D180 Preamp Tubes.

[fuzzbass]

Greetings, all. I posted the following in the Bass Amps forum. No feedback yet, so I thought I'd repost in this forum. Here's my original post:

I just picked up an old D180 (no-graphic eq version) and was wondering if anyone could speak to this - I've seen other D180s where DYMO labels were used by Boogie to note where the 12AT7 goes, as opposed to the 12AX7s. This amp doesn't have any labeling inside. Any thoughts? I seem to remember reading something where if there was no label, all 4 tubes should be 12AX7s, but I'm not sure.

Also, it seems like the feature where the Master Volume knob changes the voltage to get fuzzbass at low volumes doesn't work. Could this be from improper tube placement or a wrong tube in the group (it came with 3 Ruby 12AX7s & 1 SPAX7)? Thanks all for your assistance.

and here's the post I remembered reading - from the Talkbass messageboard:

joetiii
01-04-2006, 08:45 PM
Spoke with George from Mesa Boogie today. He cleared things up for me on the D180 preamp configuration. Apparently there are three versions... one had 2 12AX7's in the preamp, a 12AT7 phase inverter and one more 12AX7 in the final master stage. A second version had one 12AX7, 2 x 12AT7's and a final 12AX7. The last version had 4 12AX7's. The amps were only labeled if/when they had different tube sockets.

Can anyone speak to any of this? Thanks for your time and help.

--- Victor

 

[Meson Boogie]

Sounds like it's probably 4 12ax7s then.
I haven't much of a clue, but it might be an idea to open it up and find the build date on the circuit board, then you could determine how likely it is to have been one of the later models (if you can find production dates).
On the other hand, it wont hurt the amp at all to put a couple of 12at7s in, if you want to play it safe.

 

[stokes]

AX7's and AT7's can be used safely in place of one another.The basic difference is that the AT7 has a lower gain characteristics.It would be likely that you have the 4 AX7 version,and it wont hurt anything but your tone using either tube in any of the positions.The schematic I have for that amp is incomplete,but what I see marked as "master" looks more like the limit knob on a S.O.B., not very effective,most people think it is a waste as far as adding distortion,it actually just cuts the gain of the P.I. tube.I would think you would do better using input 1,cranking volume 1 and then using volume 2 as the master to control the overall volume to get more distortion.I really have no experience with this amp,and the schem I have doesnt show the output section,so I am basing these thoughts on what I see in this schem.

 

[fuzzbass]

Thanks for your input, guys. I'm probably safe with the 4 AX7s, and I'm starting to wonder if the "master" issue might have something to do with the power tubes. They are mesa 6l6gc with the color-coding. Should THEY be in some particular order? I think I might have a mixed bag of "green" and "red" tubes.

 

[Randall D-180]

My D-180 still has the original labels.
Looking from the rear the left tube is a 12ax7 with a metal can over it. The second tube is also a 12ax7. The third tube is a 12at7. The last tube is a 12ax7.
So 3 12ax7 and one 12at7 tubes.
Hope this helps anyone.