Fender Pro Reverb - hiss

[lowell]

Yes, the dreaded noise of an old Fender. This is an AA165. I've worked on many of these Fenders over the years and every time it seems to take hours and hours to find the noise culprit. This particular amp has hiss coming from before the Vibrato channel volume pot. The bright switch makes it worse. I've replaced Rk, Rc, and Ra to no effect. Tried a new preamp tube as well. The filter caps and dropping resistors look fairly new - definitely not original.

Veterans - what are your tricks for approaching this repair?

[Leo_Gnardo]

You've tried different tubes too I imagine. Some noise comes from leaky black cardboard the circuit's built on. Sometimes the cloth-insulation on wires running under the board gets leaky too. Generally not steady hiss, leakage noise tends to sound like what you'd expect from a tube going bad, occasional spitty noises, crumble, rumble, squeeks & kissy noises, like that. I use a tie strip or 2, build the circuit up off the board. Short of that, one of the worst offending leakage noise spots seems to be the pot side of the treble capacitor. You can yank that end out of the board, run a separate wire to the treble control with a bit of heat shrink at the junction with the cap, and solder the now abandoned wire to ground at the pot end. Or just remove it.

Lowest noise plate/anode R's are metal film, then carbon film. It's unusual for a cathode resistor to exhibit noise, the parallel cap tends to shunt that to ground. But it does happen from time to time, sounds more of a rumble than hiss. Fender's pair of 68K's at input definitely contribute noise, surprisingly this isn't often cured by replacing them with metal film. In fact you're overall better off with carbon comp as a "stopper" resistor. If you don't really need input #2, you can reduce the value of the "stopper" and also move it to the tube socket where it's going to do the most good. Typically 1K or more is good. If you start with a low value & find you're picking up radio, increase until you don't. When I do this typically the stopper winds up 10K to 22K.

To keep down hum not hiss of course it helps to have the input jack(s) tightly bolted to the chassis (you'd be surprised how many come in loose), and nice fresh clean shiny Switchcraft jacks are also a plus.

Gain builds noise too. You may be able to keep hiss noise down with lower gain preamp tubes like 5751 or 12AY7. Besides NOS, there's some freshly made ones out there by EH & JJ if you care to experiment. No need to have more gain than is really necessary, and that will be different for each player & their style. Keep in mind tubes further along in the circuit, for instance V4 in your Fender, are running at full gain for the dry signal. Swapping in a lower gain tube here can help keep the overall noise level down. The other triode in V4 is amplifying reverb - there's always too much reverb anyway isn't there so a dab less gain here can be a blessing.

[The Dude]

Often works- often doesn't, but as a matter of course, whenever you open up one of these "oldies", it's a good idea to just go 'round and tighten everything- jack nuts, ground screws/nuts, potentiometer nuts, transformer screws and nuts, etc. I find often times it's just a loose ground connection that's made by something on that list.

[lowell]

I did the following with no change in the level of hiss:

Lifted the pot side of the treble cap and the pot wire from the board
Replaced plate resistors
Replaced 68k input grid stoppers
Added 68k grid stopper physically on the grid pin of the input tube

I ended up adding a 100k (10khz -3db) grid stopper to the eq recovery grid. That helped a bit. I then put a 12ay7 in and this helped more than anything. The vibrato channel has an extra gain stage so it makes sense that it's noisier. It also makes sense that it should operate just fine, volume wise, with a lower gain tube such as a 12ay7. I'm a fan of reduced gain at the input in order to "gain stage" the amp in a way that the power tubes or PI are the first to overdrive. It's the warmest, tubiest sound IMO.

[pdf64]

Do the tone control settings, esp mid and treble, affect the hiss?

[lowell]

Yes Treble increased the hiss. I used a BFC on the grid of the input stage and most hiss went away. If I put the BFC on the plate of that stage the hiss was pretty much gone. So definitely was from those points or earlier in the circuit...which only leaves the grid stoppers, 1M grid leak, input jacks, input cable (shielded and grounded at input jacks). Wasn't convinced the input cable could be the problem, but I didn't try replacing it.