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BrianA
Junior tube assistant
Posts: 7
(2/17/04 3:29 pm)
 tube question
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 What is the effect of direct parallel coupling of both halves of a twin triode (ie 12ax7) in a single preamp gain stage? Any benefit or harm? What adjustments to the plate load and cathode resistors?
THX
 
eddiejg
Senior tube assistant
Posts: 228
(2/17/04 4:07 pm)
 tube question
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 Brian-
If you connect 2 stages in parallel, the gain is added (50+50=100).
As opposed to 2 stages in series where the gain is multiplied (50x50=250).
If you change the plate load or cathode resistor, each stage's gain will be affected accordingly, and the total gain for each stage will again be added together.
I've found that this fact makes it a lot easier to manipulate the gain level & shape the tone of a preamp using 2 parallel stages without overdoing it.
Matchless's Spitfire is a perfect example of it's use.
Take a look at the schematic & look at the fact that they use a 220K plate load resistor with a 1.5K cathode resistor & 22uF cathode cap in parallel.
-Ed
 
Dbltlk44
Junior tube assistant
Posts: 6
(2/18/04 12:50 am)
 Re: tube question
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I know it was a typo, but...the 2 stages with a nominal gain of 50 each in series ends up being 2500.

Now, don't you pretty much have to have 2 tube stages in series SOMEWHERE in a guitar amp? I'd think that a single gain stage, even 2 in parallel, wouldn't put THAT much juice to the PI ( or the power tube if it's SE ).

I'm a real newbie at this stuff, so please feel free to set muh straight if I ain't ezakly makin sense here...
 
eddiejg
Senior tube assistant
Posts: 229
(2/18/04 1:07 am)
 Re: tube question
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 Hey, at least I got the addition right!
And what you say makes sense & is correct.
Sure, there's going to be at least 2 stages in series.
The 5F1 or 5F2 tweeds use 1 tube- 1 stage preamp & 1 stage driver for the single power tube.
And they're in series.
So, using the hypothetical 50, you would have 2500 after the 2 stages.
But, now take an amp with 3 stages & a phase inverter, all in series, you would wind up with 6,250,000.
A lot of gain is cut or thrown away in each stage to keep the gain under control.
But, with the Spitfire, 2 stages in parallel + the PI is 5000 instead of the 250,000 it would be if the 2 stages were in series.
Of course, Matchless used a 220K plate load resistor to up the gain on that stage.
-Ed
 
BrianA
Junior tube assistant
Posts: 8
(2/18/04 11:14 am)
 Re: tube question
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 Thanks Ed,
That's what I wanted to know. I've got an extra 1/2 tube in a prototype I'm working on and wasn't sure if I could do anything with it without jacking up the gain too high.
 
eddiejg
Senior tube assistant
Posts: 230
(2/18/04 11:51 am)
 Re: tube question
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 Brian-
In that situation, it makes a great gain boost to make the stage switchable in & out of the circuit.
Similar to what Marshall has on the 18 watter- plug into the single jack- 1 stage; plug into the parallel jack- 2 stages.
If you use an SPDT switch, you can make it switchable.
Also, keep in mind that if you use the same cathode resistor for both stages, the actual resistance in the circuit will be double the actual value of the resistor, so you may want to modify this. Or you can use a separate resistor & capacitor so each stage give a slightly different sound & combined makes for a third, higher gain sound.
-Ed
 
 
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