Output Transformer advice for 6V6 Design

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, Jul 13, 2020.

  1. NSB_Chris

    NSB_Chris Tele-Meister

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    Hello All
    I would benefit from some opinions/advice on selection of an OT for another amp I will eventually build with a pretty straight forward 6V6 push-pull power section. I want to purchase the transformers now because I want to get a choke to play with a modification that I am doing with one of my old kit amps and I figure consolidating the orders may save on a little shipping cost.

    Some options I see are:
    Hammond 1760H 20W 6.6k (4, 8, 16 ohm)
    Hammond 1760F 15W 7k (4, 8, 16 ohm)
    Hammond 1750PA 18W 8.4k (4, 8, 16 ohm)
    Classic Tone 40-18045 15W 8.5k (4, 8 ohm)
    Classic Tone 40-18090 20W 8k (4, 8, 16 ohm)
    Classic Tone 40-18038 20W 6.6k (4, 8 ohm)

    I expect to use the Hammond 270EX PT, so my preference is to use a Hammond OT to purchase from the same distributor.

    The amp will have a decent amount of preamp gain/distortion I would most likely pair it with a Vintage 30 or a Warehouse 25W Green Beret, because I already have them.

    Difference would probably be subtle as I am not likely to get the amp up to full power often, but I would be interested in opinions/advice...
    Go with lower impedance or stay on the higher side toward 8k?
    Go for a lower wattage to get a little compression at higher output?

    What do you think?
     
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  2. printer2

    printer2 Poster Extraordinaire

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    I have the 1760H and the 1750PA. I have not had the chance to use them yet but the PA is bigger than the H, tossed them on a scale and got 970g to 712g. The H does have end bells on it so that can be an advantage if you are building in a way that it can be damaged, but the PA just looks to have more meat on its bones.
     
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  3. andrewRneumann

    andrewRneumann Tele-Meister

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    I'll be watching this thread! I haven't designed a push-pull yet so have many questions myself. On my SE design, I went with 5k because that's what the Champ had. But I regretted that after buying the OT. Once I dug into the tube datasheets and plotted some load lines, I wished I had gone up to something more near 8k.
     
  4. printer2

    printer2 Poster Extraordinaire

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    The voltage you run at also helps dictate your impedance. At lower voltages you would use a lower impedance transformer to get the most power out of the voltage you have. With higher voltages a higher impedance. But guitar amps do not necessarily follow the datasheet. Then there is the case of trying to squeeze every last watt out of a tube. I doubt most people would notice a 20% drop in power. Running them more conservatively would extend the life of the tubes.
     
  5. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Good points from @printer2!

    I have used the 1760E, 1760H for 6V6 PP so far. Both will work great, in your case I would probably go with the smaller OT.
     
  6. andrewRneumann

    andrewRneumann Tele-Meister

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    What’s the deal with the idea that the closer to red plating the better the sound? I don’t have clue as to why that would be, but some swear by it.
     
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  7. printer2

    printer2 Poster Extraordinaire

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    It could make the internals of the tube more microphonic. Not good in a preamp tube but at the end of the chain and not a whole lot of gain who knows. When metals heat up they loose some of their strength and are more flexible. Possibly they 'sing' more?
     
  8. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    I have used the 1750PA in a 5E3 type. Worked great with a Jensen Falcon speaker. The Jensen is kind of the opposite of the Green Beret, but it worked well with the dark 5E3 circuit.

    I think you could make a PP 6V6 work with either of those speakers. You would just have to voice the amp to the speaker.
    Since you will not be pushing the amp often, I think the 8k4 primary will help to get a little drive character when a 6k6 primary would not. Maybe a little boost on the front end will push the preamp drive so it could get cookin' at moderate volumes. I think you could create a killer 6V6 amp with the V30, but you may want the lower spl of the Green Beret.

    Looking forward to see where you will take this build.
     
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  9. NSB_Chris

    NSB_Chris Tele-Meister

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    I have made some attempts to plot the load characteristics for both the RR800V that I built which I really like and this project draft. I realize that the plate and screen voltages end up being high for our amps these days with SS rectifiers and available PT options but I didn't realize the calculated power output of the 6V6's was on the order of 30W. In Rob's write up for the RR800V he states that the power output is limited by the OT, but I thought he was referring to only the EL34 option. I think now he must have been indicating that both are limited, just more by the higher output EL34. I think now I am leaning toward the 1750PA.

    Draft configuration 6V6 tubes and 1750PA OT
    Plate =385V
    Screen = 381V
    Iq = 20mA
    Ra-a = 8400 ohm

    Load line intersects the 0V grid line at about 33V, 166mV
    6v6 draft.png


    My RR800V build with 6V6 tubes (I did not record the actual screen voltage)
    Plate =432V
    Screen = 428V
    Iq = 19mA
    Ra-a = 7000 ohm
    Load line intersects the 0V grid line at about 54V, 215mV
    RR800V II.png
     
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