Grainger49
New member
Mike, is that battery on a cathode?
Bottlehead C4S as a differential pair constant current sink?
- Thread starter Mikey
- Start date
Grainger49
New member
Mikey said:
Got'cha! There was a mod of the Paramour where you biased the driver, changed to a 76, with a 9V rechargeable. Once it was charged the circuit kept it charged.
Should this be quieter?
Paul Joppa
Moderator
It's not a transistor, it's FET - MOSFET to be specific. It's enhancement more so it needs several volts of positive bias, more than can be supplied by an LED. I don't know to what extend the cascode arrangement will reduce the potential for distortion from the high and nonlinear capacitances. We never experimented with FETs, though I know Tucker and the late John "Buddha" Camille tried several circuits a decade or more ago and rejected them as inferior sounding.Mikey said:
Hi Paul,
I got some time on the newly C4S'ed preamp, and I liked what I heard! In fact, I liked it enough
to tackle the next iteration...
In a previous post, I mentioned the desire to try a 'hybrid' C4S, using the existing 6SN7 triodes in
place of the MJE340s. Well, I perfomed the modification this evening, and it works like a charm!
Here's a photo:
The swap required a bunch of 'cut-and-splice' to the circuit board, but it was worth it. Now the
vacuum tube can dissipate the heat when I bump up the current thru the circuit. Although the
C4S board is dangling from the wires at the moment, I'll be able to mount it on the underside of
the existing circuit board...without the MJE340's, it is a very low profile assembly!
Thanks again for all of your 'hand holding' throughout this project, I couldn't have done it without
your assistance!
Mike
I got some time on the newly C4S'ed preamp, and I liked what I heard! In fact, I liked it enough
to tackle the next iteration...
In a previous post, I mentioned the desire to try a 'hybrid' C4S, using the existing 6SN7 triodes in
place of the MJE340s. Well, I perfomed the modification this evening, and it works like a charm!
Here's a photo:
The swap required a bunch of 'cut-and-splice' to the circuit board, but it was worth it. Now the
vacuum tube can dissipate the heat when I bump up the current thru the circuit. Although the
C4S board is dangling from the wires at the moment, I'll be able to mount it on the underside of
the existing circuit board...without the MJE340's, it is a very low profile assembly!
Thanks again for all of your 'hand holding' throughout this project, I couldn't have done it without
your assistance!
Mike
Hi PJ,
I wanted to wrap up this thread with a few photos of the completed installation,
and also ask one additional question...
I mounted the C4S boards to the underside of the existing circuit boards. To bolt
them in place, I drilled a couple of holes in the existing board, and used 1/4" long
nylon spacers with some #4-40 hardware. Jumper wires were made to splice the
C4S into the existing curcuit:
The C4S certainly helped reject any last vestige of power supply noise from this
circuit....al that remains is a bit of tube rush.
With the 6SN7 in place of the MJE340, I think I'm going to be limited in the amount
of current I can draw through this arrangement. With both triodes (of the 6SN7) in
use, the RCA data sheet specifies the maximum dissipation of the tube is 7.5 watts,
or 3.75 watts per triode. By my reckoning, that equates to a current of 15mA
through the C4S.
Does that sound about right? Is 15mA the upper limit of this scheme?
Mike
I wanted to wrap up this thread with a few photos of the completed installation,
and also ask one additional question...
I mounted the C4S boards to the underside of the existing circuit boards. To bolt
them in place, I drilled a couple of holes in the existing board, and used 1/4" long
nylon spacers with some #4-40 hardware. Jumper wires were made to splice the
C4S into the existing curcuit:
The C4S certainly helped reject any last vestige of power supply noise from this
circuit....al that remains is a bit of tube rush.
With the 6SN7 in place of the MJE340, I think I'm going to be limited in the amount
of current I can draw through this arrangement. With both triodes (of the 6SN7) in
use, the RCA data sheet specifies the maximum dissipation of the tube is 7.5 watts,
or 3.75 watts per triode. By my reckoning, that equates to a current of 15mA
through the C4S.
Does that sound about right? Is 15mA the upper limit of this scheme?
Mike
Paul Joppa
Moderator
With a 250v drop, then yes, 15mA is your maximum without over-heating the 6SN7. You can draw up to 20mA, the 6SN7 maximum rating, if the low voltage is reduced to maintain the maximum heating.
To be careful, check the 6SN7 bias voltage and compute the dissipation in the small transistor. There are clip-on heat sinks for those tiny metal cans, and I recommend them if you exceed 1/2 of the free-air dissipation limit.
To be careful, check the 6SN7 bias voltage and compute the dissipation in the small transistor. There are clip-on heat sinks for those tiny metal cans, and I recommend them if you exceed 1/2 of the free-air dissipation limit.
Paul, you wrote:
"To be careful, check the 6SN7 bias voltage"
What voltage am I measuring?
In other words, where do I place the voltmeter probes?
"and compute the dissipation in the small transistor"
Earlier, you showed me how to calculate the dissipation in Q2, but I didn't see
anything in the C4S manual about the dissipation in Q1....how is it calculated?
Mike
"To be careful, check the 6SN7 bias voltage"
What voltage am I measuring?
In other words, where do I place the voltmeter probes?
"and compute the dissipation in the small transistor"
Earlier, you showed me how to calculate the dissipation in Q2, but I didn't see
anything in the C4S manual about the dissipation in Q1....how is it calculated?
