Importance of PI tube for PPIMV amps

[Billy Batz]

That may be true. And it has to do with how much you play the amp the tubes are in. My only problem is that, yes the PI tube is worked the hardest but do we really know to what degree of difference that makes? V2 is also worked harder then V1. But who says that difference in work correlates to any large difference in tone vs the other 12AX7s in the usable time theyre in the amp. That extra voltage etc.. may seem a lot on paper but does that mean the tube really goes faster? I just dont know and I can only use my experience to judge which tells me no. At least not to degrees that make a real audible difference. If anyone actually finds that to be the case Id be interested in hearing all about it. What if the tube wear is more or less the same for a givin range of voltages which doesnt start to taper off until that is exceeded or below that range? Maybe there has been tests done about thi. Who knows. Either way changing V3 just because your changing power tubes (the original point) seems silly to me for somebody to tell you to do. And like I said, I would treat it no different then the other preamp tubes and if you feel so inclined keep a known good spare around to try swapping them around every so often and see. Nothing bad can come of doing that. Theres nothing wrong with discussing this stuff of coarse but in the end I wouldnt start spending more money on tubes because of some tale like that. If I come to find its true in my experience with the amp thats different.

[rjgtr]

Dan,

I think wear is dependent on the circuit the tube is in. Some will be harder on tubes than others, just due to the fact that more energy is transfered in some circuits. Current causes heat and heat results in wear. However, I don't really know if it is harder on a tube to be a cathode follower, PI or high gain stage. I just think that people need to check their tubes periodically to make sure they're getting the most TONE from an amp.

So far I think we basically agree. I certainly agree with your first premise. There's no need to toss a perfectly good PI tube because you're changing power tubes. But maybe it should be checked at the same time to make sure it is still toneful.

[Billy Batz]

Dan,

I think wear is dependent on the circuit the tube is in. Some will be harder on tubes than others, just due to the fact that more energy is transfered in some circuits. Current causes heat and heat results in wear. However, I don't really know if it is harder on a tube to be a cathode follower, PI or high gain stage. I just think that people need to check their tubes periodically to make sure they're getting the most TONE from an amp.

So far I think we basically agree. I certainly agree with your first premise. There's no need to toss a perfectly good PI tube because you're changing power tubes. But maybe it should be checked at the same time to make sure it is still toneful.

A responsible man would check all tubes often on principal. Thats a responsib;e man. Maybe Im not that because I cringe at the thought of spending $80 on a set.

[Dai H.]

Q: Do I need to replace my driver tube every time I replace my power tubes?
A: No. This is an "old wive's tale" propagated by the tube vendors who want to sell you more tubes. The "driver" tube, or phase inverter as it is more appropriately known, is under no more stress than any of the other preamp tubes. The output stage of a normal guitar amplifier is AC-coupled, class AB1 or class A1. The "1" suffix indicates that the tubes do not draw grid current during any portion of the input cycle. A "2" suffix indicates grid current during some portion of the cycle, as in class AB2. Since almost all guitar amps are class AB1 or A1 (it takes a DC-coupled cathode-follower or interstage transformer to be able to drive the power tubes into the positive grid region), the phase inverter does not have to supply any current to the grids (it can't really source current anyway - it only sinks current through the plate load resistor, which is quite large, and inherently limits the plate current to a few mA). The AC coupling (capacitor coupling) between the phase inverter and the output tubes precludes any flow of grid current anyway. The output tube grids will merely act as a grid clamp, shifting the bias downward as the output is driven harder. This in no way stresses the phase inverter tube. In addition, the plate load resistors and the bias current in a typical phase inverter are identical to the preamp stage values, indicating that they are operating in the same area, dissipation-wise, so they cannot be "wearing out" any faster. In fact, the reverb driver tube on a typical amplifier is dissipating more power than the phase inverter, and should be replaced more often, if anything. Don't be misled by the higher plate voltage on the phase inverter either, because the cathode is usually sitting somewhere around 30V - 100V above ground, which lowers the plate-to-cathode voltage by that amount. The plate-to-cathode voltage is what determines the power dissipation of the tube, not the absolute plate voltage.

source:

http://www.aikenamps.com/TI_Aiken_Q&A.html

[Billy Batz]

I read that before several times. I forgot where. Good job finding it.