Thunderbass amps: Convert 8417s to 6L6s

capnjuan

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As President Reagan once said "There you go again" :oops: :oops: :oops: The fact is that in the land of the blind, the one-eyed man is king; this applies equally to amps and choosing a good laundary detergent. I switched out the bias resistors shown in the last pic and pretty much thought I had the amp straightened out but never got around to firing it up. When you expressed interest, I turned it on and got a puff of white smoke and the plates in the 6L6s - large vertical metal pieces in the tube - started to 'red-plate', glow red hot.

Needless to say this was a source of some stress :evil: At the time, I was so busy, I made contingent arrangements to ship it to default in Philadelphia and see if he could straighten it out. By the time I got to manning up over it, I found I'd connected the bias caps in backwards. In the image below, the green box shows the filter caps for the bias supply; the red box, some of the primary DC filter caps; notice the difference in the symbols. As a practical matter, all bias voltages are negative so the (-) end of the cap is connected to the supply and the (+) end to ground; the opposite is true for the primary DC which is always a positive voltage. Anyway, I had the bias caps in backwards thereby smoking the 1st filter cap (both bias caps and the dropping resistor have since been replaced) and letting the 6L6 run wild thus the red-plating.

guildbias.jpg


Despite your complimentary remarks, capnjuan (President and Chief Engineer of Juanzamps), stumbles and falls too .... :mrgreen: CJ
 

john_kidder

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capnjuan said:
Despite your complimentary remarks, capnjuan stumbles and falls too ....CJ

Well, I guess. Ain't nobody poifeck, after all. But at least you stumble and fall in ways that you then decode for those among us who tend to stumble, then to pick up the bad foot to see what caused the stumble thereby overweighting the foot that's still on the ground, thereby stumbling again a little harder, then finding ourselves trying to examine both feet at the same time, thus turning what might have been a minor stumble into a major pratfall, then cartwheeling end-over-end down the originally light grade to the steeper portion where descent is frightfully difficult to arrest and finally revolving way too fast and disappearing over the precipice to oblivion.

So thanks again.

John
 

john_kidder

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capnjuan said:
Despite your complimentary remarks, capnjuan stumbles and falls too ....CJ

Well, I guess. Ain't nobody poifeck, after all. But at least you stumble and fall in ways that you then decode for those among us who tend to stumble, then to pick up the bad foot to see what caused the stumble thereby overweighting the foot that's still on the ground, thereby stumbling again a little harder, then finding ourselves trying to examine both feet at the same time, thus turning what might have been a minor stumble into a major pratfall, then cartwheeling end-over-end down the originally light grade to the steeper portion where descent is frightfully difficult to arrest and finally revolving way too fast and disappearing over the precipice to oblivion.

So thanks again.

John
 

capnjuan

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My pleasure John; once I calmed down, I realized that I hadn't blown the fuse. Thereater it was ... as you say ... downhill from there. But my hands do get a little raw from brushing myself off so much :wink:.
 

capnjuan

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My pleasure John; once I calmed down, I realized that I hadn't blown the fuse. Thereater it was ... as you say ... downhill from there. But my hands do get a little raw from brushing myself off so much :wink:.
 

somecallmetim

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First let me say thanks to capnjuan & everyone else here, I've already learned quite a bit reading this forum...

I just picked a thunderbass on craigslist for $75. I'm thinking about doing the swap, but I'd like to see if I can get it running first. The PO thought that the speakers were blown, Right now it turns on but is quiet & fuzzy. The speakers ohm & look good (about 7.5 for the pair) to me. Further inspection revealed that one of the (mismatched) 8417s cathode resistors is cooked & the other ohms out at 12 in the cirucut, its marked 6.8 - 1%. The closest thing I have on hand are 10ohm 5% 1watt. I'm tempted to toss them in there & see what happens, but I'm not shure if that is a good idea or not. I need a proper schematic, the on I found online is for the 6l6 version...

is it likely that the cooked resistor is just a symptom of a large problem? What should I be checking before I go further?

thanks again, Tim

Now for some amp pron...

