[jamesgarvin]

I was hearing a pretty loud hum after listening to music this evening. I pulled out the volt meter, and discovered an output tube was bad, so I replaced the tube. I still have a hum, albeit lower in level. All the output tubes are biasing fine. Any ideas as to the reason for the hum. I'm pretty sure it was not there before, as the amp was dead quiet.

[rbwalt]

has anything changed in your system? have you moved any equipment around prior the the hum? have you checked the driver tubes? it could be a transformer that has gotten noisy. can you hear it where you sit or do you have to get right up to amp to hear the hum? i had a mk2 that had a hum. i could reduce the hum by moving my interconnects around but it would not go away totally. it also could be a grounding issue. look into these and let us know how you make out.

[Ericus Rex]

Also, is the hum coming from the speakers or the amp itself?

[jamesgarvin]

The hum is coming from the speakers. Before I replaced the output tube, I noticed the hum when I bent over to turn off the amp. I did not measure the distance the hum was audible from the speaker, but I conjecture it would have been audible from about six or seven feet. The speaker drivers were actually vibrating when I touched them. I replaced the tube, and there is still a hum that is audible from maybe seven or eight inches, but it used to not be there. I've not changed anything in the system. I have an Adcom 555 amp that I use for home theater, and I hooked up the Adcom amp, and there is no hum. I made sure my pre-amp was off, and it was, and then I switched  the pre-amp from the phono output to the line output, and still there is hum.

I've never replaced the either of the driver tubes. How do you know when they go bad?

[tubegroove]

But before you do anything, its useful to first short the inputs and listen for the hum so that you eliminate the possibility of the hum emanating from the pre or source (if you haven't done that already).  As far as I know, driver tubes supplied by Roger last pretty long and are quiet but a bad connection can create the hum.
While it may not be strictly comparable, I experienced hum in one channel of my RM10MkII sometime back.  Turned out to be a faulty solder on one of the pins of that channel's driver tube, which probably fractured over time as I periodically removed the driver to clean the pins.
If you feel comfortable, worth peeking under the hood of your RM200 and using a wooden chopstick, probing each of the pins of the offending channel's driver tube while playing music softly, to see if you can identify the source of the hum.

[rbwalt]

try taking the driver tubes and the output tubes out and put them back in. sounds like a bad connection some where. might be a bad ground. have you tried the different taps on the back of the amp? does it hum on both the 4 ohm and 8 ohm taps? was the replacement tube one from roger? was it matched to the others?

[jamesgarvin]

Well, I took out all the tubes and put them back in. I disconnected the interconnect cables. I changed the speaker cables from the 4 ohm taps to the 8 ohm taps. Nothing worked. The hum is in both speakers. The replacement tube was an RAM tube, but not matched to the others, though I've not previously had this hum with other non-matched tubes. So I ordered a set of four matched tubes from RAM, and I'll see if that changes anything.

[tubegroove]

Did you short the inputs? Sometimes you could be picking up airborne hum through the inputs
I am not familiar with the insides of the RM200 but sometimes faulty (leaky) filter caps placed just after the AC input could result in a hum.  Worth asking Roger for more guidance on where to look

[Ericus Rex]

Your output tube could have taken another internal part with it to the grave (or at least compromised it), which could then mean that part could ruin the brand new tube you've just put in and one of the new quad you just ordered.  I recommend calling Roger and getting his advice before you play the amp any more.  Tubes be expensive!

[eeramos]

I also have a RM200II with the same hum problem. When first powering up the unit the hum is quite loud through the speakers but quiets down in a few seconds so that you can only hear the hum if you have your ear about eight inches from the speaker.

I've re-routed the interconnects but the hum remains. The tubes are the original MR tubes from when I purchased the amp new and have very few hours on them.

Since the hum is so low I've learned to live with it.

[avahifi]

I am a bit suspect that when the tube failed it caused some damage to the power supply.  Assuming that most of the power supply is common to both channels, that could be why the hum is showing up on both channels now.

With the amp off, unplugged from AC, and given an overnight rest to make sure all the the high voltages have bled down, you might think about looking very closely at power supply components for any sign of excess heat (burned resistors, capacitors with swollen tops, etc.)

A friend with an oscilloscope and knowledge on how to use it safely would be a valuable asset.

Regards,

Frank Van Alstine