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'51 Deluxe (5A3?) restoration advice


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6/2/2000 1:07 AM
Dr Rico
'51 Deluxe (5A3?) restoration advice
Hi folks. I scored an interesting amp last week. Its a transitional model between a 5A3 and a 5B3 with some twists. The preamp tube is a 6SC7 and the PI is a 6SL7. I drew out the schematic and compared it to the closest schematic I could find (a 5C3) and found a few differences. It looks to me like all the parts are original (and wired in by Lupe 49 years ago) except one electrolytic, one heater wire and the cord was swapped with a 3 prong. Here's how it differs from a 5C3:  
 
1) The PI is a 6SL7 instead of a 6SC7  
 
2) There's no negative feedback (and no 1 Meg resistor)  
 
3) The speaker is wired directly to the OT  
 
4) The 6V6 grids are strapped across 250K, 7.5K, 250K instead of 1Meg, 25K, 250K (the 6V6 cathodes are tied to a 250ohm-5W resistor bypassed by a 25uF-25V cap that ties between the 250K and 7.5K resistors and one of the 6SL7 anodes ties to a point that splits the other 250K from the 7.5K).  
 
5) The caps that tie the input jacks to the 6SC7 are 0.05uF-600V instead of 0.1uF-600V.  
 
6) There is an unmarked ceramic disk cap that ties the anodes together on the 6SL7.  
 
Could any of you all point me to a 5A3 schematic so I can check out what I have? Do you recommend keeping these values or should I drop in the 5C3 values (except the negative feedback)? Some of these values are difficult for me to chase down. How close is "close enough"...10% ok? I'm thinking of swapping more modern values for the inputs (68K instead of 75K to the grids, 1Meg instead of 5Meg to earth). And what should I do with that little ceramic cap?  
 
Opinions warmly accepted. Thanks muchly for your time.  
 
Hasta -> Rico
 
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6/3/2000 7:21 PM
Steve Ahola

Rico:  
 
    As for the schematic for a 5A3 have you looked over at the Fender Field Guide site? I think that I have seen a schematic for a very early Deluxe around here someplace...  
 
    As for the unmarked ceramic disk cap between the anodes of the 6SL7, do you have a DMM with capacitance ranges? If not you could probably bring it to Radio Shack to "test out" one of their meters... :D  
 
--Good luck!  
 
Steve Ahola
 
6/4/2000 6:16 AM
Dr Rico

Thanks for the shout, Steve. The closest schematic at the FFG site is a 5C3, which is a pretty different beast (negative feedback, etc.). I've been using an analog meter...better buy a DMM I suppose (kids' shoes come first). A net acquaintance from Weber's site sent me a 5B3 schematic that looks closer than the 5C3. On this schematic, the cap says "0.0001??-1000" I can't make out if its uF, nF or whatever. I suspect its 100pF, but dunno. I'll ask our bassist to bring his DMM to practice manana (he's an electrician).  
 
Best Regards -> Rico  
 
P.S. Do you want the 5B3 schematic and the schematic I made of my circuit?
 
6/5/2000 1:31 AM
Graywater

Dr R,  
 
The 6SC7 and 6SL7 are the same tube with the former having a common cathode - see if the 6SL7 cathodes are tied together. This early amp is so rare that I would leave it as stock as possible and I would expect that even the master hackers would agree,.  
 
GW
 
6/5/2000 2:48 AM
Dr Rico

Yup, there's a jumper between the cathodes on the 6SL7. Was that done so the 6SL7 could be used in this slot? Why didn't they just drop in another 6SC7? The tube chart is modified to note the change, too!  
 
"Ours is not to reason why, ours is but to stand and stare in mute astonishment." - Cheech Wizard  
 
Hasta -> Rico
 
6/5/2000 2:38 PM
Graywater

Dr R,  
 
For many years after WWII the market was glutted with surplus tubes (I remember buying stuff in the late 60s) and both these tubes were used by the military in quantity - so my guess is Fender left their options open and used which ever tube was the cheapest at the time of mfg.  
 
Not too many of us left who remember Vaughn Bode'.  
 
GW
 
6/6/2000 1:23 AM
Steve Ahola

Rico:  
 
    Thanks for e-mailing me the drawings! They have been uploaded as:  
 
http://www.ampage.org/blueguitar/5a3_sch.pdf  
 
http://www.ampage.org/blueguitar/5b3_sch.jpg  
 
http://www.ampage.org/blueguitar/5b3_sch.jpg  
 
 
Steve Ahola  
 
P.S. So with the jumper on the cathodes you could use either a 6SL7 or 6SC7- right? I had read here that a 5751 is very similar to the 6SL7... but if they are that similar why would Mr. Allesandro use a 6SL7 in his Hounddog Redbone? Just wondering...
 

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