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Old 4th October 2011, 05:26 PM   #21
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Quote:
Originally Posted by Joshua_G View Post
If you'd use 2 volume controls, 1 after the first stage and the 2nd after the 2nd stage, the first one will determine the amount of distortion and the 2nd one the overall volume.
Isn't that how gain knobs are implemented? I'm not worried about distortion at the moment. I just don't want to feed about 20v from the second stage into the preamp of my solid state amp (which doesn't have preamp bypass for pedals). so how do i reduce the output from 20v to 100mv. (20v is just an random number).

Like i said i was contemplating a voltage divider after the coupling cap, but im not sure what effect the impedance of the guitar amp's input will have on the voltage divider.
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Old 4th October 2011, 06:35 PM   #22
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Quote:
Originally Posted by Alexrmc92 View Post
Isn't that how gain knobs are implemented?
Right now, the existing volume control may cause distortion by clipping, if turned up too high volume. There is no separate control over distortion and separate control over volume.

Quote:
Originally Posted by Alexrmc92 View Post
I'm not worried about distortion at the moment.
To your question:
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Originally Posted by Alexrmc92 View Post
My new question is if i were to make a tube distortion pedal (essentially an overdriven preamp in a pedal) what way is best to reduce the output of the second stage back to guitar level, lets say 100mv?
I replied:
Quote:
Originally Posted by Joshua_G View Post
If you'd use 2 volume controls, 1 after the first stage and the 2nd after the 2nd stage, the first one will determine the amount of distortion and the 2nd one the overall volume.
My reply meant to save you an extra overdriven preamp at the cost of an extra volume control.
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Old 4th October 2011, 11:27 PM   #23
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Originally Posted by ChrisA View Post
The results of your bread board experiment are valid but the breadboard does not represent a Fender Champ. NFB changes "everything" and you left it out of the breadboard.

I just happen to have a couple Champs. One is a 5E1 clone I built and the other a Fender Champ 600. ( I built the 5E1 because I was slightly disappointed with the 600) Just this last weekend I took the feedback off the 5E1 in order to trouble shoot a problem. While I had it disconnected I thought I'd try plugging in a guitar. It dramatically changes the amp.


It also makes a huge difference it you are playing chords or a single string lead. It should be clear that six strings are louder than one string.

My theory is that the champ does not distort so easily because there is less gain than you'd think because only the first triode is outside of the NFB loop.
Never said that the gain that I achieved is what you would get with a Champ, I gave a best case scenario on the amount of gain two 12AX7's would give you with Fender resistor values. I gave the negative feedback and the tone stack in later Champs as more things that reduce gain from the theoretical gain a 12AX7 is said to have. I could have bypassed the cathode resistors for more gain but then to be fair to the Champ I would have to put in a lossy tone stack. Rather than bat around a bunch of 'it should be this and this effects that' I decided to give real numbers using real parts. If I had a Champ I would have given the whole gain structure.
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Old 4th October 2011, 11:33 PM   #24
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Quote:
Originally Posted by Alexrmc92 View Post
Isn't that how gain knobs are implemented? I'm not worried about distortion at the moment. I just don't want to feed about 20v from the second stage into the preamp of my solid state amp (which doesn't have preamp bypass for pedals). so how do i reduce the output from 20v to 100mv. (20v is just an random number).

Like i said i was contemplating a voltage divider after the coupling cap, but im not sure what effect the impedance of the guitar amp's input will have on the voltage divider.
Use a volume pot on the output of your tube preamp and put a couple of back to back zeners across the output in case you have the volume up accidentally feeding too much voltage. That or a bunch of diodes in series at the voltage level of your choosing. Don't bother worrying about the loading effect on the pot from your amp, just set the volume where you want it.
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Old 5th October 2011, 11:08 PM   #25
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I just ended up doing this. 1vrms at max volume shouldn't any damage to a preamp. It will defiantly distort, but that's what the volume pot is for. I just didn't want to push 60vrms into a preamp. and thats 1vrms with no load, so a low input impedance input should reduce that max voltage by quite a bit.
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Old 12th October 2011, 08:51 PM   #26
shanx is offline shanx  Canada
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Quote:
Originally Posted by Alexrmc92 View Post
i agree as well, i never considered the negative feedback. That changes things a lot. Honestly i feel much better understanding that guitars aren't really outputting 1v constantly. I using this knowledge to design my own preamp and i just wanted to understand why the champ doesn't clip at all volumes with 100v into stage 2. I knew it wasn't 100v but i wanted to know why.

I guess my only question is how far positive can you safely go on the grid, or should you never go past 0v?
My attempt at helping with these questions (IMO): The first stage will have a voltage gain of about 30. Fender schematics in later years models show test points and the expected voltages at those points, so you can get a good idea of what to expect.
For Blues Jr, which has sort of similar 1st /2nd stages, they test with10 mV signal generator at input. 1st stage should get 273 mV at the OP. But then goes through a filter/volume network and that drops it down to 33 mV where it get (re) amplified through second stage to 1.35 Vac. Most subsequent preamp stages are there to drive tone shaping, reverbs, and to add deliberate distortion. Hitting the preamp with a 1 volt signal will cause clipping, but real guitar signal will only peak like this at the beginning of the note, not a continuous signal. But you can expect in the order of 5% THD at 20 mV input in the blues junior. Hope that helps.
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