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Thread: Blackfacing a 72 Twin Reverb questions

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    Supporting Member Randall's Avatar
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    Blackfacing a 72 Twin Reverb questions

    Correction: it is a '77 SF Twin Reverb in for a cap job and blackfacing. This one is close, but not exact to this schematic. It has two 100uF/350v reservoir stacked caps, the attached schematic shows 70uF. Does anyone see an issue using 80uF here?

    Also, the reverb drive tube has no cathode bypass cap, just a 820 ohm resistor, the schematic shows a 25uF cap and a 2200 ohm resistor. Should I go with the schematic?

    The second cathode on the trem tube has a 5uF cap, should I change it to 25uF?

    The bias supply has two 80uF caps separated by a 3.3K resistor, why was this done, and is it a good thing? I am going to change the bias control back to blackface and lose the balance thingy.

    Any other recommended changes?

    https://www.thetubestore.com/lib/the...-Schematic.pdf

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    Last edited by Randall; 11-12-2019 at 12:50 AM.
    It's weird, because it WAS working fine.....

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    Not that it's necessary, but thelater Twin Reverb schematic will probably be closer. I can't link it or paste it from my phone but Fender Amp Field Guide has it. It's listed under 100W Master Volume. It doesn't show up in TR searches cuz Fender just lists it as "100W RMS Amp w. Reverb & Tremolo."

    So if you want to see how it should have been, that one is right.

    Justin

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    Supporting Member The Dude's Avatar
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    My .02: None of those things will effect the "Blackfacing" part.

    1) the 100UF totem pole caps are close enough to the older 70UF. In fact if you measure a lot of the old caps, their actual value is much higher anyway. Unless they are old and in need of change, I'd leave them. Otherwise 80UF is fine.
    2) If the reverb sounds good, I'd leave it alone.
    3) The bias supply is just a static reference voltage supplied to the tubes. The tubes wouldn't know if it was a battery or a complicated class D supply. Again, I don't see a reason to mess with it. It won't have an effect on tone.

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    Last edited by The Dude; 11-12-2019 at 01:32 AM.
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    Quote Originally Posted by Randall View Post
    the reverb drive tube has no cathode bypass cap, just a 820 ohm resistor, the schematic shows a 25uF cap and a 2200 ohm resistor. Should I go with the schematic?
    I'm with The Dude on this one, if it sounds good, it is good. FWIW when a Fender owner says he's got "too much reverb" - can't turn the knob up past 2 without getting too far back in the tunnel - one of the first things I do is nip that bypass cap and install a 2K2 cathode resistor. Without the bypass cap, the gain on the reverb drive tube is reduced noticeably. And there's plenty of bias current for the 12AT7 with a 2K2 Rk. Just makes life a little easier for that tube, and hopefully on the transformer too. Golly, now we can turn the reverb all the way up to 3, maybe 4, before it gets out of hand.

    The second cathode on the trem tube has a 5uF cap, should I change it to 25uF?
    On an amp of this age, you should be replacing all the bypass caps anyway. It won't make a hoot in hell whether that cap's 5 or 25 uF or anything in between.

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    Supporting Member Jazz P Bass's Avatar
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    "Black Face" a Silverface amp?????

    WTH is that supposed to mean?

    What, remove all of that gadly wire?

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    Supporting Member Randall's Avatar
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    So far I have changed the filter and bypass caps, put the bias circuit back to AB763 except for a 100uF/100v bias cap, changed the PI plate resistors to 82K/100K, and changed the power tube grid leak resistors to 220K. I did also put a cathode bypass on the reverb driver. I know some of you guys wouldn't have done much of this, but it is what the customer asked me to do, so I gotta do something.

    I'm on the fence over disabling the master volume, but I'm leaning on leaving it if it isn't going to hurt anything.

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    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    I'm on the fence over disabling the master volume, but I'm leaning on leaving it if it isn't going to hurt anything.
    On my own Silver-face Twin master volume, I got rid of the mostly useless (and hum attracting) pull-for-boost wiring & installed a plain old 1M A taper pot. I don't find much attraction to dialing it way down. BUT it's good to dial down to 8 or maybe as far as 7, just to reduce overall hiss noise. There's still enough gain to slap the output into clip at those settings, so nothing lost in terms of clean power.

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    How does this master volume even work in this amp? Is it doing it with negative feedback somehow? Would it have any effect with the pot full down? Is there any advantage to just leaving the pot down versus taking it out?

    https://www.thetubestore.com/lib/the...-Schematic.pdf

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    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    How does this master volume even work in this amp? Is it doing it with negative feedback somehow? Would it have any effect with the pot full down? Is there any advantage to just leaving the pot down versus taking it out?
    Just a volume control between channel mix point (the typical pair of 220K) and the input to the phase inverter. What's unusual is that Fender lands what would be the ground end of the pot onto the "hot" end of the 100 ohm resistor in the feedback network. Why, Fender? Why do you land what should be the ground end of the pot onto an "almost but not quite" ground point? It still works as a master volume, but I say bollocks to Fender-CBS's kooky idea, and bring the no-signal end of the pot to proper ground, say at the other end of that 100 ohm.

    Somehow Fender's MV never hit the sweet spot, the way it did on Marshalls. A great big fizzle. What they expected would work as an overdrive, didn't. In Fenders it sounded wretched instead, with the channel volume cranked and the MV just cracked open, like you would do with a Marshall. So, that's why I don't dial the MV below 7, using it only to reduce hiss.

    Of course, leaving out the MV entirely is more or less like having the MV dialled up all the way. The difference is, a little bit of signal lost due to loading the mix point with a 1 M pot. Perhaps a tiny dab of high treble clarity lost, for the same reason. Plus all the noise those long wires pick up, as well as some hum from being jam up next to the power transformer.

    Another place to lose hi frequencies is that pair of 330K resistors at the input of the PI. Swap 'em out for 1M in case you already haven't.

    When confronted with the vexing things CBS engineers wrought on otherwise perfectly fine amps, think "What Would Leo Do?" Then do that. Dial the clock back to 1964, and simplify! Works for me.

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    Stray Cap DrGonz78's Avatar
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    It also appears to me that when the MV boost is engaged that something is changing from the gain stage from reverb stage tube. Not saying I understand it but I do see a switch 5 (in a triangle shape) MVS. So in one position it sends signal to ground through a 1 meg resistor and in other position it shorts out that resistor. So is a portion of the MV boost coming from the varying gain stage of that reverbs circuits tube v4b?

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    Another place to lose hi frequencies is that pair of 330K resistors at the input of the PI. Swap 'em out for 1M in case you already haven't.
    Only increasing to 1M will mainly boost unwanted low frequency content. Mind that the BF TR used a .001µ instead of the 0.01µ PI coupling cap in the siverface, which more than compensates for the increased bass.

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    Quote Originally Posted by Randall View Post
    The second cathode on the trem tube has a 5uF cap, should I change it to 25uF?
    If you change it to 25uF, the intensity will increase as you decrease the speed. The 5uF cap is there so that the intensity stays the same across the entire speed range.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    "Mind that the BF TR used a .001µ instead of the 0.01µ PI coupling cap in the siverface, which more than compensates for the increased bass. "

    Yes I changed that cap .001 also.

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    It's weird, because it WAS working fine.....

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