Different Tubes and general purpose?

I’m trying to get a better midrange push or clarity out of my Line Magnetic 501ia tube amp.   Can someone help me understand what the specific function each type of tube performs in a power amp?  

I would like to upgrade several or all of my “stock” tubes but would like to understand where to start to beat enhance each aspect of the sound spectrum.   On my 501ia i have 4x KT120 and i believe those are power tubes, but please help clarify if wrong.  There are 2x 6SN7EH 1x 12AU7 and 2x 12AX7 (but the AU7 and AX7 look interchangeable to me).   As i look to upgrade where to i look to start, and why?  
Thanks in advance for help. 

Showing 3 responses by oldhvymec

Wow, way down here..  I haven't used your particular amp.  But I have heard The 805. quite a few times.. From the stock tubes he went to
RCA and Amperx (vintage) both improved the bass and low mids. Amprex better all round. He used Sylvania too. A lot more sparkle to that valve. Better highs for sure. NOT BRIGHT.. Tame sylvanias with good copper wire...

I have used countless others. Mac. VTL, CJ, DIY, ect.  Matched signal tubes can be quit a sonic nugget, if, they are not matched well to begin with, or have degraded.  L/R.

The big tubes, Unless there is a problem, USE them..KT120 hefty valve.

Again matched tubes go a long way on how a STEREO amp sounds make no mistake about it.. depth, clarity and cleaner phantom speaker resolution. 

Hope this helps..

Input stage: 2x 12AX7  Preamp tubes in this case, pre to the power amp section. Used to to amplify the signal from the driver tubes (my guess). In the actual preamp.

Rectifier: 1x 12AU7 Converts AC to DC, so the rest of the tubes can do their thing. Starts the whole thing off.

Drivers: 2x 6SN7 usually to drive a higher impedance load, normally into another valve, medium gain tube. Great preamp tube, so Dac, phono stage, guitar pick up, are amplified and handed to the next stage, each stage cleans and boost the signal (normally). In this case to the 12AX7s,
(again a guess) in the power amp section..

Power stage: 4x KT120 final stage, high gain valve. To the load, 

This guy explains a valve amp operation very simply, and thoroughly.
Just great..

How a tube amp works GREAT!!!

imhififan979 posts03-25-2020 8:03pm
"Can someone help me understand what the specific function each type of tube performs in a power amp?"

This has yet to be answered...

Input stage: 2x 12AX7
Rectifier: 1x 12AU7
Drivers: 2x 6SN7
Power stage: 4x KT120

I read above the power tubes have some issues also. Nothing I've ever owned used KT120, or 150.  I have always been a 6L6 (lower wattage) 6550, KT88 and 90 user, when it comes to power tubes. I'll bet if they were bios different, the problem might go away. AUTO bios can be an issue..for some valves.

6SN7s have been around a long time, there has to be some great valves out there for that one too. Very good valve. Use to call um double trips,
actually two valves in one.. Can't tell me matching this valve isn't important.

The 12AX and AU7 will cost the least. TONs of options. MATCH the tubes. If there is no way to bios the signal tubes, matching makes a big difference. The quality of the valve, prone to vibration (BIG ISSUE).
(Valve tap with a thin wooden dowel), if you don't have a tester.  An old
school, valve amp guy, showed me that one, the tap test. (Lightly), not a snare drum. Hold the top, one finger, tap the side.
I've  also gotten serious changes with HOW the unit was and is bios,
there is a range say 200-300 (just a number). It can change how the amp sounds A LOT.. The 20-50.00 usd on signal tubes, bios options (if there is any), matching, unit vibrational control (good idea) TWEAKS in the valve world for sure, and dirt cheap..

These, along with, placement, treatment (heaver drapes), controlling the bass usually cleans up EVERYTHING.  just a bunch of stuff  you could do for just a few bucks, in the name of MIDS...You'll get it..

Respectfully The OldHvyMec..