100_5610.jpg


sn 5311

100_5613.jpg


100_5607.jpg


100_5603.jpg

100_5606.jpg
 

capnjuan

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Hi Tim and welcome to LTG; send me a PM including your email addie - I have a schematic for the 8417 version, happy to send it along. T'were my amp, I'd probably start at the bias supply end - note the 'X' in both pics below. That is the design bias supply - you are looking for -20vdc out (from the 8417 schematic, 6L6 shown) where the voltage is set by the 3.3K resistor. If you want to try biasing with 6L6s (see below), you may need to change the dropping resistor to 2.7K. If it's off or the bias supply filter caps are no good, then you won't have what you need here - can be checked with amp on and no tubes in sockets. If the supply is good, check the cold and hot values of the resistors; 1Ks, 91Ks, and the 2.7K. I found 6.8ohm resistors (5%, not 1%) but did not find the 91Ks ... the values and layout of this circuit are identical in both the 6L6 and 8417 models. I'd suggest replacing the 6.8ohms and, unless they were off, I'd leave the rest of them alone ... at least for now.

If you get it stable and values within range, you can try to bias with the 8417s you have recognizing that one may be shot. Note also that the design cathode voltage is .525vdc; that's the target value to achieve by spinning the bias pots and reading the voltage at the test points; the amp can take un-matched 8417s. If you only have one known-good, you can always bias the other side by moving the tube.

bias.jpg



If you decide to rebuild the power supply to support 6L6s, the pic below compares the design voltages for the 8417 amp v. 6L6 layout.

6V6v7417.jpg



The 6L6 and 8417 share the same pinouts; physically the 6L6, provided nobody has cowboyed something on to your pin 1s, is a drop-in replacement for the 8417 and will run at the lower voltages provided by the existing 8417 supply ... it just won't reach optimum performance.

6L6v8417.jpg



My model would have looked like the head below; the cabinet had the bottom holes for the stand-off bracket / carrying handle.

tbassbigrig-1.jpg



Not sayin' this is what yours looks like but every amp thread is better with porn. :wink:

bigboy.jpg



The 8417, like the 7027, 7189A and others, has pretty much vanished; NOSs are expensive as are the used models. Finally, check the driver tube; the schematics call for a 7247 (a/k/a 12DW7) but 12A_7s share the same pin-out and will fit and function but you'll get better performance if you can find a 7247; the driver half of the 7247 is built to dissipate the 300+ plate voltage ... a 12A_7 can do it but not over a long period of time. Let me know what I can do to help. CJ
 

capnjuan

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somecallmetim said:
Thanks again, mine is the goofy upside-down handle model.
My goofy handle was gone when the amp arrived. I patched the holes with dowel and plastic wood in anticipation of fresh tolex ... it was painted day-glo magenta ... which would have been fine but my black light burned out long before "...the legend ever did..." Sir Elton "Issenda" John

My suggestion for handling this is a standard strap mounted on one of the cab's ends. Can be carried straight-down at your side particularly if all the preamp and power tube clamps are working correctly .. like many of the straps on later models which relied on rubber feet for friction - not brackets - to hold them in place ... No application to The Guild Amplifier Authenticity Society necessary ...

Full view of this style amp and matching cab ... said to have been used in several Star Trek episodes and, fitted with a Neutron Emulsifier, will kill bugs in a 50-yard radius ... :wink:

tbassbigrig.jpg
 

somecallmetim

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Last night I did the checks recommed, things are looking good. I've got about -18.5v bias supply. 310v to the screens 410v to the plates. I replaced the 6.8ohm cat resistors with the 10ohms i had on hand, biased to .525ish with no trouble. :)

Sounds pretty darn good for an amp I have $75.10 in, but has some hum & isn't nearly as loud as I expected. I've pretty much decided that I'm going to do a full recap & retube with the 6l6s.

Thanks again.

-t
 

capnjuan

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somecallmetim said:
... has some hum & isn't nearly as loud as I expected. I've pretty much decided that I'm going to do a full recap & retube with the 6l6s.
Hi Tim; You can do as you see fit but I re-built the entire DC power rail. Although my wife considers it a character fault in me, I am not the kind of guy who takes issues that I had a chance to fix beforehand very well. That is, apart from the massive power-dropping resistors, I replaced all the dropping resistors and rebuilt the bias supply too .... If I were a mature adult, I could probably tolerate down-stream resistor failure ... but I saw it as something I could take measures to prevent when I was doing so much work anyway but ... like I said, it's your call.

