The sunn Forum
Sunn Musical Equipment => Q & A => Topic started by: desotoslo on August 31, 2015, 03:06:42 pm
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Hi ya'll,
My tech is currently trying to replace missing SM-1 module on a 1968 Sunn Sceptre.
The one major thing he needs to know in order to replace it is what lamp number is used in the circuit. He tried a bunch of different things but to no avail.
Does anyone have an original parts list for the Sceptre, with a detailed description of the tremolo circuit components, particularly the lamp used?
Many thanks for any help.
David
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I don't think parts lists exist for that circuit.
I replaced the lamp in the module on one of my Sceptre heads a few years back.
I didn't write down what lamp I used, but for some reason I think it was a Neon bulb.
Did you do a search here and see if anyone mentions replacing that unit?
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Thank you! Yes, I did many searches on the server but only was led to some threads that didn't contain the info I need.
like: http://sunnforum.ampage.org/index.php?topic=6453.0
and http://sunnforum.ampage.org/sdp/index.php/topic,4772.0.html
Neon bulb, you say? Does it work well?
Does someone out there must know which bulb belongs in the SM-1...?
Thanks again!
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It was either a Neon or an LED, I just cant remember which and I don't have the amp in my possession anymore.
The first thing for your tech to do is to make sure the Tremolo oscillator is working correctly with a scope. Then once that is known to be working he should be able to sub in various bulbs, neons, or LED's until he finds one that lights up bright enough to work the photoresistor.
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Hi, thanks again. The oscillator is working correctly. He has tried subbing in various bulbs many times, but they have burned out because they're the wrong value. He's quite a fantastic tech, actually, but he can only go so far without knowing the lamp part number. :/ Oh well, this seems to be one of the universe's great mysteries!
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If the bulbs burned out, then they were poorly selected. The voltages should be easy enough to determine, and possible bulbs selected accordingly. In fact, those of us with operational amplifiers could easily measure the voltage and current. That should narrow down the possibilities.
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Bumping an old thread... but would anyone out there in the internet-sphere be willing to snap a photo of a working opti-tremolo circuit on a Sunn Sceptre (or other Sunn amp that uses the same circuit?) Or help out with the actual bulb #?
I'm trying to avoid the temptation of buying a second Sceptre just to find the circuit so I could fix my other one. Currently my Sceptre also has a loud hum at idle that is a symptom of the bulb-less tremolo circuit (?)
I'd be willing to PP you some dough for your time. Thanks.
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You may take a look at this thread for some options.
Greg
http://sunnforum.ampage.org/index.php/topic,10195.0.html
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No much new here but I'll throw it out there...
I recently asked Conrad if he recalled anything specific about the tremolo circuit's bulb and resistor for my Sentura II.
He replied "We custom made the photo cell devices from purchased components. They were each hand tuned for maximum swing. I have no idea what the part numbers were for the incandescent bulb and photo resistor. No LED's back in the day.
Conrad "
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If someone would put one of the SM-1 modules on the bench and do a voltage-to-resistance plot, that module could be duplicated. That is, plot a curve where 1v = X1 ohms, 2v = X2 ohms, 3v = X3 ohms. While I have a working SM-1 module, I'm not brave enough to try to take it out of the circuit to measure it. Like Conrad said, it's nothing special, but you do need the right values of incandescent lamp and CdS photocell.