Thunderstar Bass Schematic 3-tube Pre-amp version

capnjuan

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Free to a good home: PDF of a schematic for the 2 X 12AX7 / 1 X 7247 / 2 X 6L6 / ss rectifier version of this amp. Exact model or revision # unknown.

For more on the 7247, go here: http://tdsl.duncanamps.com/show.php?des=7247. Its pinouts are the same as a 12AX7 but it may not function the same in the circuit.

SFIII was kind enough to put one in play for the 4 X 12AX7 version, also w/ diode rectifier and 2 X 6L6.

Soooieeeee...... These amps operate @ 470 volts on the outputs .... makes my stuff look like table radios...

If it helps in ID'ing your model:

The 3-tube pre-amp version has 2 inputs, 2 pre-amp outs, and 2 8-ohm outputs, primary and auxillary.

The 4-tube pre-amp version has 4 inputs; 2 normal and 2 bass, no pre-amp outs, and the same pair of 8-ohm outputs.

Still confused? Open it and figure out how many of what kind of tubes are in the amp... :wink:

Post here or PM for copy.

cj
 

john_kidder

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Just noticed that someone has swapped out one of the (4) 12AX7s in my Thunderbass for a 12AT7. Web search tells me this may be to get a cleaner sound from the pre-amp stage? If so, why would the swapper do it for only one pre-amp?
 

john_kidder

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So, on a little reflection, that may have been a dumb question. Maybe the desire was to have one clean channel for (say) a bass, and one that was more prone to overdrive for (say) a lead guitar?
 

dklsplace

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I had tried using 12AT's in my Hotrod Deville to take some of the edge off & warm up the tone. Ultimately used 5751's which did the trick.
 

capnjuan

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The 4-tube model (SFIII's schematic) indicates 3 X 12AX7s and 1 X 7247 driving the 6L6s; my earlier post suggested that all 4 were 12AX7s; my error. The 7247 now is more commonly known as the 12DW7:
Here

The 12AT7 has a lower gain factor than the 12AX7; same 'sound' just less of it assuming all things are equal...a 12AT7 could take some edge off and represent tweaking; same pinouts, same function; drop-in substitutes.

5751: you might be on to something Don: according to the folks at the Tube Depot:
"...If you want to reduce the gain of your amp so you can reach that creamy tone without blowing the windows out, then this is THE tube to get. Fully interchangeable with a 12AX7 tube, the US made JAN Philips 5751 will allow you to drive the power stage harder with less volume.".

According to this version of the Hot Rod Deville: http://www.blueguitar.org/new/schem/fen ... deluxe.pdf This amp uses .1uf coupling caps and metal film resistors; more on both below.

ThunderCat posted some pics of his 2 X 12AX7 model in which Guild used metal oxide/film resistors, mylar caps, and relatively large value coupling caps ahead of the outputs; the Guild T1 RVT has .15ufs and the Thunderbass has .27ufs; larger value coupling caps plus metal film resistors is a recipe for 'clean'.

By contrast, my Gibson 6V6 amps have .02ufs in the same position even though both the Guild and Gibson driver tubes are operating at nearly the same voltage and, similar to the Thunderbass, the Fender 75 (2X6L6, 75 watts) has .022uf coupling caps.

Caps: re/ your suggestion John about 'clean' for bass and something closer to 'dirty' for lead in the twin-input Thunderbasses, I offer the following example of the effects of .15uf caps and larger-value caps in general between the driver and output tubes.

I have a working copy of a T1 RVT (.15uf caps), a top quality Gibson GA18 (.02uf caps - one seventh the value of the corresponding Guild cap in the same position), and a Marshall BluesBreaker II distortion pedal - and a really fine pedal it is. Both amps are 2 X 12AX7 and 15 watts nominal output.

Using the BB2 and the GA18, I get a sweetly gritty output. Putting the BB2 in front of the T1 RVT, I get clean; turn the BB2 pots, I get clean; turn the T1 volume up, I get clean; not Spam a la SNL, but 'Clean Clean Clean Clean...' Even though I am 'injecting' a disorted signal into the T1, I get clean out the speakers which are used Celestions in good condition; that is, any 'dirty' coming through the amp isn't being damped by sluggish speakers. The 'dirty' is being filtered out ahead of the output tubes; they never see the distorted signal due to the effects of large-value coupling caps.

Metal film resistors: the 'holy grail' of stereophiles; less noise, better resistance to heat and 'drift', and can be manufactured in tighter tolerances. The stereo crowd (I used to belong but lost my card...) likes these because the last thing they want is grit; okay if in the signal, not ok if generated locally.

Carbon composition resistors and lower-value coupling caps contribute to the the typical sound of the 1960s amp/era; raspy, gritty, twangy. Maybe Guild was after a particular sound the way Leo the Legend chased his or because Guild sold more jazz guitars and bass amps that needed 'clean', who knows but I wonder whether any of the mid/late 60s Guild amps can do 'dirty'.

I have to watch it here because Matsickma gets annoyed when I hit on Guild amps. My T1 does what it does very well; musical, versatile, decent tone, and a nice wide soundstage but if grind is the goal with a mid/late 60s Guild amp, better warm up the soldering iron...

Thanks for the tip on the 5751s DKL; just might buy a pair and do a little tube rollin'. Based on its schematic, I could see where the stock Hot Rod Deville might tend towards the 'dry' side.

Any news on the Thunderbird John?

cj
 

dklsplace

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Based on its schematic, I could see where the stock Hot Rod Deville might tend towards the 'dry' side.

They also came from the factory biased extremely cold. I had replaced the stock Groove tubes with JJ's, replaced the stock 2x12's with Reverend Alltone 1250's & found the sweet spot in cranking up the bias adjustment. Turned out to be an almost entirely different animal than stock. But still too freakin' loud to serve me well.
 

capnjuan

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The Fender 75 had a low / high power switch on it; took it from 15-odd watts to 75, useful thing; didn't see one on the Hot Rod. The F75 also had 3 cascaded volume controls; volume, master, and lead.

It got a little like the old Bob Newhart show: "This is my brother Darryl and this is my other brother Darryl..." Turn this one for louder, turn this one for louder, turn this one for louder...turn the wrong one abruptly and Whoa Nelly.

Have you tried fooling with an external attenuator? 500K pot in a little plastic box; 1/4" jacks in/out wired via preamp-out / power-in. Would allow you to turn the volume way up in pre-amp then back down again - get those 5751s screamin' although I guess that's what all the drive / more drive stuff is supposed to do.

What made you choose the Reverend Alltones? Are they ceramic or alnico?
 

dklsplace

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I remember a lot of talk & someone posting simple plans for a DIY volume box, but I never did. The HR is long gone. Aside from the volume, I just got tired of lugging 65+ lbs out of the basement every time I played out. Glad I didn't have the 4x10! :roll:

I believe the Alltones are ceramic. I tried one on the recommendation of a friend who builds his own amps. It was such an improvement I bought another directly from Joe Naylor. Wish he was still making them! Even though the amp is long gone, the Alltones remain. I have one in my '60 Martin, & another in a '49 Bell & Howell projector amp housing with a custom class A amp. Very sweet!
 

capnjuan

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10-4 on Fenders:

My F75 also achieved that fearful symmetry; 75 watts @ 75 pounds. Play out? A few hundred for the amp and a couple of thousand for the forklift truck to move it...

I'll bet your class A amp is killer; output, 6V6?

When it comes to tone, less is more; reaching someone seated in the 40th row outdoors is something else.
cj
 
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