Hi,
i'm making another 2ch tube preamp (Redd 47), this time it is for really talented singer who benefits from such preamp, the other channel will be for different sources. I find this quite rare as most musicians only get better sound in the studio, if at all. Her voice also live, where letters in high frequencies like s, c and similar are finally as sang, beside other advantages like tight bass, a lot more details, much more natural sound, etc. It was very surprising hearing such difference for better.
Group has at least two concerts per week, often four, two per day if acoustic... Gear is stored in place with temperature from ~ -3 degrees C to + 30 degrees C, used outside live it can be in ambient from -10 degrees C to ~ +40 degrees C. This is big challenge for me, of course it takes a lot of thought getting it inside standard rack, allow for modifications, turning transformers, getting as needed in general (made many preamps myself), etc. I love it a lot anyway, sound is so much better after test on a concert and comparisons to mixer.
PSU is first thing i need to make, properly and small, hope to manage dual mono with one power trafo, i see no reason for separating phantom. Requirments are 290V/18mA HT, 6,3V/500mA heaters and 48V/10mA phantom per 1ch.
For HT i used TL783 as Gyraf's G9, Maida with LM317, capacitance multiplier with bipolar transistor-zeners into base and only a few times RC filters. Maybe one or two other similar with this.
1. TL783 needs cooling with heatsink, two quite big ~150u caps where smallest are snap in (not too useful for veroboard), datasheet show quite a few precautions against desctruction, simple discrete design instead IC can be advantage, minimum current for regulation might not be a problem with proper resistor.
http://www.boemboemklap.nl/diy/b_plus_psu.gif
2. Maida is more complicated, needs heatsink and again several precautions against shorts. Smaller components as advantage here because of space. I don't have much experience with it, beside making it for much less current than 18mA at similar HV. Datasheet: http://www.ti.com/lit/an/snoa648/snoa648.pdf
https://i.stack.imgur.com/pBaHV.jpg
3. Capacitance multiplier with bipolar tr. works well every time and has noise in uV, without heatsink. Attachment show it without cap values, normally they are 150u input, the rest 47u. Judging by simple RC filters and compare to cap multi it has two RC filters, not quite enough for what i would like, can output 47u filter enough to get me into low enough range to power one channel? Can't remember if input and output cap were both 150u and others 47u for 290V/18mA.
http://static.flickr.com/139/324693062_1bcc2f4ae2.jpg
4. Capacitance multiplier with IRF840 instead of TIP50 as suggested by TV. Circuit should be pretty small and simple, without heatsink, i don't expect output shorts, although it would be nice if it would survive them for short periods like 30 seconds max as TIP50 one does. I tried hard but couldn't calculate even aproximate gm of FET to find out myself how much it magnifies capacitance and pick right cap for the job. This are TV's propositions:
-Each R/C section (there are two of them - R1C2, R2C3) should have "corner" frequency of cca 1-2Hz. The "gate stopper" could be 330R-1K. Add the gate protection zener as well.
-You will have to calculate the first RC section with the Zener current in mind, i.e. the R1 will have to pass the proper current to the HV zener. R1 will have to be a 2W type.
-In theory, if you use 56K resistors, you could use 2,2uF for the C3 and 22uF for C2 (or possibly C4). C1 and C4 you will have to come up with your values.
5. PRR suggests simple RC filters for projects like this, says it takes about 1uF per mA and tells it is time proven concept. It probably means each such cap in any of three filters because 22u seems pretty low. So looking at classic like V72 it has 8uF after gretz, the rest are two RC filters at 2k5R/8uF.
V76 with gretz into 32uF two RC filters at 8kR/2x32uF-8k/32uF, HT current is 20,7mA, not that far away considering German engineers.
This is very simple, higher current output tube is taken from first RC filter at 8kR/2x32uF and 14,6mA/300V, the rest goes into other 3 EF804 tubes with total 6,1mA current and lower than 300V . I like this approach, 47uF caps are small, high quality and cheap, circuit can be small without big resistors. Ian's psu confuses me with RC filters consisting from 220uF caps for one channel and lower current than V76 or Redd47. Might be designed so to power more channels, can't find other reasons.
Heater and phantom are easy, small and cheap, so no problem to add them. It takes heatsink for something like LT1085 LDO regulator for heaters. I could use one with two regulators because each channel takes only 500mA, plus some extra for relay if needed, LEDs are in mA range.
Tried several drawings to get it into 2U rack, it doesn't seem possible with 4 tubes. Another thing to try is having two L profiles each for 2 tubes and two turret boards because they allow getting closer to tube socket and making current loop small and properly wired. This way secondary wires of IT into grids can be very short, front panel for wiring switches is close, output trafos can be between profiles. Tubes will need covers because of moving, despite tight pin Micalex sockets. Those take ~20mm more to get them out if put horizontally. I will most likely have to take max depth 3U, so i don't run out of space as it happened once with input trafo...
A lot of abuse is expected because of moving, compared to Chandler good build will last a lot longer, won't need all those extras to distract soundguy. I was able to find good ergonomics with several revisions, i'm sure original engineers designed for this too, instead of fader it will have output pot. One option is Bourns T-pad, i have two old back ones left, dual pot seems to interfere with the signal too much, also have a lot of mismatch in log curve. Short runs with 1k ohms pot ala API with two small value resistors (NYD wrote schematic) in front seems best option. Have to check it some more before deciding.
I know this is a lot of writing and ideas, if someone feels it would be better splitting between different posts i will do it sooner or later. Space between sections is left because for easier reading and no one will reply to all of it, feel free to respond with thoughts or experiences about any part. Similar examples are very welcome, i could only find some without OT, or other formats, builds meant for not much moving, etc.
