New member with new toy ( Thunder 1)

Brendan

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Hi,
Nice place you have here. :) Someone pointed me in the direction of this forum after I mentioned a Craigslist purchase I made on Saturday. I bought this '67 or '68(?) Thunder 1 (part no. T1-12, serial number 4515) for $50. With a very grainy picture and a description that merely said "powers up", I was expecting a bit of a basketcase...and that was fine. When I saw it in person, I was shocked at how clean it was. Aside from the missing back panel, nothing looks touched and it still has the factory tubes. Apparently someone's grandfather passed away and this was in his apartment. The seller said they borrowed a guitar to test it and it made some noise, but then began to smell funny and they shut it down. The best pics I have at the moment:
thunder1.jpg

thunder2.jpg

thunder3.jpg


I guess my first question concerns the schematic. I've been inside an amp before for minor component changes and understand the safety issues at play, but I really want to understand how it all works and part of the reason for buying was to learn. If I understand correctly, the RVT schematic is pretty close, but I have found two sites that claim to have the T1-12 for a price. Has anyone actually seen one of these? I've seen some really bad schematic scans before....but I guess it's only 5-15 bucks, right? :wink:

Can caps - I see one on the chassis and it looks like there's one at Antique Radio Supply with these specs: 40/40/40/40 UF @350 VDC That seems to match what I'm seeing on the RVT schematic. Would this be a suitable replacement?

Last question - any recommendations for a 12" speaker? I understand that some of these had 10" speakers and this one is definitely a 12". I'm looking for something that breaks up quite late, so if anyone has recommendations, I'd love to hear it.

I'm removing the chassis next weekend for a visual inventory, but will not begin work until after Christmas. I have a bit of reading to do in the meantime. Any help appreciated!

Brendan
 

coastie99

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Congratulations on your "bargain" Brendon, and welcome to the Forum.

I can absolutely guarantee that someone will be along real soon to talk to you. In fact, I can sense capnjuan stirring as we speak !!
 

capnjuan

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Hi Brendan and welcome ... indeed it is I, capnjuan stirring from me slumber ... PM me and include your email addie, I'll send you the schematic for your amp. Subject to confirmation, your amp has the one 'brown cigar' 40uf cardboard cylinder cap near the rectifier tube and a 4-section can cap. That first cap needs to be rated 40uf/450vdc; either a 40uf Sprague Atom although they're sort of bulky or a smaller 40uf F&T will work.

The Antique Radio Supply 4-section @ 350vdc will do fine provided it's the same diameter as the one you have in there. If so, don't forget the mounting hardware; wafers or clamp depending on ... Once you get inside, you'll find that the dropping resistors are mounted on the hot posts of the can cap. You'll have to decide whether to re-use or replace them ... but for another $2 ...

If the speaker code is 137XXX, then it's a CTS ceramic and, given that everything else is pristine, then it's likely the original. Unless the cone is shot or you know that the voice coil rubs, you might want to wait messing with a new speaker until you get the amp stabilized; find out what it sounds like and then make a move. Good luck! ... more pics of the chassis please when available. CJ
 

Brendan

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Thank you, gentlemen!

Capnjuan, I really appreciate the offer. Yes, from what I can see, it has one cap near the rectifier and the 4-section can cap. I will post some chassis shots when I pull it this weekend. PM coming...
 

capnjuan

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Hi Brendan; email w/ schematic sent. Good luck! CJ
 

john_kidder

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We all just love to hear about $50 deals for clean Guild amps, even though they may tickle a not-too-deeply-buried jealousy bone. And with your apparent expertise and Capnjuan on side, if it isn't singing now it soon will be.

Nicely done.
 
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Rather than start a new thread, I thought it would be best to add to this one.

I too am a new member who finds himself with a Thunder 1 (non-reverb) that needs a recap. I just bought it last night ($150). Mine looks just like Branden's, albeit not as clean. It is complete, original and has the back panel. The date codes on the filter caps put it at 1964. Unfortunately it does not power up. The tubes all check out ok. So does the fuse. One of the dropping resistors on the can is fried though.

My first step is going to be to replace the filter caps, the fried resistor, and the power cord (with a 3 prong).

I plan on starting as soon as I can obtain the parts and a schematic. I look forward to photographing and documenting the process and posting it in a new thread. I plan on doing this before Christmas so perhaps this could be of help to Branden.

My dad is a tube circuit guru and with his advice (via email and phone) I've repaired multiple amps over the last several years (Acoustic G60T, Fender Hot Rod Deluxe, Crate TV120, SonicCord Bantam). This is the first Guild amp I've ever owned. It is really great that there is a resource such as this site with such knowledgeable and helpful folks.

I would be grateful if someone could forward me a copy of the schematic.

Thank you very much.
 

Brendan

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I removed the chassis today to take a look. Some of the shielding on the cabinet ceiling was stuck behind the chassis faceplate and it took more force than I expected to remove it. I'm pretty sure this chassis had never been pulled before. I took some pics of the chassis today:
G1.jpg

G2.jpg

G6.jpg


Nothing really jumped out at me - no carnage visible. I was sort of hoping that I would find one "smoking gun" component. :) At this point my plan is to replace the filter caps and the dozen or so other caps. The ceramic disc caps should be fine, yes? I'll finish it off with the 3-prong plug and see what it does.

