preamp tubes

[mikejanbohn]

I have 4 preamp tubes in my Mesa...Subway Blues amp from left to right what does each tube do???????? thanks for the info in advance....JMB

[chipaudette]

Hi Mikejanbon,

Your question is pretty general...there are many different ways of answering it. The snide answer is that they all light up, get nice and warm, and glow pretty. That's what they do. That's probably not the answer you're looking for, though. I'd like to be more helpful.

Do you have the schematic? If you can find one on the web, that would show you (or help us tell you) what each tube does.

Of course, that assumes you have looked at a schematic before. Do you know how to read one? If not, that's fine, but let us know so that our answers have the right level of detail and use the right types of words (ie, what jargon to avoid)...

Chip

[mikejanbohn]

I don't know shit.......just need a general answer....for example ..I think the first 2 preamp tubes "run" the power tubes....is this right and if it is what do the others do??? if you don't know , give me a guess. I just want to understand basics not tech stuff. jmb

[chipaudette]

OK, I got the message. I checked out the schematic for the Subway Blues, which is spread out over several pages (http://www.tubefreak.com/mesa.htm#blues). From this, one can see the function of each part of each tube.

As important background, each pre-amp tube used in your amp (and most guitar amps) has two completely independent sections within it. Therefore, one tube can perform two functions simultaneously. So, your 4 pre-amp tubes actually perform 8 functions. Here is what your guitar signal sees as it flows through your amp

V1a = initial gain of guitar signal up to workable level
V1b = make-up gain after losses in your tone controls
V2a = more gian, presumably to give the sound some grit and distortion
V4b = driver to the reverb tank
V4a = gain to recover from losses in reverb tank
V2b = step-up gain after effects loop
V3a and V3b = phase-inverter which drives the power tubes

So, there are the eight functions performed by your four pre-amp tubes. In terms of layout within your amp, usually the tubes appear in order V1 -> V2 -> V3 -> V4. Usually, V1 is the one closest to the input jack.

FWIW, I was surprised to find a transistor (Q1) in the main signal path. It's used as a cathode follower for driving the effects loop. Tube-purists would cry "heresy!", but I bet that one substitution (a very minor one) saved $100 on the price of the amp.

Hope this helps,

Chip

[chipaudette]

Also, I just noticed that the voltages in the pre-amp for the Subway Blues are surprisingly low...especially for a Mesa amplifier. The schematic shows that most of the pre-amp is running off a 168V rail. Mesa has a history of running its pre-amps up around 400V. That's a pretty big difference.

Of course, you won't care...but, to anyone else reading the thread, why might they have done that?

Chip

[stokes]

They were going for a more vintage tone in the Subway Blues.FWIW they dont run any of their preamp tubes at around 400v's.The rail that feeds them is in the 390v zone but it drops to about 250v's at the plate.The actual voltage on the Subway is around 110-115v's at the plate.You are reading the volts on the wrong side of the plate load resistor.Lower plate volts will give you a "browner" tone,more like the lower voltage Tweed Fenders.

[Robert M. Martinelli]

Also, I just noticed that the voltages in the pre-amp for the Subway Blues are surprisingly low...especially for a Mesa amplifier. The schematic shows that most of the pre-amp is running off a 168V rail. Mesa has a history of running its pre-amps up around 400V. That's a pretty big difference.

Of course, you won't care...but, to anyone else reading the thread, why might they have done that?

Chip

What Stokes said.

The max ratings for 12AX7 are 300 VDC on the plate and 100 VDC between the heaters and cathodes ( important in long tailed PI design where cathodes get lifted many volts above zero, though you can overcome this lifting heaters as well ), and max plate current is 1 mAmp, so, even if Mesa guys have a reputation of driving tubes hard in their designs, even they cannot have gone THAT high

Regards

Bob