Like the thread title says, I'm hoping someone can tell me which (if any) 7-pin miniature pentode tubes are equivalent or very close to the old 6SJ7 metal can, octal pentode.
Usage is for the input tube in a guitar amplifier. 'Tube flavor', 'tone' and reasonably low noise are the goals, not low distortion.
6AU6A?
6CF6?
6CB6?
5879? (that's a 9-pin mini...)
Others?
The circuit I'm working with has B+ = 300V, Rp = 100k, Rg2 = 1M, Cg2 = 0.22uF, Rk = 2.2k, Ck = 22uF.
Gain should be about 60X.
Or is the answer that any small signal, sharp cutoff pentode would sound the same if set up to make 60X gain with about 1mA of plate/screen current?
Usage is for the input tube in a guitar amplifier. 'Tube flavor', 'tone' and reasonably low noise are the goals, not low distortion.
6AU6A?
6CF6?
6CB6?
5879? (that's a 9-pin mini...)
Others?
The circuit I'm working with has B+ = 300V, Rp = 100k, Rg2 = 1M, Cg2 = 0.22uF, Rk = 2.2k, Ck = 22uF.
Gain should be about 60X.
Or is the answer that any small signal, sharp cutoff pentode would sound the same if set up to make 60X gain with about 1mA of plate/screen current?
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6AU6 seems the best.
Other options (not taking care of linearity, being for guitar):
https://frank.pocnet.net/sheets/129/e/EF93.pdf
https://frank.pocnet.net/sheets/129/e/EF95.pdf
Other options (not taking care of linearity, being for guitar):
https://frank.pocnet.net/sheets/129/e/EF93.pdf
https://frank.pocnet.net/sheets/129/e/EF95.pdf
Thanks!
The US equivalent of EF95 is 6AK5, which is actually cheap and easy to get.
The US equivalent of EF93 is 6AB6, which is a bit more expensive, but not too bad.
6AU6 has more gain than 6SJ7, and it seems make about twice the distortion I get from 6SJ7 at similar operating points. But that distortion would be almost pure 2nd harmonic, so it might actually sound good. I'm not going for a distorted rock or blues sound. I'm actually going for a cleaner but still 'warmed up' sound for jazz guitar, so I'm a little worried about THD, I guess... But not overly much.
The US equivalent of EF95 is 6AK5, which is actually cheap and easy to get.
The US equivalent of EF93 is 6AB6, which is a bit more expensive, but not too bad.
6AU6 has more gain than 6SJ7, and it seems make about twice the distortion I get from 6SJ7 at similar operating points. But that distortion would be almost pure 2nd harmonic, so it might actually sound good. I'm not going for a distorted rock or blues sound. I'm actually going for a cleaner but still 'warmed up' sound for jazz guitar, so I'm a little worried about THD, I guess... But not overly much.
The 6AU6 is the closest and was the tube that took the 6SJ7's place in the venerable 5 tube AM radio. There is a long list of tubes that are pin compatible. See the UNSETURATOR tab in my long running "spreadsheet of tubes." All of the 7 pin tubes listed have been tested in the UNSETURATOR circuit and verified to work, including the 6AU6 and 6AK5.
I have been experimenting with "reinventing the guitar amp" using something other than the usual FMV clone circuits. There will be at least one preamp channel in whatever I build that uses a pentode in the first stage.
There was a Hundred Buck Amp Challenge contest here that ran back in 2011. My low buck entry used a pentode input stage. After the contest ended, I proposed an elimination of the "no sand" rule and suggested that the thread be allowed to remain a place to post low cost guitar amp designs. It is now a sticky at the top of the instruments and amps forum. The 4 tube low buck design got refreshed with some design changes some silicon and reposted. It is the only working guitar amp that I still have from that challenge. The schematic of the amp is included. The 18FW6 is a 6AU6 with an 18 volt 100 mA heater. R9 is a pot, and it directly controls the gain of the input stage as the load resistor R5 is bootstrapped and appears to be nearly infinite. Many pentodes are microphonic at large gain values since there are three wire grids instead of one. This amp can go from very clean to overdriven mayhem at about half throttle on a 1 meg pot. I stuck a 2 meg pot in mine and I get acoustic feedback from the speaker to the amp head at full throttle. Talking into the back of the cabinet puts your voice in the speaker with some strange sounding reverb effects. I called this circuit the saturator since the input tube can be fully saturated from any guitar I tested at less than full gain.
I devised another circuit I called UNSET which was intended to get triode like curves from a TV sweep pentode without violating the screen grid specs. In the just to see what would happen department, I blended the saturator and the UNSET together into one stage and created the UNSETURATOR. I was expecting overdriven guitar amp mayhem, but was surprised with the cleanest vacuum tube gain stage that I have ever made. Gains in the 50X range can be obtained at THD numbers under 0.2%!
Some testing here:
I have been experimenting with "reinventing the guitar amp" using something other than the usual FMV clone circuits. There will be at least one preamp channel in whatever I build that uses a pentode in the first stage.
