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Single ended Octal Amp

Discussion in 'Shock Brother's DIY Amps' started by Nickfl, Dec 29, 2017.

  1. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    I found another beat up old radio console to salvage on craigslist recently, an admiral 9B15 for $10. It was a single ended 6v6 amp and while the cabinet was in rough shape, the only thing it was missing was the speaker. I got all the tubes, including a grey glass 6v6, a 5u4g, and a 12sq7, along with a bunch of 7 pin rf tubes that I probably can't use for anything. I figured it was worth buying just for the tubes, but I've also got a PT, OT, and a choke and even some nice sockets. I've started thinking about what I could build with all of this and came up with an idea for a single ended, octal preamp tweed Princeton inspired thing. Here's my first draft of a schematic:

    Single Ended Octal build (1)-001.jpg

    As always, comments and suggestions are welcome!
     
    Snfoilhat and sds1 like this.

  2. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    I like it but am confused at something. You have two 6V heater tubes and one 12V? The 25k PS resistor might drop your voltage a tad.
     

  3. Teleterr

    Teleterr Friend of Leo's

    Feb 7, 2011
    Lewes De.

  4. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    Good point, somehow that didn't register in my head... The tube chart in the radio and the schematic I found both call for a 6sq7, but the tube I pulled out of the chassis is a 12sq7, so someone must have stuck it in there as a substitute at some point. I wonder if that worked ok? Anyway, I'm not going to try to run a 12 volt tube off 6volts in a new build. It seems like the only option for using the tube properly would be to build some sort of voltage doubler to supply that tube, which is probably not worth the effort for one old tube that has been abused for who knows how long with incorrect heater voltage. Or a seperate 12v transformer, also not worth it. Especially since it looks like NOS 6sq7 tubes can be had for $5.

    So the question is, should I still use that tube (or rather its 6v equivalent) in that position? I chose it because I had one, but if I'm going to have to source something else anyway maybe there is a better choice?
     

  5. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    6J5 would be a popular choice, and cheap. Check old Gibson schematics for them. (It's half of a 6SN7). Otherwise, a dual triode could get you another input, like a 6SL7. They've started making them again too.
     
    Nickfl likes this.

  6. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    The 6sq7 has a gain of 100 (theoretical, in circuit it is lower) and is the right tube for what you are doing (in my mind). The 6J5 is used to drive output tubes but it only has a gain of 20 and you may find it lacking.
     

  7. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Not really, I have two Gibson GA-20's from 1948 and the only gain stage is the 6SJ7 going into a 6J5 paraphase. You'll get plenty 'Tweed' if that's what you are looking for.
     

  8. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    Yeah, that is kind of what I was thinking as well. Looks like the 6sq7 is more or less half of a 6sl7 with a couple of diodes included so it would work best in the schematic I posted.

    However, Fender Lover's mention of a second input got me thinking... I'm now considering the possibility of using a 6sl7 and using half of it as a cathode biased input. So, I will have both a grid biased pentode input and a more modern cathode biased triode input. Another thread recently mentioned the idea of combining both cathode biased and grid biased inputs in one amp and I thought it was a cool concept. I'm going to work on an updated schematic and I'll post it later.

    Edit: It was @screamin eagle 's post in this thread that gave me the dual input type idea.
     
    Last edited: Dec 30, 2017

  9. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    Here's and updated schematic with the second cathode bias input. I also measured the resistance on the salvaged choke and did the math on the power supply to confirm my B+ and plate voltages.

    Single Ended Octal build, dual input version.png

    If anyone sees any problems, please let me know. I would particularly appreciate comments on the way I am bringing the two inputs together before the volume and tone controls. I just put in some mixing resistors instead of adding another volume control, but I suppose I could do a three knob layout and put it in a tweed deluxe size chassis.
     

  10. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    If you are only going to run one input at a time then the one volume control is not an issue but I would go to a 470k mixers. I wouldn't go with a common cathode resistor for the two stages, in parallel stages maybe but not in series. Just looking for instability in my mind. Also you want to take your second stage grid from the middle wiper of your volume control not the top of it.
     

