1965 Fender Bandmaster - Hiss When Treble is on 7 or Higher

[Cornbinder]

Hello:

I am new to the forum and I am hoping someone here can help track this issue down.  Please forgive me if this has already been posted.
I have a 1965 Fender Bandmaster that sounds great except I get quite a bit of hiss when I turn the treble up on the normal channel.  This happens even if nothing at all is plugged into the amp.  Turn the treble down and the hiss goes away.  Maybe this is normal however I nothing to compare my old amp with.

Here is what I have done this far:

All electrolytics have been replaced, filter, coupling and rectifer cap last year.  Note amp wouldn't work at all until I replaced these.
Replaced a pair of crappy Radio Shack speakers with 2 Weber's. (The speakers are awesome).
Replaced all the original RCA's and Mullards with Reissue Tungsol 12AX7's and 6L6's. Replaced the phase inverter with a 12AT7 EVH.
All the original RCA's and Mullard's still sound good however I have set them aside.
Have substituted all my 12AX7's in V1,  Still have the hiss when the treble is at 7 and up.
Have replaced the 100k plate resistors and slope resistors with MOS resistors.  No change.
Have replaced the 470 ohm and 1.5 k ohm resistors on the power tube sockets.  No change.
Changed the phase inverter 100k & 82K plate resistors. No change.

I know anything beyond V1 and the tone stack probably wasn't necessary but there was no real cost involved.
Should I be looking at the treble pot itself?  All pots have been cleaned with DeOxit, no scratchiness.
Should I suspect a tone signal cap or the grid resistors?

Thanks in advance!

[HotBluePlates]

Consider 2 things:
1.  If the hiss level changes when the Treble pot is rotated, then the hiss originates prior to that pot (this means it's in the 1st gain stage for that channel).
2.  It's normal for hiss to increase/decrease with a Treble pot change, as much of the hiss sounds like a treble frequency.

Tubes can be noisy, but so can resistors. Quite a lot of things can contribute to overall noise floor.

However, since we are considering just the first gain stage, you might consider replacing (one at a time, so you know which had an effect) the series 68kΩ resistors from the input jacks, then 1MΩ to ground at the input jacks, the 100kΩ plate resistor, the 100kΩ slope resistor in the tone stack, and the 1.5kΩ cathode resistor. Metal film will have the least noise of readily available types in these values.

You mentioned having already replaced all 100kΩ resistors with (I presume) metal oxide resistors.

That's fine and you can move on from there. If you want to test the input jack resistors before wrestling with them, you can use a short jumper with alligator clips to ground the point where the two 68kΩ resistors are tied to the input stage grid wire. That will eliminate the 68kΩ & 1MΩ resistors from the circuit.

I'd probably do the above to see if replacing the 1.5kΩ cathode resistor seems warranted.

You'll have to decide how quiet is "quiet enough". Carbon comp resistors will be noisier than carbon film, which themselves can be noisier than metal film, which are again noisier than wirewound (where the resistance is low enough to permit use of a wirewound resistor). And older carbon comps will be hissy compared to new carbon comps. That said, lots of part-swapping can impact resale value of a vintage amp, and most old amps are noisy compared to a new build.

Only you can decide how low the noise floor must be, and when to call it "good enough". Note also there is some lower limit of noise which you simply can't reduce, which is due to thermal noise and how tubes function.

[Willabe]

Metal film will have the least noise of readily available types in these values.

You mentioned having already replaced all 100kΩ resistors with (I presume) metal oxide resistors.

Have replaced the 100k plate resistors and slope resistors with MOS resistors.  No change.

If you used NOS carbon comp. 100K's try metal film or metal oxide resistors.