NAD: Thunderstar Bass Amp

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Whelp, I broke it. Powered it up today and the lights all come on and the tubes all light up, but there ain't no sound. Time for some diagnosticatin'!
 

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On those, the standby switch grounds out the signal, it does not switch the high voltage on and off. Probably not your issue, but something to keep in mind.
 

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In my limited testing I found that the volume pot had completely seized. It had been very stiff and a nice application of contact cleaner/lube didn't seem to help. It's a double pot for reasons I can't figure out looking at the schematic.

1969-Guild-Thunderstar-Bass-Turret-Pots.jpg



Thunderstar-Bass-schematic.png



While we're on the subject of schematics, this amp has two 12AX7 tubes, but from what I can see each is only using one of the dual triodes, with V1-a using triode #1 and V1-b using triode #2. What's the reasoning for not using both triodes in a single tube? Ease of wiring? Lower Noise? Making people buy another tube? :tennis:

Also I can't seem to find a replacement for that pot so far and looking at the amp I don't think it's going to be fun to replace if I do. :moody:
 
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BTW I think it's the volume pot because a) it's seized and b) when I slide the tone switch I get popping in the speaker. Just my luck that the vol pot failed on 0 instead of 10. To test this theory, I used the cool "powered speaker" functionality of the amp since it completely bypasses the entire preamp including the volume. I plugged in my little battery-powered Roland Micro Cube line out into the Power Amp In jack, hooked the cab up through my RockCrusher Recording Load Box/ because it's almost 11pm here, and grabbed the closest guitar. Success!

This is the current state of GAD Labs. The fusion of vintage, modern, and crazy appeals to me.

1969-Guild-ThunderstarBass-Testing-GADLabs.jpg
 

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Might be transitional. Here's the Maverick Bass, which also has the dual pot. There should probably be a 7247/12DW7 in the phase inverter position.

Guild%20Maverick%20Bass.jpg


Dotted line is the other pot, so it looks like half the signal is bypassing the tone switch circuitry. You might have to disassemble the pots and try freeing them up that way. IIRC, they aren't available.
 
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Yes, that should definitely have a 7247 in the phase inverter. Take a gander at the 15k, 2 watt resistor in the phase inverter. That side would be the 12AU7 side, and it's dumping a lot of voltage on that side of the tube.
 

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I think you nailed it; the second pot is wired directly to one of the lugs on the standby switch.

There is a 7247 in the phase inverter. There are two 12AX7s, a 7247, and two 6L6GCs.
 

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I think I may have dorked up the volume pot. The stuff i used is not DeOxit. I bought a can of contact cleaner without even thinking about it because I couldn't find my DeOxit. As it turns out, it was a big old can of exactly that - contact CLEANER. It contains no lube and probably worked so well that it mucked up the innards of the pot. Gonna get me some proper pot lube (heh) and see if I can get it going again without having to remove it. It's possible I damaged the pot with the cleaner, too. I'm hoping that's not the case.
 

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Yes, that should definitely have a 7247 in the phase inverter. Take a gander at the 15k, 2 watt resistor in the phase inverter. That side would be the 12AU7 side, and it's dumping a lot of voltage on that side of the tube.

We love it when you talk dirty!
 

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I think I may have dorked up the volume pot. The stuff i used is not DeOxit. I bought a can of contact cleaner without even thinking about it because I couldn't find my DeOxit. As it turns out, it was a big old can of exactly that - contact CLEANER. It contains no lube and probably worked so well that it mucked up the innards of the pot. Gonna get me some proper pot lube (heh) and see if I can get it going again without having to remove it. It's possible I damaged the pot with the cleaner, too. I'm hoping that's not the case.

This is precisely why I like reading your reviews and stories. I learn things. I am also reminded of the definitions:
Experience is what you learn by making mistakes. Wisdom is what you learn from someone else's mistakes.
 

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This is precisely why I like reading your reviews and stories. I learn things. I am also reminded of the definitions:

Thanks! I have a military friend who says, "Wisdom comes on the other side of pain." I like that a lot.
 

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The pots on the amp are split-shaft knurled. Good to know there's an option out there, though.
 

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Update - I got some real pot/fader lube and worked it into the pot and now it turns again. Whew! After putting in the proper 9' power cord and strain relief, I fired up the amp again and it works! But only on the middle switch position and not at what I would think is full gain based on how it sounded before. That leads me to wonder if one of the pots in the stack has stopped working and only the "bypass" pot is engaged which (if I'm reading that Maverick schematic correctly) might mean that one of the preamp tubes is getting no signal.

Edit - I was wrong. The volume *is* working on the outside switch positions - they're just much quieter. That tells me that the pot is working but something else is amiss. Off to look for gremlins.
 
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I'm stumped so I'm walking away for now. The volume pot seems to work just fine. It also appears to be a 1M/500K from what I can tell, though I can't really trust the measurements in such tight quarters and with the pot connected to the circuit. I can't find a broken solder joint anywhere and i don't see anything grounding anywhere. I've replaced both preamp tubes with three different pairs and even swapped the positions and still get the same results. Gonna turn my brain off for a while.
 

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If you run the line out of the transistor Guild into the preamp in, does it get both loud and awful at the same time?
 
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