Name: Steve Kennedy
Email: steve@pacair.com
Subject: 12ax7 vs. solid state preamp
Thread: 158
Time: Sat, 8 Sep 2001 19:39:16 UTC
If you are talking about the early Music Man amplifiers, the 12AX7A was NOT used in the front-end as a preamp tube, it was used at the BACK-END as the phase-splitter/driver for the output tubes.
The models with the 12AX7A phase splitter are somewhat warmer sounding and easier to overload (and more "bluesy" when you get to overload), but they are more prone to catastrophic failure.
By this I don't mean to imply that these models fail more often, I just mean that there is a chance that the right type of failure of the 12AX7A tube can cause complete output stage failure which can take the plate resistors, output tubes and output transformer with it!
This is only likely if the tube fails in the right way and if the amp is not turned off IMMEDIATELY once it stops working. It would probably hum loudly for a number of seconds before any major damage could occur and the fuse blow.
Music Man reportedly saw enough of these types of failure under warranty that they designed a more reliable solid-state phase splitter and retrofit warranty repaired amplifiers with it. I have talked to several owners who sent their "12AX7A" amps in for repair and got back a "solid-state" unit. Some of them didn't like the sound as much as the original, but it depends on how the amps were used and driven.
Music Man also used this same "retrofit" in production as well (with the 12AX7A chassis hole covered with a plate). The solid state phase splitter has higher headroom and doesn't overload as easily, but it is more aggressive sounding once overload is achieved.
Before overload is approached, the amps still sound quite similar to my ear. I have 2 MM410-65 amps, one of each type. The older one with the tube phase-splitter does sound a bit warmer, especially as you get close to overload conditions.
If you want to get picky, the 12AX7A-models almost always win the "warmer sounding" vote, but the trade-off in tone for the more reliable solid-state phase-splitter circuit is relatively small unless you are really particular.
As long as you know that you should immediately shut down the amp anytime something obviously changes in your volume or tone then you should be OK and shouldn't have to worry too much about the potential consequences. Common sense and knowing this information will keep you clear of a melt-down. It is usually the guy who leaves the amp on while he slowly troubleshoots his cables, etc. that might run into this sort of major failure mode. It doesn't happen all the time or everytime a 12AX7A fails, it just CAN happen if the tube fails in a certain way... I think the plate has to short circuit to a specific tube element for this to happen (I am not sure which one as it has never happened to me personally).
In the later 112RD-50 and 110RD-50 models, Music Man DID use a 12AX7A in the preamp as a Limiter/Overdrive element in the preamp (which could be switched in & out). Outside of these models, all Music Man amps have completely solid state preamps to my knowledge.
Steve