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5f2a with a 6SL7 instead of an 12ax7

Discussion in 'Shock Brother's DIY Amps' started by Mongo Park, Nov 25, 2020.

  1. Mongo Park

    Mongo Park Tele-Afflicted

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    I want to build a 5F2A with a 6SL7 for the preamp. This will be a slow learning process for me I am wanting to use this as a try at drawing tube load lines for the 6SL6 tube. This would be the next quantum leap for me opening the door to ursine all kinds of tubes or just getting the commonly used tubes dialled in. I am using the most excellent information provided free of charge by Rob of Robrobinett fame.
    https://robrobinette.com/Drawing_Tube_Load_Lines.htm

    So the data sheet for the 6SL7 gives a max plate voltage of 300volts.

    https://frank.pocnet.net/sheets/049/6/6SL7GT.pdf

    below it gives an operating voltage of 250 volts. In an ideal setting would 250 volts be a good starting voltage to use. I want to amp to have some nice dirt on it and don’t want to set up the preamp tube to be clean. So would 250 volts get me there or do I want to start with some other voltage , like going t to the 300volts being a guitar amp.
    Cheers Ron.
     
    Last edited: Nov 25, 2020
  2. radiocaster

    radiocaster Poster Extraordinaire

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    The 6SL7 is somewhere in between a 12AX7 and a 12AY7, thus less gain and more current than a 12AX7, but not quite like a 12AY7.
     
  3. Mongo Park

    Mongo Park Tele-Afflicted

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    My plan would be to hot bias V1 and then cold bias V2.
     
  4. Mongo Park

    Mongo Park Tele-Afflicted

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    Thanks Radiocaster, I haven’t quite got to understanding the sonic effects of the various voltage/current combinations
    Cheers Ron.
     
  5. Mongo Park

    Mongo Park Tele-Afflicted

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    My next decision which may affect the plate voltage is the plate resistor, is something above 250k in the ball park.
     
  6. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    6SL7 is a very fine tube. The 5B2 Princeton used it as a preamp tube, but in grid-leak bias mode at the input. The 5751 is probably the closest 9-pin tube in characteristics.

    When designing a gain stage, you should decide how much gain you want first. That determines the plate resistor (usually about twice the plate resistance). Then decide your source voltage, which in the 5F2A is around 280 volts, comfortably under the 300 volt limit. Then use the cathode resistor to set the bias. Don't use the plate resistor to chase a particular idle plate voltage, that's doing it backward. :)

    If nothing else, a 6SL7 works nicely with the classic 100k plate resistor and 1.5k cathode resistor.

    Hope this helps.
     
    Last edited: Nov 25, 2020
  7. radiocaster

    radiocaster Poster Extraordinaire

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    Really it's probably the closest to a 12AX7, not sure you;d need to mod the circuit. If you put a higher plate resistor you'll lower the voltage, and I doubt you could get even close to 250V in V1, although I haven't researched the circuit that much.

    5751 has a similar gain factor, but less current than even a 12AX7. You might be able to get more distortion with a 5751.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I assume you mean the two different sections of V1.
    The 6SL7 has the same mu as the 5751 9 pin tube. (I see ThermionicScott beat me to it.)
    The plate resistor is a choice. IIRC, typical plate resistors for 5f2a type amps range from 100k to 470k.

    The 9pin tubes were designed to replace the older larger octal tubes. There is not a lot of difference in sound, other than what one hears with normal tube rolling IMO. The octals are known to be become microphonic easier due to their size. They do look nice when looking at them. lol.
     
  9. radiocaster

    radiocaster Poster Extraordinaire

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    To be fair, I'm not sure I understand them from theory either, just from tube rolling.
     
  10. Mongo Park

    Mongo Park Tele-Afflicted

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    Yes thanks I guess I should call it V1a and V1b.
    Well I have a nice glass 6SL7 and wanted to give it a try as an exercise of figuring out how to use different tubes. I most likely have a 6Sn7 with a metal shell. I can look in my bin and see what else is there.
    OK I will start around 208 volts,and when I draw this as load lines to get the cathode resistor will this reflect the gain. Sorry for asking such questions I am just starting out along this path.
    The whole idea is to draw the load lines so I can begin to use other tubes. I want to be able to get a small step beyond following someones schemattic.
    I will check the tube bin anyways.
    Thanks for the help.
     
  11. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Nice! And good on you for playing around with load lines, you can really understand what's going on a lot better once you have a handle on those.

    Merlin Blencowe has some great articles about designing tube amps on his site, if you haven't been there yet: http://www.valvewizard.co.uk/index.html

    Spoiler alert: once you've got a good set up for 6SL7s, you may be a little disappointed with 6SN7s in their place. They're also fantastic tubes (great for hi-fi and phase inverters), but the gain is so much less that your 5F2A may sound a little "blah."
     
  12. Mongo Park

    Mongo Park Tele-Afflicted

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    I do have some 6SJ7 tubes, I am sure there are 6SN7 9FD3907C-F7B3-4BA7-9849-9E046F3D8D42.jpeg
     
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  13. Mongo Park

    Mongo Park Tele-Afflicted

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    I have looked at his site, and others in trying to put it all together, I will visit it again. I will stick with the 6SL7 for now to go through the "process"
     
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  14. Nickfl

    Nickfl Friend of Leo's

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    Go thought the process and then compare the values you come up with to some of the classic, proven circuits. A lot of single ended valco amps used 6sl7 tubes. A couple of early fender amps did as well and many early fenders used 6sc7 tubes which are, aside from having the cathodes tied together internally, close enough in specs to be considered equivalent to the 6sl7.
     
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  15. Wally

    Wally Telefied Ad Free Member

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    I have never worked on an early tweed Princeton with the 6SL7, and the schematic does not give voltages. I will observe that the early Fender amps with octal on which I have worked ran voltages in the preamp that were much lower than the voltages Leo used later for the noval preamps. The 5B5 Pro I restored ran just under 60 volts to the plates in the preamp. Magnificent amp.
     
  16. Mongo Park

    Mongo Park Tele-Afflicted

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    Thanks for the advice. I am still poking away at it, as recommended I am going through the Balzac wizard info. Concentrate knowledge which needs more than one pass to soak it all up.
    I will l come back with something when I have it triangulated.
     
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  17. chas.wahl

    chas.wahl Tele-Meister

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    @Wally has real-life experience with these and I don't, but I will note that a circuit I've been interested in, the Ampeg Reverberocket from early 60s, uses both 6SL7s and 6SN7s (it has both onboard tremolo and reverb), and the voltage chart for the 6SL7s is: 310[!]/6.2 for preamp stage 1 (with trem inoperative), 150/1.8 driving trem, and for the paraphase PI pair 200/2.2 and 195/2.0. As pointed out above, the mu is 70 rather than 100. That amp is reported to be fairly "round" sounding, and one of the Ampegs that features more dirt than usual. Personally, I think that trying a 6SL7 as the preamp tube for a 5F2-A would be really interesting.
     
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