Musicman 2100 RP questions: new tubes, oscillations, etc.

[bibi]

Hello to all,


As the title states, I have a Musicman 112 RP 100 or 2100 RP chassis (the one with the "phasor" circuit) here.  This model uses 6CA7/EL34 tubes and came with one of the original Sylvania tubes left in the amp.  First problem is that this amp has the massive magnet EVM12L in it.  That means there is very little clearance between the original tubes and the speaker.  When I tried to put in a pair of new EL34s in, they simply do not fit!  With the chassis all the way in, the tubes are pulled out of the socket slightly, sitting at an angle.  I have looked at all the datasheets for current production tubes and I can't find any that are so short as the original Sylvanias:  these stand around 94cm from base bottom to top of the glass, just enough to pass above the magnet.   



Has anyone else had this problem and found a solution (beside buying a pair of NOS 6CA7 Sylvanias!)


Second problem I have on this amp is that when the master volume is at full volume and the gain is around 7 or 8 (on a resistive load of course!) there is a very nasty spike of oscillation that appears.  I know from experience that these amps have a bad habit of taking out speakers-- very quickly and efficiently I might add.  I wonder if there may be some correlation between the two phenomena?  There are quite a few protection add-ons and snubbing caps on the output tubes but still the problem exists.  I could update the components as they are nearly 40 years old now but again, I would be curious to know if others have seen similar problems...


I have attached a schematic that is similar enough to the amp just without the phaser section. 


Thanks for any thoughts!

[92Volts]

These use crazy-high plate voltage, right? The schematic doesn't have them listed but I remember hearing they use close to 700v.

The sockets would need rewiring but the JJ 7591S is small enough and has a 550v plate limit... which is higher than most other options... lower power rating but maybe it could work with bias changes?

Other types that could "usually" substitute (same pinout as EL34) would be the 6L6GC/5881. 5881 are smaller, but rated for 400v... Now, some 5881s are just short-bottle 6L6. New-production JJ 5881s have the same 500v rating as 6L6GC, which is better than older 5881s, but might still be too low.

This amp has screens at 1/2 voltage compared to the plates, which goes a long way towards keeping tubes safe which otherwise appear close to their limits. It would help to know actual plate voltage though because if it is 700v I'd be hesitant to use anything other than EL34/6CA7 types.