I wasn't aware we were butchering a guitar for this discussion
Presumably it's far higher than you could possibly need?.
I wasn't aware we were butchering a guitar for this discussion
trying to look at it as a whole
and now looking at mu-follower tube pre
thank you
btw, reading TheValveWizard, there's a mu-follower ecc81 curcuit claimed to have 6M input and 300R output
sounds perfect, and I really like the ecc81
don't know why I didn't see that before
but maybe I wasn't looking fore it
searching on mu-follower, reading AlanKimmel describing mu-stage with top penthode, and how to replace it with a mosfet, the hybrid mu-stage
and look what popped up right under....http://www.diyaudio.com/forums/blogs/geek/104-underappreciated-hybrid-mu-follower.html
and look what popped up right under....http://www.diyaudio.com/forums/blogs/geek/104-underappreciated-hybrid-mu-follower.html
its funny
somethimes, the stuff you failed to find
suddenly pops up when not looking fore it 
and here it is, straight from our own forum
mind you, I haven't read it yet
http://www.diyaudio.com/forums/headphone-systems/175047-mu-follower-ecl805-output-xformer.html
somethimes, the stuff you failed to find
suddenly pops up when not looking fore it and here it is, straight from our own forum
mind you, I haven't read it yet
http://www.diyaudio.com/forums/headphone-systems/175047-mu-follower-ecl805-output-xformer.html
Attachments
just now I read that in a mu follower, the lower triode should be one that works well on lower voltage
and that you basicly want a high mu for the top triode, and a low mu for the bottom triode
some of those among the Compactrons too
but seems a major problem with many of them are the tube bases
hmm, it may become boring to hear about my C3g and 6C19n, but I wonder if they would form a nice mu stage
a 300B driver with D3a mu stage is not hard to find
but can I put in the 6C19n
and that you basicly want a high mu for the top triode, and a low mu for the bottom triode
some of those among the Compactrons too
but seems a major problem with many of them are the tube bases
hmm, it may become boring to hear about my C3g and 6C19n, but I wonder if they would form a nice mu stage
a 300B driver with D3a mu stage is not hard to find
but can I put in the 6C19n
6C19n data says resistance in grid curcuit 0.5Mohm
does this mean that 500Kohm will be my max possible input
Well, this means the maximum value of the grid resistor is 0.5 Mohm, although there is a note on the datasheet saying it can be pushed higher for at least one application. Isn't your 6S19P on the top of the mu-follower though?
But what is the point of all this? High input input impedance is easily and trivially achieved by using any number of input tubes, a FET, or a FET opamp as is commonly done in commercial amps. Where are you trying to go? Maybe I'm slow but I just can't read that from your posts.
if at all using a tube for this, I would like it to be easy and cheap to service
it will be driving a 100watt SS power amp
how about a single ecc82 with one half as cathode follower output ?
ups, and back again
Well, I still don't quite get it, don't know why you would go to a transformer coupled cathode follower, so I'll just throw out recommendations until you like one.
If you just want to make a simple high-impedance front end for a SS amp, just use a single cap coupled JFET.
If your SS amp needs a balanced input and you like tubes, just make a long tailed pair with a dual triode.
If your SS amp has a lowish input impedance, you can try a dual triode with 1/2 as common cathode and 1/2 as cathode follower.
Or why not just copy an existing design, like an Alembic F2B with the tone stack left out?
If you just want to make a simple high-impedance front end for a SS amp, just use a single cap coupled JFET.
If your SS amp needs a balanced input and you like tubes, just make a long tailed pair with a dual triode.
If your SS amp has a lowish input impedance, you can try a dual triode with 1/2 as common cathode and 1/2 as cathode follower.
Or why not just copy an existing design, like an Alembic F2B with the tone stack left out?
Alembic looks nice
hmm, ok, lets try
if I said it doesn't matter how
I need gain
I have enough of 2sk170 (non matched)
and I might use balanced classD power amp
how would you do it ?
Another way to look at the input is that impedance is all relative.
What is the output impedance of the driving device.
