I have never once had any problems with either pre amp or power tubes in any of my Diezels. (VH4, Herbert, Einstein combo)
However, I never use nearly the amount of saturation offered in any of these amps, so I change out my preamp tubes to re-voice the amp.
I prefer NOS Mullards, NOS Telefunkens, NOS 5751's, and NOS RFT preamp tubes.
The Mullards are my first choice. They offer a sweeter tone and wider sound, with a very sweet break up. I have 3-4 varieties of NOS Mullards to play with.
The Telefunkens offer the same, but with a bit more high end that is noticable. This is almost like switching out the power tubes to 6L6 from EL-34's but not totally the same.
The RFT's are similar to the Mullards but not as clear sounding overall to my ear. Very close, however, and as stated, a bit thicker sounding.
What I have done and at this point am satisfied with is using a NOS Mullard 12AT7 in V-1 and V-3, a 5751 in V-2, and a perfectly matched 12AT7 preamp tube in the phase inverter position.
I am running two JJ KT77's in the power section of the Einstein combo. The Einstein I have is dead stock for everything else, but I did put a beam blocker in front of the Vintage 30 Celestion speaker.
I have not installed the Einstein mod as I am incapable of doing this type of work, and find it unnecessary for how I use the amp.
The result of changing out the preamp tubes to lower powered types is that I have more use of the gain control on both channels, and get a distortion that is clearer sounding, with just as much sustain. I set the gain on channel two to about 9 o'clock, and the channel two volume to about 1 o'clock. On channel one, I use mode 2 and set the gain to about 12-2 o'clock and the channel volume to about 11-12 o'clock.
I am using a LP or other humbucking pickup guitar and set the tone controls to about twelve o'clock, back off the bass to about 11 o'clock, treble at 12-1 o'clock on both channels, mids to about 11-12 o'clock on both channels, deep to 12 o'clock, and presence to 2 o'clock.
Channel one master volume set to room levels as needed, with master two volume higher as a volume boost which can be used for either channel.
This gives me complete control over all tones from the guitar. If I need absolute clean, I simply change the mini toggle to mode I on channel one and hit the master volume boost if needed.
I play all types of music except for shred and have found these settings to be the most versatile and adjustment-free. Because I have lowered the preamp tube value, channel one, Mode 2 can stay clean or get just a touch of hair on it if I want some character added to the sound of a clean setting.
I have used these same preamp tubes in the VH4 nd the Herbert with similar results.
I hope this helps out.
