NAD Thunderbass head

AcornHouse · Dec 30, 2014

While my amp tech is sorting out my Masteramp, I thought I'd pick up a little something for me to work on, a '60s Thunderbass head. It's got the 8417 power tubes. Oddly, all of the front switches (power, standby, and polarity) are broken off in the on position, but that's a fairly easy fix. It was advertised as working on the bass channel, not on the "normal" channel. Will also need a 3-pronger and 1 or more replacement knobs.

Interesting upstairs/downstairs layout of the components. Very tidy compared to the Masteramp.

Default · Dec 30, 2014

Ah, that's the model without the voltage regulator tube.

mavuser · Dec 30, 2014

Nice! Looks like now u just need a bass :triumphant::excitement:

AcornHouse · Dec 30, 2014

mavuser said:
Nice! Looks like now u just need a bass :triumphant::excitement:

Actually I have a couple (non-Guild), but I also have student bess players.

AcornHouse · Dec 30, 2014

AcornHouse said:

Ok, the pots are all dated '65 or '66, so is this a '66 model; is that consistent with what we know of its history?

Land for the techie/owners: if you look carefully in the center of the above pic of the underside, there appears to be a missing component. There is a jacketed copper wire leading to nowhere, and another wire coming out from around the bottom left of the perforated area that would be at the other side of the missing piece.
Can anyone figure out what should be there?

AcornHouse · Jan 5, 2015

Here's a close up of the missing (?) component. It's going to ground so it makes me think resistor. Anyone with an 8417 tubed model that doesn't have the extra power regulator 6GF7 tube? Can you take a pic of the undersides of the missing component (?)?

And, here's a fun little nest. I sure hope that it didn't come like this from Guild. There are supposed to be 2 6.8ohm resistors here; you can see the one on the left. On the right, instead of the 6.8, someone used 4(!) to hopefully equal the 6.8.
I'm not looking forward to unsoldering that mess.

killdeer43 · Jan 5, 2015

I can enjoy/appreciate the tech and the pics but this is WAY over my head....but more power to you (no pun intended). :apple:

It's no wonder I prefer acoustic,
Joe

mavuser · Jan 5, 2015

i have the same amp. but on several levels, it will be a while before i'm looking at the underside of the inner chassis. i'll see how adventurous I am when I dig it out of storage for my friend to change the speakers, at some point hopefully soon. maybe I can get a gut shot for you, with his assistance.

those switches are a little flimsy, it's no surprise they broke. I think one of mine was replaced with a "real" switch. Or at least its different from the others.

AcornHouse · Jan 5, 2015

Yeah, just a pic of the underside should help.

It looks the switches are plastic where they broke off. I have new power and standby switches coming in tomorrow with regular metal. Won't bother with the polarity since I'm putting a 3-prong grounded cord on and getting rid of that cap, so it's non-functional.

AcornHouse · Jan 12, 2015

After installing a grounded 3-prong cord, replacing the broken off power and standby switches, and getting rid of the 6.8 ohm resistor mess in the pic above, I finally fired it up for the first time. Surprisingly, both channels were working, as opposed to the seller statement that the guitar "normal" channel didn't work. Wasn't getting full volume on either channel, anything past 1/3 up started to get raspy. A quick check of the power tubes showed that only one of them had any glow, and that wasn't terribly strong. So, definitely needs new 8417s (on the way.)
May still need some caps replaced, but I'll need my tech to check those, unless someone knows how to check for bad caps using a digital multimeter.

59Panhead · Jan 26, 2015

I believe that I watched the very same amp, but did not want another total overhaul project. I have one of those, and I installed new 6.8 ohm resistors. I also added my Bias Level modification, so that I can adjust the total Bias, and balance the Bias to the individual tubes. Really works great.

I don't have mine at home, as I took it to leave at Church, when I ironed out all the little issues and made the modification to the Bias. A truly fine sounding amp. One of the best Guitar channels that you could ask for.

Still, I prefer the ThunderStar Bass amp, as it has an Effects Loop. I wonder why they didn't include one in this model.

Jack

AcornHouse · Mar 10, 2015

Finished the rehab on my T-Bass head, and its sounding good! Replaced all filter and electrolytics caps, and one lumpy brown turd coupling cap. I was able to find replacements for 2-3/4 of the can caps, but had to find space for 3 caps for the remaining 1-1/4 cans. A couple of resistors, the power and standby switch (didn't bother with the polarity switch since I did away with the death cap), and a new 3-prong cord.

Got a couple of Westinghouse 8417s; one of the ones it came with was toast.

I hope to post a video later this week.

ejudasf · May 16, 2015

I can't upload pix yet.

NAD Thunderbass head

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