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It's a late 70's/early 80's mini brute (with felt cover, no distortion knob). It was recently serviced and has no grounding issues. It's got a hum with nothing plugged in. Sometimes the hum turns into a very auditable radio signal. Like you can listen to the broadcast if you sit close to the speaker. Looks like a shielding issue. It's present in the preamp output as well. Is there anyway to shield this thing so it doesn't pick up radio interference?
Last edited by Tal_175; 02-03-2023 at 07:17 AM.
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02-02-2023 10:47 PM
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When Patty Hearst was kidnapped by the Symbionese Liberation Army I heard it first from a radio broadcast feeding into my Polytone. Strange event, haven't thought about that for many years. Good luck resolving the grounding issue.
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The aliens talk to me through my Hiwatt.
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Find the right station and it might make for a good backing track!
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I use a Bugera PS1 to eliminate the noise when I'm practicing quietly. It's a passive resistive attenuator. The amp becomes dead silent without any effect on the tone. It'd be ideal if there is a way to shield the electronics of the amp but I can live with my currently solution if shielding is not practical.
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You have to diagnose before you can treat. Does this happen only when plugged into a specific outlet or does it happen in many outlets? Is it only in your home, or is it the same in other locations? Does plugging a guitar in change it in any way? Do the controls on guitar or amplifier affect it in any way?
Check with a continuity tester to be sure the grounding pin on the power cord is connected to the chassis. You don’t need a tool - you can use wire, a D cell and a flashlight bulb. You say it has “no grounding issues”, but that may not be the case since I assume you didn’t have this before the service. What did the tech do? Was it anything that involved the power wiring? Did he or she do any work on the input wiring? One of the solder joints in the signal ground path could be cold enough to have finite resistance or even be discontinuous.
Make up a shorting plug by bridging the two terminals on a 1/4” plug and plug it into the input. Does the noise change or disappear? Inspect the preamp board carefully with magnification to look for a bad joint, a broken trace, or signs of a bad component (eg misshaped or discolored cap, resistor).
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I had the issue before the service. I was using the amp in a different location before the service. It had a strange hum that to me sounded like radio interference but I wasn't sure. In the location I'm using it now, there is still the hum but the hum turns into radio broadcast sometimes. So it seems like in the new location it's tuning to a radio channel in whatever frequency that it's prone to pickup.
Originally Posted by nevershouldhavesoldit
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OK more details. My tech did a general service back when I first got the amp. It was a reverb purchase and the amp didn't work properly upon arrival. The service was covered by the reverb insurance.
The hum was there before and after the service. It actually gets louder as the amp warms up. After 10-15 min use, it'll get noisier than any of my amps but still manageable. The hum sounded a little different than a regular amp hum to me. Now I think that it was radio interference all along as it's occasionally becoming radio broadcast in the new location.
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Some good advice here. Usually this kind of thing Is a grounding issue, though not exclusively. One thought is that the input jacks ( Switchcraft jacks, on a Poly) can cause trouble; they are 'shorting jacks", which means that they are supposed to ground the pre-amp input when nothing is plugged in, but ocasionally the shorting mechanism gets dirty or even gets bent out of shape though use over time, and doesn't ground the input, making it noisy and susceptible to radio interference. You might be able to check this visually ( when unplugged of course). I have found that Polytone solder joints are pretty rugged, although anything's possible. The input jacks are not situated on the PC board, which removes one common issue with modern amps. Originally Posted by nevershouldhavesoldit
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I have a highly regarded Hermida Audio reverb pedal that picks up what seems to be some sort of Spanish ham radio and hard core fire and brimstone gospel. I guess it's possible that I'm getting Spanish gospel but it just doesn't sound angry enough. Could it be the power source instead of the pedal? I get no issues from any other gear, four amps, another reverb and clean boost pedal. Fwiw, the same results in Florida as now in my home in Oklahoma.
