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Old 2nd July 2012, 02:08 AM   #31
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Quote:
Originally Posted by brilliantblue View Post
does 100k on the PI plates seem a bit high to you? would the headroom decrease or increase if I went to say 47k? In preamp stages I've built, the higher the plate resistor the more "nasal" and "pinched" the sound seemed to become - well, particularly getting around 220k and higher on a 12ax7.
It does seem rather high and only drawing less than 1.5mA of current for a 12AU7, as if it's designed with a 12AX7 or 12AY7 in mind. If you change to 47K plate resistor, you will need to reduce the value of the cathode resistor so plate voltage maintains the same. You're essentially changing the operation point of the tube. I don't think you need to mess with the original circuit too much. Just adding AC balance is a nice touch. The PI circuit does not have a lot of gain and further reduced with global feedback so the preamp section is very important to have the necessary gain. It's rather strange that it would clip so easily to begin with.

Anyway, I am not a guitarist or deal with guitar amp too much so I am not equipped to answer all your questions. Good luck with your project and, most important, have fun.
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Old 2nd July 2012, 02:29 AM   #32
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thanks DD! i don't have much room in this chassis for the balance pot (its a cramped thing) without possibly inducing noise as I'd have to cross from the back of the chassis to the faceplate - though I may try it with shielded wire.

i did drop the 220k to 100k on the 12ax7, and will see how that opens things up.
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Old 2nd July 2012, 03:41 PM   #33
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Quote:
Originally Posted by brilliantblue View Post
i don't have much room in this chassis for the balance pot (its a cramped thing) without possibly inducing noise as I'd have to cross from the back of the chassis to the faceplate - though I may try it with shielded wire.
Trim pots don't take up space at all. You can place it right next to the other parts, no need to mount on anything.
Click the image to open in full size.
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Old 2nd July 2012, 06:02 PM   #34
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oh yeah, right you are - i had the standard panel knob in mind, but I suppose a trim pot hidden away in there would work

one thing bothers me about the schematic you posted though - with the pot turned all the way toward ground, wouldn't the signal be nil? isn't that equivalent to putting the two resistors (470k & ~470k) to ground and pulling a lead from ground to pin 7 of the PI? I think the PI pin 7 needs a ground reference resistor - which is what the original 240k was doing. I'd be inclined to put the pot on the other side of the 390k resistor you suggest, so that it always sees at least 390k as ground reference. doesn't that make sense?
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Old 2nd July 2012, 07:15 PM   #35
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Quote:
Originally Posted by brilliantblue View Post
with the pot turned all the way toward ground, wouldn't the signal be nil? isn't that equivalent to putting the two resistors (470k & ~470k) to ground and pulling a lead from ground to pin 7 of the PI?
First, you don't ever want to turn it towards ground. You can start at midpoint. The 390K resistor and the wiper to ground forms a voltage divider that sends a signal voltage that should be equivalent to unity or same as the input signal and that depends on the mu or gain factor of the tube. Pin 7 is seeing ground through the pot. Once you find the right value, you can replace the pot with a fixed value resistor if you want. But having a pot there is very convenient for tweaking. Besides, the pot is not for you turn all the time, just there to dial in the AC balance and then you leave it alone.

Quote:
Originally Posted by brilliantblue View Post
I think the PI pin 7 needs a ground reference resistor - which is what the original 240k was doing. I'd be inclined to put the pot on the other side of the 390k resistor you suggest, so that it always sees at least 390k as ground reference. doesn't that make sense?
Again, one end of the pot is connected to ground and that's your ground reference already. There are many ways to do this and since the 6V6's are self biased, this is the easiest and simplest way to do this. If you place the pot on the other side of the 390K, you will not get the correct signal voltage to the grid, it will be too high no matter where the wiper is.

Here's a schematic of the famous Pilot 232 amp (best sounding 6BQ5 amp ever) and perhaps give you an idea of one way of balancing a paraphase circuit and in this case two fixed resistors forming a voltage divider to the grid:

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Old 2nd July 2012, 07:54 PM   #36
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Here's a calculator for values of all the parts in a paraphase circuit. But it assumes the circuit uses no feedback and the cathodes are bypassed with big caps so the gain is higher than many circuits, including yours.

Paraphase Inverter Gain and Resistor Calculator

But you'll get the idea.

Last edited by directdriver; 2nd July 2012 at 07:57 PM.
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Old 2nd July 2012, 08:34 PM   #37
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Right on! Thanks for your input and energy. Now that you say not to turn the wiper all the way to ground, it obviously makes more sense. I was considering the pot to cover a range of useful tweakyness, and how to achieve that -

i guess still caught up in the control panel knob mindset

but your inclusion of the schematic helps illustrate things

the one element i'm a bit confused is this - in the original schematic, the 390k and 260k (or whichever, can't remember exact figure) came together at pin 7 - instead of 1 leg (390k, in your suggestion) going to ground. isn't the signal to pin 7 a summation of the signals through these two resistors? it visually appears as though taking one of them off of pin 7 and grounding it would reduce the signal that appears at pin 7.

i'll review your calculation link as well. thanks for that!
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Old 2nd July 2012, 09:25 PM   #38
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Quote:
Originally Posted by brilliantblue View Post
the one element i'm a bit confused is this - in the original schematic, the 390k and 240k came together at pin 7 - instead of 1 leg going to ground. isn't the signal to pin 7 a summation of the signals through these two resistors?
The original circuit is using a floating paraphase. When the two dissimilar resistors form at one point to grid or pin-7, it also has another 240K resistor to ground. The idea is again to have a portion (unity) of the first tube's gain to the second tube. Regardless of how you do it, the bottom-line is to feed a tiny bit of the out of phase signal to the second tube, which really has no gain at all. Try to think of the paraphase circuit as TWO single ended circuits with opposite phases.

Quote:
Originally Posted by brilliantblue View Post
it visually appears as though taking one of them off of pin 7 and grounding it would reduce the signal that appears at pin 7.
Don't think of it as summation of of two signals. Think of them as subtraction. When two identical level but out of phase signals join together they cancel each other out. That's why the 390K and 240K resistors are not the same value. Regardless of approach, at the end of the day you need a portion of the first tube's signal feeding into the second tube so second tube's gain is unity. Getting this out of phase signal in the right portion is what the pot is for. Some circuits use a fixed value through calculation or measurement but, to me, dialing the pot is the most accurate. The original circuit might not be balanced at all and that might contribute to its sound but having a pot there makes life easier and the best of both worlds as you can purposely make it imbalance to get a more distorted sound. Either way, you have some control.
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Old 2nd July 2012, 10:21 PM   #39
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I get it now. I didn't look thoroughly enough to perceive the phase cancellation/neg feedback. thanks for your insight!
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Old 5th July 2012, 05:14 AM   #40
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Thumbs up Thanks again!

Much better now - turns out I had a pretty good imbalance there - almost twice as much signal on one 6v6 than on the other. I followed your advice and now they are very close to each other - it changes slightly with the volume knob position.

I do still have significant distortion with the guitar volume all the way up, and the amp volume above about 30% - I'd like to get a little more range. I also wonder if I'm not hitting the output section a little hard - i get about 60v signal at grid 1 when cranked, and it hits that point pretty fast when turning up the knob (about 30% of the way, as mentioned above). This amp may be a good candidate for a master volume. Or for turning one of the preamp stages into a cathode follower.

But, the correcting the imbalance made the distortion much more musical, which is a step in the right direction!

Thanks again for your help Direct Drive!
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