Switchable Hi-Z input impedance, how ?

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Hi Guys

Using a center-off switch to control three LEDs is easy.

hey, Kevin

trying to follow your describtion

like this :confused:
 

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Ex-Moderator R.I.P.
Joined 2005
ah, I understand now, the middle LED is connected directly to power supply
but only goes on when the other LEDs are off, because of the diodes

thanks Kevin, thats great

btw, i have mounted a small 3V supply
read somewhere that its optimal voltage
ripped it out of wall plug thing
hope it will do
 

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Ex-Moderator R.I.P.
Joined 2005
thought it would be nice to have a green LED close to output att/pot, together with the red one

but realized it could present a problem
output LEDs are controlled by the tube curcuit bypass/OFF/ON switch
so it will have to ON in both positions 1 and 3
but now we are on to the diodes, I found something about logic curcuits, and thought it might be useful

but will it work this way ?
any suggestion what diodes to use ?
 

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Hi Guys

If you want a LED that is on for both the 1,3 positions merely lift those two LEDs from ground, link them together at the cathode and insert the fourth LED.

The fourth LED will be on in both up/down positions but not in the middle.

The other LEDs will function just as before.

If you further want control over this fourth LED, you can parallel it with a switch/relay contact or BJT to shunt it off when desired but the other LEDs will still work as before.

Have fun
Kevin O'Connor
londonpower.com
 
Ex-Moderator R.I.P.
Joined 2005
input att/pot/buffer and SS gain stage could be 'fixed'

and the only selectable option would be, tube output or no tube output
it does look simpler and better this way
at least on att/pot and buffer is gone this way

bummer, that was the pot to 'balance' the channels
 

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Hi Guys

In the tube section of the preamp, there is only one gain stage. This will provide very little tubiness.

Have you in fact tried wiring the tube as a standard fender pre? That is the basis for every tube bass preamp ever made except the Rumble which was a copy of my Spectrum preamp front end. I think you will find it is nice sounding without excess colour. With an output level you can overdrive it a bit before the power amp. You would likely find that you do not need any other switching, etc.

Have fun
Kevin O'Connor
londonpower.com
 
Ex-Moderator R.I.P.
Joined 2005
hi Kevin
isnt a Fender preamp is supposed to drive a tube power amp, and works better without the buffer

Im driving solid state power amp, and need output buffer

but Im not really after tube sound as such
thats not my reason

anyway, I have made the box and layout like this to make it easier to make curcuit changes
I might start like this, maybe
hope to order a few more parts soon
 

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Hi Guys

The basic Fender circuit can drive the PA input. If you add an output control as suggested it will still drive the PA from the pot wiper, or a bit better if you add a buffer.

Why add a tube if you do not want tube sound?

Maybe you should do some reading about tube guitar amps to see how they function and how to change things to get alternate sounds. "The Ultimate Tone" series covers this very well. Check out TUT3 for a practical construction-oriented guide including five bass amp. See my site for detailed descriptions of the books.

You seem to be working with only hifi information and that is very limiting when it comes to amp design for musical instruments.

Have fun
Kevin O'Connor
londonpower.com
 
Tinitus, I think you really want the tube sound(!) And I like the layout potential of 2 jfets going in to 2 tube stages that you show on previous schem. You have enough room and switching possibility to really play around with the staging and tone stack position. Just suggest be careful with the tone stack impedance and previous stage or you are going to get a lot of overall signal loss at the tone controls. That is a potential spot for a j-fet buffer/driver circuit to handle
 
Ex-Moderator R.I.P.
Joined 2005
You have enough room and switching possibility to really play around with the staging and tone stack position. Just suggest be careful with the tone stack impedance ......

thanks, shanx :)

yeah, Im trying to keep the att/pot away ;)

but there are two resistors I have doubts about
one is the the tone out grounding resistor
I have marked it with a red dot
the other is tube input grounding resistor

not sure I need both
looks to me like they do the same thing :confused:
 

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