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Old 16th March 2008, 03:36 AM   #1
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Default Need help with Tremolo LFO!

Hi all,
I usually figure things out pretty well but this one has me stumped. This is part of the Tremolo LFO in a Sears Silvertone 1484 guitar amp, and as you can see it is not of the ordinary LFO circuits. Maybe someone can shed some light for me and help me understand this one. The circuit consists of a 12AX7 where one section is the LFO and the other they are using for the noen light driver, which is part of a opticoupler. All values are in Mf and even though the schematic shows 205v for pin 8 plate voltage it actually measures 320v. I think this is print a error due to the fact that R53 (330K) is fed by the 340v 6L6 grid supply. Cheers
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Old 16th March 2008, 09:25 AM   #2
poynton is offline poynton  United Kingdom
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Location: A life on the ocean waves when I'm not at home in N. Wales (but I'm not Welsh so no sheep jokes!)
The LFO looks standard to me.

What's the problem?



Andy
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Old 16th March 2008, 11:05 PM   #3
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The bias arrangement is a little different from what I am used to seeing. Actually this is the first time I have delt with a LFO oscillator so I am in the learning stage. And of course the fact that it doesn't work. I just wanted another opinion because there are a few miss prints on this schemematic. Actually when I got this amp someone had wired it R56 to B+ pin 8, then tied them to ground. Now way this would have worked. Also all the designs I have seen so far go with 300v plate voltage and this circuit shows 205? I just like to know a little theory before swapping parts out. I do know the Neon lamp is very weak but shouldnt I still get the oscillator to work? Cheers
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Old 17th March 2008, 06:01 PM   #4
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Quote:
Originally posted by Toobstheway
The bias arrangement is a little different from what I am used to seeing.

Actually, that's the 'normal' method of biasing, in use before auto-biasing via a cathode resistor appeared.

I'm presuming you've checked the 330K anode load resistor?, that's a very common fault in valve gear, going high or O/C.
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Old 19th March 2008, 12:10 AM   #5
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Figured it out!!

Even you pro's missed this one. Look at the schematic real close and notice plate and cathode pins labeled wrong! Just goes to show you can't always trust schemo's you get on the net. I did however learn a lot about how oscillators work and how do design them, so over all a positive learning experience. Cheers
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Old 19th March 2008, 09:50 AM   #6
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Why would having the pin numbers printed wrong on the diagram stop it working?, it's also uncommon to even have them on diagrams.
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Old 30th March 2008, 05:12 AM   #7
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When I initally got the amp, it was in shambles and needed to be wired up and I went by the schematic, which in turn led me to wire it up wrong. And of course therefore it wouldn't work.
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