Tube amp with excessive noise. - Page 1
Forum section: electronics / repair

author: mkiijam
author_rank: Regular Contributor
timestamp: May 26, 2025, 07:59:22 pm
content: Hi Guys,I'm working on a Mesa Boogie Studio 22+ amp that I've got working again, but it seems excessively noisy to me. I really have nothing to base this on, except just gut feeling over the years of playing other amps, as I have not seen this one before.Here is the schematic and my white noise floor is up to about -60dB at node A.It got me curious as to where this white noise is actually coming from and what makes one amp "noisier" than another with this same layout and topology.I guess I'm kinda guessing on "Node A" because I have removed V1 and the noise is still there, and the noise is affected by the EQ controls and lowers with the Volume control that feeds V2B.Thoughts?

author: TimFox
author_rank: Super Contributor
timestamp: May 26, 2025, 08:07:22 pm
content: I didn’t see any attachment, but individual tubes can go noisy during their operation, due to gases, degradation of internal insulation, heater-cathode leakage, etc. that get worse over time in the hot tube.An advantage of stereo amps is that you usually have identical tubes on the two channels that can be swapped for diagnosis.Can you temporarily replace V2 with a new one?

author: mkiijam
author_rank: Regular Contributor
timestamp: May 26, 2025, 08:25:55 pm
content: Added

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 26, 2025, 08:44:30 pm
content: What type of tubes are used? My guess would be 12AX7/ ECC83 types.Your assumption that the noise is being generated at or near node “A” is correct one.

author: TimFox
author_rank: Super Contributor
timestamp: May 26, 2025, 08:48:16 pm
content: Are both V1 and V2 12AX7/ECC83?If so, and you swap them, that could blame either V2 or the circuitry around it.

author: mkiijam
author_rank: Regular Contributor
timestamp: May 26, 2025, 11:42:02 pm
content: I've replaced the tubes. Same noise levels. They are 12AX7

author: Shock
author_rank: Super Contributor
timestamp: May 27, 2025, 02:07:04 am
content: Provide a link to the schematics. An amp of this age if any caps are used in filtering they may have become leaky. If not physically leaking aka venting, internally leaking.This is known as a noisy model so perhaps with eq and tone turned up and on lead channel (etc) it's going to sound like any non gated loudly driven amp.Do you have an oscilloscope and have you worked on mains powered circuits before safely? If not don't mess with it further.Aside from perhaps running a guitar direct from a pedal into the external pedal loop if it has one (bypasses a lot of the preamp in most models) there is not much you can do without a disassembly and potential of exposure to mains or higher voltages.Some other hints are lead dressing, intermittent connections and joints. Bad grounds, introduced noise. There is mention online of swapping the input tubes to lower gain, so that sets some of the expectation ahead of time.

author: mawyatt
author_rank: Super Contributor
timestamp: May 27, 2025, 02:07:44 am
content: As a kid early on put myself thru school by repairing all sorts of electronics things including audio/guitar tube amps. For finding noisy components the spray cold freeze in a can proved quite helpful, often a carbon comp resistor or coupling cap was the source.Best

author: mkiijam
author_rank: Regular Contributor
timestamp: May 27, 2025, 03:08:23 am
content: I've recapped it.30 years tech experience, just not much on vintage guitar amps, and nothing with this particular model. As I repair tech, we sometimes skip theory and "just get it done." I'm finding myself curious...4 scopes in da house... :-)Freeze spray is great... quick tech tip - canned air upside-down is freeze spray and freeze spray upside down is canned air!

author: xgabrielx
author_rank: Newbie
timestamp: May 27, 2025, 09:57:26 pm
content: Are the transformers bolted down securely? If they are even slightly loose, that can cause a lot of noise, but thats 50/50 cycle hum, not white noise. What about the ground connections? Often there's just a physical connection, like a bunch of eyelets bolted to the chassis, so corrosion can get in and you get a crappy connection. It can also be as simple as the ground connection in the input jack - another solely physical connection. Often you get a lot of white noise, but if you give the end of the guitar cable a sharp tap, you get a pop or crackle, and the white noise goes away.It sounds like a sketchy ground connection with some corrosion.Tube pins/sockets can also cause this noise if you get corrosion in there. Take each tube out, put some contact clearner on the pins, and push it in and out of the socket a bunch of times, to get a good connection.The most common cause of white noise, I'd say is the input jack not making a good connection with the ground of the guitar lead. Try giving it a whack, or put some contact cleaner on the jack plug and shove it in and out of the socket a few times.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: May 28, 2025, 03:39:51 pm
content: Anode resistor of V2A would be my next candidate to inspect / exchange. vintage carbon composition resistors have a tendency to get "hissy" over time. you may want to try a metal film type of same mechanical (!) size instead.