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  • #31
    FWIW schematic attached re: what I was talking about using just the one input jack. Note the lowered coupling cap value(s) for the other channel as well. Or you could even just leave the other plate resistor at 100k and see what tonal variations arise with that.

    Or if you maybe want a 2nd input, see dumb idea V2 using 2 input jacks, a DPDT switch, and the usual 5E3 compliment of tubes and knobs that I came up with the other day (which I posted somewhere on another thread at the time). 2CW

    Take all this with a pinch of salt BTW, these are just suggestions.
    Attached Files
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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    • #32
      Success!

      OK, I jumped the input to the grid on the second side of the input tube and reconnected the associated circuitry I had previously removed. I still have some serious hum that I need to deal with. That said, I love the sound, especially withj the gains cascaded. I can see this replacing my pedalboard and Pro Reverb for gigs--just guitar-wire-amp!
      The amp was star grounded; I relocated the preamp and filter cap grounds to another chassis connection, probably need to get that filter cap ground away from the preamp ground now that I think of it. I re-routed the OT secondaries away from the filament wires. If I still have the hum issues after separating the filter cap grounds from the preamp grounds I'm thinking about trying the artificial center tap. Should I ground this or elevate it with voltage from pin 8 of the power tubes? I've seen both suggested. I think the CT was elevated when I started, i just didn't understand it.
      Macdillard, there's no resistor at pin 2, just heat shrink. Ohmed out at 0.
      Tubeswell, I'll get some resistors ordered for next week. No electronic parts nearby.
      Bruce, finding a 12AY7 around here is about like finding an honest politician. I can't even buy a 12AX7 within 70 miles. I'll place an order next week.
      I rechecked the volume controls against the layout diagram. It seems all the adjustment is in the first 20% of the pot; after that the hum increases dramatically but the amp volume doesn't change much if any. I need to check the pots to see if they're log pots I guess. Anything else to look at?

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      • #33
        The artificial CT for the filament will help a lot as to the noise. Some say that elevating them by the 20 volts does not help much. Needs to be higher voltage but I just go ahead and do it on my builds. Sure does not hurt anything. Grounding, well all have their favorite method. I generally go with buss grounding system and ground the buss to the chassis right at the input jack. Mains grounding on its on ground lug where the mains enter the chassis. Mostly I use a modification of Hoffmans system that you can find over on the el34 site or do a search for hoffman amps. The last high gain amp I built I actually had three ground points to the chassis and that is suppose to be a real no no. I had one ground for the mains, one ground for all of the ct's, the power tubes cathode ground and the ground for the 1st and second filter caps. This ground was right at the first filter cap. Then a third ground at the input jack for rest of the grounds, pre amp, input jacks etc.. It works very well. The amp is very quiet. You would think that it would have ground loops but it doesn't. If you want to see it go to www.ddawgamps.com , gallery, then Mac's Lead II. Anyway, what works works.
        Last edited by macdillard; 08-08-2010, 01:07 PM.

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        • #34
          Wow!

          Thanks again everyone for the help. I sorted out the grounds and elevated the heater supply and have ended up with a great little amp. It's very quiet now and the volume responds normally. I'll definitely play around with some resistor changes in the preamp section and get a proper 12AY7 in V1 but I'm already in love.
          Now that I've learned a little, I'm going to go back and rewire some errors on my first build, an STF Champ.
          I also have another 5E3 style amp where the builder just wired all the components flying above the chassis with no tag board. It's 2 6V6 and 3 12 AX7 and a 5Y3; any ideas what I might do with that extra preamp tube?
          Thanks again; you guys are great!

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