Thunderbass advice wanted

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Hello LTG.
Your Guild amp forum is unmatched on the Internet. Thanks to y'all, the web no longer has a dearth of info on Guild amps. Rather, there is just a shortage.

So here's my story and my reason for posting. About a decade ago I received a Thunderbass head. It didn't work due to some burned up resistors in the bias circuit. After my main amp was stolen ('63 Bandmaster) I had it repaired (and got a grounded cord) and have been using it since. I like it a lot. I suspect I could truly love it if it was what I imagine it could be.

I have a plan for this amp: I want it sound as good as it looks. (I think the head looks great.) I know I should overhaul this thing. I would like to do the work myself. I don't normally play in old electronics, but I know my theory, can solder well, and am very afraid of electrocution (read know I must drain the filter caps). Eventually, I might want to mod one of the channels (I'm a guitarist for the most part). And there are some problematic noises that come out from time to time. I'll start with some pron. (I have the cabinet which is 3/4 inch plywood, black tolex, and two vents on top. I just didn't include it here.)

Front.png

Rear.png

Underside.png



The serial no. is 7197.
The intended tubes are (3) 12ax7, (1) 7247, (2) 6l6.

I identify this amp as a Thunderbass 1-A. Because of the cabinet styling and serial number I guess it is late in the run and from 1969. Does anyone think I'm wrong?

First question: What are the dangers and problems presented by using a 12ax7 instead of the 7247 as the inverter driver? That has been the case for the past few years and nothing noticeable has changed in the amp. I know the 7247 is half 12ax7 and half 12au7, so I suspect half of the signal is being over driven and the push-pull doesn't sum back up as it should.

Second question: What is the right schematic? I have downloaded the 'Guild Thunderbass 4 Tube Preamp' that is at prowessamplifiers.com. From everything I've traced, this is the right schematic. Specifically the filter caps. This leads me to my third questions.

Third questions: Caps… I see no damage or bulges in any caps. Should I replace all the smaller caps in the preamps or just the filter caps?

Fourth question: Sometimes, the amp will start to make noises. The noises are intermittent, loud, and sound like I am plugging in a cable without the standby on. These loud throaty clunking noises last longer than that though, more like a couple of seconds. They can accompany a slight change in tone too. They happen even if the standby is on. From my thought experiments on troubleshooting this I find it might be a bad solder joint or accidental ground just before the output stage. Any thoughts?

Fifth question: In closing this long first post, in what order should I do following?
a) troubleshoot the noise
b) replace caps
c) replace tubes
d) bring it to a professional


I'll thank anyone and everyone in advance... Thank you.

-E
 

capnjuan

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Hi ekwon and welcome to LTG. It's good to be concerned about electrocution ... just don't wet your finger when you put it in your ear ... :wink: Some of these (early ones?) were 8417-powered, Only differences are in the bias and power supply; link to a thread where I converted the same model 8417 amp to 6L6s Here. Yours is missing the vertical metal shield that goes between the guitar channel circuit board in the middle and the output transformer; bolt holes visible in the pics. Your model has the big dropping resistor on top to dissipate heat, in my copy, it was underneath.

No guess on the year of mfr. Yes, the driver half of the 12AX7 has more gain than the 12AU7 but you can see by the schematic that that 15K R is dropping nearly 300V on the plate of the back half ... that's close to the top of the capability of a 12AX7 ... that's the reason for the 7247; the voltage handling ability of the 12AU7 driver half ... that tube is a 'worker bee' contributing very little to the tone of the amp ... and you might be right about the overdriven thing ... but mostly it's a about having a tube than can safely dissipate the the energy. Yes; that's the right schematic, I put it on Prowess Amps; this BB received it as a courtesy from FMIC.

Caps… Just the filter and bias caps. The green coupling caps are polycarbonate ... and signal caps are, as one of our BBers described ... like the taste of cheese. Did you know that France has 246 different registered types? If they aren't bad and unless you like Mallories or something, I'd suggest leaving them alone for now ... not to mention that it's a serious PITA to loosen and flip those little circuit boards off their standoffs ... it's tedious, time-consuming, and risky so ... you better have a strong preference ...

Noise ... Last time anybody cleaned the tube sockets and pins? Sometimes corrosion forms between the pins and sockets crudding things up ... sometimes crackly spittly but start there and clean the controls too; CAIG DeOxit (red) for the pots, blue or any other non-lubricating contact cleaner for the sockets / pins; squirt once or twice, push in / pull out tube several times. If there's inadequate pressure from the sockets, you might have to re-tension them. Can search net for lots of good directions. Dental tool works best; drain ps caps, follow directions, and when your done, pick your teeth ... your dental hygenist will love you.

Sequence ...
b) replace caps; power supply and bias.
a) troubleshoot the noise; clean sockets, pots, and jacks
c) replace tubes; only maybe; you need to rebias and play ... see what happens
d) bring it to a professional

Do yourself a favor though and make large-scale free-hand drawings of exactly what you have. Pics are nice but you need something with you so you can make notes, mark up what you've finished and stop and start without relying on memory to tell you what you have and haven't done.

Good luck with you amp and welcome to LTG! CJ
 

bassmyf

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CJ, you`re one of the reasons I love this place, and half the time I don`t even know what you`re talking about :oops:
 

capnjuan

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Thanks ... but it's mostly 'In the land of the blind, the one-eyed man is king' ... I'm just a maintenance guy but this stuff comes easier to people like me who've spent time :D and made more mistakes :cry: than others but thanks again. CJ
 
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Thanks Cap'n. I'm excited to start ruining my amp. I'm going to follow your advice, except I'm not going to replace capacitors with cheese.