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That's why I suggested trying a grounding plug in the input. If that solves it, your thought is correct and the fix is easy. It's also possible that the power cord ground is no longer connected to the chassis. If this were the case, I'd have expected a decent tech to find and fix it, though. The problem has been there since the amp was first bought by the current owner and the amp's been serviced at least twice without eliminating it or even making it somewhat better. Either the tech missed something or the amp is functioning to its specs. I've heard many complaints about noisy Polytones over the years, but I've never owned one and have no first hand experience. Some amps are just inherently noisy - my first SS amp was a 1970 Kustom 150 that hissed like mad from the day it was born. Originally Posted by Franz 1997
It helps to identify the frequency of the hum. 60 Hz hum is usually caused by cable or wiring problems such as poor or inadequate shielding, a low level signal wire running too close to a source of EMF (e.g. anything that generates a magnetic field around it, a power wire carrying line level AC), Grounding problems (especially loops) often cause a 120 Hz buzz. The RFI component will vary with radio frequency generating sources near the location, along with variation in the many conditions that affect radio transmission. So the fact that the radio station is intermittent may be cause by external factors that you can't control. For example, it may vary with time of day because of atmospheric conditions. Some radio stations only broadcast during certain hours on certain frequencies, e.g. HD radio channels.
The hum and the RFI could be from separate problems, but Occam's razor says they both probably have the same cause. There should be no RFI at all if the amp is well designed and functioning properly per its design specs. The answers to the questions I asked in my first response will help guide the search for a cause. Going further, does moving the amplifier around in the room and/or rotating it through 360 degrees while it's turned on affect the noise at all? Does shaking it around or whacking it with your hand while it's on have any effect? What happens when you insert a shorted 1/4" plug into one input at a time? What happens if you insert shorted plugs into both inputs? How about plugging a guitar into one input and shorting the other?
The cause is usually findable with a sufficiently careful and thorough search. In the unlikely situation in which no specific cause is found, there are hum "filters" (like the one pictured below) that preserve the electrical safety of the grounded power cord and do not lift the ground connection. They're under $100, if that much is worth it to you - and there's no guarantee that one of these will reduce the noise enough to matter. Ferrite beads around input and line level signal wires can suppress RFI - they're easily added by a tech, and they sometimes work.
Last edited by nevershouldhavesoldit; 02-03-2023 at 01:54 PM.
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I just noticed something. I connected the polytone to another cab. So the speaker inside the polytone cabinet is disconnected. Yet I'm hearing a hum coming from (what seems to be) power transformer. Can this be related to the hum that comes through the speaker and radio interference?
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70s and 80s Polytones ( ie before the "sonic circuits" versions) are quite noisy amps when turned up to any degree; the circuit doesn't have a master volume and the main volume control is somwhat upstream from the pre-amp output. This is not a problem in use unless the player prefers to control volume from the guitar, in which case it can be. The solution is simple enough; a master volume control installed immediately before the output. It bothered me enough to do just that.
This is more of an FYI; I'd doubt that's what's causing the RF noise unless turned right up to max. As for the transformer picking up RF noise, it seems very unlikely. Some slight hum, yes, but hardly a radio station's worth.
Whe I started playing, many decades ago, hearing radio stations through the amp was commonplace; often, Radio Luxembourg, an evening rock & pop station. The introduction of compulsory grounded wiring in clubs & houses, and plastic moulded & grounded amp leads ( and now IEC sockets) means that it's nearly unheard of
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I love the fatness of polytones. The high notes especially sound very full. I don't think it's just an EQ thing. I suspect something in the circuit is creating the impression of fullness. It's just a simple, dry SS tone but very musical.
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Rumour has it that it's an accidental but useful side-effect from the original chips ( integrated circuits) used in this 70s/80s circuit. They are said to have quite high levels of harmonic distortion ( a defect, by today's standards ), whereas today's chips are super-clean. I have briefly owned later Polytones, and can confirm that, although there are extra bells & whistles, they don't sound as fat as the more primitive earlier circuits. Originally Posted by Tal_175
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A "point contact" that is just a hair under allowance of
conducting (cracked solder, granule of contaminant in
a potentiometer, too soft input jack contact pressure)
may readily rectify AM broadcasts. These are just the
kind of things that may come and go from moving the
item, plugging/unplugging connections, turning knobs.
If pedals are used, check those - often, it is the pedals.
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