ok, managed to get the supply voltage down to 14.3V
drain resistor 8.2V/1K3, 6.3ma
source res 47.2mV/7R5, 6.3ma
DC before output cap dropped 2V to now 6V
and sounds real good, best yet
feels like I lost some gain
and no doubt that all controls works much better now
will try to up the voltage to 15V
and btw, hum is gone now
but only when I shut down signal on bass guitar
drain resistor 8.2V/1K3, 6.3ma
source res 47.2mV/7R5, 6.3ma
DC before output cap dropped 2V to now 6V
and sounds real good, best yet
feels like I lost some gain
and no doubt that all controls works much better now
will try to up the voltage to 15V
and btw, hum is gone now
but only when I shut down signal on bass guitar
got it to 14.9V
nothing else seems to change
and doesnt seem possible to get higher bias with this voltage
but wow, it certainly cleaned up the sound
gave it a little touch of more power
seems like endless headroom to play with
I can live with this for a week or two
ready to move into a fine home now
ok, it continues with attenuators, and hopefully buffers
I found a 'generic' simple one with a jfet and bjt
unfinished and a few parts missing
and juma posted a schematic I found interesting
and I have plenty of the BC547
lets see how it goes
I found a 'generic' simple one with a jfet and bjt
unfinished and a few parts missing
and juma posted a schematic I found interesting
and I have plenty of the BC547
lets see how it goes
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If you look at how a vacuum tube is made the grid is literally isolated in a vacuum, not touching anything. The impedance is infinite because it is an open circuit. But grid can collect an electrostatic charge and must be connected to ground through a bleeder resister to keep the grid from being charged. The impedance is determined by this "grid leak" resister. You can get away with a 10Meg resister although "everyone" uses a 1 meg.
This does not apply only to tubes but you see this more on tube preamps, You can use up to a 10:1 audio transformer and transform the impedance any way you like. Remember that impedance ratio is the square of the turns ratio so a 10:1 turn ratio is a 100:0 impedance. In theory you can get up to a Gig-ohm but in the real world all you'd hear is noise and hum because 1Meg is already high enough to pick up noise.
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thanks Chris
I know the title have become a bit 'old'
the hi-Z thing is more or less sorted
experimenting with the gain stages and buffers now
but you have a good point with having a tube as input
tho I have decided to save the tubes for alternative output
and since I have decide it should full functional with tubes pulled out, I must also have alternative SS output
and even worse, I decided to have two channels
but how it ends I dont know yet
and I really need them tone controls
I know the title have become a bit 'old'
the hi-Z thing is more or less sorted
experimenting with the gain stages and buffers now
but you have a good point with having a tube as input
tho I have decided to save the tubes for alternative output
and since I have decide it should full functional with tubes pulled out, I must also have alternative SS output
and even worse, I decided to have two channels
but how it ends I dont know yet
and I really need them tone controls
hi juma
yes, gain stage-att-buffer
input signal ? Im not sure
but in any case it will end with having multiple gain stages
the jfet boz I use now is more than enough
but will also be attenuated at input
maybe 12db will do
and maybe even the output 'buffer' could have moderate 6db gain, and still have low impedance
I dont think it have to be a unity gain buffer
whatever will be convenient too
z-in ? about 1Mohm at first input stage
at other places through the amp, Im not sure
attached a couple of my messy ideas, just to show where I might be heading
and btw, its basicly the jfet related curcuits I want to look at right now
yes, gain stage-att-buffer
input signal ? Im not sure
but in any case it will end with having multiple gain stages
the jfet boz I use now is more than enough
but will also be attenuated at input
maybe 12db will do
and maybe even the output 'buffer' could have moderate 6db gain, and still have low impedance
I dont think it have to be a unity gain buffer
whatever will be convenient too
z-in ? about 1Mohm at first input stage
at other places through the amp, Im not sure
attached a couple of my messy ideas, just to show where I might be heading
and btw, its basicly the jfet related curcuits I want to look at right now
Attachments
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