But seriously, I know that a 4 cap can that is spec'ed right for this amp is no longer available. I recall from your output tube conversion from 8417s to 6l6s, you said you were just going to replace the 4 cap can with 4 individual caps. Is that what you did with that amp? The space will get mildly crowded with 4 separate caps and I was wondering if you took a photo of the finished topside that you might be able to share.

-E
 

capnjuan

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Hi E: You are correct; the 4 X 40uf X 450V can cap is not longer available. I used two JJs and a 3X40ufX450V (on the left) ...

newcaps-1.jpg



... and found a home underneath for the 4th 40uf/450V section ... the blue Sprague Atom at the bottom. I had two large, self-disassembling 25W power dropping resistors the replacement of which produced the handy clamp used to fasten it in place:


tbasssupply07.jpg


The other replacement 25W resistor is that black one at the upper right; could have been stud-mounted like yours on the top but it's okay there. Finally, my amp had a 2-section bias filter can cap where the yellow arrow is pointing ... you have a 2-section paper cigar. I replaced the original can with the fresh caps in the yellow box. I also noticed that you amp doesn't have a dropping rail to the right of the caps like the one shown in the pic above.

Everybody feels differently about how much is too much but I re-did the entire supply including all the dropping resistors; partly because the values for the screens were different for the 6L6s and because I like myself too much to have to go back in there and replace an original $.75 part because it failed when I could have done it when I was messing with it before. Same goes for the two diodes; okay .. they don't fail too often but they don't cost much or take much time to replace/upsize either.

This is a link to my bass amp photobucket album. Lots more pics of my amp and many other Guild models including some that will look similar to mine and yours except there's a funky extra tube in the middle of the chassis. Those are pics of the 'Quantum' version of this amp; pretty much the same audio circuit and controls but that tube is a 6GF7 that was used to regulate the screen voltage supply. Several members here have Quantums and they are prized by their owners; of this series, it's the one to own. Good luck with yours.

Good decision on avoiding the cheese choice too ... :wink: CJ
 

fab467

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Don't know how you keep all that info. in your brain Capn. And access it too! My challenge? Wonder where I parked my car? :? :)
 

capnjuan

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Hi Fab and thanks for the compliment but it's just a different form of obsession ... that plus I threaded my Thunderbass head here giving me my own notes to peek at. Best wishes, J
 
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So I'm working on following the Capnjuan's advice. Since it will be a few days before any caps arrive, I started by checking some component values and decided to bias the output tubes. I used the bias ports to check the voltages across each tube. I swear I know how to use my multimeter but I was getting crazy low values. I assume that the schematic is correct in that .414vDC is the intended bias, and others threads here mention setting the bias to .410v or similar. Well, one tube read .031vDC and the other read .022vDC. That's an order of magnitude. That's clearly not right. I turned the trim pots up and down and got ranges of ±.01v from the readings I was getting. I balanced these readings to .029v. I played for a minute and the tube that had started at .022v burned out. The amp was turned off immediately.

Now that I'm thinking about it, maybe the repair that was originally done to this amp changed the wiring to make sense to the amp tech, but doesn't match the schematics... I'm going to go trace some circuits, but in the meantime anyone have any guesses other than I'm totally incompetent? I studied physics in college, so I know I'm incompetent as an engineer.

-E
 
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wrong component values

I was right about the bias circuit being repaired incorrectly. The 6.8? resistors that go to ground from pin 8 of the 6l6s are only 2? in my case, and the third resistor that comes of the bias trim pots that goes to ground at the same point should be 2700? but in my case was 8200?. So I see now why the cathode voltage read so low, and that explains why the amp has always sounded thin. I wonder what these amps sound like if they are wired like they were intended?

Additionally, in my amp, the 2? resistors were each piggybacked in parallel with a small diode. I don't see any parallel diodes on the schematic, so I suppose I'll pull those when I replace all the wrong resistors. And now I've confirmed why I want to do this myself. I think a trustworthy well qualified amp tech is as hard to find as a great mechanic. I'll admit that the repairs to my amp were done without the schematic, so there was some guesswork done with the circuit, since these components were fried and probably unreadable. So maybe I should back off of that claim and say that the correct schematic is gold.

Either way, I can't wait to hear what my amp will sound like when I'm done with it.

-E
 

capnjuan

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Hi E: well ... you're getting there. Can either test the bias supply voltage from the transformer or at the pots. I'd trust the schematic ... since it isn't clear what the other guy was trying to do. Supposed to look like this.

bias01.jpg



Confirm that when you are turning the amp on, you have speaker or a dummy load connected to the output transformer ... must have a load of some kind or you will fry the OT. I know you are tied up but it would help if you could post a better pic of the chassis ... taken while it's laying flat upside down ... closeups of both ends ... then we can see what you see. CJ
 

capnjuan

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Your amp and the schematic; Don't forget to look at the supply end. Should have -73V coming out of diode, -44V leaving ... if you don't have it, it won't matter what you do at the other end ... it will never be right until this end is straightened out.

tbassbias.jpg
 
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I'm just replying to say thanks so far and apologize for being so slow to respond. I'm buying a house right now and it's been taking up all the time that should be devoted to taking pictures of bad circuits and soldering. I'll post the screwy pics of the old repair job on my amp and more thoughts on it soon. Then I'll be ready for some more 'advice'.

E
 

capnjuan

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Hi E: thanks for the note ... good luck with your house; happy to help as much as possible. J
 
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