Have you seen the schematic I attached??
I think it's a bit odd because the buffer is located before the volumecontrol, so the buffer doesn't provide a signal directly to the 6C45 grid, but it provide a signal through a potmeter and then to the 6C45.
fdegrove said the buffer is needed, but do you think I can drop it?
Will a CD-player be avaiable to drive the high input capacitance?
I think it's a bit odd because the buffer is located before the volumecontrol, so the buffer doesn't provide a signal directly to the 6C45 grid, but it provide a signal through a potmeter and then to the 6C45.
fdegrove said the buffer is needed, but do you think I can drop it?
Will a CD-player be avaiable to drive the high input capacitance?
I don't understand the given value for the input capacitance of the 6C45. (I've read 120pf or even 240pF here) Can anyone explain ?
According to the datasheet this input C is about 12 pF, just a little bit higher than the 5842 (9pF) wich I'm jusing right now with a 100K potmeter. I'm not interested in rebuilding the preamp into a two driver stage.
maximum anode voltage rating of the tube is according to the datasheet indeed 150V but the tube seems to work properly with voltages up to 180 or even 200V
According to the datasheet this input C is about 12 pF, just a little bit higher than the 5842 (9pF) wich I'm jusing right now with a 100K potmeter. I'm not interested in rebuilding the preamp into a two driver stage.
maximum anode voltage rating of the tube is according to the datasheet indeed 150V but the tube seems to work properly with voltages up to 180 or even 200V
Sjef you are right the Cga is about 11pF, but we have the Miller effect which increases the input capacitance according to the amplification. So actually the input capacitance is dependend on the amplification.
I don't know the exact Miller formula, but I this one will be an good approximation:
Cin=Cag(G+1)+Cgk
//eeehh, something is wrong with this formula, EC8010 calculated the input capacitance to be 120pF with 20* gain.
Maybe EC8010 will give you the right formula.
with a gain of 20 you will have a input capacitance of:
Cin=11pF*21=231pF
and the input capacitance would be even higher for higher amplification.
Sorry for my english, but I hope you understand anyway
I don't know the exact Miller formula, but I this one will be an good approximation:
Cin=Cag(G+1)+Cgk
//eeehh, something is wrong with this formula, EC8010 calculated the input capacitance to be 120pF with 20* gain.
Maybe EC8010 will give you the right formula.
with a gain of 20 you will have a input capacitance of:
Cin=11pF*21=231pF
and the input capacitance would be even higher for higher amplification.
Sorry for my english, but I hope you understand anyway
If you use a tube in a cascode configuration the Millereffect will no longer exist and the actual input capacitance will be ~11pF.
But I don't know if 6C45 is a good tube for cascode configuration.
In fact the author behind TubeCAD once said that the only tube good for cascode is the ECC88.
316a... wouldn't it be difficult to use a cascode configuration in a differential amplifier?
What about stability?
But I don't know if 6C45 is a good tube for cascode configuration.
In fact the author behind TubeCAD once said that the only tube good for cascode is the ECC88.
316a... wouldn't it be difficult to use a cascode configuration in a differential amplifier?
What about stability?
I've gone back to your diagram. Any CD player should be perfectly capable of driving a 10k volume control, so I don't see why you shouldn't be able to drop the buffer. No doubt Frank will say why he felt you ought to keep it.Gunderz said:
The formula is correct, but where did the 11 come from? My data sheet says Cag = 5pF, hence 120pF (OK, strictly 125pF + 11pF = 136pF, but the gain was given as between 15 and 25).Gunderz said:
6C45 ought to make an ideal cascode, but you would definitely need a good oscilloscope standing by to hunt down and kill any oscillation.
I am also surprised by the stated 150V Va(max.). I have heard rumours relating to accelerations experienced by fighter aircraft as "explanations" for this figure, but does anyone have any hard facts? Given that mu is proportional to the ratio of anode/grid to grid/cathode spacing, higher mu valves should be able to withstand a higher anode voltage. As a reverse example, the PD500 has Va(max.) = 25kV and mu = 1050. This is a log-winded way of saying that I think it should be safe to consider Va(max.) = 250V.
cascodes
...well I had no problems running 6SN7 in differential cascode , apart from the fact I did not have enough gain and needed more HT . Allen Wright uses uses differential cascodes in his PP1C amp and various others . The ECC88 is the preferred cascode valve as it has high H-K ratings , also low Ra . With 6C45 the H-k ratings won't matter as separate triodes are used therefore a floating heater supply would be used for the top pair , Ra is also lower with 6C45 than with ECC88 . It might work , then again could end up as an oscillator . Thoughts anyone ?
