Mesa Boogie Roadster 2x12 Combo irritating HUM... HELP!

[jetsonsgg]

hi guys i apologise in advance if this is the wrong place to post this but im new.. be nice please

well my roadster combo just started to make a low hum when i switch to channels 3&4 but it happens randomly.
i have browsed a few forums but i couldnt find anything with a similar problem.

anyway i have tried to do a few things first before posting here:
when the HUM started to happen i had a 12ax7 custom set in there (and the hum was a lot worse than in the video below) so i changed it for the mesa stock set of 12ax7's, the result was minimal but it did go from a constant HUM (custom12ax7's) to a broken up random HUM as seen in the videos that comes and goes at it pleases. I have also tried it on all switches diode/recto 50/100watts and its still present.

the weird thing is the HUM has never came on on channel 1&2. The Hum is also there if i unplug the guitar and cable, and if i turn all volume controls down even the master.

as you can probably realise this is my first top amp and my knowledge is limited so excuse my not so accurate terminologies.

another thing to note is that after recording the video in the link i sat behind the amp thinking what to do next, so i slapped the top left of the amp (if you are looking from the back it would be the area which the manual channel selector knob is at the back, that type of area), and it made it a little worse by slightly increasing the HUM, so i did it again and it stopped. no HUM.

I have spoken to 2 tube amp techs (as their not many in my small town here in Australia but one in particular that i spoke to was very informative and is ringing the mesa dealership interstate for some extra info also)
when talking to both different techs i was told that it may be one of many things including anything from a bad power tube, ax7 or soming called a fillament cct issue. So im bringing it in to them in the next few weeks as i am strapped for cash right now.

I thought maybe one of you guys here on the boogie board may help and assist my issue, or if anyone has had the issue lead me in the right direction?

this the link of the short video i made is below, it will link you to youtube.
also here is the times in the vid where the HUM can be heard.. please turn up volume to hear HUM properly

https://youtu.be/Qfn4pyyU3ms

first hum comes in at the start then i change channels then comes in at 0:29, stops, comes back at 0:32 stays till i switch to channel 2.
comes back after switching thru to channel 4 at 1:11 stays for a second, comes back in at 1:16 till the end of recording


sorry about my dog hehe

and i do apologise for the long post.
please help.
J. 8)

 

[afu]

If it never happens on 1 and 2, I would suspect V3. Those channels share everything with 3 and 4, except V3. That tube is a coupled cathode follower with a large voltage on the cathode. Many 12ax7s are not designed to handle that voltage, which is around 200V. The Russian tubes are known to fail there and V4. Some JJs or similar Chinese tubes should fix it in that case.

Good luck.

 

[jetsonsgg]

If it never happens on 1 and 2, I would suspect V3. Those channels share everything with 3 and 4, except V3. That tube is a coupled cathode follower with a large voltage on the cathode. Many 12ax7s are not designed to handle that voltage, which is around 200V. The Russian tubes are known to fail there and V4. Some JJs or similar Chinese tubes should fix it in that case.

Good luck.

thank you afu for taking the time to reply.

it is weird because i changed the "custom set which were in there when i got the amp" the set is from dougs tubes, they were in like this:

v6 = sovtek 0303
v5 = penta 135
v4 = penta 194
v3 = penta 158
v2 = jj ecc835
v1 = tung sol 1301

the numbers after the names in the list were just numbers on the tube.

so as stated in my original post i change all of them out for the mesa stock 12ax7's, this minimised the consistency of the hum but the hum still comes and goes.

so by changing all the tubes as i have - i thought that should of got rid of the hum? thats what makes me think its not tube related? but again im a total noob when it comes to tubes and amps of this caliber, as it is my first mesa and first major tube amp.. anymore insight afu?

i also mentions i hit the top of the amp with my palm on the top left (looking from the back) and it stopped.. for now.. but im sure it will come back as it comes and goes

 

[domct203]

Are you using the tube rectifier at all?

Dom

 

[jetsonsgg]

Are you using the tube rectifier at all?

Dom

thanks for the reply domct203
i use tube recto on all channels accept ch 4 its diode. the hum is still present if i switch it either way for all channels. but the hum is only on 3&4. ive tried switching all to 100w then all to 50, hum is still there.

it wasnt making this sound before just suddenly happened

 

[domct203]

Does it still hum with the FX loop HARD BYPASSED?

You have somewhat eliminated the preamp tubes as the issue, now we need to look at the rectifier & output tubes.

Even with the amp set to diode rectifier the 5U4GB tubes are still in the circuit. With the amp turned off, switch all 4 channels to silicon diode (solid state) rectification. Remove the 5U4GB rectifier tubes from the amp. Power up the amp as usual giving 60 seconds of warm up before coming out of standby. Does it still hum? Be sure to shut down the amp before re-installing your 5U4GB's.

 

[jetsonsgg]

Does it still hum with the FX loop HARD BYPASSED?

You have somewhat eliminated the preamp tubes as the issue, now we need to look at the rectifier & output tubes.

Even with the amp set to diode rectifier the 5U4GB tubes are still in the circuit. With the amp turned off, switch all 4 channels to silicon diode (solid state) rectification. Remove the 5U4GB rectifier tubes from the amp. Power up the amp as usual giving 60 seconds of warm up before coming out of standby. Does it still hum? Be sure to shut down the amp before re-installing your 5U4GB's.

i havent tried that, but i will try what you suggested, should i switch it to hard bypass? because its switched to loop system in.. note i have no effects running in the loop just guitar>amp

 

[domct203]

Does it still hum with the FX loop HARD BYPASSED?

