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The amp in question here is a SF Princeton Reverb but my remarks/question are really for all Fender amps with built-in tube driven reverb.
Now, I know that the reverb circuit is one of the weaker links, noise-wise, in a Fender amp. The transducers can be affected by proximity to the transformers, the cables connecting the reverb tanks to the chassis must have a very good ground connection and we're amplifying very small transducer reverbed signals here where if noise/hum is present, it can get amplified to noticeable levels.
Such is the case with my late 60s Princeton Reverb. It has a hum (yes, I know, it doesn't know the lyrics) and combined with the fact that the tank in this amp seems to need a little more recovery volume than most, I have a noticeable, distracting hum. If I leave the reverb at low levels, it's passable but if I'm playing a song that calls for a strong, deep reverb sound, then I hear this unwanted hum. I know I could change the tank out with another but reverb is about good tone and this reverb unit sounds very good so I'm reluctant to do that as a first remedy.
Now, normally when I encounter a reverb circuit in a Fender amp that has too much noise or hum, the first thing I do is change out the tubes to see if it makes a difference and because it's easy to do. The driver tube is usually a 12AT7 and the recovery tube is a 12AX7a/7025. If that doesn't help the situation, I usually bench the chassis, check the grounds, check the voltages and go over the reverb circuitry with a fine tooth comb (not literally), looking for something that does not seem/look right. What has worked, or helped in the past is the use of shielded grid wires in the reverb circuit, especially on the long runs to the front panel reverb amount control. I know this is not usual to see in a Fender amp reverb circuit but I'm trying pretty much anything now. Paying attention to grounding and not just grounding the reverb circuit willy-nilly, as so many older Fender amps were, can help too. In some extreme cases, I've de-coupled the reverb power supply from the usual screen supply with its own resistor and filter cap.
None of these measure have seemed to help in the case of the hummy 60s SF PR, as mentioned above. Is there something else I've missed that I could try? I'm speaking here to guys who see alot of amps on a regular basis and have a wealth of experience with getting noisy reverb circuits to sound right. Of course, I know I could go out and get that newest digital Gothic Church Times 10 but when a Fender reverb sounds right, it is very right and I like THAT sound very much. Plus, the fewer gizmos I have to plug in at a gig, the happier I am.
So I appreciate any insights you care to pass on about how to improve the Fender reverb circuit, with an emphasis on noise/hum reduction.
There's one other thing that's I've wondered about with the Princeton Reverb reverb recovery circuit, slightly off-point but close enough to bring up: In all Fender combo amps with reverb (and amp heads with reverb) from the blackface and silverface eras, in the recovery circuit, the reverb pot wiper connects to a 470K resistor. On the other side of this resistor a 220K resistor is connected and then goes to ground. The Princeton Reverb (both models AA1164 and B1270) does not have this 220K resistor to ground; it is just eliminated. I've wondered if it's just a cost cutting move by Leo on a glorified practice amp and I've wondered if there is any audio difference if the 220K resistor there or not. I have sometimes added this resistor to some PRs but I don't have a real opinion if I'm just wasting a part or bettering the amp. Any opinion or analysis here is certainly welcome.
Thank in adv,
Bob M.
Now, I know that the reverb circuit is one of the weaker links, noise-wise, in a Fender amp. The transducers can be affected by proximity to the transformers, the cables connecting the reverb tanks to the chassis must have a very good ground connection and we're amplifying very small transducer reverbed signals here where if noise/hum is present, it can get amplified to noticeable levels.
Such is the case with my late 60s Princeton Reverb. It has a hum (yes, I know, it doesn't know the lyrics) and combined with the fact that the tank in this amp seems to need a little more recovery volume than most, I have a noticeable, distracting hum. If I leave the reverb at low levels, it's passable but if I'm playing a song that calls for a strong, deep reverb sound, then I hear this unwanted hum. I know I could change the tank out with another but reverb is about good tone and this reverb unit sounds very good so I'm reluctant to do that as a first remedy.
Now, normally when I encounter a reverb circuit in a Fender amp that has too much noise or hum, the first thing I do is change out the tubes to see if it makes a difference and because it's easy to do. The driver tube is usually a 12AT7 and the recovery tube is a 12AX7a/7025. If that doesn't help the situation, I usually bench the chassis, check the grounds, check the voltages and go over the reverb circuitry with a fine tooth comb (not literally), looking for something that does not seem/look right. What has worked, or helped in the past is the use of shielded grid wires in the reverb circuit, especially on the long runs to the front panel reverb amount control. I know this is not usual to see in a Fender amp reverb circuit but I'm trying pretty much anything now. Paying attention to grounding and not just grounding the reverb circuit willy-nilly, as so many older Fender amps were, can help too. In some extreme cases, I've de-coupled the reverb power supply from the usual screen supply with its own resistor and filter cap.
None of these measure have seemed to help in the case of the hummy 60s SF PR, as mentioned above. Is there something else I've missed that I could try? I'm speaking here to guys who see alot of amps on a regular basis and have a wealth of experience with getting noisy reverb circuits to sound right. Of course, I know I could go out and get that newest digital Gothic Church Times 10 but when a Fender reverb sounds right, it is very right and I like THAT sound very much. Plus, the fewer gizmos I have to plug in at a gig, the happier I am.
So I appreciate any insights you care to pass on about how to improve the Fender reverb circuit, with an emphasis on noise/hum reduction.
There's one other thing that's I've wondered about with the Princeton Reverb reverb recovery circuit, slightly off-point but close enough to bring up: In all Fender combo amps with reverb (and amp heads with reverb) from the blackface and silverface eras, in the recovery circuit, the reverb pot wiper connects to a 470K resistor. On the other side of this resistor a 220K resistor is connected and then goes to ground. The Princeton Reverb (both models AA1164 and B1270) does not have this 220K resistor to ground; it is just eliminated. I've wondered if it's just a cost cutting move by Leo on a glorified practice amp and I've wondered if there is any audio difference if the 220K resistor there or not. I have sometimes added this resistor to some PRs but I don't have a real opinion if I'm just wasting a part or bettering the amp. Any opinion or analysis here is certainly welcome.
Thank in adv,
Bob M.
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