need a Thunderbass head or output transformer

Timi

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Hey y'all-
just checking to see if anybody might have a older model Thunderbass head they want to let go of- it doesn't need to be complete or functioning.... Mainly I need a replacement transformer for mine. If you just have the transformer, that would be awesome too. I used my Thunderbass as my main gigging amp for years til the transformer blew (right before the biggest show I ever played, by the way... 29,000 people.... jeez!)

Anyway, it hurts that I don't have this up and running. My tech put in a new transformer but it destroyed the sound I knew and loved about the old head. I gave it back to him so now the amp is sitting on a shelf in my house and those lovely 8417 tubes are just getting no play at all.

Thanks for reading- let me know!
all the best-
Timi
 

capnjuan

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Timi said:
... just checking to see if anybody might have a older model Thunderbass head or a replacement transformer If you just have the transformer, that would be awesome too. I used my Thunderbass as my main gigging amp for years til the transformer blew ... Anyway, it hurts that I don't have this up and running. My tech put in a new transformer but it destroyed the sound I knew and loved about the old head. I gave it back to him so now the amp is sitting on a shelf in my house and those lovely 8417 tubes are just getting no play at all.
Hi Timi; welcome to LTG .. sorry about your amp. Transformer ... power or output? This is a link to an eBay Thunderbass head. The auction doesn't say whether it's a 6L6 or 8417 version but you might ask the seller who, based on the auction text, sounds like one of those drop-it-off/I-sell-it type eBayers. The auction amp is in pretty good shape including a very rare complete set of knobs.

I know this amp pretty well; I used to think the 8417 models were the earlier ones with no metal corners but ... never say never with Guild ... I'm saying this one might be an 8417 model. Even if it isn't and you are looking for a power transformer, you might consider buying a complete amp with both transformers. This is a study I did comparing the power supplies of the 8417 and 6L6 model heads ... the result of converting an 8417 model to 6L6s Here:

6V6v7417-1-1.jpg


I don't know how technically inclined you are but the 8417 and 6L6 have the same pinouts. The 6L6 models run at higher plate and screen voltages but bias a little cooler. For example, if the auction amp were a 6L6 version and found to be running at the higher voltages, you tech can de-rate the supply using higher value dropping resistors. The Tube Data Sheet suggests not more than 300V on the screens. While these ratings are typically conservative and considering how rare ... dinosaur-like .. the 8417s have become, this would be no time for fooling around when it comes to exceeding mfrs' ratings.

Anyway, if the auction amp turned out to be an 8417 model, you'd be home free; use it as is or cannibalize it for parts. If it's a 6L6 model, you'd have to wait to get it in the house to see what the voltages are. In either case and if you have the caps re-done, that'd be the time to re-work the power supply to support 8417s ... wouldn't add that much money to a cap job. I think you're going to find it's a move like this or learn to love 6L6s. Good luck. CJ
 

Timi

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Thank ya, Cap'n!

That Tbass must have just been listed... thanks for the link! I might just bid on that thing. Those definitely look like 8417s in the auction pics, but boy are they glowing! Scary. Mine's older- the thinner cabinet wood with no metal corners... of course I found it at a dump and it looked like a family of bats had been living in it for the last 15 years, but I got it cleaned up, re-capped and re-tubed and well,,, it was rock n roll til the transformer died. It's the output transformer, btw.

I will admit this amp was a little tricky- it had a habit of the bias increasing mysteriously over time and cooking 8417s, and those are not cheap to replace. Love the sound of those big fat tubes though. I'd like to keep it the same... though it isn't COMPLETELY clear that this amp was ORIGINALLY an 8417 model...? The output transformer did look a little small to handle the wattage of those tubes. That's what my tech seemed to think anyway- that the transformer appeared to be a 30-ish watts deal, whereas the tubes seemed to be producing 50-60 watts. I found it with 8417s in it though, so that's what I've been using since.

Thanks for the info on conversions too. Maybe that's the route to go, but I keep thinking a good set of 8417s should last 10+ years if I could only just get everything else right! Ah, we shall see. Thanks again!
Timi
 

capnjuan

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Timi said:
... I might just bid on that thing. Those definitely look like 8417s in the auction pics, but boy are they glowing! It's the output transformer, btw ... it had a habit of the bias increasing mysteriously over time and cooking 8417s, and those are not cheap to replace ... though it isn't COMPLETELY clear that this amp was ORIGINALLY an 8417 model...?