Mike
Paul Joppa
Moderator
In normal use, the bias on Q2 is +0.7v, leaving about 2.5 drop across Q1 (2*1.6v across the LEDs, minus 0.7v base to emitter on Q2). If the 6SN7 bias is (for example) -8v then the drop across Q1 is 11.2v. Measure ground to the 6SN7 cathode in you case. Then if the current is 15mA, the dissipation in Q1 is (0.015A * 11.2v) or 168mW. The 2N2222 is rated fro 500mW without a heat sink, so I would not run it at more than 250mW without a heat sink. The above calculation says that's OK but the margin is not large.
Paul Joppa said:
PJ, did you mean to write:
"Measure B- to the 6SN7 cathode in your case"
If I measure from ground to the cathodes, I get some funky numbers.
Assuming you meant B-, I measure 10.0v between B- and the 6SN7 cathode.
Plugging into your equation, the drop across Q1 is 13.2v. At the moment, this
C4S is set for 8mA, so the 2N2222 is dissipating 105mW the way things sit today.
Of course, that assumes that I measured the 6SN7 bias correctly.....
Can't I measure the voltage drop across Q1 directly? Sure, it may be somewhat
cumbersome because the 2N2222's three terminals are out of reach, but I can
follow the circuit board traces to points that are more accessible. It would be
nice to compare the results both ways to see if they match!
Mike
Paul Joppa
Moderator
The 2N2222 emitter is connected to B- (yes, that's what I meant) through R1 which drops about 0.855 volts. Its collector is connected to the 6SN7 cathode. So B- to cathode is a slightly conservative estimate of the voltage drop, and much easier to measure.
Horns Forever
New member
I know this is an old thread, but I didn't want to hi-jack someone else's thread off-topic, so this seemed like the right place to start.
I am designing a pre-amp using a balanced differential pair of tubes. The two kathodes will be tied together and will feed a CCS current sink to ground. I'm curious to know whether the C4S will work in my circuit. The voltage drop between the kathodes and ground will only be about 9 or 10 volts, so there will be very little across the CCS. Would this be enough for the C4S to operate? There shouldn't be much voltage swing in this part of the circuit. Its the nature of a differential pair to hold the kathodes at a nearly constant voltage.
Thanks for any replies or comments.
I am designing a pre-amp using a balanced differential pair of tubes. The two kathodes will be tied together and will feed a CCS current sink to ground. I'm curious to know whether the C4S will work in my circuit. The voltage drop between the kathodes and ground will only be about 9 or 10 volts, so there will be very little across the CCS. Would this be enough for the C4S to operate? There shouldn't be much voltage swing in this part of the circuit. Its the nature of a differential pair to hold the kathodes at a nearly constant voltage.
Thanks for any replies or comments.
The other strategy that I've used when I don't have enough compliance from cathode to ground is to put a voltage doubler on the heater winding, then ground the positive side to make a negative DC rail for the CCS.
Horns Forever
New member
Thanks to both of you for the quick reply. I'll likely buy a C4S kit to experiment with. I bought one 10 or 15 years ago, but I know I lost the booklet when we moved and I suspect the parts are somewhat changed since then. Do the current kits include the appropriate parts for a current-sink application?
I'm not sure I understood the solution offered by Caucasian Blackplate. Is this sort of like "filament bias" where the extra voltage is consumed in a resistor between filament and ground? (but in this case, consumed by the C4S).
If it helps any, I'm going to use a Coleman Regulator on each filament.
Thanks again!
I'm not sure I understood the solution offered by Caucasian Blackplate. Is this sort of like "filament bias" where the extra voltage is consumed in a resistor between filament and ground? (but in this case, consumed by the C4S).
If it helps any, I'm going to use a Coleman Regulator on each filament.
Thanks again!
Not really. If you bought something like the SEX C4S kit, you could cut the board in half and then substitute an MJE-340 in for the MJE5731A/MJE350, as well as a 2N2222A for the PN2907. The LED's also fit in the other way, and for a current sink, the ground terminal goes to B+, then the "I" pad is the input (ground or negative rail) and the "O" pad goes out to your cathodes.Horns Forever said:
I didn't consider the possibility that you're using a differential DHT. In that case, my alternative will not work. There's also a third alternative that I credit to PJ (not sure if he came up with it or ran across it elsewhere) where you can add a resistor between ground and the negative output of your power supply. Given that you're drawing constant B+ current, you can use Ohm's Law to drop whatever voltage you like across that resistor. One end of the resistor will be at ground potential, and the other end will provide a negative bias voltage.Horns Forever said:
In your case, I'd just try running it with the bias voltage you're intending to use.
Horns Forever
New member
I'm sorry, I should have described what I was doing better. I try to use DHTs whenever possible.
Is this what you are describing for the C4S? (See attached concept) Each tube will pass 6 mA from the plates (total of 12 mA), so there won't be much heat.
I could probably push the tube bias up so there is 10 volts across the C4S if I increased the Vp a bit, but I think the #26 sounds best near where I've specified it.
Thanks again for your advice. You guys are the best!!
Brian
Is this what you are describing for the C4S? (See attached concept) Each tube will pass 6 mA from the plates (total of 12 mA), so there won't be much heat.
I could probably push the tube bias up so there is 10 volts across the C4S if I increased the Vp a bit, but I think the #26 sounds best near where I've specified it.
Thanks again for your advice. You guys are the best!!
Brian
