6sl7 & 12ax7 in parallel &nb

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punkykatt
Junior tube assistant
Posts: 90
(5/23/05 9:14 pm)
Reply  6sl7 & 12ax7 in parallel
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 Hey Guys, I am building a 5F1 Champ type amp and i want to use a 6sl7 and a 12ax7 wired in parallel for a preamp. My question is. Should i parallel the two triodes in each tube individually? Or should i mix them up with between both tubes? Or will it make any difference? Has anyone ever tried this?
Thanks for your thoughts and your opinions.
Punky
 
HotBluePlates
I only work on Fender's
Posts: 1187
(5/24/05 7:26 am)
Reply  Re: 6sl7 & 12ax7 in parallel
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 Do you want both tubes running in the amp at the same time? Or did you want the option of swapping one type for another?

If you're using both, and the goal is to parallelthe 2 section of each tube when in use, then your layout will dictate what you can get away with.

The easiest way is to parallel both section of one tube, and parallel both section of the other tube, then cascade one pair into the other. That keeps wiring neat around the tube sockets.

If your goal is to parallel a 12AX7 section with a 6SL7 section, and do this x2 to use both sections of each tube, then you'll need to figure out a wiring layout and socket layout that allows for neat wiring, without long run or having the circuit cross over itself or cross back and forth through the chassis. To do so creates a strong risk of oscillation.

I haven't compared their data sheets in a long time, but the 6SL7 and 12AX7 are very similar electrically, although their curves may be a bit different. Just for ease of design, you can use a circuit that normally uses 12AX7's for the 6SL7.
 
triode3
Junior tube assistant
Posts: 28
(5/24/05 7:31 am)
Reply  Re: Parallel 6sl7, 12ax7
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I have not tried that personally, but I know that Angela has a
diagram where they show that you can switch from one to another,
and it is in (basically) a 5F1 circuit... I think you could get some ideas
on it from there (www.angela.com, in the howto section)

Asking if you should parallel them in each or between is a real
design choice... you will have to think about what kind of gain you
want out of each stage or the preamp as a whole. I know it can be
done in one tube, but when you parallel between the two, you will
have to be careful (plate resistances, interconnection capacitances,
etc)...

A novel idea, I have a small question: the single 12ax7 does not
give you enough gain in the 5F1? I would test out using both triodes
in the 12ax7 (running one stage into the next) first, and then see if
you need more gain than that, or see how that sounds first.


 
wayneosdias
I will work on all amps
Posts: 379
(5/24/05 10:52 am)
Reply  parallel drive
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 Im currenty doing something similar. Im following KOC's TUT1 parallel drive topology pg 5-29. Basically you can parallel both sections of each tube and then take the sum of both(again both tubes paralleled) tubes to another gain stage or to the poweramp.

When paralleling sections the resultant gain is the sum of the sections, if you put a pot on each section you can adjust the value of each section to mute, max or any level between for each section.

wayne


 
wayneosdias
I will work on all amps
Posts: 380
(5/24/05 10:54 am)
Reply  or
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 or you can just use the sum of 1/2 of each tube

wayne
 
punkykatt
Junior tube assistant
Posts: 91
(5/24/05 7:56 pm)
Reply  Re: Parallel 6sl7, 12ax7
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 Thanks for the response guys. Info well taken. Im always a believer of simple is best.
Punky
 
 
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