You have somewhat eliminated the preamp tubes as the issue, now we need to look at the rectifier & output tubes.

Even with the amp set to diode rectifier the 5U4GB tubes are still in the circuit. With the amp turned off, switch all 4 channels to silicon diode (solid state) rectification. Remove the 5U4GB rectifier tubes from the amp. Power up the amp as usual giving 60 seconds of warm up before coming out of standby. Does it still hum? Be sure to shut down the amp before re-installing your 5U4GB's.

i havent tried that, but i will try what you suggested, should i switch it to hard bypass? because its switched to loop system in.. note i have no effects running in the loop just guitar>amp

Do one test at a time. First try the amp with the FX Loop HARD BYPASSED. If no change with the hum, move on to the rectifier tube test.

Dom

 

[jetsonsgg]

Does it still hum with the FX loop HARD BYPASSED?

You have somewhat eliminated the preamp tubes as the issue, now we need to look at the rectifier & output tubes.

Even with the amp set to diode rectifier the 5U4GB tubes are still in the circuit. With the amp turned off, switch all 4 channels to silicon diode (solid state) rectification. Remove the 5U4GB rectifier tubes from the amp. Power up the amp as usual giving 60 seconds of warm up before coming out of standby. Does it still hum? Be sure to shut down the amp before re-installing your 5U4GB's.

i havent tried that, but i will try what you suggested, should i switch it to hard bypass? because its switched to loop system in.. note i have no effects running in the loop just guitar>amp

Do one test at a time. First try the amp with the FX Loop HARD BYPASSED. If no change with the hum, move on to the rectifier tube test.

Dom

ok thanks dom i wil try that when it hums again, as i said i hit her on the top of the amp and the hum stopped yesterday. but i plugged in just then but no hum so i cant try this at the moment, i will try again tomorrow and report back.
cheers mate for the info, appriciated

 

[afu]

If slapping it made it quiet, somewhere there is likely to be a poor contact being made. It probably is a tube socket. Depending on the results of the two tests, you could try a tap test to find the culprit, but it would be perfectly fine to just get electronic deoxidizing solvent to clean all the tube sockets and all 1\4 inch jacks altogether. I do it periodically as preventative maintenance.

Otherwise, I've had hum from damaged cords, guitar wiring, and corrosion on pedal jacks (which doesn't apply to you, but may for a future reader). Those were all quiet on clean channels, because the gain is not anywhere near as high as the dirt channels. You can identify them about the same as an amp by tapping the guitar electronics with a nonconductive material (pencil) or wiggling the cord. Just finish troubleshooting the amp before moving on to the other parts of the signal chain.

 

[jetsonsgg]

Does it still hum with the FX loop HARD BYPASSED?

You have somewhat eliminated the preamp tubes as the issue, now we need to look at the rectifier & output tubes.

Even with the amp set to diode rectifier the 5U4GB tubes are still in the circuit. With the amp turned off, switch all 4 channels to silicon diode (solid state) rectification. Remove the 5U4GB rectifier tubes from the amp. Power up the amp as usual giving 60 seconds of warm up before coming out of standby. Does it still hum? Be sure to shut down the amp before re-installing your 5U4GB's.

hi sorry for the late reply but i have been waiting anxiously until the low hum/noise came back before i could test your suggestions, well the low hum/noise came back today whilst i was playing which game me the chance to try your methods.
my results are as follows:

Q: Does it still hum with the FX loop HARD BYPASSED?
A: yes and when switched to hard bypass makes is horrible hiss which is very prominent (i have nothing in the loop and FX is switch to off when i tried)

Q: Remove the 5U4GB rectifier tubes from the amp.. Does it still hum?
A: yes even with all channels switched to diode and with the 5u4gbs removed it still makes the hum/noise coming out of standby.

i also tried a few tings a tech asked:
raised both vol main output and master - hum/noise does not increase

hum/noise goes away on standby.

tested switching to recto from diode - hum/noise still present

tested switching to 50w and 100w - hum/noise still present
--
the only change that has drastically reduced the hum/noise was changing the 12ax7's as stated in my OG post it went from really bad when it came thru the custom set, then when i changed to stock mesa it reduced the hum.
Its weird cause the hum/noise it cuts in and out?

im all out of ideas any help would be greatly appriciated. i spoke to a tech and hes ordering me all new tubes in the next month or so when he gets stocked so maybe that might help? but any other ideas and or suggestions im all ears. thanks let me know

 

[jetsonsgg]

If it never happens on 1 and 2, I would suspect V3. Those channels share everything with 3 and 4, except V3. That tube is a coupled cathode follower with a large voltage on the cathode. Many 12ax7s are not designed to handle that voltage, which is around 200V. The Russian tubes are known to fail there and V4. Some JJs or similar Chinese tubes should fix it in that case.

Good luck.

hi afu i have tested a few things suggested by domct203 and i have posted my results in my prev post..

also as for your reply should i change v3? i am unsure what you mean for me to try sorry for the noob question

 

[afu]

I've been busy and ill. Just try changing V3 with another good tube from China. See if it disappears. Russian tubes often have a spiral filament which becomes damaged by the circuit at V3b. My mind is a bit hazy, but perhaps someone can suggest common tubes for you, other than Mesa, Ruby, or JJ.

Also, I still think Deoxit, or another electronic solvent, would be a good idea for the jacks and the tube sockets. Spray a little inside or on a spare jack/tube. Then insert/pull the hole(s) about 20 times or so. Give it awhile to dry and put the tubes/cord back in to operate it. The instructions will indicate how long evaporation will take.