Switchable Hi-Z input impedance, how ? - Page 6 - diyAudio
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Old 24th November 2011, 06:45 PM   #51
tinitus is offline tinitus  Europe
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ecc82, gain stage and cathode follower(AC) output

and maybe possible 'frequency boost' adj/pot on autobias bypass cap

all comments appreciated
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Old 26th November 2011, 11:32 PM   #52
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hey, I chopped this out of a probably nice amp

but I wonder, running this at just above 100V+, maybe it would make a nice buffered distortion preamp
only part of it ofcourse

btw, I night have found my preferred bass sound
adding just a little touch of natural growl, and its almost similar to the gnarly sound of a real acoustic double bass
which I happen to like a lot
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Old 26th November 2011, 11:56 PM   #53
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also, this might be cool

both gain and frequency adjustable

still need output buffer to drive a solid state amp
either the same mosfet
or I have some 6C19n that ought to run fine on this 'lowered' voltage

well, easier said than done

hey, maybe the 6C19n would make nice distortion with even lower voltage

hmm, I would need two input options, with a switch
one with all mosfets, and one with tube coupled in
but thats the least of my problems
is this completely nuts
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Old 27th November 2011, 05:19 PM   #54
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any reason why it wouldnt work ?
that is if I can figure out the values etc
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Old 28th November 2011, 02:06 AM   #55
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using P-channel output ?

some reading

http://www.datasheetcatalog.org/data...shay/70611.pdf
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Old 29th November 2011, 01:47 AM   #56
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should be something like like this
still dreaming
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Old 29th November 2011, 02:35 AM   #57
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The answer to getting hi input Z at AC whilst maintaining that lower max Rg1 value for the input tube was in an earlier post (D96 was it?). Divide the cathode bias resitor, bypass the top resistor and connect the RG1 to the junction of the 2 resistors instead of 0V.
That is BOOTSTRAP the Rg1. DC wise you still meet the max Rg1 requiremnt (and it is at DC where it is important) but you also increase the effective AC impedance.
Cheers,
Ian
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Old 29th November 2011, 03:26 AM   #58
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Quote:
Originally Posted by gingertube View Post
The answer to getting hi input Z at AC whilst maintaining that lower max Rg1 value for the input tube..........
maybe I understand what you are saying, but only slowly

it bothered me that the 6C19n might not be suited fore Hi-Z input

btw, I removed one att

I think D96 said that I could use the cathode bias like a gain control
but now I want to use as distortion gain control

not yet sure how much gain I will have, or how much I need
I got a nice DIY stereo power amp yesterday
built it a long time ago, and now got it back

will try a few experiments
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Old 29th November 2011, 04:08 AM   #59
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Quote:
Originally Posted by gingertube View Post
Divide the cathode bias resitor, bypass the top resistor and connect the RG1 to the junction of the 2 resistors instead of 0V.
That is BOOTSTRAP the Rg1. DC wise you still meet the max Rg1 requiremnt (and it is at DC where it is important) but you also increase the effective AC impedance.
Cheers,
Ian
something like this

I remembered now that max grid resistance of 6C19n would be 500K
but should be no problem to have a jfet in front of it
apart from the 'problem' with different voltage
I was trying to avoid that by using a mosfet instead(2sk2013)
at least I suppose it would be ok with this higher voltage

btw, I plan to have a switch with 3 different cap size fore the cathode/gain control, and pot adjust will be on front, or as rotary switch with fixed resistors
if possible at all
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Old 29th November 2011, 04:42 AM   #60
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Yes,
Something exactly like that.
The signal voltage at the cathode follows the signal voltage on the grid. For AC signals the signal across Rg1 is therefore about 1/10th what it would be if the Rg1 were tied to 0V instead - for AC signals therefore Rg1 looks to be about 10 times as large. So if you use a 500K resistor for Rg1 it will look like about 5 Meg Ohms as far as its AC input impedance goes.

Of-course there is nothing wrong with the proposed JFet buffer either - the boostrap arrangement might just be a little easier (less parts).

Cheers,
Ian
Cheers,
Ian
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