Bias for Dummies: 210 Sixty Five (2275-65 12AX7 model)?

hc803

Anyone willing to open up their chassis (or use an existing photo) and give me a step-by-step guide on how to bias my 6CA7s?
I keep seeing different posts on multiple boards about how to properly do this and want to get a MM owner's perspective before I start one way or the other. Otherwise I may just break down and buy an in-line tool from Eurotubes. Thanks in advance if you're willing to do this. I'll be happy to document my procedure for anyone else once I'm comfortable.

Full Name
Bill Moore

Member for

11 years 2 months
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bill_moore Tue, 02/21/2017 - 10:01

Check the downloads, and resources drop down at the top of the page.

Full Name
Lars Verholt
Music Man Equipment
HD-130 head, 210X cab, 210RH cab, RD-50 110 combo, Sixty-Five 112 combo, RP65-112

Member for

11 years 2 months

lmv Tue, 02/21/2017 - 16:14

Is this a new set of tubes? If so, then yes the bias point will need to be verified and probably adjusted.

1. Unplug amp from outlet
2. Open amp, take out chassis
3. Place chassis so it is safe to work on while under power (very important!)
4. Connect dummy load resistor to output
5. Connect test leads to VOM (and scope, if applicable)
6. Switch amp on, set to 'hi' power, let warm up for 5mins
7. Check voltage at cathode(s) - adjust if necessary
8. Leave for another 20 minutes
9. Fine-tune voltage if necessary
10. Optional: Test with tone generator and scope or test with guitar and speaker in place
11. Switch off, let cool down (go have a coffee or listen to American Pie once or twice)
12. Close up amp
13. Test with guitar or other instrument as appropriate

There's no other voodoo here.

Personally I'd rather spend time playing a Music Man amp than fixing one, but I don't always get to make that choice ;->

Don't take this the wrong way: If the above directions are not detailed enough for you, I would suggest you take the amp to a qualified technician. It's not worth risking your health (or your amp's ditto) over. Not with 725V running around inside....

If the amp will not allow you to adjust the cathode voltage to 500mV DC at zero signal, or if adjusting to this voltage yields distortion, red-plating or other issues, you either have defective output tubes or a problem elsewhere in the amp.

Sincerely,
Lars Verholt

Full Name
craig hern
Music Man Equipment
112rd 65

Member for

8 years 3 months

craig Thu, 02/23/2017 - 12:36

MM 112 65 RD 6CA7 tubes..... Recapped and installed new JJ 6CA7 tubes. I can only get 15mV and 10.2mV testing with a "Amp-Head Dual Bias Tester" with the bias pot maxed out. Amp is quiet and sounds great. Do I need to run the tubes at 25mV or am I okay whit what I have? thanks for any comments