316a
...well I had no problems running 6SN7 in differential cascode , apart from the fact I did not have enough gain and needed more HT . Allen Wright uses uses differential cascodes in his PP1C amp and various others . The ECC88 is the preferred cascode valve as it has high H-K ratings , also low Ra . With 6C45 the H-k ratings won't matter as separate triodes are used therefore a floating heater supply would be used for the top pair , Ra is also lower with 6C45 than with ECC88 . It might work , then again could end up as an oscillator . Thoughts anyone ?
316a
Ah...
Hi,
Unfortunately I was wrongly assuming that the figure you had given before was the actual Cag.
So after some quick calcs I obviously came to the conclusion that this valve was very hard to drive without a grunty (CF) stage preceding it.
Sorry to lead anyone astray with this and to make up for it here a link to some circuit theories:
PAENG
Hope this helps,
Hi,
Unfortunately I was wrongly assuming that the figure you had given before was the actual Cag.
So after some quick calcs I obviously came to the conclusion that this valve was very hard to drive without a grunty (CF) stage preceding it.
Sorry to lead anyone astray with this and to make up for it here a link to some circuit theories:
PAENG
Hope this helps,
10k volume control without buffer??
I think it will be a bit problematic, the input resistance will then be about 10k, but many CD-players will have problems driving low impedances.
The instructions manual for my CD-player says that the loadresistance should be higher than 47k, so what is it like to drive my CD-player with a 10k pot?
Well, my CD-player isn't very expensive, it's a Sony CDP-XB920 or something like that, can't remember the exact number.
Ok, my formula was correct then, but I took the wrong numbers... Thanks anyway
I think it will be a bit problematic, the input resistance will then be about 10k, but many CD-players will have problems driving low impedances.
The instructions manual for my CD-player says that the loadresistance should be higher than 47k, so what is it like to drive my CD-player with a 10k pot?
Well, my CD-player isn't very expensive, it's a Sony CDP-XB920 or something like that, can't remember the exact number.
Ok, my formula was correct then, but I took the wrong numbers... Thanks anyway
Re: Ah...
Wow! Thanks Frank. My printer is now busy...
Most CD players use an op-amp at their output. Although distortion rises for loads below 2k, most op-amps are quite happy with 10k loads. It may be that there is some passive circuitry that causes the 47k specification. Unfortunately, if you use a 47k volume control, the output resistance will form a low-pass filter in conjunction with the input capacitance of the 6C45. You could always put the volume control before the buffer, as a cathode follower has very low input capacitance, and you could even use a 1M volume control if you wished...
fdegrove said:
Wow! Thanks Frank. My printer is now busy...
Gunderz said:
Most CD players use an op-amp at their output. Although distortion rises for loads below 2k, most op-amps are quite happy with 10k loads. It may be that there is some passive circuitry that causes the 47k specification. Unfortunately, if you use a 47k volume control, the output resistance will form a low-pass filter in conjunction with the input capacitance of the 6C45. You could always put the volume control before the buffer, as a cathode follower has very low input capacitance, and you could even use a 1M volume control if you wished...
Eeehh, I'm a bit confused....
EC8010 said that he would use a 25k pot or less for my configuration(gain= 20). Then I said I'm planning to use a 10k pot if you used the schematic I attached.
Then EC8010 said it was okay, but then I hear that I don't need the buffer because of some misunderstanding.
So now is it a problem to use a 10k pot because of the driving capabilities of my CD-player(and many others).
EC8010 said that he would use a 25k pot or less for my configuration(gain= 20). Then I said I'm planning to use a 10k pot if you used the schematic I attached.
Then EC8010 said it was okay, but then I hear that I don't need the buffer because of some misunderstanding.
So now is it a problem to use a 10k pot because of the driving capabilities of my CD-player(and many others).
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