'63 Brown Super on the bench...

[SleepLess]

Hi there!
A friend of mine recently purchased a 1963 Brown Super amp and gave it to me yesterday for a thorough check. He just said that he turned it ON, it emitted a loud noise and he turned it OFF. I have opened it up and found a few surprises (click on the pics):







Yes, it is from November 1963 with an Export PT (I'm in France) and it doesn't have a GZ34 but diode rectification!!! What the hell!!! I figured out I might bypass the diodes and install a GZ34 in the socket but I don't see the two yellow wires coming from the PT that would need to go to the GZ34. The two red wires do go to the diodes. It is supposed to be a 6G4-A but that schematic shows a tube there, not diodes!!!

If you have any idea, shoot!
Thanks!

[eleventeen]

If there's no 5 volt winding you'll have a physical issue mounting a 5 v transformer to power the GZ34 heater.


The barrier strip & pin plug seems to be the way of selecting line voltage.


"Certainly" the export transformer can be made to work but you had best work out what you have going there before getting too involved.


The bias section appears to have been changed to fixed bias (I dislike that, the proper terminology, because it isn't "fixed"--it's "variable" with that pot there!)

[SleepLess]

Absolutely eleventeen! I love that line voltage selection strip, I think it is very clever and also safer than the switch Fender used on the backplates on later models. I don't see any 5V winding on this PT, that's a PITA! I guess I'll have to stick with the diodes then. The OT was already changed for a Hammond so I don't want to change the PT as well or it will lose too much value. I may replace the Hammond for a nice Magnetic Components OT though.

So, so far I have turned it ON, all knobs on 5 except volumes on 0 and no pops. Some strange crackling at times though but nothing major or that had to make me flip it OFF. The B+ is 422V and the amp was biased at 32mA. I added 10mA to that, it's now at 42mA. I had a huge hiss when the bias reached 47mA so I took it down...

[Willabe]

Can you post the #'s off of the PT? Maybe I can look them up in a couple of books I have on Fender iron and see what that PT is really for.

Should be stamped on the bell housing.


                  Brad     

[SleepLess]

Can you post the #'s off of the PT? Maybe I can look them up in a couple of books I have on Fender iron and see what that PT is really for.

Should be stamped on the bell housing.


                  Brad     

Absolutely Brad, I even have a picture!
Numbers are:
125P32B
606 337
I know the second means made by fender the 37th week of 1963, which means September. The tube chart is stamped MK which means November 1963. So it's an original PT. Glad you can help Brad!

[Willabe]

Nope, can't find it, sorry.

But I will say;

1. Most (more than 50%) PT's for the brown amps had a 4 digit #. The BF amps had the 125P..... #'s, only a few (that are listed) brown face amps had the 125P.... #. (Maybe that's when Fender was switching over to Schumacher from Triad?) 

2. Most PT's listed for export (there weren't many) had an X as a last/2nd suffix.

(Gerald Weber/Kendrick amps and Dave Funk books.)


               Brad        

[eleventeen]

Probably the infamous 100K plate resistors are causing at least 75% of the hiss. Looks like you have had some of the cathode bypass caps replaced. (and one or more NOT replaced) Otherwise, the parts board seems reasonably stock.


You are apparently NOT getting the "loud noise" eg; the source of the owner's complaint?

[HotBluePlates]

The Pro of the same era had a solid-state rectifier. Maybe the export transformer was sized for the solid-state diodes (and lacked a 5v winding) so that is what Fender used. There are no PT voltages given on the brown Super or Pro schematics, so we don't know for sure.

[SleepLess]

So I fired it up today and played it a little. The sound is ok but not as good as it should be, it's a little trebly and skinny sounding. I played my 5F4 replica through the cab and it sounded great so the speakers are definitely good (and original!). All settings on 5 except Presence on 7. I had a huge squeal when I turned the presence knob down to 4 and below... It's the first time I experience noise when I turn things down... lol.

I'm gonna try and change the Presence pot blue molded cap to see if that squeal stops, unless you have other tips!
Thanks!

[6G6]

From all I have heard, the blue molded caps are hardly ever bad.

[eleventeen]

Before changing caps a little bit "at random" I would go through the exercise of cleaning the pots (something you would do as a piece of normal maintenance) and then seeing if you get any kind of static noise turning them---to see if you have DC leakage. This is on the theory that you want to be non-intrusive if you can, you are investigating the general quality of a few caps doing this.


