Fender power supply caps and resisters

[Rob Livesey]

Hi All,

I have a '73 Pro Reverb which is about to get a cap job.

The value of the resisters across the 20uf caps under the can, varies from schematic to schematic through the years.

The AA165 shows 4700 ohm and 1k.

Mine has 10k and 2.2k respectively. These are stock, and the combination does not show up on any of the published scematics.

What effect would changing the values of these resisters to AA165 have on the amp, ie is it desirable when performing a blackface mod?

Cheers,
Rob.

[martin manning]

This larger values will lower preamp, reverb recovery and PI voltages. I think I would try both ways, and also 1k and 10k to see what sounds best.

[Stevem]

Whatever value dropping resistor you pick/end with liking,make sure to step up to metal film type resistors to cut down on hiss and make them a higher wattage than stock by a factor of two or three.
To do this you may have to use two resistors in serise, or 3 or more in parrallel.
You can play around with crappy carbon types to get the values you want and then order up good metal film types.
In regards to the hiss issue I even go so far has replace every preamp stage 100k plate load resistor with a 3 watt metal film type, the ones I get are not much larger than a 1 watt type carbon.
Why guys and builders put up with all the added hiss from carbon types just to get 1/2 % old type resistor distortion when a high volotage is placed thru a carbon type is beyond my comprehension!
While you are in there its good time to blackface the PI section and get all the brown crappy cupuling caps out of the amp and if nothing eles change out the ciramic shit cap at the input to the PI.
If you play the amp above 4 or 5 and you do not want the bottom end to fart out for clean sounds you should change out the two main 70uf filters for 220 uf at 350 volts.
Also while you are in the filter house, check where the ground for the output tube screen filter is landed, if it is landed at the brass strip in the front of the amp under the controls, clip it off, extend it and ground in with the other power transfromer grounds over by the PT.
This will take a good amount of 120 hz idle hum out of the amp.

[Rob Livesey]

Thanks everyone for the good advice.

I have 3w Metal Films for the power supply in the stock AA165 values, I'll try those to start.

By saying the cap at the input to PI, that's the .01 ceramic that's there right now right? I have a Silver Mica .001 which I was going to use during the BF mod, lucky I already bought Silver Mica !

Re the 100k plate loads, I'll see what the noise is like first, but it's good advice. The amp is my own, perhaps if I was doing this for someone else I would do that right away so I wouldn't be seeing the amp back anytime soon.

Cheers,
Rob.

[Firestorm]

Is this a master volume model? That's the stock dropping string in almost everything Fender built with a master. No need to keep it, tho. As Martin said, see what you like. I you move the screen supply ground to where it should be, you may also have to fiddle with the reverb grounds, since the reverb tranny comes off the same node. Gerald Weber theorised that Fender had trouble with reverb oscillation and moved the screen supply ground/reverb send supply ground. Whether that's true or not ...

[pdf64]

Whatever value dropping resistor you pick/end with liking,make sure to step up to metal film type resistors to cut down on hiss

Stevem, are you sure about this?
As I see it, for a B+ dropper resistor to add hiss to the signal path, the B+ smoothing / decoupling caps would need to have developed massive ESR, ie be faulty.
Generally, any suitable, functional B+ cap would be likely to have an ESR of maybe 10 ohms max over the audio band, most probably well under that.
Which would massively attenuate any noise created by a dropper resistor.

However, I agree that their replacement, using a type with flame retardant properties eg metal oxide, would be beneficial.

If you play the amp above 4 or 5 and you do not want the bottom end to fart out for clean sounds you should change out the two main 70uf filters for 220 uf at 350 volts

Have you considered the <=40uF of reservoir capacitance that is typically used to protect the 5U4?
If it was perceived to be beneficial to nearly tripling this value then some mitigation for the additional plate current required to charge it could be considered, eg add some current limiting resistance in series with the rectifier plates.
See http://www.mif.pg.gda.pl/homepages/fran ... /5U4GB.pdf for further information on this topic.
Pete

[rp]

...and get all the brown crappy cupuling caps out of the amp...

I don't agree w/ this. After everything else has been done (new filters etc) if it sounds Fender 'right' - bright but sweat and not ice-picky, clear and lively not dark or muddy then leave well enough alone. If you do take them out do it carefully so you can put them back. I'm my mindless butcher days I would shotgun all the supposed 'crappy' brown and blue turds always for the worse. If you come across SFs full of holcos and MIT or Kimber caps that was me. But I'm far enough away now that you can't get me!

[Stevem]

The ESR on those epoxy soild brown or blue caps is pretty poor even when compaired with the older white label tubular blue caps that Fender used earlyer.

[martin manning]

I don't think ESR is important to coupling caps at audio frequencies. Unless it's so low that they are considered leaky ;^)