A general "Rule of Thumb" is for Hi-Z keep the input impedance greater than 10X the driver Output impedance. This gives a 9.09% reduction in the input signal over the open circuit output voltage.
If the input impedance is 100X the output impedance you will have 0.99% reduction, etc.
The figures I've found for guitar output impedance indicate it is generally between 20K and 40K ohms.
IF this is so, then 1M/20K=50 and 1M/40K=25 and the typical ratio for a 1Meg input impedance is 25 to 50X the output impedance which is light loading.
Once you switch to Low-Z (in your circuit it is approximatly 136K because the two 68K resistors are in series) your ratio drops to 136/20=5.4 to 136/40=3.4 which is significantly greater loading and will reduce the signal level as well as effecting frequency response. Furtheremore the two 68K resistors are in series dividing the voltage by 2 which is the main gain reduction.
Rough answer is greater than 400K is high-Z for guitar amp, Once you get down to 150K you are loading the output of the guitar and in the Lo-Z area. Somewhere in between is the transition range.
What is the output impedance of the driving device.
A general "Rule of Thumb" is for Hi-Z keep the input impedance greater than 10X the driver Output impedance. This gives a 9.09% reduction in the input signal over the open circuit output voltage.
If the input impedance is 100X the output impedance you will have 0.99% reduction, etc.
The figures I've found for guitar output impedance indicate it is generally between 20K and 40K ohms.
IF this is so, then 1M/20K=50 and 1M/40K=25 and the typical ratio for a 1Meg input impedance is 25 to 50X the output impedance which is light loading.
Once you switch to Low-Z (in your circuit it is approximatly 136K because the two 68K resistors are in series) your ratio drops to 136/20=5.4 to 136/40=3.4 which is significantly greater loading and will reduce the signal level as well as effecting frequency response. Furtheremore the two 68K resistors are in series dividing the voltage by 2 which is the main gain reduction.
Rough answer is greater than 400K is high-Z for guitar amp, Once you get down to 150K you are loading the output of the guitar and in the Lo-Z area. Somewhere in between is the transition range.
that is great info
I'm learning
hmm, if using passive pickups, and if I left out the passive volume pot on the guitar, I would have lower impedance, right ?
actually, with passive pickups, controls on guitar may be the worst of the impedance 'problem' (?)
edit, actually, I have considered using a chip/volume 'pot', with a buffer
but with needed gain, balanced output etc etc, it seems complicated
You could, but you have to realize there will probably be a rather extreme peak in the response of the guitar pickup. Part of the reason for the tone section is to achieve critical dampening of the response by placing a low pass filter in the appropriate place frequency wise in the output. You could add such a filter with a simple RC combo after measuring the response to locate the resonance and dampen it with a pot and cap to find the best values.
There is a good explaination here:
BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups
I'm not sure how the chip pot would work, as I've not used any. Theoretically they should be fine as they are used in A/D circuits for calibration and many of the A/D circuits are up to 16 bits or more.
My understanding is the employ a MOS-FET as the pass device. Varying the gate to source voltage results in a varying drain to source resistance. Using it with a buffer would make sense, but as you say it becomes complicated quickly.
There is a good explaination here:
BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups
I'm not sure how the chip pot would work, as I've not used any. Theoretically they should be fine as they are used in A/D circuits for calibration and many of the A/D circuits are up to 16 bits or more.
My understanding is the employ a MOS-FET as the pass device. Varying the gate to source voltage results in a varying drain to source resistance. Using it with a buffer would make sense, but as you say it becomes complicated quickly.
nice even more tricky than I was aware of, yes
as I understand, the peak isn't removed, but moved in frequency
critical Q may change too, yes
since last time I looked at bass pickups a few thing seem to have changed
passive are really no longer passive, as such
but are also sold with 'active curcuit'
seems like others are learning too
also, that paper indicate that a load into lower impedance will dampen the peak
or maybe it was in the link at bottom, to commercial product
it needs further consideration
balanced or se power input
I'm still indecisive on that
like say if I used big power amp, but without needing it
maybe I wouldn't need that much preamp gain
but balanced power amp input is low impedance
'single ended' usually higher impedance
but still not quite enough
and so it goes round and round
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