12AX7 Question

[Molly]

I'm putting fresh tubes in the Boogie F-50. The manual states that each 12AX7 contains two separate triodes. I've always assumed that one 12AX7 is the same as another. Is that not the case? Apparently this is the signal path.

V1 A= Input Stage both Channels
V1 B= Clean Channel
V2 A= 2nd Input Stage bothChannels
V2 B=Channels 2 & 3 Boost Stage
V3 A=Channels FX Send
V3 B= FX Return
V4 A=Phase Splitter
V4 B=Output



Any clarification appreciated.

[jeremyb]

My limited experience, Tung Sol in V1, JJ's in the rest, the tung sol just sweetens up those cleans

[finn61]

Yep, a 12ax7 is a dual triode tube, essentially meaning it acts like two individual valve switches sharing a physical container. Each side can be used in different parts of the circuit, as described above, where one side of the first tube is on the input before the signal is sent two each channel, and the second side is used solely on the clean.

I have absolutely no idea whether this explains anything extra that's helpful, feel free to question anthing I wrote for more clarification :]

Edit: Reread your original post and realised you weren't after an explanation like that :S My bad. A better response:
Yep, valves are all kinda different to each other. Because of their non-linear nature, they can vary between manufacturers, and even between tubes from the same manufacturer (though probably not as much). This may or may not make a difference in tone depending on where in the circuit they are eg. input stage valve will change the tone more than one used in the phase splitter as far as i'm aware...

[Molly]

Thanks for the replies.

I have a vague memory of buying a dual-somethin' or other 12AX7 and being told I couldn't use it as a direct replacement for the one I was taking out... I've still got it at home so will put up a pic for clarification. I suppose my main concern was that I didn't have to source a particular type of 12AX7 for this amp but could just use the spares I already have.

Thanks.

[finn61]

Oh, I think i know what they're talking about. Normally if your amp isn't cathode bias, you have to adjust the valve bias when you put different valves in, because of their different tolerances. Mesa Boogie chose to not allow biasing, and tell you to only purchase their tubes, which they source within their chosen tolerances to work with the same bias. You may not be able to use just any 12AX7 as replacement.

[Dharmajester]

Oh, I think i know what they're talking about. Normally if your amp isn't cathode bias, you have to adjust the valve bias when you put different valves in, because of their different tolerances. Mesa Boogie chose to not allow biasing, and tell you to only purchase their valves, which they source within their chosen tolerances to work with the same bias. You may not be able to use just any 12AX7 as replacement.

No that is a point entirely related to the power amp section and relevant tubes.
You're pretty safe with any 12ax7. You will however require one that's balanced (ie the two sections are equal) if it's going in the phase inverter position. Short and long plate 12ax7's have differences in output and some amps prefer not to have long plates ( hotter) in certain positions. I think that may be cathode follower spots. I'm sure someone with thorough technical knowledge will jump in and clarify, my knowledge on the topic being fairly thin.

[finn61]

Oh, I didn't realise that only power tubes are arranged in a fashion requiring manual biasing I learnt something today too! Haha

[Bg]

yes. and no.
preamp valves are biased too, but they are generally forgiving and within tolerance anyway. You'd need to change resistors to bias them.

Anyway... power valves.
3 types of bias
Fixed bias - despite its name, theres usually a pot in there to bias how hot and cold.... sigh

Cathode bias - these are biased by big power resistors. you can bias them, by swapping the resistors, adding them in parallel etc.

Fixed adjustment (Fixed bias with no adjustment) - Mesa and peavey classic 50, 30, delta blues. They're the same as the fixed bias but with no means of adjustment. Saying they don't need to be adjusted is bollocks - unless you buy mesa's specified valves they may be too cold or too hot. They're usually biased cold from factory though so the chance of doing damage is less likely. I've added bias pots to mesa's in the past, just because I like to hear the difference and play.

[sizzlingbadger]

In answer to your original question you can use any 12AX7,ECC83,7025 valve in any pre-amp position in your Mesa amp. Some suit certain positions better than others but they will all work and cause no harm to the amp.

I like a TungSol in V1, its a nice open/bright tube and can help remove some darkness, esp. in Mesa amps.

I avoid Sovtek (russian) tubes in cathode follower positions (i.e. v2 in an AC30) because they die quickly due to the elevated cathode voltage.

You don't have to have a balanced valve in the PI position. In a long tailed pair circuit (like the F50) your unlikely to notice any difference. Most PI circuits are purposely built un-balanced as it adds extra "colour" to clean tones that are missing in push-pull power sections (2nd harmonics get cancelled)

I also avoid using tubes with long plate construction in combos as they tend to get microphonic due to all the vibration.

Tube Depot / Tubstore etc. will supply power tubes within Mesa's specs if you tell them what amp you have.

[Molly]

Oh, I think i know what they're talking about. Normally if your amp isn't cathode bias, you have to adjust the valve bias when you put different valves in, because of their different tolerances. Mesa Boogie chose to not allow biasing, and tell you to only purchase their valves, which they source within their chosen tolerances to work with the same bias. You may not be able to use just any 12AX7 as replacement.

No that is a point entirely related to the power amp section and relevant valves.
You're pretty safe with any 12ax7. You will however require one that's balanced (ie the two sections are equal) if it's going in the phase inverter position. Short and long plate 12ax7's have differences in output and some amps prefer not to have long plates ( hotter) in certain positions. I think that may be cathode follower spots. I'm sure someone with thorough technical knowledge will jump in and clarify, my knowledge on the topic being fairly thin.

Thanks chaps. Is this what you mean by balanced? I think I may have got this from you Steve when I bought the Reinhardt or Mk.1. Can't recall exactly. If I'm not using this a phase inverter position can I use it elsewhere? Excuse my ignorance but would this one be better in V1 for example? Would it be quieter?

[Conway]

Just bung 'em in, what could go wrong? LOL

[sizzlingbadger]

You can use it anywhere in the preamp, balanced doesn't mean its necessarily quiet.

[Molly]

Thanks.

Am really going to have to sort out my valves. I've quite a few here and there. When I read the Boogie forum and they talk about this or that type of tube in the preamp section I don't always know what it is they're achieving with their particular mix.

[Bg]

balanced just means that both halves of the valve have the same gain. It usually doesn't matter as each side has a different purpose. One side could be preamp, the other reverb or trem etc...

As conway points out. shove it in and take it out when its spent.

[Molly]

balanced just means that both halves of the valve have the same gain. It usually doesn't matter as each side has a different purpose. One side could be preamp, the other reverb or trem etc...
...

I never knew that until today.