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  • Justin Thomas
    replied
    Here's what I cooked up for the inputs...
    I can do the rest once I get it twiddled to perfection.

    Justin
    Attached Files

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  • Justin Thomas
    replied
    As far as I can tell, yes I can. The switch selects either Both Channels For Me or Either Channel for Me/A Channel Each For Two. Of course, neither is big on power or "dialable distortotronics." [PATENT PENDING!] I mostly made it to have different options as a solo thing or to share with a friend. And I still have my Concert & Bassans for all that clean stuff.

    I do feel like there's still some "interaction" when I've got it set up as a solo-player & I'm using both channels; I'm not sure how to isolate them using just 2 input jacks & a slide switch; I guess I could draw up a schem of how I did it, for public scrutiny. And there's not much room left for extra parts, cuz True PTP is fun!

    I did make a hole in my Bassman cab for two jacks & separated the speakers into L & R; don't tell anybody! But I put the hole behind the original serial number plaque & used an old Crate cab jack plate. I can put the Fender badge over the hole & it'll look original if I ever sel it.

    I'm more partial to the side with the cathodyne. And my previous favorite build also had a cathodyne, driving EL34s. I think I like that design. Just seems crisper.

    Jusrin

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  • Chuck H
    replied
    Originally posted by Justin Thomas View Post
    Still plonking away with this thing. I abandoned the whole dual power supplies as unnecessary, since one seems to work fine. I gave it 4x6V6 to free up heater current also.

    Two channels, each with a triode & pentode, and a different PI per channel. One is a simple Fender Harvard but with a 6U8 pentode at the front, & the other is more like a Deluxe Normal channel but with an EF86 2nd stage. The LTPI is also using a 12AU7.

    Both are tweaked a tiny bit, mostly for smaller caps to trim bass. I also managed to rig up a way to switch between using both channels in parallel myself or having 2 players use one each, separately. I'll figure out how well that works when I get someone else to play with. Otherwise, it seems to work quite nicely. Still loud, but some warm overdrive in there, especially on the Harvard channel. Nine whole tubes... I'm proud of myself. Thanks to everyone here for 20 years of education!

    Justin & Jusrin
    Can you parallel the channels? That would be my first thing!!! Set up the one with the best OD for some grit and set up the one with the best clean for definition and use any kind of Y splitter to use them simultaneously. Stevie Ray did a lot of that sort of thing for both recording and live, but using multiple amps. Ground loops and hum along with additional noise, etc. Totally incidental to him

    So?..

    I always thought that the ability to do this sort of thing in the same amp would be cool. You have two amps in there. You've got to at least try it.

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  • Justin Thomas
    replied
    Still plonking away with this thing. I abandoned the whole dual power supplies as unnecessary, since one seems to work fine. I gave it 4x6V6 to free up heater current also.

    Two channels, each with a triode & pentode, and a different PI per channel. One is a simple Fender Harvard but with a 6U8 pentode at the front, & the other is more like a Deluxe Normal channel but with an EF86 2nd stage. The LTPI is also using a 12AU7.

    Both are tweaked a tiny bit, mostly for smaller caps to trim bass. I also managed to rig up a way to switch between using both channels in parallel myself or having 2 players use one each, separately. I'll figure out how well that works when I get someone else to play with. Otherwise, it seems to work quite nicely. Still loud, but some warm overdrive in there, especially on the Harvard channel. Nine whole tubes... I'm proud of myself. Thanks to everyone here for 20 years of education!

    Justin & Jusrin

    Leave a comment:


  • Chuck H
    replied
    Originally posted by Justin Thomas View Post
    Just gotta remember to fetch another OT from another carcass in the attic.
    We should coin a phrase for this sort of thing. I'll nominate "grave robbing".

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  • Justin Thomas
    replied
    So I did some pinout checking & found the 7687 & a 6HL8 (I think; maybe 6LH8) I had on hand were the same pinout. Both sounded just as good as any of my 6U8As, & that 7687, zero hum, even at 10. It's an old pull from that Stromberg-Carlson PA that was my first build. Wow, what tube! The 6LH8 was nice, too.

    Time to get away from the same-old 12A_7s! Loads of cheap tubes out there - most of what I have on hand are cheaper than a JJ 12AX7!

    I've elected to stay with 4x6V6s to spare the heater current, so I can goof around with more preamp tubes. Just gotta remember to fetch another OT from another carcass in the attic.

    Justin

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  • Leo_Gnardo
    replied
    Originally posted by Justin Thomas View Post
    From what I can find, 6U8s are the recommended replacement for a 7199, just needing a rewiring of the socket. It's said they don't need any resistors changed or anything.
    There are "scrambler" adapters that let you sub 6U8 or 6GH8A for 7199. I've used a couple, they work fine. Have to make sure there's enough room inside a chassis to allow for the extra height - only half an inch but that might make the difference between a fit or not. Antique/CE plus a couple other sources have them, typically $20 each. Or Ebay direct from China, they're cheaper but shipping takes up the difference.

    Also - not every 6GH8A or 6U8 works perfectly in this application. You'll need to select the best from a batch. To be expected, 7199's never were a guaranteed success straight out of the box, lots of them had excessive hum and/or any number of unfriendly noises. Even more so for the Russian made subs from EH/Sovtek.

