Guild Thunderbass Head

Hey, Cap'njuan!

I just happened across this. Has it already been scrutinized here at LTG?
Guild Thunderbass Head

Saw it. Cap's got one already.

Don't want the wife to deepsix me.

Auction Link Here's a pic of the auction amp: This is the somewhat rarer 8417 model (3X12AX7 and 1X7247), maybe from the earlier series - believe that it is the later models w/ 6L6s. My project copy arrived configed for 8417s. Several BBers have this model; John K, RWood, and another guy. I don't know if there's a meaningful difference in tone but there is more than a meaningful difference in the price of 8417 and 6L6 tubes because the 8417 is no longer in production.

I am staggering along in another thread with my copy; except for tolex and bits, about done but I rebuilt the power supply for 6L6s; plate, screen, and bias supplies are different. The amp's biggest virtues are its big clean output, one channel each for bass and guitar, and separate channel and Master volume controls allowing the player more variation in tones. Also has a bright switch and another set of filters on it.



Link to a recently closed Thunderbass auction Here, $274.00 Beige tolex and funky hardware on the bottom that mates with the hardware on certain Guild speaker cabinets. Seller didn't ID whether this one was an 8417 or 6L6 model. The caps in both these amps are 35 - 40 years old and anyone considering messing with one of these has to take that into consideration.

cj

Standard caveat:



"it's not a Fender"


Make sure your tech isn't fixated on Fullerton.

What does one do with a Thunderbass that sounds like a hissing cat? I played King_pizza's at one of our last shows and toward the end of the night, it was having a whole lot of trouble staying focused. My guess caps or power tubes, but I'm guessing you guys have more experience.

capnjuan said:

The amp's biggest virtues are its big clean output

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this is why I ask.

danerectal said:

What does one do with a Thunderbass that sounds like a hissing cat? I played King_pizza's at one of our last shows and toward the end of the night, it was having a whole lot of trouble staying focused. My guess caps or power tubes, but I'm guessing you guys have more experience.

capnjuan said:

The amp's biggest virtues are its big clean output

Click to expand...

this is why I ask.

Click to expand...

Hi Dane: always risky trying to diagnose via the ether but generally when power supply caps start to fail, they leak DC to ground which is picked up by the audio circuit and amplified as hum. Not saying it isn't the caps but for diagnostics, I'd suggest the 'Get Lucky' approach. Starting with a known good 12AX7, switch it in/out to determine whether one of the preamp tubes is f'd up. If all good and a known good 7247/12DW7 driver tube available, same thing. If all good, then same for the output tubes. If lucky, then it's a tube and all is good. Among other ailments, hiss is also a characteristic of a failing resistor the fix for which requires intervention with a meter and possibly a soldering iron. Try 'getting lucky' first... :wink:

Bump: all the Thunderbass amps and all/nearly all of the Thunderbirds (v2) / Thunderstars use a 7247 / 12DW7 as the driver / phase inverter; physically closest to the output tubes in that amp. Because a 12AX7 will work in that position and the 7247 is less common, alot of people sub in the 12AX7.

Every Guild amp using the 7247 has a 15K / 2W plate resistor and 14K / 2W cathode resistor. The 'back half' of the driver tube is expected to dissipate alot of voltage - hence the 2W ratings on the resistors. Although the 12AX7 is able to dissipate in the range of 300v, the 7247 is a considerably more rugged tube that was specified for the application. For test purposes, you can put a 12AX7 in the driver position but a 7247 belongs there; in good supply, not especially expensive.

capnjuan said:

If all good and a known good 7247/12DW7 driver tube available, same thing.

Click to expand...

This could very well be the issue. I happen to have a few dozen old Baldwin 12AX7s, so I'll try substituting preamp tubes and then go for the phase inverter. I don't think I have a 7247 around and the one in the Thunderbass is a deathly old Telefunken, so that's where I'm putting my money without the amp directly in front of me. The 6L6s are old too. They come from my Harmony 420 and are probably on their last legs. I think I'll try to convince Pizza to go the all new tube route if I can isolate the problem first. Otherwise it may have to see the inside of Marguerite's.

Hi Dane: shown below, the 7247/12DW7 driver from the TBass 2, 3, or 4-tube preamp models and several other Guild amps. The 'B3' tap on the power supply is providing 335 volts. That voltage is being dropped across the 100k plate resistor on the front half and the 15K resistor on the phase inverter back half; there is more current at higher voltage in the back than there is in the front.

According to the RCA data sheets, each half of a 12AX7 can handle as much as 300vdc and the plates can dissipate up to 1.1 watts max. By contrast the 7247 'unit 2' - the back half - can handle 330vdc but dissipate as much as 3.1 watts; it's a much more ruggedly-built tube.

danerectal said:

Otherwise it may have to see the inside of Marguerite's

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Is Marguerite a local girl?

capnjuan said:

Is Marguerite a local girl?

