Randall RG80 Hum

[JHow]

I finally got to look at this again this weekend. Status is unchanged. It is 60hz hum, not the 120 hz buzz. All the electrolytics were replaced in August. I double-checked the polarity of each this weekend, just to rule out a backwards-installed cap. The filters c38 and c39 are grounded to their own lug on the chassis. Nothing else is grounded to that same point. The pcb is grounded at several spots (six screws that look like they ground to frame and a line to the input jack and a line to the reverb footswitch jack). Maybe I am being picky, because the hum is not really noticeable if I am standing and the amp is on the ground, but I can hear it if I put my head down next to the speaker. But I feel like I have tube amps that I built myself that are quieter - so I would like to get rid of it. Any thoughts anyone would have would be appreciated.

[ChS]

Sorry I can't help much about further reducing the hum, but just to let you know my '86 RG100ES also has a hum. I'm thinking it's normal for these amps.

[JHow]

Thanks for the reply. I think you might be right. I really don't hear it if the amp is on the floor and I am standing up. I just have quieter amps, that's all. When you crank the amp, it doesn't matter!

[Chuck H]

Yup... Probably just the nature of the beast due to the ground scheme. Not much you can do on a PCB. You could try isolating some things and dick around with it, but the returns may be minimal. I know that once I'm into a problem it becomes easy to hear it even when it's almost not there. Sometimes you just have to look things over and have the confidence to know you've done your best short of re-engineering the amp.

[oc disorder]

Stop me if I'm being obsessive but in regard to 52 Bill's question "Have you checked all of the grounds for good connections?" I would remove the jacks and clean the chassis with emery paper and refit with a star washer with a dash of deoxit etc and make sure all the other grounds are likewise.
Looking at the scope picture it seems like inadequate filtering how far are points
A on the diagram and B apart I mean where the positive leaves the 6000uF capacitor and goes to the collectors of the amp ...I'm suggesting at that point
A & B on the Amp output portion of the diagram try tacking say 2200uf to ground there and see if it makes any difference.
There is a possibility the new replacements are maybe duds ..does happen some times.

[JHow]

Well, oc disorder, it's worth a shot. I tightened the jacks but didn't emery the contact surfaces. It would be easy to try your idea on the caps, too. I'll play with it some more and let y'all know if I get things better.

[DrGonz78]

Sorry to resurrect a thread that is long past... However, I have this same amp assembly date 1986 and it behaves in the same exact way as mentioned throughout this thread. Actually, the hum is not that noticeable in most locations in my house except for one room where it just goes ballistic! Probably a bad grounded jack in that one room. Had a jazz band guitarist use it in my living room and it was fine. In my bench room it works the same but in this other room where the ground is not as good the hum is insane!! Took it over to my brothers house for a jam and man oh man the hum was horrible. Brought it home that night and the hum in my bench room was much better. The bass amp that was in my brothers house had a hum too, but my hum on the Randall was just god awful!

The guy I bought it from said the hum got worse over time...??? I am about to recap the sucker and do all the stuff that can reduce some stray capacitance etc. However, something just occurred to me about the power transformer on this amp. It has those shoelace type wires instead of insulated rubber wiring. My gut is just telling me, that like Enzo said, it has a shared ground copper winding to return path of the PT. That I think is what is going on with this amp. However, tonight I just thought that the wiring is like shoestring material. Just like you find in an old style vintage single coil pickup from a Telecaster. My thought is now to insulate those wires before doing any cap job. Isolate the problem right?

My question(s) here is... Do you think this might be the reason the power supply is adding hum to the amp by way of shoestring wires? Do you think insulating these wires might help reduce some hum? If I do this what would you recommend to wind around the wire to add some insulation to these suspect wires from the PT?

Thanks in advance as always you guys give such great responses. Even when I have a silly thought like tonight or most nights lol.

Edit: Just wanted to say one more thing... I just recently acquired a Randall RG30 and it is from 1981 with all original components. This amp is now my studio amp!!! Totally silent no hum! It's a great amp man. Not sure that is even related but just ironic in a way.

Double Edit: All of the wires form the PT are shoestring insulated wiring...

[louloomis]

I don't know if this is helpful, but I used to have an 80's era RG-80 combo. While I don't remember it specifically humming, I do remember that it was not a particularly quiet amp. Might have been hiss+hum, but mostly hiss? That's the way I remember it...

LL

[DrGonz78]

If you turn the reverb all the way up to 10 you can here a hiss... However, this is a 60hrtz ground hum and occurs w/ all knobs turned to zero w/ no guitar plugged in. The two black wires from the on/off switch that lead to the power transformer are of the shoestring rag wire types. Also, these wires pass right over the filter caps and not sure if that has any effect to increase a hum? Then the two primary and center tap wiring is all the same rag wire type insulation. So, just got me thinking that the PT's on these amps have these wires and might create an increase in hum.

