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Thread: Mod'ing a tube circuit (from early 60's stereo turntable)

  1. #106
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    For sure. Just want to throw out there that it's all on a PCB with the PT in one corner and the OT's in another.

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  2. #107
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    Re-wiring the long un-shielded cables with short shielded ones... will report back after that!

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  3. #108
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    Replaced the long un-shielded wires (that run from the amp PCB to the control panel controls) with shorter shielded wires. Grounded them all together (only grounded on one side - at the control panel). No more ice pick problem!! I can crank all the dials now and it holds steady and sounds rippin'!!!
    Lead dress - one of the most tedious things is one of the most important, for sure.

    Chuck - Those 100pf bright caps on the volume controls that you suggested, I put them on toggle switches - and maybe i've lost my high frequency hearing but I'm not noticing any difference. I've already drilled the holes for these suckers, so i gotta put SOME kind of mod in there :-) heh hehhhh.
    So, should I go for a smaller value cap maybe, or....?

    Hope everyone had a great turkey day and a long weekend!

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  4. #109
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    @ChuckH: Hey, also I noticed that on your schemo for the mid section you changed C11 from .01uf to .022uf. This wasn't on the one full schemo you posted, and I didn't do it because it slipped past me.

    What's the story on that mod? Boosting fewer lows in the mid-section controls?

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  5. #110
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    More questions on my mind:

    I've just ordered a Cap tester - but, should I just go ahead and re-cap the PS? It's three 50UF caps.
    If I luuuuuv the sound of the amp, should i try to match their current ratings or just go with the stock values?

    What about the SS rectifier - is it something that should be replaced out of hand, or will it not jack stuff up too bad if and when it goes out on me?

    The cathode bypass caps (16uf and 25uf) have a little orange foamy bulge out of one end. I'm assuming that's not a great sign. I have no problems replacing them, but am I tripping that its hard to find those two exact values at low voltage?!

    The speaker jacks seem to ground when nothing's plugged in. They're very proprietary jacks so I'm replacing with regular jacks. Am I correct that by using shorting jacks (as they do) that the amp won't blow out the OT if no speaker is plugged in? I'm very careful about this but just curious about the logic.

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  6. #111
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by TubeNoob View Post
    I've just ordered a Cap tester - but, should I just go ahead and re-cap the PS? It's three 50UF caps.
    If I luuuuuv the sound of the amp, should i try to match their current ratings or just go with the stock values?
    Great question! You've been paying attention But there are other aspects than value to affect the tone of the amp WRT the filter caps. The impedance and ESR will affect how much NFB and potentially positive FB can occur in the power rail. But there's honestly not much to be done about that in practical application. So..

    I'd say mach the value. Those old caps can be +/- by quite a lot from their spec. So to keep things as close to what you have now I would match the value.

    Quote Originally Posted by TubeNoob View Post
    What about the SS rectifier - is it something that should be replaced out of hand, or will it not jack stuff up too bad if and when it goes out on me?
    Is it silicon or selenium? If it's selenium I'd replace it. If it's silicon I wouldn't worry about it.

    Quote Originally Posted by TubeNoob View Post
    The cathode bypass caps (16uf and 25uf) have a little orange foamy bulge out of one end. I'm assuming that's not a great sign. I have no problems replacing them, but am I tripping that its hard to find those two exact values at low voltage?!
    15uf and 22uf would be close enough, provided that those caps measure close to their spec'd value. They probably don't. Further, if they are exhibiting any shorting it could be affecting the bias of those stages. That and the actual value will affect the tone of the amp more than differences in the power supply filters so you should test for these things. Measure cathode voltage at those stages and then lift one leg of the bypass caps. Then measure cathode voltage again. If it measures the same then you only need to replace the caps with something close to their measured value. If it measures different then you'll need to adjust the cathode resistor for those stages to get similar voltage that you had with the old caps. The caveat always being, if that voltage keeps the tubes operating in a safe zone.