Both supply designs show a 10ohm/10W resistor ahead of the rectifier. In my re-do and despite duplicating the 6L6 layout, I did not get the 460V primary. Since it couldn't have been me ( :wink: ), I think that whoever adjusted the ps design for 6L6s didn't take that 10ohm/10W resistor into consideration ... that would provide a reason (not-CJ's fault) for the limited voltage ... I guess I'm saying that when you make your move, consider buying a 5ohm/10W resistor for that location - put the 10ohm in if you choose but .... just sayin' about that resistor.

I hope the pics help ... recognizing that you already know your beans. Guild had a lot of respect for that power supply design and transformer; it's common to many models from the era ... speculating, it was an in-house 'design-it-once' project that only needed tweaking a few resistor values to work. Bearing in mind my amp was running at 410V primary, it was plenty brawny, warm, clean, airy, glassy; did not test the bass side of the equation. Good luck! John
 

capnjuan

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Bump: similar Guild head on eBay: Auction Link not the best pics, converted to use 6L6s. Pretty high opening bid compared to recent sales.


!BZdynFw!mk~$(KGrHgoOKkMEjlLmV,uSBKm04qd9Eg~~_1.JPG
 

capnjuan

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capnjuan said:
Bump: similar Guild head on eBay: Auction Link not the best pics, converted to use 6L6s. Pretty high opening bid compared to recent sales.

!BZdynFw!mk~$(KGrHgoOKkMEjlLmV,uSBKm04qd9Eg~~_1.JPG

Sold: $381 ... Wow! ... best recent price for one of these heads, and in considerably better cosmetic condition than this one, was $340 :shock: :shock:
 

somecallmetim

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I'm finally getting back to this project, I've got a big shopping list going at AES, just a couple of questions before I send my order...

1) It looks like in the pics you have 550uf/385v JJ caps where the 250/300s go. The closest AES has are 250/500v or 220/385v. Right now I have the 250/500s in my cart.

2) A 12dw7 is the same as 7247? I'm buying a full set of tubes, probably JJs unless they are crap...

-t
 

capnjuan

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somecallmetim said:
1) Right now I have the 250/500s in my cart. They will be fine.
2) A 12dw7 is the same as 7247? I'm buying a full set of tubes, probably JJs unless they are crap... Yes; 12DW7 same as 7247.
Hi Tim; the sad fact is that buying new tubes is a pig in a poke thing. There are as many people who hoot and holler about Brand X as there are people who dis Brand X as nothing but crap. If it were mine, I'd probably buy mid-priced 6L6s then go one of two ways; either 3 new JJs or mid-price 12AX7s or I'd get used / cheap 12AX7s off my workbench or eBay, get the amp up and running, and then slowly upgrade the tube set one at a time.

Apart from the output tubes and if you're a guitar player, the two tubes you want to spend your money on are the guitar channel preamp (on left viewed from rear) and the driver / 7247. eBay search string for 7247s The guitar preamp tube has everything to do with the amp's tone and you want a stout-hearted 7247 facing the outputs; Amperex / Mullard / Siemens / Brimar ... don't skimp on the money here. Good luck with your project. CJ
 

capnjuan

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Ok; good luck! CJ
 

somecallmetim

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I spent some time on the TB today, here is where I'm at.

Power supply...

1) Changed the 10ohm 10w to 5ohm
2) Changed the 2k25w & 6.8k25w to 6l6 values, 1.5k10w & 20k25w
3) The resistor between B5 & B3 is also diffrent. 4.7k for the 8417 10k for the 6l6. I put in a 10k.
4) Fresh caps & diodes, new 6l6s.

I'm getting...

b5-413v
b3-302v
b2 - 253v
b1 - 220v
b4 - 370v

Bias supply...

1) New caps. The 8417 were 500uf/50v & 50uf/50v I put in a 100uf/100v & 50uf/50v per the 6l6 print.

2) I missed the 2.7k resistor when I was ordering parts. The 8417 has a 3.3k. I stuck another resistor (15k iirc) in parallel to get a measured 2.7k. Kinda hokey, I know, I'll fix it later...

3) New Diode & 5ohm 5w resistor.
4) Replaced the 10ohm resistors I had on the cathodes with the proper 6.8ohm ones.

Now in only getting -21 v (right for 8417), not -44 as shown on the 6l6 print. This make sense to me as I'm only getting -34v after the diode, only 25vac out of the transformer. How did you deal with this?

When I check the bias points I'm getting about .85vdc turned all the way down, nearly a full volt the other way.

I didn't run the amp for long, given my lack of understanding about the bias circut.
 
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