On that last pic there's a pot on the bottom right. What's that for? I know the tremolo strength pot is on the opposite corner, but I'm not sure what this one is. I'm still working on interpreting the schematic (thanks, capnjuan!)
 
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The pot is for bias. It controls how much current the power tubes draw. Too little and the amp sounds sterile. Too much and the power tubes will be saturated which might sound good but ruin the tubes. There is a happy medium. Often this is set by ear. Sometimes there is a recommended measurement. Which brings up a good question. Anybody out there know of a recommended test points and/or current values?
 
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I take that back. It looks like a "Strength" setting for the tremelo. Anybody have a recommended way of setting this?
 

Brendan

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Interesting. There's another pot near the other corner. You can see it next to the filter cap in the first pic. Is that bias?
 
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Brendan,

I'm looking at the schematic now. The other pot is not for bias. It's for hum balance. This you would adjust by ear until you hear the lowest amount of hum.
 

capnjuan

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Blues and Jazz said:
... the other pot is not for bias. It's for hum balance. This you would adjust by ear until you hear the lowest amount of hum.
Hi B&J, welcome to LTG, congratulations on your Guild amp, and you're doing fine so far. Other than manipulating resistor values in the bias supply, these amps have no means to adjust bias. Pics when ya got 'em. Welcome. CJ

Edit: ignore the remark above about bias. The schematic I sent Brendan is for a fixed-bias amp; I don't know about yours B&J but Brendan's is a cathode-bias version ... it doesn't match the schematic I sent him ... however I don't have a schematic for a cathode-biased amp. CJ
 

capnjuan

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Hi Brendan and B&J: Brendan, your amp is a cathode-biased amp and it doesn't exactly match the schematic I sent you. B&J; maybe you already know but if yours is like Brendan's, well, it's cathode-biased too. Pic of Brendan's amp showing, on the left, the HV secondary AC, rectifier heaters in green, and DC leg running to the multi-section can cap; there is no bias tap. On the right side, there's a resistor bypassed by an electrolytic cap running from the cathode of one output tube to ground. There's a jumper from the other output tube cathode to the + side of the bypass cap; this is how the bias of the output tubes is achieved; not by injecting -dc on the grids but by tying the cathodes together and connecting to ground via a bypassed resistor; this isn't a fixed-bias amp like the schematic, it's a cathode-biased amp. No cause for alarm ... just no applicable schematic. CJ


GuildT110or12.jpg
 
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Thanks so much, capnjuan. While I have a bit of practical experience, you are obviously much more knowledgeable than I am about theory. My amp does seem to be like Brendan's. Your info is applicable and welcome.

Well, part of a schematic is better than none at all. When I get a chance, I'll see if I can edit the schematic we have to reflect the actual circuit.

SonicCord went out of business and despite my Bantam being a wonderful sounding hand-built amp, there don't seem to be any schematics at all for them.

One positive development with my Guild amp. I found a suitable resistor in my parts bin and replaced the one I found that was charred. Then I powered the amp up again to trace the line power to the filter caps. I wasn't getting any readings at all. Then I traced the power cord connection to the plug. One of the connections was open.

I already have a suitable 3-prong cord that I will be installing tomorrow. Too late now. As my dad taught me, and I'm sure you agree, never work with electronics when you are tired.

Brendan, check your power cord. Yours might be bad too.

I'll start a new post tomorrow with pictures of the new line cord in place.

Good night, everyone.
 

Brendan

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capnjuan said:
Hi Brendan and B&J: Brendan, your amp is a cathode-biased amp and it doesn't exactly match the schematic I sent you. B&J; maybe you already know but if yours is like Brendan's, well, it's cathode-biased too. Pic of Brendan's amp showing, on the left, the HV secondary AC, rectifier heaters in green, and DC leg running to the multi-section can cap; there is no bias tap. On the right side, there's a resistor bypassed by an electrolytic cap running from the cathode of one output tube to ground. There's a jumper from the other output tube cathode to the + side of the bypass cap; this is how the bias of the output tubes is achieved; not by injecting -dc on the grids but by tying the cathodes together and connecting to ground via a bypassed resistor; this isn't a fixed-bias amp like the schematic, it's a cathode-biased amp. No cause for alarm ... just no applicable schematic. CJ


GuildT110or12.jpg

Thank you capnjuan...you've gone above and beyond! I think the schematic still helps. I'm putting a basic shopping list of caps I will be changing and look forward to digging in after Christmas. In the meantime I may create a layout to help me keep this sorted.
 

capnjuan

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Brendan said:
.... I think the schematic still helps ... In the meantime I may create a layout to help me keep this sorted.
Hi Brendan; you're welcome. The schematic is probably accurate for the preamp and power out. The 6GW8s are triode / pentodes. One 6GW8 triode half acts as the phase inverter / driver and the other triode section of the other 6GW8 is the tremolo oscillator that's connected to the 6GW8 pentode grids via the 39(?)0K resistors and jerks the grid voltage/volume up and down. If you don't know, the 6GW8 has long been out of production; you might want to consider contacting Coastie about replacements / backups. CJ
 
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