Then, pull V1 (again, I am calling the 12AX7 most distant from the power tubes "V1" even though there is no such designation from Fender) and get an idea of how much hiss ch 1 is making. Do the same with the 2nd 12AX7.


Finally, pull V5 (1 tube distant from power tubes) and isolate the output section from all input and mixing circuitry. Check noise, rotate presence control.


"Quick survey" is the operative word. I have not worked on many Brown Fenders but looking at the schematic, there sure looks like there are lots of caps in there. (said as an observation, not meaning anything this way or that)

[sluckey]

The OT was already changed for a Hammond...

...The sound is ok but not as good as it should be, it's a little trebly and skinny sounding.... I had a huge squeal when I turned the presence knob down to 4 and below...

You may have wrong phase on NFB loop. I would verify correct phase by temporarily disconnecting the NFB. If squeal goes away then reverse the OT plate leads.

[SleepLess]

Will do so guys! Thanks a lot, I'll keep you posted!

[goldstache]

On a related note.  I have what I think is a 62' 6G4-A brown super opened for it's first service/recap.  It's by far my favorite fender I own.  It started blowing fuses about 6 months ago.  Stuck it in the pile of broken amps I have, till I got around to diagnosing the issue. 
Hooked it up to the variac without any tubes installed.  Replaced the fuse.  Brought the AC up slow and she didn't blow after about 30 seconds.  It used to blow immediately upon powering up.  The guts are all stock except for the bias section I rebuilt when i bought it 6-7 years ago.  I tested all the tubes in it and they checked (good) on my old EICO tester.  After the stability with the tubes out I decided to pop a new recto in despite the old one checking good on the tester.
Voila' amp is back and sounding like her old self, however I do have some blistered filter caps and decoupling caps.  Gonna change them.  Any suggestions for a similar sound. 
Not to hijack your thread, but I am wondering a few things:


1.  Silkscreened on the backplate of my amp it says "2A slo-blo" 
In the schematic it says 3A
What should I do here?


2.  Also my bias term is chugging pretty hard in the low end throb when intensity is past 6.  What parts should I focus on in that section, to help this.  Im used to opto repairs and am not sure where it's stemming from.


Not sure if we have the same beast on the bench, but if you need any photos or gut shots or serial number comparisons, feel free to ask!
Thanks!


Hope she is still so dark and warm after the new caps.  I know there will be a shift, just want to get parts in there that capture its existing sound.   

[John]

For the fuse, imo only, if the 2A doesn't give you problems blowing, use that. Nothing wrong with a little extra margin of error.


On replacing those caps,if possible measure the capacitance of them. Most decent DMM's will have that on there, even my $50 Klein has one. If you're hoping to keep the same sound, replace with modern caps that measure about the same as the old ones (that have probably drifted 1 way or the other)


ONLY my 2 cents, which ain't worth halfa dat. 

[DummyLoad]

i'd use the fuse rating that's printed on the amp, not the schematic. 


--pete

[SleepLess]

Thanks for your messages. Interesting to know goldstache!
So I did Sluckey's test and it turns out that the OT wires were reversed. Swapped them around and no more squealing! Thanks!

The amp sounds gorgeous and I don't have any cratchy pots. The only flaw is that the amp is really noisy. Everything's fine even on 8 on the volume knob as long as you don't plug a guitar into it, then when you do it is really noisy (I'm not talking about playing... lol). I have tried all 4 inputs and 3 different guitars (Strat, Tele, LP) and the hum is really big on all of them. I have tried to replace all the preamp tubes one by one for a known good one but the hum stays the same.

It's not a big deal as long as you play but when you stop playing, I'm telling you you'd better bring the guitar volume knob down to 0 or it's hell! Any tips about that?
Thanks!

[John]

I'd check the grid resistor feeding V1a, if that's not it try shielded wire going to the input valve. After trying a different patch cord from guitar to amp, of course.    It sounds as if your guitar is acting as an antenna, so I'd look at that input resistor first.


Other guys will have better ideas... trouble shooting is not my strong suit!

[shooter]

"everything's fine til You plug in the guitar" 
I just fixed a problem like that, was the guys cord(s), the shield was broke on one n she just screamed, the other cord seemed fine electronically but got real noisy when plugged in.

[goldstache]

+1 on trouble shooting the input to grid of V1A
And shielded cable perhaps.

Glad it's sounding better!

[SleepLess]

About 19 months ago, some one on this forum found an amp with a similar p/t 125p32b, try a search on this forum for : 7355/6L6 circuit,

Thank you!