  11. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    Good catch on the volume control, I was pulling the tone stack directly from the 5b2 schematic but didn't notice that I got that part wrong. I corrected that and implemented your other suggestions. I went ahead and removed the bypass cap from the second stage, since I figured it was probably unnecessary there (and easy enough to add one later if I change my mind). I also decreased the size of the bypass cap on the cathode biased input.

    Single Ended Octal, dual input v2.png
     

  12. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Oh, sorry, missed the tone control caps. You have them reversed.
     

  13. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    You're right agian, this simple control schematic is really giving me a hard time for some reason!

    Single Ended Octal, dual input v3.png

    I think I have the schematic pretty much were I want it, next I need to test the PT and OT and confirm they still work and will suit this build.
     

  14. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    The PT checks out good with no shorts and ballpark correct secondary voltages, I'm getting 279-0-279 on the unloaded HT secondary, which is about what I'd expect for the 260v loaded spec.

    The choke appears to be good too, 125 ohm DC resistance and a rough measurement of 4.6hy using the method using a 3.75K resistor described halfway down this page http://www.diytube.com/phpBB2/viewtopic.php?t=3334 Which is about what I would have expected based on the size and original circuit and it should be perfect for my build.

    I still need to check the OT, the bolts were pretty well frozen in place on that one and I couldn't find the right size nut driver or socket to break it free so I'll have to come back to that.

    Overall this is looking pretty promising for my next project, it looks like between this radio and my parts bin I will have everything I need except for the chassis and some of the small parts like caps and pots!
     
    awasson likes this.

  15. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    I got the OT out and tested it this evening. It appears to have a turns ratio of 37.5 to 1 so an impedance ratio of about 1400 to 1. I'm assuming it was meant to be used with a 4ohm speaker in the original radio as that would give a primary impedance of about 5.6K which seems like about what radio amp designers would have been looking for with a SE 6v6 amp. I was hoping to use it with an 8 ohm speaker I already have, which would give me a primary impedance of about 11K. From what I have read the primary impedance recommendations for early SE 6v6 fenders are all over the place, from the tube manual recommended 5K all the way up to the 17K classic tone apparently measured with a vintage champ OT they reverse engineered.

    So does anyone have any idea what the primary impedance on the early tweed princetons might have been? I am using the 5b2 and 5c2 princetons as inspiration for this build, but I am having a hard time finding transformer info on those.
     

  16. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    8k from what I find using Google, if it is a 5k then an 8 ohm should be fine. Depends on the plate voltage, if you are around 250V then you might want to use the 5k impedance.
     

  17. Nickfl

    Nickfl Tele-Holic

    918
    May 24, 2016
    Florida
    You're right, 8K looks pretty good as a compromise for a commonly available value.

    I played with some load lines and it looks like ideally I would want around 6.8K for center biased class A. I'm estimating 285v plate to cathode and 280 on the screen, with around 16 volts across the cathode resistor (I've got a 500ohm resistor on the schematic, but it should probably be 360ohm).

    Unfortunately the 11K I'd get from the salvaged transformer and the speaker I want to use looks less than ideal. I'd like to use the transformer from the old radio, but a new 8K single ended OT is only about $20 and I already have an 8ohm Weber 12A125 speaker sitting in a box that would be perfect for the build so it makes more sense to just get a new OT.

    Here are the output transformers I will consider for this amp (just listing them for my own future reference when I get around to actually starting the build):

    Classic Tone 40-18030
    Edcor XSE10-8K
    Weber W022905M

    Edcor is the cheapest (and probably the best made), but a long lead time and they don't sell anything else I'd need so I can't combine shipping with other parts. Weber is oddly the most expensive, but they do sell other parts I'd need and that transformer has a 7K primary, so it is theoretically a slightly better match. Classic tone falls in the middle on all fronts and probably won't be my choice this time around.
     

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