manuals for newer models state that tube matching is not required. But it certainly won't hurt, except maybe your wallet. So like many things, this tube matching is a personal decision. OK, I may have just opened a can of worms and probably gonna get yelled at by Mike and Lars. Good luck. -mgriffin
Not difficult but you HAVE to
Not difficult but you HAVE to be careful . Remember,770v bouncing around in there. Navigate to MM service bulletins and read number one. That pertains to 65's, 130's AND 150's with SS driver circuit. BE CAREFUL. Mike.
Thanks all! Probably go to a
Thanks all! Probably go to a qualified tech.......
What is in there are some Sovtek 5881 WXT's......I know these are 23 watts, but take high voltage......I don't think these are the best for this amp and might be why it sounds a bit sterile .....
What would happen if I got a matched of JJ's 6L6GC's for example, and try em out....make sure they are not getting too hot and see if they sound good plug and play?
Go Stock
Hi Woody, Your amp was designed for 6L6GC tubes. They handle more plate power than the 5881's. Hopefully, a previous owner didn't change any resistor values around the tubes. When you go to the tech for bias adjust, have them verify the resistor values that are connected to the tubes against the schematic. http://pacair.com/mmamps3/sites/default/files/docs/2475-150_and_2275-150... They are all in the upper right corner of the schematic. Another question for you: Has this amp been re-capped? When these amps age, the electrolytic capacitors will degrade...even if the amp was never used. If the caps are original, it would be wise to replace all of them. That includes the power supply caps as well as the interstage coupling caps. I mention this because a cap job on my 112-65RD made a huge improvement in the tone. It went from sterile squeaky thin to big fat sound. Perhaps you could post some pictures of the amp's innards and mm210 will be able to tell if they are OEMs or newer replacements. If in doubt, change them out. Keep it coming. -mgriffin