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Robert Herndon said:
Would a ECC83S potentially give tighter low end in V4 compared to a 5751???
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Personally Robert, I would try the Ken Rad's in V4. The Ken Rad 12AX7 is a GE designed tube so should spec better than any current product type (not sure if yours are NOS or "pulls" that test good). Either way, a listen test will show if they are any good there. Don't worry about "balanced triodes" for the PI, the long tailed pair PI is unbalanced to start with, so a "balanced" tube won't help. If you want to swap tubes in the V4 position to find which fairs best, you want to check each tubes "plate differential voltage". This is the voltage difference between the two PI (V4) plates. Now you'll need to be inside the amp with the amp powered up to do this, so take great care.
Set your meter to the 20VDC range. Now put one probe on pin 1 & the other on pin 6 of V4. It doesn't really matter which probe goes on which pin, the meter will read the voltage as a either positive or negative voltage, but it'll be the same figure either way. You want the tube that gets you closest to a 7.2VDC difference between the two plates. No matter what "type" 12AX7 you have in the PI you want to shoot for that differential voltage. Note, this voltage only applies to this (common Marshall) LTP PI design. The closer you get to this 7.2VDC differential voltage the better balanced the PI outputs are. As a result, applied negative feedback (NFB) does not have to try to "correct" the discrepancies, resulting in a tight, powerful & sustaining output section.
I know this is a long rant & you probably dont want to go to this much trouble, but in case you do (or just for "interest"), there's the low down.
After all, the PI is arguably the most important & definitely the hardest worked pre amp tube. Cheers
Edit; keep your presence & resonance controls low to keep the power amp nice & tight. Hope all this helps. Cheers
 
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ivan H said:
Personally Robert, I would try the Ken Rad's in V4. The Ken Rad 12AX7 is a GE designed tube so should spec better than any current product type (not sure if yours are NOS or "pulls" that test good). Either way, a listen test will show if they are any good there. Don't worry about "balanced triodes" for the PI, the long tailed pair PI is unbalanced to start with, so a "balanced" tube won't help. If you want to swap tubes in the V4 position to find which fairs best, you want to check each tubes "plate differential voltage". This is the voltage difference between the two PI (V4) plates. Now you'll need to be inside the amp with the amp powered up to do this, so take great care.
Set your meter to the 20VDC range. Now put one probe on pin 1 & the other on pin 6 of V4. It doesn't really matter which probe goes on which pin, the meter will read the voltage as a either positive or negative voltage, but it'll be the same figure either way. You want the tube that gets you closest to a 7.2VDC difference between the two plates. No matter what "type" 12AX7 you have in the PI you want to shoot for that differential voltage. Note, this voltage only applies to this (common Marshall) LTP PI design. The closer you get to this 7.2VDC differential voltage the better balanced the PI outputs are. As a result, applied negative feedback (NFB) does not have to try to "correct" the discrepancies, resulting in a tight, powerful & sustaining output section.
I know this is a long rant & you probably dont go to this much trouble, but in case you do (or just for "interest"), there's the low down.
After all, the PI is arguably the most important & definitely the hardest worked pre amp tube. Cheers
Edit; keep your presence & resonance controls low to keep the power amp nice & tight. Hope all this helps. Cheers
Click to expand...

Thank you, Ivan. Indeed, I am inexperienced in these things so I appreciate the information greatly.

I think I have both NOS and "tested pulls" in the Ken-Rads....
 
Saw an interesting post on the Marshall forum where a fellow was using a JJECC832 (12AX7x12AT7) in V3.

I did not try that and I have either an 823 or 832 on hand...
 
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