Randall rg75d humm

[cmanningjr]

Hi all, new guy here.

I just got a Randall RG75 G3 amp. As soon as you turn it on it had a constant hum. Knobs have no affect and it does it if a guitar is plugged in or not. Everything seems to work fine except for this hum.

Any ideas on how to find the source of this problem??

[52 Bill]

As soon as you turn it on it had a constant hum. Knobs have no affect and it does it if a guitar is plugged in or not.

Welcome to the place.

When any solid state amp has a constant hum that is not affected by the controls there are usually two causes. One is a bad or disconnected filter cap and two, a power amp problem that is causing the speaker output to be energized with a dc voltage.

When you turn on the amp, does the speaker cone push in or out and stay there? If it does, there is a dc voltage on the output of the amp. If this is true, do not turn the amp on again until the speaker has been disconnected. The constant dc voltage will cause damage to the speaker's voice coil.

If the speaker does not stay pushed in or out, then the likely problem is a bad or disconnected filter cap. Turn down all of the controls and try giving the amp a good thump with your fist. Try hitting the top and the sides to see if the hum will stop even for a short time. If it does stop or get better, then the solder connections for the filter cap(s) has probably cracked and needs to be resoldered.

I don't know what sort of skill set you have, but either of the above repairs will require opening up the amp and probably removing the circuit board. Let us know what you find out.

[cmanningjr]

Thanks for the response! I haven't dug into it yet but will check the speaker like you described. Mainly looking for a direction to start looking.

I'm an electrician by trade and as a hobby tinker with a little of everything. My soldering skills are fairly decent. Just have never attempted to trouble shoot an amp before. But am very interested in going as far as I can.

I did however send an email to Randall and they said I may need to replace the pre amp tube.

[J M Fahey]

Email that shows that Randall is clueless on their own stuff.
A bad tube hum would be affected by the tone and volume controls.

[cmanningjr]

Welcome to the place.

When any solid state amp has a constant hum that is not affected by the controls there are usually two causes. One is a bad or disconnected filter cap and two, a power amp problem that is causing the speaker output to be energized with a dc voltage.

When you turn on the amp, does the speaker cone push in or out and stay there? If it does, there is a dc voltage on the output of the amp. If this is true, do not turn the amp on again until the speaker has been disconnected. The constant dc voltage will cause damage to the speaker's voice coil.

If the speaker does not stay pushed in or out, then the likely problem is a bad or disconnected filter cap. Turn down all of the controls and try giving the amp a good thump with your fist. Try hitting the top and the sides to see if the hum will stop even for a short time. If it does stop or get better, then the solder connections for the filter cap(s) has probably cracked and needs to be resoldered.

I don't know what sort of skill set you have, but either of the above repairs will require opening up the amp and probably removing the circuit board. Let us know what you find out.

Ok, so today I took the back off the cabinet turned all the controls to zero nothing in the input turned it on and the speaker didn't move. (Pull in or out) While I was there, don't know if it matters or or not, turned the amp off pulled the tube to see if it looked normal (it did) left it out powered the amp back up..(No Hummm) Turned the amp back off, reinstalled the tube turned the amp back on it took a couple of seconds, but the humm came back..Did the "Thump Test" with my fist on the top of the amp, for like a half a second the humm went away...

I have not pulled the chasis out yet to inspect the components, (plan on looking at that tomorrow)

So this is where I stand as of tonight....Anymore thoughts??? Sound like a Filter Cap???

[52 Bill]

Turned the amp back off, reinstalled the tube turned the amp back on it took a couple of seconds, but the humm came back..Did the "Thump Test" with my fist on the top of the amp, for like a half a second the humm went away...

The two things that you have proven with the tests are one-the hum is being amplified by the tube so it relates to the preamp and two-the problem is probably a bad connection or solder joint.

Time to pull the chassis. Have you found a schematic for this amp? Try and pull out the chassis without disconnecting anything, so that you can power up the amp with the pc board exposed. If this is possible, carefully poke around the circuit board with an insulated probe, like a wooden dowel. See if pushing on any of the components will stop the hum. Pay particular attention to the filter caps.

Remember that working on a live chassis can be dangerous.

[cmanningjr]

The two things that you have proven with the tests are one-the hum is being amplified by the tube so it relates to the preamp and two-the problem is probably a bad connection or solder joint.

Time to pull the chassis. Have you found a schematic for this amp? Try and pull out the chassis without disconnecting anything, so that you can power up the amp with the pc board exposed. If this is possible, carefully poke around the circuit board with an insulated probe, like a wooden dowel. See if pushing on any of the components will stop the hum. Pay particular attention to the filter caps.

Remember that working on a live chassis can be dangerous.

As a matter of fact, I took a chance and emailed Randall yesterday. Within a couple of hours they emailed me back the schematics for the amp...Do I need to post them?? They are PDF files..


Attached them for entertainment purposes...

[J M Fahey]

Hi cmanningjr.
You did the right thing by attaching the schematics.
Otherwise, how could we have helped you?
Now to the point: this amp, contrary to 99.9% of hybrid amps out there, does *not* have a tube in the preamp but in the power amp.
Put a new 12AT7 there as suggested by Randall (which seems to be not as clueless after all) and tell us what happens.
And if hum stays, then we'll put our troubleshooting hat on and go ahead.
Good luck.
PS: post your results, whatever they are.

[cmanningjr]

Thanks for the reply!

I haven't done a lot of research on this amp, I've only had it a few days. Is there a particular tube that is better than the other?? I forgot who made this one, I do remember it saying made in Russia..

I'll get a new tube and go from there...It may take a couple of days. Not many music stores local to me...

[cmanningjr]

Ok, went back and read the email from Randall about the tube again...It said ECC81/12AT7 Preamp tube...

Does that change anything???

[J M Fahey]

No, just get the same one
And really don't get carried by any cork sniffer hype, the work this tube does is quite plain and basic.
The important parameter is whether it works or not.
Different brands and types won't make your amp "bluesier" or "warmer" or even "make it longer and thicker".
I mean the hair on your scalp, of course.

[teemuk]

Ok, went back and read the email from Randall about the tube again...It said ECC81/12AT7 Preamp tube...

Does that change anything???

This is one of those amps where it matters to put exactly the same type of tube for replacement. Randall has encountered plenty of issues with people swapping in a higher gain tube (such as 12AX7) and then complaining their amp distorts or acts funny in other ways. Furthermore, higher gain tube won't get you "tube overdrive" from this particular circuit. Put in the tube type the design calls for.

[cmanningjr]

Got a tube ordered, no one stocks them around here.

Didn't take the chasis out yet. it's been a LONG day..

[cmanningjr]

New tube installed, quiet for about 5 seconds humm came back.

Filter caps??

[52 Bill]

New tube installed, quiet for about 5 seconds humm came back.

Filter caps??

Earlier on, didn't you say that the hum responded to physical stimulation?