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Old 16th March 2007, 06:45 PM   #1
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Default 12AX7 how many is enough?

Been playing guitar through solid state for as long as i can remember. Want to upgrade to a nice tube amp. Electronics has been my living for about 17 years and would eventually like to build my own tube amplifier. I notice that more of the high end guitar amplifiers have at least 4 or 5 12AX7 tubes in the preamp section and usually use 6L6 in the power section. Where do you draw the line with how many pre amp tubes you put in an amplifier keeping input power requirements out of the equations?

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Sambeaux
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Old 16th March 2007, 07:34 PM   #2
kevinkr is offline kevinkr  United States
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They don't use enough power to make that a particularly relevant concern, (except in cheap amps) it has more to do with the designer's end goals for sound and feature set.
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Old 21st March 2007, 12:07 AM   #3
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I don`t exactly understand what U`re asking, but I hope this would be helpful somehow.
Many many amplifiers use 2 or 3 preamp tubes, which act as "the tone maker", they create such sounds like crunch or distortion, it depends, how hi the gain is set for the particular stage to ovedrive the next stage.
the last-preamp/one-and-only-power-preamp tube act as inverter, which is needed to drive the output power tubes to gain overall out power.
Tube buffered fx loop is optional...
All this means, that you will need 4 to 5 preamp (like 12AX7`s) tubes in your amp, countless power tubes of course.
a minimum would probably be 2 preamp tubes, but just for clean tone.
power consumption would not be a problem (I suppose), `cause 12AX7 has current drain about 1.2mA at 250V, and 150mA at 12,6V for filament, so one tube up/down is somewhat irrelevant, I think.
I don`t know, how are you experienced with tube design, but I personaly preffer building a prototype amp of a particular brand like MB, peavey, and then experiment with values (with in limiting values of course) of components in front of the power stage. Power stages are not very different, most brands just use a trustful types, but the quality of op tranny is important.
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Old 21st March 2007, 12:42 AM   #4
kevinkr is offline kevinkr  United States
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My remark was predicated on the idea that the only significant difference in power consumption between a 3 tube and 5 tube pre-amp circuit using 12AX7A would be in the filament category.
A five tube design will probably give you more flexibility in tweaking the sound to your liking...
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Old 21st March 2007, 06:49 AM   #5
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Hi,

Keep in mind that the more preamp tubes you use, the harder it will be to keep the amp stable.

If you're looking for that Marshall Rock & Roll sound, try 3 or 4 before your phase splitter. If you're into bluesy, cleanish sound, use 2.

Another great place to glean great amp design info at is www.18watt.com.

Cheers!
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Old 21st March 2007, 12:53 PM   #6
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That`s true, it`s hard to say how many tubes in your preamp will you precisely need without knowing, what kind of distortion you`re looking for. I design and mod tube all-tube amps for 6 years and I never got a horrible result, wether I was using 2 or 5 tubes, keeping in mind a final product character I aspecialy want from jazz to death metal.
Try to establish some kind of guidelines of what are you looking for, that might help us help you..
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Old 21st March 2007, 01:45 PM   #7
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Default tubes

Thanks gentleman for the replys.

The type of sound im looking for is the metal distortion sorta like the Peavey Van Halen amp which is now the Peavey 6505 head.....

I have access to a circuit board router and will be looking for a schematic to build.

Another question i have is, would it be good to use point to point wiring instead of using a PCB? any information would be appreciated.

Thanks
Sam
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Old 21st March 2007, 04:37 PM   #8
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I would deffinitely not use P2P wiring, because that would generate realy high noise (or if worse, heavy oscilations) as this amp have significant distorting capability. I built this amp myself and so I can say, that using a PCB will deffinitely have the best result. I personaly don`t use nothing else, than pcbs, because you don`t have to worry about how to make it, design it (or not that long). If you would be THAT interested in using eagle to generate the board, I could send U the redrawn circuit in it (sorry, not a finished board...) and then you will design the board yourself.
btw, as far as I know, van hale signiture head is 5150, not 6505, but thats the quite same one...I redrawn 5150 and modded a little...but this way my amp have practicaly no noise and the hiss is irrelevant while sitting and playing it in front of the head, so no hiss while in the exercise room. I published some photos on my site...but, well, it`s not the best perfomance (the site), so sorry for that, it`s just a personal site..
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Old 21st March 2007, 05:49 PM   #9
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Default 12AX7

Thanks Ragnar. If memory serves, Peavey does not make the 5150 anymore due to their "contract" with Eddie. Ive heard that they call it the 6505 now since EVH is no longer "with" them. I would greatly appreciate you forwarding me the redrawn circuit that you speak of. I would love to build one...

Thanks for your expertise and advise. Im sure I will have more questions to ask on here. I appreciate any help at all...

Sam
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Old 21st March 2007, 06:04 PM   #10
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Indeed, the number of necessary preamp tubes depends on your goals. In my opinion, the minimum for a metal distortion preamp channel is two tubes (four gain stages) and another tube for the phase inverter to drive the power tubes. Then you may need another preamp tube for an effects loop. Some high-end amps also use separate tubes for the clean and drive channels, separate effects loops for each of the channels, a reverb driver tube, etc Ö

One thing Iíve noticed when experimenting with guitar preamp designs is that having a tone stack after the input stage (such as for example is in designs of the Peavey 5150 evh and the Mesa Boogie Mark IV) provides way more versatility to your distortion than having just a drive/volume pot. It acts as pre-distortion EQ of sorts.

Hereís a link to a great site which somebody recently suggested to me. http://www.aikenamps.com/ .... go to "Tech Info".....Very helpful!

Good luck to ya and be careful with the voltages!
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