Oops, I blew it up!

chrisb

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Hi All,
I made a little boo-boo and I'm hoping you can advise me on how to proceed:
I have 3 of those record players you used to find in schools (you know the ones, they have lids that hinge off). One of them has a mic input, so I plugged my 360/12 in and it's a great little amp (6L6). The other ones have different tube configurations, so I thought I'd set about adapting them to use as guitar amps also.
Starting with the EL84 model, I decided to bypass the motor since it hummed like a beehive and didn't turn. The wiring goes AC mains>fuse>motor>trannie primary on one side, while the other side goes AC mains>power switches (1 main, 1 motor)>trannie primary. It seemed to me that bypassing the motor should be as simple as cut & splice, but in my typically rank amateur manor, I didn't count on the fact that the motor has a resistance that might be important (100K as I measure it now). So when I turned it on after wiring my bypass it popped and now won't fire up again.
Do you think I blew up the trannie, or just some section of the quad can cap?
I'd like to get it working, but I don't think it's worth a trip to the tech and I've now exceeded the boundaries of my testing knowledge... I can change the caps but the trannie is a different story.
Any advice would be greatly appreciated!
Chris
 

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Did you check the fuse? Outside of that, it's hard to troubleshoot what I can't see in person.... :(
 

capnjuan

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Hi Chris; default is right, gotta have some pics. Have a schematic? Make / model of the equipment? Pure guesswork but if the fuse blew right away, you're transformer is probably ok but you've wired yourself up a nice short. If motor is in parallel with the transformer, what you did shoulda worked ... the resistance across the AC is nothing ... although small consolation. If the fuse is intact then chances are one or more of the filter caps or the rectifier is shorted to ground. More likely to happen to the caps when old gear sits for long periods of time. J
 

chrisb

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Fuse is fine, never blew. The motor is in parallel, and looking at it now it seems I could disconnect it altogether with no consequences (it even has its own switch, which happens to be a switched pot for volume). I'll take it in to get some caps to replace the quad can & see if that helps--the guys at the local shop can be quite helpful too... I'll try to get some pics also.
 

coastie99

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chrisb said:
Do you think I blew up the trannie

Wish I could help ya Chris, but I've only ever had experience of blowing up stuff like this................

sheep_front.jpg
 

capnjuan

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chrisb said:
Fuse is fine, never blew. The motor is in parallel, and looking at it now it seems I could disconnect it altogether with no consequences (it even has its own switch, which happens to be a switched pot for volume). I'll take it in to get some caps to replace the quad can & see if that helps--the guys at the local shop can be quite helpful too... I'll try to get some pics also.
Hi Chris; agree, motor can/should be disconnected ... the combination on/off-volume controls are still around .. check the back of the pot section, might have mfrs ID/date code/value stamped on it. If not and if aftering checking the on/off switch contacts for continuity, you find the switch contacts are burned, then you'd need to know the value of the pot section. If it doesn't say and it's disconnected from the circuit, you can measure it's resistance characteristics to figure out what you need. Good luck! J
 

chrisb

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All right, problem solved! It was the power switch all along (you can laugh at me now), which had apparently opened for some reason. I simply switched the mains over to the motor's switch after disconnecting the motor, and voila!

I've also successfully converted it to guitar use, removed the cap/resistor on the input and changed the filter caps. To increase gain, I've been advised to put a 100K resistor to the plate of the 6AV6, but I'm not sure which pin that is and don't read tube schematics... anyone know which pin???

Here's a handy tube chart: http://www.qsl.net/wd4nka/TOOLS/Manual/manual.html
I think it might be pin 7, but I don't want to blow anything up!
 

chrisb

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Thanks cj! Nice slab of info too.

And then I laugh at myself again! There already is a 100K on pin 7. So if I want even more gain, can I put in a different value? Even cranked, it won't distort & is barely loud enough to hear well, much less to bother the wife.
 

capnjuan

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Yes; if you want to increase the gain, you have to reduce the resistance of the plate resistor. The next smaller commonly available resistors are 82K, 75K, and 68K; I wouldn't mess with anything smaller. Changing the plate resistor will also change the bias of the triode section of the tube. The 68K will give you the most gain however it will also have the most change on the bias. That said, I'd probably try the 82K on the plate with the hope of getting around having to mess with the cathode resistor. CJ
 

chrisb

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So what would be the drawbacks/consequences of changing the bias? I'd like to get the most gain (within reasonable limits) out of the preamp in hopes of driving the power tube harder. Which leads to the next question: can I change any resistors to the 6BQ5 to increase its gain?
 

capnjuan

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chrisb said:
So what would be the drawbacks/consequences of changing the bias? ... can I change any resistors to the 6BQ5 to increase its gain?
Hi Chris; if you get the tube too far from its bias point, it won't work correctly; crappy sound, bad frequency response, and if too hot, it will blow. The only resistor of consequence connected to your 6BQ5 is its cathode bias resistor at pin 3 and I don't recommend fooling with it. Messing with preamp voltage/gain is one thing, messing with the bias of a tube that works correctly is another. You can hot-check that resistor to make sure it hasn't drifited and replace it if necessary. CJ
 
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