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  • Justin Thomas
    replied
    From what I can find, 6U8s are the recommended replacement for a 7199, just needing a rewiring of the socket. It's said they don't need any resistors changed or anything.

    I've got so many triode-pentodes it's stupid. 6AN8s, a 7687, other odd ones that seemed potentially useful. Main issue is the triodes, only having μ of 20-45.

    Justin

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  • Chuck H
    replied
    Originally posted by Justin Thomas View Post
    It's not metal-type gain, just a nice overdriven squishy sound that cleans up when I roll off the volume on the guitar.
    You can ask the Kinks if that qualifies as "metal" or not

    Originally posted by Justin Thomas View Post
    My most "complicated" project so far, & aside from a few real Dumbis wiring errors, I'm surprised this has come out so easily. Layout gets hard in these organ chassis, though. But it's fun.
    That'll happen to you if you do this at least once in a while and spend time on this forum You've probably already had that WOW! moment when you fired up a build or extensive mod and everything worked right from the flip of the switch.?. That's a good feeling, though I even questioned the first time it happened. I thought there must be an error or something because it was so quiet and well behaved

    Originally posted by Justin Thomas View Post
    If anyone knows if the pentode side of 6U8s are hummy as first gain stages, that's cool too. I had about 10 NOS to try, got 3 that were "good" to "acceptable". The others might do better as not-first-stages, I think. Just trying to look into using some of these weirdlings I have...
    I can't comment on the 6U8's because I've never even seen one. But I understand about trying to use what's in your stash. They're all tubes and they're all awesome when you treat them (or mistreat them) right

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  • Justin Thomas
    replied
    I got bored & decided to make a second parallel preamp feeding the PI & output that was good. Ill worry about the rest later, after I cash my "stimulus" check. But right now I've got 2 nice channels, simple Vol/Tone. Will tweak the voicing later, but right now they both sound pretty good, minimal hum unless I crank them way up, and a reasonable volume & a good mix of grit on both channels, which is what I was going for - something where I wouldn't need a distortotron pedal!

    It's not metal-type gain, just a nice overdriven squishy sound that cleans up when I roll off the volume on the guitar. It actually behaves a lot like my old 79 Champ, with the slurry undertone which suits my playing really well.

    My most "complicated" project so far, & aside from a fee real Dumbis wiring errors, I'm surprised this has come out so easily. Layoit gets hard in these organ chassis, though. But it's fun.

    If anyone knows if the pentode side of 6U8s are hummy as first gain stages, that's cool too. I had about 10 NOS to try, got 3 that were "good" to "acceptable". The others might do better as not-first-stages, I think. Just trying to look into using soe of these weirdlings I have...

    Jusrin

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  • Justin Thomas
    replied
    Well, half is working now & showing great potential, after fixing a gross wiring error where I fed the cathode side of the cathodyne back into the volume control, forgot to ground the preamp filter caps, & didn't solder the 6U8A screen! DUMBIS!

    I gotta get my pot wiring theory down.

    Jusrin

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  • Justin Thomas
    replied
    Well,

    I'm glad I checked before I got TOO far along in this build. One of my OTs had an open winding on one half of the primary. I dissected it & found the break, in the second layer down. Me being me, I kept unwinding out of curiosity or boredom. There was another break in the other half of the winding, in the 3rd layer from the middle, even though that half of the winding had checked decent with my multimeter. The 2nd break had a little tiny hole punched through the insulating paper, surrounded by a corrosion-green-tinged ring, very tiny. I ran across a few other green rings, though no further breaks.

    I'd taken trannies apart before, but never this methodically. Though I think with some minimal equipment investment I could have rewound it. Oh well.

    So I'll work on one half first & gank another OT from one of my other 5 chassis I have in the attic.

    Justin

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  • HaroldBrooks
    replied
    Originally posted by Enzo View Post
    It isn't the voltage that makes plates red hot, it is the current.
    Right, thanks Enzo, that's my understanding too.

    Once I raised the screen grid voltage by dropping the power supply node resistor, and pdf64 jumped in and reminded me that it might change the tube plate current and upset the bias a bit. I hadn't the experience to make that association yet, but he was correct and it now measured high, so I added some cathode resistance and all was back to normal.

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  • Enzo
    replied
    It isn't the voltage that makes plates red hot, it is the current.

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  • HaroldBrooks
    replied
    Originally posted by Justin Thomas View Post
    I am trying to stick with what I have on-hand for tubes & other pricey parts here. I'm not ruling out 7189s or the original 6973s at some point, but with EL84s & a couple 6V6s I can use tubes I have on hand.

    Justin
    Sure, you probably won't need them. I have another amp with ordinary EL84s and I am running the plates at 383vdc and the screens at 350vdc, and no red plating and no screens glowing (1k screen grid resistors and a reasonable 10 watt per tube plate dissipation), even with a highly boosted signal and horrific distortion. If you get a good set of old EL84s, those tubes can take it I find. Some old Hi-Fi schematics show those types of voltages, even though the spec sheet limits are less.

    I like EL84s, they quickly have become my favorite smaller tube, better than 6V6s or 6973s, and least the way work in the Zenith Hi-Fi amps I've converted so far (might be the particulars of those circuits). Good choice for a modest power amp.

    Good luck with your build and I think your making good choices, and I will now copy your one amp cutout scheme, so thanks for the good ideas Justin.
    Last edited by HaroldBrooks; 04-14-2020, 02:59 AM.

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