Excessive Amp Noise Level

[Tbone55]

Hello All,

I have a Trinity Tramp amplifier that I built several years ago thats been just sitting during the pandemic and several days ago I fired it up and noticed that it seemed unusually noisy when you turn the master volume or volume control up.

I don't recall the amp being that noisy before. I would describe the noise as a hum. The noise level is dependent on what positions the master volume and volume control are at. The noise is noticeably less if only one of the controls is turned up while the other is off and increases when both controls turned up.

While I was able to build this kit as well as doing the Hoffman Blues Junior conversion I'm not as adept at trouble shooting. I'd sincerely appreciate some guidance on how to proceed. If you require any other information or a copy of the schematic just let me know.

Thanks.

[uki]

If you post the schematic and some high resolutions pictures that will help.

[Tbone55]

Thanks for the reply. I'll see if I can get a pdf of the schematics or take a picture of the one I have. I may try to do a video so you can hear what's going on.

Much appreciated.

[Tbone55]

I didn't have any pdf's so I had to take some pictures. I would have scanned them but they're too large. Hope they're good enough.

Thanks.

[Tbone55]

Okay, lets see if it works now.

[T Wilcox]

If the 6.3vac on the PT you used does not have a center tap then you might need to add (2) 100R resistors for an artificial center tap.
The layout you posted shows neither that I can see

look at the artificial center tap section for reference
http://www.valvewizard.co.uk/heater.html

[PharmRock]

This amp comes with a heater supply centertap.  Its the dashed green wire in the layout, right below the red/yellow HV-CT.

[T Wilcox]

I missed that. probably because its not labeled Lol.
But yeah if that is landed on ground then its another issue causing hum most likely

[Tbone55]

Hey Guys,

Thanks kindly for having a look at the schematic. It's been a while since I looked at it myself so I'm trying to re-orient myself to it. I happened to have a 5751 pre-amp tube kicking around so I replaced the 12AX7 in the amp with it. The noise level dropped dramatically back to what I remember it being. I don't have another 12AX7 right now to try but will get one. Maybe my 12AX7 is faulty? What would be going on in it that would cause the high noise level?

Thanks again.

[PharmRock]

about 10 years ago I built the combo version of this amp.  There are a couple of "finicky" areas that took some time to troubleshoot during the build, but I ultimately got it sorted out.  Not saying any of these would result in the symptoms your amp is experiencing, but may be worth looking into.

1. VRM - The way the MOSFET was integrated with the board was a little tricky in getting geometry of the MOSFET "legs" set correctly.  Also, wiring of the VRM components on the board can get a little tight, with close positioning of the leads and possible contact.
2. Some builders have reported that the under-board 220K resistors R19 and R26 come loose.
3. The pull "tude" switch on the volume pot has some wiring on the DPDT that could make it easy to have cross contact.  This switch is also susceptible to a lot of noise and I believe the build manual suggests short wire runs as possible.
4. An option on this amp is to wire the the OT primary switch which allows for EL34 use.  Builders (including myself) have experienced arcing across this switch at full power levels.  Trinity now recommends painting on some of that "liquid electrical tape" onto the contacts.
5. Check the speaker output jack wiring...it can be very tight as it is next to the tube sockets as well as the cathode resistor switch and (optional) OT primary switch.


It took me a while to get this amp quiet, but I finally did.  There's a lot packed into that chassis.  You may want to chopstick the wiring to see if the hum changes.  Check all solder connections.  Switch in a different pre-amp tube (just noticed your post that you've already done this...maybe over the time you haven't played it, it got bumped around a little?)

[Tbone55]

Thanks for the great information. You've brought up some excellent points. All of the areas you mention are critical in this build. The VRM wiring was a bit tricky and some of the components around the MOSFET are really close. I had to replace the 100ohm 5W resistor. It started to burn up while I was playing and created a great smokescreen 

I want to get another 12AX7 tube to try and see if the noise comes back. My understanding is the 5751 I used has a lower power value so maybe that's why the noise subsided so much. When I get another 12AX7 I'll post back the result.

Thanks again for your help. Cheers!

[Bieworm]

I believe a 5751 would be a tad less noisy, but not to a WOW level of difference. There's a good chance that 12ax7 is the source of the noise...

[Tbone55]

Finally able to post back some info. I replaced the the 5751 pre amp tube with a 12AX7 and it's a bit more noisy but nearly as bad as it was. I'll have to do a bit of checking around of the different areas that might affect the noise level as suggested in an earlier post. I'm running an EL34 power tube in it at this time. Once I've gone through checking the "suspect" areas I'll post back my findings.

Thanks again to you all for the great help.