There was a Hundred Buck Amp Challenge contest here that ran back in 2011. My low buck entry used a pentode input stage. After the contest ended, I proposed an elimination of the "no sand" rule and suggested that the thread be allowed to remain a place to post low cost guitar amp designs. It is now a sticky at the top of the instruments and amps forum. The 4 tube low buck design got refreshed with some design changes some silicon and reposted. It is the only working guitar amp that I still have from that challenge. The schematic of the amp is included. The 18FW6 is a 6AU6 with an 18 volt 100 mA heater. R9 is a pot, and it directly controls the gain of the input stage as the load resistor R5 is bootstrapped and appears to be nearly infinite. Many pentodes are microphonic at large gain values since there are three wire grids instead of one. This amp can go from very clean to overdriven mayhem at about half throttle on a 1 meg pot. I stuck a 2 meg pot in mine and I get acoustic feedback from the speaker to the amp head at full throttle. Talking into the back of the cabinet puts your voice in the speaker with some strange sounding reverb effects. I called this circuit the saturator since the input tube can be fully saturated from any guitar I tested at less than full gain.
I devised another circuit I called UNSET which was intended to get triode like curves from a TV sweep pentode without violating the screen grid specs. In the just to see what would happen department, I blended the saturator and the UNSET together into one stage and created the UNSETURATOR. I was expecting overdriven guitar amp mayhem, but was surprised with the cleanest vacuum tube gain stage that I have ever made. Gains in the 50X range can be obtained at THD numbers under 0.2%!
Some testing here:
Attachments
Hey George! Thanks for your very informative (as always) reply.
I'm thinking of replicating a ca. 1960 Gibson GA50 in a small 'head' using a Hammond 14" x 6" x 3" chassis. The GA50 used a 6SJ7 into a weird tone control circuit, into a pair of 6J5 configured as a paraphase splitter, driving a pair of 6L6GA. It also made use of plate-grid parallel NFB around the output stage (the only guitar amp I've seen with that). The B+ is 365VDC, but I'll take whatever I can get from a junkbox power transformer as long as it's in the ballpark. I don't want to spend money when I have suitable parts on the shelf. I also have a Fender Deluxe OPT to use. It's relatively small and light, which is a plus in this situation. I only need to get about 20W of output.
I was thinking of using a 6AU6A as the input instead of 6SJ7, and a 6FQ7 as the phase splitter instead of the two 6J5s (or an octal 6SN7). I'll want to use 6P3S tubes instead of the 6L6GAs.
I'm not sure I want to go down a more complicated route with something like your UNSETURATOR -- as cool as it is. I was thinking of keeping it simple, simple, simple.
The thing I'm least sure of is whether or not I should retain the strange GA50 tone control circuit. I could probably do just fine with a simple tone control (treble cut only). I briefly tried fitting a James tone control circuit but I ran into some problems. I'll need to go back and figure out what I was doing wrong.
Here's what I have so far. It's still basically the original GA50 circuit.
The 'partial' NFB around the output stage only really works on the high frequencies. I think that's a lot of what makes this amp sound the way it does. It never goes into rock n roll breakup, and that's fine with me. It's dark sounding, with very little treble boost because the gain at higher frequencies is attenuated by the NFB. That's also fine with me.
I've attached the original Gibson GA50 schematic (PDF).
I'm thinking of replicating a ca. 1960 Gibson GA50 in a small 'head' using a Hammond 14" x 6" x 3" chassis. The GA50 used a 6SJ7 into a weird tone control circuit, into a pair of 6J5 configured as a paraphase splitter, driving a pair of 6L6GA. It also made use of plate-grid parallel NFB around the output stage (the only guitar amp I've seen with that). The B+ is 365VDC, but I'll take whatever I can get from a junkbox power transformer as long as it's in the ballpark. I don't want to spend money when I have suitable parts on the shelf. I also have a Fender Deluxe OPT to use. It's relatively small and light, which is a plus in this situation. I only need to get about 20W of output.
I was thinking of using a 6AU6A as the input instead of 6SJ7, and a 6FQ7 as the phase splitter instead of the two 6J5s (or an octal 6SN7). I'll want to use 6P3S tubes instead of the 6L6GAs.
I'm not sure I want to go down a more complicated route with something like your UNSETURATOR -- as cool as it is. I was thinking of keeping it simple, simple, simple.
The thing I'm least sure of is whether or not I should retain the strange GA50 tone control circuit. I could probably do just fine with a simple tone control (treble cut only). I briefly tried fitting a James tone control circuit but I ran into some problems. I'll need to go back and figure out what I was doing wrong.
Here's what I have so far. It's still basically the original GA50 circuit.
The 'partial' NFB around the output stage only really works on the high frequencies. I think that's a lot of what makes this amp sound the way it does. It never goes into rock n roll breakup, and that's fine with me. It's dark sounding, with very little treble boost because the gain at higher frequencies is attenuated by the NFB. That's also fine with me.
I've attached the original Gibson GA50 schematic (PDF).
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