Edit: Have read about rag wire losing it's ability to fully insulate wires over time. So, in this case I just assume this could be why the previous owner said the hum got worse over time.

[52 Bill]

Edit: Have read about rag wire losing it's ability to fully insulate wires over time. So, in this case I just assume this could be why the previous owner said the hum got worse over time.

So you're thinking that cloth covered transformer wires can leak 60 hz hum into the power supply filter caps?

Please explain.

[Chuck H]

Sorry I didn't re-read thoroughly... So stop me if I'm being redundant.

Any high current power lines around sensitive leads can induce hum. If it's 60Hz it's mains AC. If it's 120Hz it's HV ripple. But even if it's not ripple that doesn't mean the filters aren't old, and perhaps have high ESR. High ESR means the leads on the filters carry more signal sensitivity. So I think it could be possible to induce 60Hz hum on them. I know that replacing the filters in old JC's can cure hum. If the electros are old I don't see a down side to replacing them. If it fixes the hum you're done.

[g1]

DrGonz: I think you are getting insulation mixed up with shielding. Plastic, cloth, or rubber insulated wires are all unshielded. The insulation is for safety reasons, not noise.

[JHow]

As OP here, I still have my RG and it still hums, but only if you really stick your head down and listen to it. Standing up, five feet away you really don't notice it. Use the red channel and I stopped worrying about it. Oh, mine also hisses a bit when you turn up the reverb.

I will be interested to know if you find a fix, however.

[DrGonz78]

So you're thinking that cloth covered transformer wires can leak 60 hz hum into the power supply filter caps?

Please explain.

Bill, well it sounds like an odd question and was figuring it was worth asking to see if it makes any sense to anyone here... Since you all have much more experience than I do. But yeah it seems like it might be just time to get on with filter recapping.

Any high current power lines around sensitive leads can induce hum. If it's 60Hz it's mains AC. If it's 120Hz it's HV ripple. But even if it's not ripple that doesn't mean the filters aren't old, and perhaps have high ESR. High ESR means the leads on the filters carry more signal sensitivity. So I think it could be possible to induce 60Hz hum on them. I know that replacing the filters in old JC's can cure hum. If the electros are old I don't see a down side to replacing them. If it fixes the hum you're done.

Chuck, my main thought is that some of the wax or plastic insulation could become brittle or cracked underneath some of the rag wires from the PT. So high ESR from the filters could theoretically make it possible to induce a hum. I was thinking if any of the PT wires were brittle and not fully insulated anymore that could be a cause to induce hum over such components(filter caps)? I read about how the wax or coating under the cloth rag wire can dry out and crack a bit...

DrGonz: I think you are getting insulation mixed up with shielding. Plastic, cloth, or rubber insulated wires are all unshielded. The insulation is for safety reasons, not noise.

Yeah G-One point taken here... I was only really referencing the shoe laced pickup wiring as a way to describe what this type of wiring looks like to me. Later I found out it is called rag wire. But yeah I was just a bit confused to how to describe this type of wiring. Yeah I understand that they are different sorry my writing made it a bit confusing.

Standing up, five feet away you really don't notice it. Use the red channel and I stopped worrying about it. Oh, mine also hisses a bit when you turn up the reverb.

JHOW, did you ever plug it in somewhere and had a huge hum??? I described that when I bring it to a few other locations it hums like 60 monks going at once. Yeah in my work shop it really is not humming all that much... Identical to how you describe your amp(only when you put your ear low towards the speaker). I might add some tubing on the wires that are from the PT that cross directly over one of the filter caps. Also, I will do a recap job. Then I will take it in the horribly grounded room to see if it is responding the same way. The thing that perplexes me is that I played this amp at my brothers house with a band recently. Both bass amp and guitar amps were humming as the ground in the house was not the greatest. Came back 2 weeks later w/ a little Marshall 15 watt amp and barely a hum on the same outlet. That just confuses me but will change caps and test to see if the caps are indeed the major culprit to that type of huge hum change. The amp sounds great in the few rooms that I can depend on good grounds.

Thanks for all the great feed back. Now most will respond bad grounded jack can't be an amps fault. You are right, but this one has a coupling effect that makes it hum louder than any other amp in the same room. So, my goal here is to at least get the hum to be bearable in these rooms where the amps hums a little more. I would love to have every place just be grounded better but if I sell this amp I don't want some guy taking it somewhere and having this anomaly.

[SmartAlex]

RG 80 hum

IMHO, the background hum in the RG80 comes from a design flaw in the power amp. Problem: There is no power supply decoupling between the voltage gain stages and current amplification stages of the power amp allowing ripple from the power supply to enter the voltage gain stage and ultimately the speaker. Looking with a scope, there is no hum at the effect send or "preamp out".

As OP here, I still have my RG and it still hums, but only if you really stick your head down and listen to it. Standing up, five feet away you really don't notice it. Use the red channel and I stopped worrying about it. Oh, mine also hisses a bit when you turn up the reverb.

I will be interested to know if you find a fix, however.