    Quote Originally Posted by TubeNoob View Post
    The speaker jacks seem to ground when nothing's plugged in. They're very proprietary jacks so I'm replacing with regular jacks. Am I correct that by using shorting jacks (as they do) that the amp won't blow out the OT if no speaker is plugged in? I'm very careful about this but just curious about the logic.
    Make sure there's no switching weirdness for multiple jack use going on. If the jacks are wired as just plain ol shorting jacks then do that. Does it mean the amp is safe with nothing plugged in? Not really. The tubes aren't intended to run into a dead short and will draw considerable current doing it. It's still preferable to an open load though. The shorting jacks would be the lesser of two evils. There's one old thread here where a guy was running his old Fender with the "ext" jack plugged into another amps input and had been doing it for years. That doesn't mean it's a good idea. I'll just say your amp is LESS likely to suffer catastrophic damage running into a short than an open.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  7. #112
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    Quote Originally Posted by Chuck H View Post
    I'd say match the value. Those old caps can be +/- by quite a lot from their spec. So to keep things as close to what you have now I would match the value.

    Is it silicon or selenium? If it's selenium I'd replace it. If it's silicon I wouldn't worry about it.

    15uf and 22uf would be close enough.

    Make sure there's no switching weirdness for multiple jack use going on... I'll just say your amp is LESS likely to suffer catastrophic damage running into a short than an open.

    1) Will try to match those values! Yahoo, yeah I've been trying to pay attention; i refuse to get older without trying to get wiser!
    2) They appear to be Selenium. Ugh. Ok, so what's the best option - 4 silicon diodes?
    Click image for larger version. 

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    3) Ok, I have those values (15 and 22). I'm still gonna wait till my new toy arrives and I can measure them! heh!
    4) No weirdness on the jacks that I can see. I'll def go with the switching jacks; but as I said I do take care with that sort of stuff (with the fear only a DIY'r can have).

    Thanks so much, Chuck!!

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  8. #113
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    Oh and Chuck - still really hoping for your input for those previous two questions!! (100 pf bright cap and the C11 change to .022uf).

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  9. #114
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by TubeNoob View Post
    Oh and Chuck - still really hoping for your input for those previous two questions!! (100 pf bright cap and the C11 change to .022uf).
    Ah, Right. Ok. Been a little backed up with other forum activities.

    Yes, use plain ol 1n4007 diodes wired as shown in the schem.

    If the 100p bright caps aren't making a difference then you can bump the value for more instant gratification on the switch. I promise that some others would hear it with the cap switched out. The 100p value is small, but helps preserve a little sweet chime that can get lost with no cap at all. It's a small affect, not really enough to merit a switch. Which is why I showed them hard wired. So, since you already have the holes, maybe hard wire the 100p caps on the pots and switch in another 100p or 150p cap with the switch. You really should be able to hear that.

    The change from .01uf to .022uf was only indicated on the boost/cut switch part of the discussion. If you like what's happening with the .01uf value then absolutely do leave it that way. But the reason for the value difference as it pertains to the boost/cut switch is that the .022uf value gives the switch more effect by cutting deeper into the lower mid frequencies. It would voice the preamp more like a typical BF Fender with the switch in the "normal" mode. Which would be different from what you have now.

    Did you include the boost/cut switch? And, if yes, does it give enough gratification between "normal" and "cut" settings? And, if yes, leave it the way it is.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    The effect of a bright cap reduces with increasing vol. settings. At max. volume it has no effect as it is shorted.

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  11. #116
    Bent Member Chuck H's Avatar
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    Good thing to point out.

    Also, the actual knee frequency goes up. So there is less HF in the useful, audible treble as a result of that cap as the volume is increased. It's a double whammy.

    The reduced affect of the bright cap with volume increase actually works out well most of the time because most amps start clipping well before the bright cap affect is disabled. And most amps sound better clipped if you DON'T push the very high treble frequencies. It does make for a switch on the front panel that doesn't operate at all amp settings though. I always hated that aspect for purely aesthetic reasons.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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