I recently got a box of old NOS RCA tubes, with 2 each of 75, 76, 42 and 80. I was thinking a small SE 42 amp could be fun to try, for a whopping 4 watts in pentode mode.
The 76 is apparently a very nice tube, maybe a little low gain if I'm going to use feedback or try the 42 in ultra-low power triode mode.
That leaves the 75 with a mu of 100, but with very low current capability, 1 mA seems to be the limit, with a total of 2.5W dissipation. Can the 75 be suitable as a driver for the 42? I've tried plotting some different load lines but as a total tube design noob (I've only built other's designs) I'm not sure if it's feasible.
The 76 is apparently a very nice tube, maybe a little low gain if I'm going to use feedback or try the 42 in ultra-low power triode mode.
That leaves the 75 with a mu of 100, but with very low current capability, 1 mA seems to be the limit, with a total of 2.5W dissipation. Can the 75 be suitable as a driver for the 42? I've tried plotting some different load lines but as a total tube design noob (I've only built other's designs) I'm not sure if it's feasible.
Ri=91k, so anode load resistor must be equal or greater than 270k.
Suggested anode voltage is 250V, current 0.8mA, so B+ at least
250 + 216= 466V.
Output impedance is 68k.
It's not a forcefull driver.
p.s. Similar than 6SQ7, 6SQ7GT.
Suggested anode voltage is 250V, current 0.8mA, so B+ at least
250 + 216= 466V.
Output impedance is 68k.
It's not a forcefull driver.
p.s. Similar than 6SQ7, 6SQ7GT.
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So that's almost 12AX7 territory. I could try running it at a lower voltage, The Tung-Sol datasheet also suggest another operating point with a 100V plate voltage and a 250K load resistor. The voltage gain is lower but still a fair bit more than the 76, and I could possibly get away with the 300V B+ I had planned for the 42s.
Thanks for the reminder about the 6SQ7, I've seen it mentioned as an equivalent but I haven't really looked it up 🙂
Thanks for the reminder about the 6SQ7, I've seen it mentioned as an equivalent but I haven't really looked it up 🙂
There are 100's of old radios with that combination, e.g. this one picked at random
https://www.radiomuseum.org/r/republic tl6c
The 2A6 is similar, and I have this circuit which is meant to be highly regarded, but cannot remember where I sourced it ...
https://www.radiomuseum.org/r/republic tl6c
The 2A6 is similar, and I have this circuit which is meant to be highly regarded, but cannot remember where I sourced it ...
Attachments
I know I've seen that 2A6 schematic before somewhere, but I couldn't find it now. Around 300V on the 2A6, so it definitely looks like something I could use, thanks!
I use a 6SQ7 to drive an EL34 and it sounds really nice.
I'm doing a YouTube build series on that amp right now. I think a 75 would work just as well with a few limitations. I'm running them fairly hot at 250V -1.8V bias with 1.25-1.33mA of current, they don't seem to have any problem running there. The 6SQ7 does have a higher max plate voltage of 300V vs the 75 at 250V, the diode is rated at 1mA but the plate can handle more AFAIK. As long as you are driving a tube that can run with a 500K grid leak resistor and doesn't need a ton of swing, you should be fine.
BTW the 6SQ7 datasheet has a nice chart of operating points/resistors to use for different loads and the gain needed.
Skunkie Designs EL34 6SQ7
I'm doing a YouTube build series on that amp right now. I think a 75 would work just as well with a few limitations. I'm running them fairly hot at 250V -1.8V bias with 1.25-1.33mA of current, they don't seem to have any problem running there. The 6SQ7 does have a higher max plate voltage of 300V vs the 75 at 250V, the diode is rated at 1mA but the plate can handle more AFAIK. As long as you are driving a tube that can run with a 500K grid leak resistor and doesn't need a ton of swing, you should be fine.
BTW the 6SQ7 datasheet has a nice chart of operating points/resistors to use for different loads and the gain needed.
Skunkie Designs EL34 6SQ7
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Yeah, the pinout is different but likely could use the same circuit for either. Shame they don't share the same pinout for the triode section!
@stephe :
Thanks for the tip about the 6SQ7 chart! I saw the thread about your amp and was meaning to ask you if you’ve tried it at lower voltages. Looks like I can get something useful out of it with 300V B+.
Thanks for the tip about the 6SQ7 chart! I saw the thread about your amp and was meaning to ask you if you’ve tried it at lower voltages. Looks like I can get something useful out of it with 300V B+.
Be nice if every tube data sheet had that 🙂 The RCA manuals have a neat chart like this for most tubes too. These tubes don't have a lot of info on using them in modern designs, but they sounds GREAT.