#31
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Quote:
I'll have to pick up a pack of fuses and systematically troubleshoot the amp next weekend.
__________________
"You don't have to be great to start, but you have to start to be great." -Zig Ziglar Acoustics 2013 Guild F30 Standard 2012 Yamaha LL16 2007 Seagull S12 1991 Yairi DY 50 Electrics Epiphone Les Paul Standard Fender Am. Standard Telecaster Gibson ES-335 Gibson Firebird |
#32
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Quote:
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#33
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Actually, yes. Much simpler circuit, though. At least the magic smoke stayed inside.
__________________
"You don't have to be great to start, but you have to start to be great." -Zig Ziglar Acoustics 2013 Guild F30 Standard 2012 Yamaha LL16 2007 Seagull S12 1991 Yairi DY 50 Electrics Epiphone Les Paul Standard Fender Am. Standard Telecaster Gibson ES-335 Gibson Firebird |
#34
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Been making any progress - or just a bigger pile of blown fuses?
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#35
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Not yet. Been working; I'll pick it back up Saturday. I did confirm the wiring continuity by multimeter though, and found a missing B+ connection on the Vibrato circuit. Easy fix there. That shouldn't cause the fuse issue though. Next is a slow, controlled, step-by-step startup checking voltages as I go. I've gotten a couple of votes for the theory that the inrush current from turning on the Standby switch could have caused it. We'll find out this weekend.
__________________
"You don't have to be great to start, but you have to start to be great." -Zig Ziglar Acoustics 2013 Guild F30 Standard 2012 Yamaha LL16 2007 Seagull S12 1991 Yairi DY 50 Electrics Epiphone Les Paul Standard Fender Am. Standard Telecaster Gibson ES-335 Gibson Firebird |
#36
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That sounds plausible. With a tube rectifier, allowing the tube to warm up on standby before switching on seems to increase the inrush current compared to no standby and a cold tube.
You've probably seen this already but the consensus seems to be that standby switches don't actually do anything useful in modern amps. Inrush current seems to be a common weak point in tube amp designs. Don't know why thermistors aren't standard. Maybe they introduce some noise. |
#37
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also, have you thought about a variac or an isolation transformer? https://www.youtube.com/watch?v=wRFRwOnLsZI play music!
__________________
2014 Martin 00015M 2009 Martin 0015M 2008 Martin HD28 2007 Martin 000-18GE 2006 Taylor 712 2006 Fender Parlor GDP100 1978 Fender F65 1968 Gibson B25-12N Various Electrics |
#38
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Ok, got some help from a Facebook builder's group. Turns out I had the bias rectifier diode backwards. Fender has the schematic symbol right, but added a helpful +/- symbol backwards. The layout is also labeled backwards, as are a fair number of pics of diode polarity markings. For the record, the stripe is on the positive end, it goes towards the bias AC supply, and the amp is much happier that way. Haven't blown a fuse yet, either, and I'm up to installing the preamp and power tubes now. To be continued tomorrow. I need a good night's sleep and a clear head to continue this safely. Even with insulated gloves, dealing with high voltage is mentally tiring.
__________________
"You don't have to be great to start, but you have to start to be great." -Zig Ziglar Acoustics 2013 Guild F30 Standard 2012 Yamaha LL16 2007 Seagull S12 1991 Yairi DY 50 Electrics Epiphone Les Paul Standard Fender Am. Standard Telecaster Gibson ES-335 Gibson Firebird |
#39
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That's great . Tough one to track down if the layout is wrong.
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#40
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Not totally sure it isn't intentional on Fender's part - if you really know what you're doing you'll catch the error, and it keeps the "unauthorized" from messing around inside their amps; kinda makes it tough for guys like you, though...
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#41
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Mojotone provide schematics and layouts for the kit on their website. It's odd they haven't corrected it. You'd think they'd have had some customer feedback about that.
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#42
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You know, this made sense last night but looking at it again this morning I'm all turned around again. Big picture is it is a circuit designed to rectify AC to DC, but end up with a negative instead of a positive voltage with respect to ground so the diode and cap should be reversed. Oy. Either way, I swapped the diode around and the bias voltage is right (currently about -55V) now.
__________________
"You don't have to be great to start, but you have to start to be great." -Zig Ziglar Acoustics 2013 Guild F30 Standard 2012 Yamaha LL16 2007 Seagull S12 1991 Yairi DY 50 Electrics Epiphone Les Paul Standard Fender Am. Standard Telecaster Gibson ES-335 Gibson Firebird |
#43
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You've probably got this Fender schematic with voltages relative to ground already. If not, that might help to check if everything's working as it should. Presumably these were measured with the tubes installed.
Be careful measuring a live circuit |
#44
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Moon, thanks. I was just looking for the proper point to check bias voltage.
Installed all tubes, plugged in the speakers, and powered up. Very faint white noise. Very good sign! Time to adjust bias and install the chassis into the cab after the filter caps bleed down. Exciting!
__________________
"You don't have to be great to start, but you have to start to be great." -Zig Ziglar Acoustics 2013 Guild F30 Standard 2012 Yamaha LL16 2007 Seagull S12 1991 Yairi DY 50 Electrics Epiphone Les Paul Standard Fender Am. Standard Telecaster Gibson ES-335 Gibson Firebird |
#45
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Weird again. Almost everything seems to work, but the amp is really quiet. I checked voltages against the schematic moon listed, and they check out ok. All pots have the effect they should, except the reverb one for some reason. Time to discharge the caps and follow wiring. Again.
__________________
"You don't have to be great to start, but you have to start to be great." -Zig Ziglar Acoustics 2013 Guild F30 Standard 2012 Yamaha LL16 2007 Seagull S12 1991 Yairi DY 50 Electrics Epiphone Les Paul Standard Fender Am. Standard Telecaster Gibson ES-335 Gibson Firebird |