I want to add a volume switch to this pre amp schematic. http://diyaudioprojects.com/Schematics/images/6DJ8-SRPP-Tube-Preamp.png
I do not quite understand where to put it and the value that I would use for the pot. Also do I need to consider the 1m resistor. Thank you for any help.
I do not quite understand where to put it and the value that I would use for the pot. Also do I need to consider the 1m resistor. Thank you for any help.
Add the volume control just after the input socket, with the wiper going to the grid.
The value can be anything up to 500k depending on your source, but probably 100k
is reasonable. The 1M resistor can stay. http://www.dact.com/assets/images/volume-control-potentiometer-connections-schematic.gif
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That preamp design will require some adjustments to actually be something you'd want to use.
1. The coupling cap is way, way too small. If you fed this into a 10K load (think driving a subwoofer plate amp), you'll be -3dB at 160Hz. Increase the coupling cap to 3uF.
2. I have no idea why the designer chose to run 1mA of current through a tube that could do so much more. Sure, the output impedance is 850 Ohms, but you aren't going to deliver much voltage into a low impedance load because you are current limited. Crank the voltage up a bit and run more current. (Some internet searching shows a circuit or two run off 250-ish volts and running 10mA of current)
3. Where's the power supply? DC heaters and elevating the heaters above ground are important considerations in such a design.
4. Grid stoppers and plate stoppers are absent (the grid stoppers at least need to be there, just buy a bag of 220 Ohm carbon film 1/4W resistors).
5. The cathode bypass cap is a little small. (It can be a 6V cap, use a big one there, 470uF-1000uF)
1. The coupling cap is way, way too small. If you fed this into a 10K load (think driving a subwoofer plate amp), you'll be -3dB at 160Hz. Increase the coupling cap to 3uF.
2. I have no idea why the designer chose to run 1mA of current through a tube that could do so much more. Sure, the output impedance is 850 Ohms, but you aren't going to deliver much voltage into a low impedance load because you are current limited. Crank the voltage up a bit and run more current. (Some internet searching shows a circuit or two run off 250-ish volts and running 10mA of current)
3. Where's the power supply? DC heaters and elevating the heaters above ground are important considerations in such a design.
4. Grid stoppers and plate stoppers are absent (the grid stoppers at least need to be there, just buy a bag of 220 Ohm carbon film 1/4W resistors).
5. The cathode bypass cap is a little small. (It can be a 6V cap, use a big one there, 470uF-1000uF)
You're off by an order of magnitude on cathode current, it's 10mA, not 1mA.. 😀 And the 6DJ8 sweet spot for plate voltage is about 80 - 90V on the plate, so as designed this circuit is just about right. (Go to 250V across an 6DJ8 SRPP and you may get fireworks.)
You are right about the output impedance, it's not all that low, probably around 200 - 300 ohms. (I think the 850 ohm value quoted is high)
You didn't ask what he is driving with this so a small output capacitor may be fine, or possibly something as large as 1uF. I have been hard pressed to find decent larger film caps at reasonable cost, and if the input impedance is 100K or more it's not needed in any case.
This volume pot from Khozmo is very nice and dont cost a leg:
High Quality Audio & Industrial Attenuators
I have the 100Klog see post 178 & 179:
Let's See Your Transformers!
High Quality Audio & Industrial Attenuators
I have the 100Klog see post 178 & 179:
Let's See Your Transformers!
I believe in lowest common denominator, which is the 10 Kohm IHF "standard". 3.3 μF. is the absolute minimum into that impedance. I suggest a 4.7 μF. metalized polypropylene part bypassed by a 0.47 μF. 716P series Orange Drop. For (naturally) a price, a 0.33 μF. PPFX MultiCap, as the bypass, is an improvement.
cap
Installed a 250k pot and it works fine but it has very little adjustment. Loud to quiet is a very small area. Also installed a 3.3uf cap on the output. Much better bass and richer tone. thank you for your help.
Installed a 250k pot and it works fine but it has very little adjustment. Loud to quiet is a very small area. Also installed a 3.3uf cap on the output. Much better bass and richer tone. thank you for your help.
I have added a 100K pot to HTTP Error 404 - Not Found | diyAudioProjects.com When turning down the volume I had a large 60hz hum. I am driving a 6sn7/300b amp. I added 1k resistor on both grids of both 6dj8 tubes. Noise is gone. I think it was parasitic oscillation. I now feel that the top end has been attenuated a little bit. Question is the value right. Should I use a resistor on both grids or is one resistor on the first grid enough.
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