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She's the local music store in Moorehead. :mrgreen: The only place within at least 200 miles that has a Guild that I've seen. I've also had them do work on my Princeton Reverb and Dreamlander's Bassman 100, so I know they'll treat the TB right.

Hope you get it straightened out ok. J

Rats, I could have sworn I posted a reply here. :?:

Any way. I made every conceivable tube swap with no positive results. I did happen to notice that the power tube on the right (from behind) had a strange blueish glow as soon as it was warmed up. I'm thinking it could be a resistor issue. Maybe a capacitor? I'm not really too sure what's happening, but I tried a half dozen power tubes that should've had no problem and all of them reacted the same way.

Resistor driftage, a leaky cap feeding the powertube....
No way of knowing without a meter. :?

danerectal said:

Rats, I could have sworn I posted a reply here... made every ... tube swap ... no positive results...I did notice that the power tube on the right (from behind) had a strange blueish glow as soon as it was warmed up.... I tried a half dozen power tubes that should've had no problem and all of them reacted the same way.

Click to expand...

"Working on mysteries without any clues....' Bob Seeger Hi Dane; default's right; time for intervention by a qualified meter operator. The power supply generates all kinds of voltages for the output tube; primary B+, screen power, and bias as well as supplies to the preamp and driver boards; one, several, or all of these sources could be off.



In the pic above, the red (B5) is the primary B+, green (B4) is the screen supply, and the yellow (X) is the bias. Not trying to show off but you can see there is a dozen passive parts like resistors and capacitors that control these voltages. With the amp off (and one hand in your pocket), you can test resistors except that they may drift off under load. My best guess (from 2,000 miles away) would be problems in the nest of resistors in the bias supply; the 6.8, 2.7K, and the other resistors down stream of the 'X' bias supply tie point, shown below:



If you get adventurous, this is what the cluster of bias resistors looks like. Since there are potentially lethal voltages in the amp, you must drain the DC energy from the big, cylindrical power supply caps by connecting each positive terminal to ground with a low value / 5 watt resistor; then the next, then the next.... Need a meter to confirm that voltage to ground from the hot terminals is 0.... Good luck ...

Thanks fellas. For now I think it's across the border to Marguerite's. I'm going to do some reading in the next month or so, and hopefully start some build projects starting with some fixing. Right now, I'm not quite ready to tackle this one on the inside.

What's the old Clint Eastwood quote about a man "...knowin' his limits..."? Good luck, happy to help in any way. John

Bump: Hi Dane: where to go for:

Inspiration: The Guitar Amp Handbook, Dave Hunter, Backbeat Books; excellent read and down the middle; not too technical, not too homogenized. Discusses some of the finest circuits ever produced, how they work and why they sound the way they do. Includes interviews with people like Ken Fischer from Trainwreck amps and others.

Electronics foundation; Basic Electricity and Electronics and How AC and DC Circuits Work, Howard Sams Publishing - as straight-forward as it gets. If you're interested in these, PM me with your street address, free to a good home.

Troubleshooting is the highest art in amp work; if you can't do it or if you try to fix something that has a problem you don't understand, this will lead to frustration and/or making the problem harder to locate. Suggest either buying dead gear - for which you have a schematic - and just building it back with fresh parts; unless the power tranformer is blown, by replacing every passive part and the tubes, sooner or later you will fix the problem.

Alternatively, consider scoring in the low end of the eBay tube amp spectrum; Model 2 Kalamazoos, Silvertone 1471s/1481s and so on. The Silvertones are simple, well-documented amps which, for $75, you can buy every part in the amp, including a new output transformer and 3-wire cord. Without gushing like an idiot, it's hard to describe how well Jeff Heard's 1471 came out; simple Champ-like single-ended circuit that didn't cost $500 for a cheezey, no-nuts, re-issue 'Champion 600' or $1,500 for an authentic vintage Champ ... put simply, it has wonderful vintage tone; very 50s, Buddy Holly, early gymnasium-dance. Not hard to work on thereby upping the chance of a good outcome. Bite off too much and you'll be using your soldering iron to burn your name into wood fence boards ....

Lots of help here; John

Those sound like some pretty helpful books. As of right now I'll be trying my hand on a rebuild of a Kay Model 703-C. If all goes well, it's on to building some combos for friends and coworkers. After that, I've got a friend that wants to start building boutique-style heads with mixed cabs. :shock:

Hmmm .... electronic cousin to Chris B's Dano but with an isolation transformer and a different tube set. Good luck! cj

Reference Schematic: Kay 703-C here

I've got that one saved and printed, plus I found a non-labeled one. I wonder how that happened. I suppose someone had the parts list with the schematic...