As mentioned it is important challenge for me, so thanks for any comment.
i'm making another 2ch tube preamp (Redd 47), this time it is for really talented singer who benefits from such preamp, the other channel will be for different sources. I find this quite rare as most musicians only get better sound in the studio, if at all. Her voice also live, where letters in high frequencies like s, c and similar are finally as sang, beside other advantages like tight bass, a lot more details, much more natural sound, etc. It was very surprising hearing such difference for better.
Group has at least two concerts per week, often four, two per day if acoustic... Gear is stored in place with temperature from ~ -3 degrees C to + 30 degrees C, used outside live it can be in ambient from -10 degrees C to ~ +40 degrees C. This is big challenge for me, of course it takes a lot of thought getting it inside standard rack, allow for modifications, turning transformers, getting as needed in general (made many preamps myself), etc. I love it a lot anyway, sound is so much better after test on a concert and comparisons to mixer.
PSU is first thing i need to make, properly and small, hope to manage dual mono with one power trafo, i see no reason for separating phantom. Requirments are 290V/18mA HT, 6,3V/500mA heaters and 48V/10mA phantom per 1ch.
For HT i used TL783 as Gyraf's G9, Maida with LM317, capacitance multiplier with bipolar transistor-zeners into base and only a few times RC filters. Maybe one or two other similar with this.
1. TL783 needs cooling with heatsink, two quite big ~150u caps where smallest are snap in (not too useful for veroboard), datasheet show quite a few precautions against desctruction, simple discrete design instead IC can be advantage, minimum current for regulation might not be a problem with proper resistor.
http://www.boemboemklap.nl/diy/b_plus_psu.gif
2. Maida is more complicated, needs heatsink and again several precautions against shorts. Smaller components as advantage here because of space. I don't have much experience with it, beside making it for much less current than 18mA at similar HV. Datasheet: http://www.ti.com/lit/an/snoa648/snoa648.pdf
https://i.stack.imgur.com/pBaHV.jpg
3. Capacitance multiplier with bipolar tr. works well every time and has noise in uV, without heatsink. Attachment show it without cap values, normally they are 150u input, the rest 47u. Judging by simple RC filters and compare to cap multi it has two RC filters, not quite enough for what i would like, can output 47u filter enough to get me into low enough range to power one channel? Can't remember if input and output cap were both 150u and others 47u for 290V/18mA.
http://static.flickr.com/139/324693062_1bcc2f4ae2.jpg
4. Capacitance multiplier with IRF840 instead of TIP50 as suggested by TV. Circuit should be pretty small and simple, without heatsink, i don't expect output shorts, although it would be nice if it would survive them for short periods like 30 seconds max as TIP50 one does. I tried hard but couldn't calculate even aproximate gm of FET to find out myself how much it magnifies capacitance and pick right cap for the job. This are TV's propositions:
-Each R/C section (there are two of them - R1C2, R2C3) should have "corner" frequency of cca 1-2Hz. The "gate stopper" could be 330R-1K. Add the gate protection zener as well.
-You will have to calculate the first RC section with the Zener current in mind, i.e. the R1 will have to pass the proper current to the HV zener. R1 will have to be a 2W type.
-In theory, if you use 56K resistors, you could use 2,2uF for the C3 and 22uF for C2 (or possibly C4). C1 and C4 you will have to come up with your values.
5. PRR suggests simple RC filters for projects like this, says it takes about 1uF per mA and tells it is time proven concept. It probably means each such cap in any of three filters because 22u seems pretty low. So looking at classic like V72 it has 8uF after gretz, the rest are two RC filters at 2k5R/8uF.
V76 with gretz into 32uF two RC filters at 8kR/2x32uF-8k/32uF, HT current is 20,7mA, not that far away considering German engineers.
This is very simple, higher current output tube is taken from first RC filter at 8kR/2x32uF and 14,6mA/300V, the rest goes into other 3 EF804 tubes with total 6,1mA current and lower than 300V . I like this approach, 47uF caps are small, high quality and cheap, circuit can be small without big resistors. Ian's psu confuses me with RC filters consisting from 220uF caps for one channel and lower current than V76 or Redd47. Might be designed so to power more channels, can't find other reasons.
Heater and phantom are easy, small and cheap, so no problem to add them. It takes heatsink for something like LT1085 LDO regulator for heaters. I could use one with two regulators because each channel takes only 500mA, plus some extra for relay if needed, LEDs are in mA range.
Tried several drawings to get it into 2U rack, it doesn't seem possible with 4 tubes. Another thing to try is having two L profiles each for 2 tubes and two turret boards because they allow getting closer to tube socket and making current loop small and properly wired. This way secondary wires of IT into grids can be very short, front panel for wiring switches is close, output trafos can be between profiles. Tubes will need covers because of moving, despite tight pin Micalex sockets. Those take ~20mm more to get them out if put horizontally. I will most likely have to take max depth 3U, so i don't run out of space as it happened once with input trafo...
A lot of abuse is expected because of moving, compared to Chandler good build will last a lot longer, won't need all those extras to distract soundguy. I was able to find good ergonomics with several revisions, i'm sure original engineers designed for this too, instead of fader it will have output pot. One option is Bourns T-pad, i have two old back ones left, dual pot seems to interfere with the signal too much, also have a lot of mismatch in log curve. Short runs with 1k ohms pot ala API with two small value resistors (NYD wrote schematic) in front seems best option. Have to check it some more before deciding.
I know this is a lot of writing and ideas, if someone feels it would be better splitting between different posts i will do it sooner or later. Space between sections is left because for easier reading and no one will reply to all of it, feel free to respond with thoughts or experiences about any part. Similar examples are very welcome, i could only find some without OT, or other formats, builds meant for not much moving, etc.
As mentioned it is important challenge for me, so thanks for any comment.