The output transformer did look a little small to handle the wattage of those tubes. whereas the tubes seemed to be producing 50-60 watts. I found it with 8417s in it though, so that's what I've been using since.

... but I keep thinking a good set of 8417s should last 10+ years if I could only just get everything else right!
Yes; it's a recent listing ... This is the model I had and I think you have ... we'll call it the earlier version ...

tbassbigrig.jpg



... and this one the later version ... this is the model in the auction:

thunderbass01.jpg


Yes; the tubes in the auction amp are red-plating ... the bias is way too hot. The creeping bias in your amp is also probably a function of either the cheesy little 1/2-watt bias resistor drifting out of tolerance, weakened bias caps, bad resistors at the bias pots, or some combination. Sorry if I didn't make myself clear; the square cabs (no metal corners) I believe are all / nearly all 8417 while the later models (metal corners) are all / nearly all 6L6 ... of course, never say never.

If you and your tech think the OT looks small, then chances are it's not original ... often the case when dumpster-diving for amps ... that's how I came by mine. Yes; you should get 10 years out of a pair of power tubes but not when the bias is off. Good luck; these heads get way too little respect. CJ
 

Timi

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Those are great pics! I agree these amps aren't as highly regarded by the masses as they should be, but I will tell you that everywhere I went and gigged with it, it got respect!

That being said, here are a couple pics of my head... When I got it the tolex was shot so I re-covered it with genuine brown suede. Totally badass!

tbass1.jpg


That's it, atop my custom 2x12 loaded with Eminence Red Fangs (which is now 1 Red Fang and 1 Wizard)


tbass2.jpg


Here's the back view with the blown transformer placed back in the chassis to show scale. My tech guy said it was 16 Ohm with the original output transformer, then the new one (that I didn't like and gave back) was 4 or 8 Ohm.. thus the scribbling on the left.

I SO want this to work again. I've since acquired a late 70s Ampeg V4 head- which is totally phenomenal by the way - but I miss the gorgeous, ballsy, transparent tone (and the light-weightedness!) of the Tbass a lot. This amp was also fitted with 12ax7's in all the 7247 positions by various amp techs (working without schematics, I suppose), which seems wrong. I was having a lot of trouble with the tone being crackly and spitty when turned up before I gave up on the new transformer.... I was definitely wondering if the 12ax7's were at least partially to blame for that. I pulled a 7247 out of my Hammond organ and swapped it in there in each position and it did seem to smoothen things out a bit.

I'm considering sending the old transformer to Mercury Magnetics to have them retro-engineer one for it... but looking at this, does the one in the ebay auction seem too different to use for parts? Hard for me to tell, being not so electronically inclined.
 

capnjuan

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Timi said:
... When I got it the tolex was shot so I re-covered it with genuine brown suede. Hi Timi: when I got mine, it was painted day-glo magenta! ... but I miss the gorgeous, ballsy, transparent tone .. mine was transparent too ... glass-like. ... I was having a lot of trouble with the tone being crackly and spitty when turned up before I gave up on the new transformer.... I was definitely wondering if the 12ax7's were at least partially to blame for that. Could have been many things; dirty tube pins and sockets, dirty controls, dying cathode resistors ... ... I pulled a 7247 out of my Hammond organ and swapped it in there in each position and it did seem to smoothen things out a bit. The 7247 is 1/2 12AX7 and the other 1/2 12AU7 ... yes ... you can use them anywhere but the 12AU7 1/2 doesn't have as high an amplification factor as the 12AX7. But you must use a 7247 in the phase inverter/driver position ... next to the output tubes because the circuit is designed to supply nearly 300V to the driver 1/2 ... ok ... a 12AX7 can handle it but the 12AU7 1/2 of the 7247 will take the punishment longer and sound better for it. ... but looking at this, does the one in the ebay auction seem too different to use for parts? Hard for me to tell, being not so electronically inclined.
If the eBay amp is wired for 6L6s; that is, the power supply puts out 460V or so to the plates, the OT's Zout ... load resistance ... will have been matched to the 6L6s based on their plate and screen voltages. You'd have to dismount the OT and try and determine it's Zout through either measurement or researching the part by mfr's ID #.

I have the schematics for both the 6L6 and 8417 versions; send me a PM with your email addie ... I'll send them to you. If you buy the amp in the auction and it doesn't work, you're not going to be disappointed in switching to 6L6s ... if your ears can tolerate the difference, your wallet will thank you. Good luck. CJ
 
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