Garnet Gnome

AngrySailor

AngrySailor

Veteran and General Yakker
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Oct 15, 2014
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---not quite right
Having a look at this little guitar amp for a friend. Never worked on tubes... got a schematic at least. It’s got some 60hz hum which I would figure to be the filter cap but it’s got a weird crackle from the tremolo circuit also. The speaker cone is moving quite a bit with the tremolo circuit engaged. Any suggestions?
 

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It doesn’t look that old, filters may be suspect but I’d check the input jack, guitar, and guitar cable for a bad connection. As for the crackle maybe check the input grid on the 6v6 for DC. I’d also look for cracked solder joints.
 
mr_rye89 said:
It doesn’t look that old, filters may be suspect but I’d check the input jack, guitar, and guitar cable for a bad connection. As for the crackle maybe check the input grid on the 6v6 for DC. I’d also look for cracked solder joints.
Click to expand...
I think it’s late 60’s early 70’s... rectum dryer diodes seem ok, I’ve got 23xVdc out and only Mv AC after the diodes. Looking for a 47uF cap to bridge the filter cap and see if she quiets down. The crackle seems to be related to the tremolo circuit. It’s quieter with that in the off position. It’s pot is dirty also. I’m a t00b n00b so is DC on the grid bad or??? I’LL LET THE SMOKE OUT OF THIS YET YIPPEE KIYAY MOTHER FUCKERS!
 
DC on the grid is normal, but it depends what polarity and how much. That's how tubes control how much plate to cathode current flows. Generally, grid bias is a couple of volts negative - but it depends on the tube and the application. This amp does not have a rectifier tube. I'd start by replacing all three electrolytics in the power supply (20 -20 -40 mfd).
 
Thanks everyone. I think I have some beefier diodes, I’ll check things when I get back home this evening
 
Looks like the wiper wire on the volume pot has damaged insulation and perhaps broken strands. Just off away from the solder joint.
I see the silicon rectifiers now, duh!
 
George S. said:
Looks like the wiper wire on the volume pot has damaged insulation and perhaps broken strands. Just off away from the solder joint.
I see the silicon rectifiers now, duh!
Click to expand...
That looks like someone hit it with the solder iron, the speaker wire is melted at the amp side of things but it looks like that may just be from soldering it to the jack however the solder joints at the speaker didn’t melt the insulation there, oh and it’s shiddy lamp cord for speaker wire... I don’t have any acceptable caps here to even tag on as filters. Everything I have is way under voltage and big capacitance. I don’t really want an 8500uF 100v cap exploding in my face at 360v or whatever it was... I’ll lift the diodes and check them, try and pull some junk apart see what I have for caps. I guess 22uF and 47uF would be the closest modern caps right? No issue going a little larger other than inrush current to charge them? I think this thing needs a good cleaning and deOxit for starters... pots are all scratchy.
 
Just had a real quick look this morning at this. I lifted the rectifier diodes and they test ok, if you look at the schematic they give some voltage values at the filter cap, those are all low, if I check AC after the transformer and before the diodes it seems one leg of the transformer might be pooched. One leg to the center tap is 314vac and the other is 2vac... measurements taken with diodes lifted. Fack! Good winding measure 260ohm to the center tap the other is open. She’s running half voltage...
 
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