Dual rect Multi-watt strange hum

[omnio]

After a lot of back and forth I finally got my hands on a near mint Dual Rect multi-watt. The deal also included a set of new Mesa Boogie tubes as well as amp cover.
The amp sounds excellent except a hum coming (maybe) from the right back side of the amp. This hum is constant and does not change in volume and/or status. It is low and constant. I also have (first owner) a mark iv and there is no such hum - if this helps anyhow, meaning I do not think is boogie's normal hum.
Is this normal? I read here and there that people with rectifiers faced this kind of humming when one of the rectifier tubes is bad and that if the amp is used only in diodes, then is safe to remove them (and this way to check if the hum goes away). Is this safe?

 

[SonVolt]

I had the same issue with a used Multi-Watt I picked up recently. Turned out to indeed be a Rectifier tube. Replaced both from a spare Triple and it went away instantly. Can't comment on pulling tubes.

 

[clutch71]

Lots of things can cause hum but as Son Volt suggests starting with the tubes is a good start. Based on his suggestions I would think if you placed it in bold and it continued to hum you could eliminate the rectifier tubes. I believe bold takes the rectifier tubes out of the circuit. From there you could begin the process of changing other tubes to eliminate them.If the hum persist I would take it to a certified Mesa tech. Good luck

 

[bandit2013]

Here is my 2 cents on the subject.

The MWDR has a switch on the back for power supply tracking. It is labeled as "rectifier select" Note that each channel has a rectifier tracking switch. The up position is for silicon diode and the down position is for tube rectifier. The spongy is varaic power to reduce plate voltage and Bold is full power. That switch does not do anything to the rectifier tube. You must use the rectifier select switches to bypass the tube rectifiers.

Also, if you hear this hum through the speakers, it would be caused by voltage ripple in the power supply which could indicate that one of the rectifier tubes is off or weak. Two rectifier tubes are used at 100W and only one is used at 50W.

Yes, you can remove the Rectifier tubes if you have all of the rectifier select switches set to diode. Not sure how much the tube rectifiers contribute to the power supply when the diode is set since they always remain in the circuit and will be operating in parallel. The voltage drop across the silicon diodes will be far less than the voltage drop across the tube rectifier. The one rectifier tube does get bypassed in some ways with a relay when using 100W/50W switch on the front of the amp on each channel (when active the relay shorts out the 100k resistors on the plates to that additional rectifier tube is providing power with the other, when 50W power is set, the relay opens, and the rectifier plates do not conduct much with the 100k plate resistors in the circuit and then only one is contributing to the power source.

The power transformer does make noise, but you should not hear that though the speakers. Mark amps usually have a fan and that is what you hear which is usually louder than the power transformer noise. I have a few amps where you can turn off the fan, RA100 is one, with the fan off you can hear the transformer making similar noise like the MWDR. Roadster is no different. No fan.

You can experiment with the rectifier switches before you opt to remove the tube rectifiers. The Power transformer is on the same side of the amp as the Rectifier tubes. I doubt bypassing them with the switch or removing them will affect the PT noise. If you do hear the noise through the speakers, it is probably associated with any of the tubes in the amp, I would not rule out preamp tubes. If the noise is present but goes away when the rectifier select is set to diode, that would indicate the tube rectifiers are contributing to the noise. If that does not work, power down, let the tubes cool and remove the Rectifier tubes and try again. As I said above, the rectifier tubes are still operating in parallel to the silicon diodes, so if they are the source of noise, you have to remove them to determine it. Just do not change the state of the rectifier select when the Rectifiers are removed from the amp while power is applied. It is probably best to replace them and keep them in the amp at all times.

 

[omnio]

Thank you both for your replies.
The noise is not passing on the speakers. It is noticeable (and does not change in volume or anything) even when the amp is in idle state.
Indeed in the mark the sound of fan is covering everything. You made me think to unplug the fan on the mark and check if there is also a hum there.

My idea is that is coming indeed from the output transformer that is why I thought it might also be the rectifier tubes.

I will check today-tomorrow your suggestions and revert.

 

[SonVolt]

As far as bypassing the Rectifier Tubes with the switches on the back, this made no difference in my hum. It still hummed when the switches were set to Silicone Diodes. However, it was still the Rectifier tubes causing the noise. The noise was coming from the amp itself, not through the speakers. Not sure if this helps, but it's worth replacing those tubes (if you have them) as a quick test.

 

[bandit2013]

It is possible the Rectifier tubes are making noise. I would not disagree on that. Probably best to replace them and see if that changes anything.

 

[bandit2013]

Just a thought or two....

Are you in the USA? All of my tube amps will have some hum from the PT. That is normal. Some with the small fans will mask that sound due to fan noise. I do have two amps I can turn off the fan, RA100 and the Mark V90. The PT hum is more apparent with the fan off. the MWDR, Roadster, and many other amps with the tubes facing upward and not hanging down, have more of a convective cooling and do not need a fan to circulate air. Badlander, Triple Crowns both have small circulating fans and the power tubes are on top of the chassis. I can still hear the transformer hum if I get close enough, usually more apparent if the chassis is out of the head shell.

If you are not in the USA, I assume the line frequency is at 50Hz. That will reduce the efficiency of the PT and will make more noise.

the Rectifier tubes follow the PT. Tubes can make noise. sure, I suppose that is possible. It is a bombardment of electrons onto a steel plate. The plate structure can resonate too. I get that with some power tubes, more so with a Simul-Class amp than a Class AB. That is when the tubes start to vibrate at the same frequency of the guitar sound. However, if you place some pressure on the tube and the noise goes away, it can also be related to change in magnetic field of the transformer that is near it. Even vibrations from the speaker enclosure (combo or cab under a head).