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I Finally got one of these!

I was able to find a few F&T capacitors, a 30,22, and 22uf so this morning I put them in and there's
still hum any idea's how to clean it up further?
 
Try disconnecting your ground wire. If this was originally engineered with a two prong plug, this might be the cause of your hum.
 
I just tried that it didn't work it still hums, constant level does not go up or down with the volume,
Maybe the coupling caps are shot.
 
tube-a-lou said:
I just tried that it didn't work it still hums, constant level does not go up or down with the volume,
Maybe the coupling caps are shot.
Click to expand...
How balanced are your power tubes ?
Make sure Cesar did not try star grounding . I find you can reduce the hum in these a little by moving the right hand power supply electrolytic (preamp) ground to the preamp side and tied to the other two power supply caps.

Also, check the input jacks , that they are shorting to ground when nothing is plugged in.
 
billyz said:
How balanced are your power tubes ?
Make sure Cesar did not try star grounding . I find you can reduce the hum in these a little by moving the right hand power supply electrolytic (preamp) ground to the preamp side and tied to the other two power supply caps.

Also, check the input jacks , that they are shorting to ground when nothing is plugged in.
Click to expand...
I'm sure those caps are leaking.

Also pull each preamp tube and see if it gets better. That can narrow it done.
 
Hi Bill,
I started to change those coupling caps and it seems a bit quieter, before when I turned up the
volume there was that scratching sound, I replaced four of those .05 caps I did switch the electrolytic's
but I had them lying around I'm getting new ones this week I ordered 3 of the 22uf and 2 of the 25uf
I might as well start fresh. Thanks for the help I might need it till I get this hum gone.
 
billyz said:
How balanced are your power tubes ?
Make sure Cesar did not try star grounding . I find you can reduce the hum in these a little by moving the right hand power supply electrolytic (preamp) ground to the preamp side and tied to the other two power supply caps.

Also, check the input jacks , that they are shorting to ground when nothing is plugged in.
Click to expand...

Can you show me what your talking about with the schematic, so we are on the same page
Thank You
 

Attachments

tube-a-lou said:
Can you show me what your talking about with the schematic, so we are on the same page
Thank You
Click to expand...
It already is seperated but fender grounded to the brass ground plane at multiple places , indicated on the layout. When I build one I run a ground wire from the preamp power supply cap all the way to the input side .
I'll dig up a drawing. But I doubt this is your problem as it only makes a small improvement .
Get new filter caps in . The scratchy noise could be D.C. Leaking thru the control pot. Again coupling cap leaking.
 
billyz said:
It already is seperated but fender grounded to the brass ground plane at multiple places , indicated on the layout. When I build one I run a ground wire from the preamp power supply cap all the way to the input side .
I'll dig up a drawing. But I doubt this is your problem as it only makes a small improvement .
Get new filter caps in . The scratchy noise could be D.C. Leaking thru the control pot. Again coupling cap leaking.
Click to expand...
From rob robinettes site.
IMG_8027.PNG
 
I changed the last to caps the .1uf caps and it getting better still hear it if I put my ear
against the cab but the motorboating has stop and at least the scratching of the volume
controls has stop. My next thing is for some reason someone put into the amp 220K
resistors instead of 250K's would that make a difference in output of the amp sound?
 
tube-a-lou said:
I changed the last to caps the .1uf caps and it getting better still hear it if I put my ear
against the cab but the motorboating has stop and at least the scratching of the volume
controls has stop. My next thing is for some reason someone put into the amp 220K
resistors instead of 250K's would that make a difference in output of the amp sound?
Click to expand...
I don't think it will make much difference. Mostly they are plate load or grid load . As long as they are the same on each side it will be balanced.

I imagine they were a more common value in that day than 220k .
 
I think I figured out the hum problem, the ground wire for the preamp is not making connection
with the board I have to lift the board up and get to it.
 
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Oh boy, I'm really getting into it now!, checking all the grounds and wires underneath. there
is a electrolytic on the far right which needs to be grounded and it wasn't.

P1060296.JPG
 
Pretty much done, it's really quiet now I only hear hum when I put my ear
about 5" from the speaker, taking out the board was the best thing to do and
clean it out and fix the wires in the rear of it. The electrolytic on the far
right should of been grounded but wasn't so that's fix and a few other things
done as well. the plate dissipation after being on for 30 minutes went to
12.1 watts which is good.


P1060297.JPG
 
Good job! Those old fender have been around and abused for many years.
Are those your blue molded coupling caps? those are usually still good and have the MOJO tone. Was the NO brass ground plate under the controls? thats where all the grounds are soldered to.

Maybe that was a later update. this is an old one.
 
There's a brass ground plate but it's not soldered, the Blue molded caps were in another amp I had
and they worked fine in there, plus I purchased a few from a guy on ebay and were tested to
450volts and they were good. Thanks for the help anything else comes up I let you know.
 
Yes to the old blue molded caps. In my Vibro Champ, I replaced them with Mallorys and didn't like the change in tone so I put the blues back in.
 
See there's something about how those caps are made, I've been playing it for about a hour and it sounds
different not like the thin sounding amps of today (no ice pick) still has a wee bit hum don't know
how I could get rid of that. One thing it does is play's loud I could go to two or three for a living room and
that's it, still need to change a few caps but for a sixty year old amp it's good, maybe I should of brought the
Fender Princeton Blackface but I had two of them and I like the 12" speaker better, and who knows that
one might of have more problem in it than this one. I do remember reading the ad and seeing a repair note
from a dealer listing the problems it has.
 
Never mind they're right, I took them out and measured them, they are suppose
to be 75K but! they measured 138K and 98K so I ordered a few new Allen Bradley.
 
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