Fender brownface hiss

[g-man]

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[eleventeen]

I can't speak from experience of having worked on these, but looking at the schematics maybe you have to replace the 100K resistor(s) in the tone network(s) too. Who knows, maybe you have a teensy bit of leakage through your tone-shaping caps; they are not defective to the point of being unacceptable, but perhaps they are passing enough leakage current so that such leakage current is passing thru the remaining 100Ks?


Are your tone controls silent when turned?

 

[g-man]

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[eleventeen]

That's generally something you do on an amp that old almost on a "need it or not basis", IMO. For the 30 cents each, you might as well. Please let us know if that fixes the problem (or not!) 

[Willabe]

CCR's are also flammable. Like screen grid R's it's a good thing to go with flameproof metal film R's for B+ dropping R's.


                Brad   

 

[HotBluePlates]

If it were my amp, I'd be looking at resistors over 100kΩ which connect (either directly, or ultimately) to tube grids.

The higher-resistance resistors tend to contribute the most hiss; the resistors which hiss near the input of the amp have their noise amplified by the rest of the amp. Pulling preamp tubes to break the circuit in various places will likely narrow down where the biggest offenders are if you don't have a scope or listening amp to spot where the noise originates.

[g-man]

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[eleventeen]

Great news! I myself did not think the intra-node resistors could cause hiss, but there you go.


You gotta love it when the cheapest part you can put in the amp solves the problem!

[g-man]

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[HotBluePlates]

The main reason I started there is because the hiss was identical on both channels, so I was looking for something common to both channels and before the volume controls.

Excellent reasoning!

[AZJimC]

Ahhh, nothing like the sound of a warm resistor sizzling.

Good call,
I'll have to remember that one for future hiss chases.

Jim