Could someone PLEASE explain tube positions? (V1, V2, V3, V4, etc.)

[Ephemeria]

I'm really confused. I have an amp with 4 pre-amp tubes, and another with 5. What does each of these positions represent, etc.? I know the last is always the phase inverter, but what is a phase inverter meant to do? Cheers

[Dmoney]

The last doesn't have to be the PI.
It depends entirely on the amp what each valves position is doing,  and numbers depends on how the manufacturer want to label it. For example, the 5150 preamp valves don't run V1 to V6 in a line.

[gwEm]

It depends entirely on the amp what each valves position is doing

+1

I've always found the first preamp valve, and then the phase inverter to have the most affect on tone. Clearly it depends on the amp in question though.

[darkbluemurder]

All correct. Best to get the manual for the amp in question or - more difficult to read obviously - the schematic. In most cases the tubes are numbered from V1 = first stage(s) to Vn = last power tube or rectifier tube. Example of a JTM45 or clone:

V1 (ECC83/12AX7) = 1st stage normal channel and 1st stage bright channel preamp tube
V2 (ECC83/12AX7) = 2nd stage both channels preamp tube
V3 (ECC83/12AX7) = phase inverter tube
V4, V5 (5881/6L6) = power amp tubes
V6 (GZ34/5AR4) = rectifier tube

What amp is it you want more info on?

Cheers Stephan

[dave_mc]

other more tech-minded people than me can explain it better, but basically the PI flips the phase of half of the sine wave/signal from the guitar (or i guess more accurately, the guitar signal which has been amplified by the preamp) so that the two pairs of output tubes (in a push-pull design) can work on them properly.

[Alex]

However, fortunately, the preamp tube closest to the input is almost always the V1, as this also means the shortest signal path.

The V1 are the first gain stages (each preamp tube can have two gain stages); because it is the first it has the biggest impact on tone and noise and should be of good quality and low noise. The other gain stages (V2 and V3) usually are not that important anymore, at least to my mediocre ears.

The PI should be a balanced preamp tube. Depending on your amp you might also have a tube buffered effects loop.

For example, if I remember correctly, a Peavey 5150 has 5 preamp tubes - 3 are gain stages (I think 5 or 6 in total), one is for the effects loop and one is the PI.

[Dmoney]

However, fortunately, the preamp tube closest to the input is almost always the V1, as this also means the shortest signal path.

The V1 are the first gain stages (each preamp tube can have two gain stages); because it is the first it has the biggest impact on tone and noise and should be of good quality and low noise. The other gain stages (V2 and V3) usually are not that important anymore, at least to my mediocre ears.

The PI should be a balanced preamp tube. Depending on your amp you might also have a tube buffered effects loop.

For example, if I remember correctly, a Peavey 5150 has 5 preamp tubes - 3 are gain stages (I think 5 or 6 in total), one is for the effects loop and one is the PI.

the PI balance thing is a fallacy.

each triode in a 12AX7 for example is usually slightly unbalanced. This is made up for in the amp usually. A Marshall long tail pair PI has a resistor on each triodes anode 100K and 82K. They share a cathode resistance. The difference in anode resister value compensates for an average unbalanced normal 12AX7. It actually over compensates and the values should be more like 100K and 91K.

for this reason, using balanced triodes in most amps with (at least) the marshall style PI doesn't make much difference, and often an unbalance might sound more musical. The Bogner Uberschall is one amp with a 'balance' pot, so you can use a scope to balance the PI exactly. Otherwise... i wouldn't worry about paying extra money for balanced valves.


EDIT: this page explains it a bit better. go down to balance. http://www.freewebs.com/valvewizard/acltp.html

[timski]

This is quite a helpful guide, especially if you have a Fender:

https://www.kcanostubes.com/content/fender-amp-preamp-tube-layout-and-recommendations