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Message: <blockquote data-quote="bandit2013" data-source="post: 498662" data-attributes="member: 36239">the saturation mod is not my thread... credit goes to APEMAN.  All I did was offer a suggestion for the 12AT7 in V4 vs V6 as the ice pick tone is generated by abundance of gain in the upper frequency range due to the design of the two cascaded gain stages from V5A to V4B.  Also there are some differences between the Mark V and the Mark IV of the similar cascaded gain stage section such that the Mark IV did not suffer the ice pick tone as it lacked the grid to cathode capacitor on the second gain stage but is present on the Mark V which may act as positive feedback on the higher frequency.  I did not run an analysis on it but realized if the gain was dropped in the second stage the higher order harmonics would be reduced thus killing the ice pick.  Tube choice does matter as a warmer tube will have more roll off on the higher frequencies and it is just enough to place the Mark V tone character of CH3 on par with the JP-2C.  As for the Mark V:25 and Mark V:35.  Not really sure what effect a 12AT7 would have. If I posted anything it was based on the manual and what the probable tube position that may have a similar circuit to the Mark V.  The original post was a fix for the ice pick tone of CH3 on the Mark V and everything else remains the same.  I am unfamiliar with the Mark V variants to make any claims on how the tube change would affect performance as it seems that the tube task chart seems to share more than one channel.  I am sure something similar could be implemented in the JP-2C but I see no apparent benefit as the amp has the classic Mesa tone everyone strives for.Keep in mind that the same tube may sound completely different in another amp design.  It is all based on the design of the gain circuit, operating plate voltage and amount of cathode bypass capacitance is used.  Another factor is the power tubes,  the EL84 are quite different than EL34 or 6L6 in gain characteristics and tone.  Actually it becomes more subjective than ideal.  One would need to review the schematics of the amps in question to determine if the 12AT7 could be used without issue.  For the Mark V there did not seem to be any apparent issue with the 12AT7 in V4 for the reverb send and the gain stage.  For the other amps it is uncertain.  Typically the lowest gain in the 12A series for the 12AX7 would be the 12AT7. Probably a better fit would be a 5751. Typically the JAN (military grade tubes) will perform the best vs consumer grade tubes but in the 5751 arena they will also cost more.</blockquote>

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