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    You sure about that? The pictures of JJ 7591s...

    You sure about that? The pictures of JJ 7591s that I can find look like they have beam-forming plates inside, not a suppressor grid. ;)

    As far as the old ones went, perhaps they were like...
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    Yep, generally best practice to design around the...

    Yep, generally best practice to design around the tubes that are being made now, not those of yesteryear, unless you have a stash of the good stuff. ;) And at the risk of sounding unromantic, the...
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    Wow, still a problem with other OTs... I was so...

    Wow, still a problem with other OTs... I was so sure the OT would be the culprit!

    I like Enzo's heat gun/hairdryer solution if you suspect stray currents on the board -- might be worth a try.
    ...
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    That's interesting! You've discovered another...

    That's interesting! You've discovered another reason for Fender to abandon that approach in favor of grid-stoppers. :thumbsup:

    Now you've got me wondering if my cap was in fact causing parasitic...
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    Yeah, I think you're right. It's always risky...

    Yeah, I think you're right.

    It's always risky trying to pin down someone else's thought process, but my thinking has been that Fender saw enough instability with high-gain PIs like the 6SC7-driven...
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    If you can hear a high-frequency squeal, that's...

    If you can hear a high-frequency squeal, that's one, if there's a "mysterious" loss of volume but a scope shows activity well above the hearing range, that's another. :)

    A cap between the plates...
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    What's the rating of your 1kΩ screen stopper? An...

    What's the rating of your 1kΩ screen stopper? An EL34 (I keep forgetting that that's what you have in the amp) can be expected to draw more screen current than a 6V6.

    P.S. I dug through some...
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    That sucks! :( It occurred to me later on,...

    That sucks! :(

    It occurred to me later on, while in a meeting, that you wouldn't need a "Champ" OT to tell if it's the problem. If you wire up some generic 4KΩ 40W OT and get more than a watt or...
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    Since Pete brought up that the Fender LTP...

    Since Pete brought up that the Fender LTP originally used 12AX7s, it's worth noting that the original circuit also used a 47pF capacitor across the plates to quell HF oscillation. Although Fender...
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    I'm not surprised that the DC component of the...

    I'm not surprised that the DC component of the plate voltage would always be a couple volts lower than the screen voltage in this design, but so what? It's only when the plate voltage swings close...
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    Hmm, I got the impression that the amp hummed...

    Hmm, I got the impression that the amp hummed badly enough to make the amp intolerable before goofing around with V1 in the first post.

    Regardless, can we assume that the amp hasn't been in a shop...
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    Maybe, but it's always best to take care of the...

    Maybe, but it's always best to take care of the simple stuff and make sure the amp is healthy before doing "creative" things with it. ;) You said it hums whether V1 is plugged in or not, right? Have...
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    A silverface Champ running at 380V rather than...

    A silverface Champ running at 380V rather than 420+ is something to celebrate, not "fix." :)

    Similarly, it's no big deal if the screen idles a couple volts above the plate. Hardly any screen...
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    One thing that comes to mind is that V1B and V2B...

    One thing that comes to mind is that V1B and V2B share a cathode resistor and capacitor, so if V1 is pulled, the second gain stage in the Normal channel is no longer biased properly. Not saying...
  15. I dig prototyping on breadboards, too. No chance...

    I dig prototyping on breadboards, too. No chance of UL certification with this one, though! ;)

    http://i36.photobucket.com/albums/e26/sstulken80/6AU6%20Amp/tested_800.jpg
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    Shh! I wanted to get EFK to walk us through his...

    Shh! I wanted to get EFK to walk us through his thought process. ;)
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    Are you sure that it's a lack of preamp voltage...

    Are you sure that it's a lack of preamp voltage sag that's causing the stiff sound?
  18. I've got a big box of octal relay sockets, but...

    I've got a big box of octal relay sockets, but wasn't sure if they were heat-rated for tubes or not.
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    The Jim Kelley FACS did it that way, too. If...

    The Jim Kelley FACS did it that way, too.

    If it were my amp, I'd probably switch in some high-value cathode resistors rather than leaving them completely disconnected in order to stave off cathode...
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    Let's see... my first thought was to wire one of...

    Let's see... my first thought was to wire one of the output tube sockets to the filament transformer, but then you'd be over its limit.

    I suppose you could try the old PT with 4 KT88s on it and...
  21. Perhaps we should start encouraging the use of...

    Perhaps we should start encouraging the use of radial-leaded power resistors too. I hate the look of big axial-leaded power resistors wired to eyelets or turrets that are too close together for...
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    You wouldn't want to wire the filament...

    You wouldn't want to wire the filament transformer in parallel with the larger PT's filament winding. Just use the extra filament transformer to power the preamp filaments by themselves. Easy-peasy.
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    I'm seeing a lot more (well, when the user can...

    I'm seeing a lot more (well, when the user can figure out how to upload a photo) newbie builds with components joined together in midair, and long uninsulated component leads right next to other...
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    You *could* know, couldn't you? A carbon trace...

    You *could* know, couldn't you? A carbon trace on a tube socket would be ________, right? (Fill in the blank with an electrical characteristic that could be measured.) ;)
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    As I understood it, capacitor manufacture has...

    As I understood it, capacitor manufacture has advanced quite a bit in the last few decades and the only reason Sprague Atoms are still big on the outside is to satisfy the vintage-weenies. Can you...
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    Trace it out, man! It's fun and rewarding! ...

    Trace it out, man! It's fun and rewarding! :thumbsup:
  27. +1 on the film caps if you really want to drive...

    +1 on the film caps if you really want to drive your vintage-correct buddies up the wall. I'm strongly considering going that direction once I've used up my current stock of electrolytics. ...
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    This guy? Realistic Carnival 6V6GT Ampl/Mixer...

    This guy? Realistic Carnival 6V6GT Ampl/Mixer Radio Shack Tandy,

    I haven't found a good schematic yet, but it ought to be simple enough to trace out yourself. I think that's a good exercise that...
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