Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: timmyjoe on November 29, 2025, 03:12:06 pm
-
My JTM amp developed a pretty bad hum. I've done some preliminary testing to sort it out and plan to do a few more today to see what I can figure out on the amp. I've noted the post on fixing grounding problems, and will continue to go down that check list. But hoping someone can give me a jump start on solving this. It's home built, but this noise is new. It's had it at two different houses.
Noise: I think it is 60 cycle hum. It sounds more like a highspeed propeller rather than angry humming bird of 120. Listing to online examples, this is the best I can figure.
Things I've Checked:
*Speakers swapped and same sound
*Guitar swapped and same sound
*House circuit swapped and same sound
*Tubes V1 and V2 swapped and same sound. I don't think I have a second v3. (parts are in storage)
*Removed master volume. (Amp had a type 4 master added. When turned half-way up on the master, the noise was the loudest. Down low or at full volume the hum was less annoying. Now, without the master volume, the sound is pretty consistance over the volume level. Also, the hum seems to need get start after the amp has warmed up for a bit. When still cold it isn't as loud.
*Used at two different locations. Both locations are older homes, so both might have grounding issues.
Things I'm Planning Next:
*Test at building with modern wiring.
*Chopstick test
*Review this document on grounding. https://el34world.com/charts/grounds.htm
I've attached schematic I used to build it. I don't think there's a way to upload sounds to this forum is there?
-
The kit didn't come with a schematic??
looks to be fixed bias EL34s??
me, i'd pull them 34s, verify the bias is where it was originally and rock-solid stable
IF so;
Power OFF
NO INPUT SIGNAL
PA tubes back in
hook up your DVM to measure the bias Voltage in-circuit
Power ON
Monitor the Negative VDC as the tubes warmup
verify it's stable
post findings
-
I'm going to need some more clarification on steps to take shooter.
The bias is fixed.
By pull the 34s, do you mean I should put them in a bias testing meter?
Looked at RobRobinette's page on measuring bias, but not getting the last steps. The measurements I've gotten so far are the following.
V5- Voltage is 448
V4- Voltage is 447.5
Center tap voltage is 465
V5 resistance is 89.4
V4 resistance is 109.8
Will look up what voltage in-circuit means.
What is the negative volts DC measurement? Also new to me.
What are we looking for, or is this a general test to help narrow down the problem?
-
I think he means remove the El34s from their sockets and test that the negative bias voltage is correct and stable, then reinsert the tubes and take more voltage measurements to see if the stability and/or bias voltage differs with those tubes conducting.
-
we'll start simple;
The kit didn't come with a schematic??
-
A schematic has been attached to the opening post.
But links work out better using safari https://cdn.shopify.com/s/files/1/0604/9615/0624/files/British_45W_SCH.pdf?v=1750264964
-
every time I load the original all I get is the layout, i'm not a big "link" follower, they tend to lead places I don't want to go
-
A schematic is definitely attached :dontknow:
To include audio / video clips, upload them to an external host eg YouTube soundcloud and provide links / paste the bbcode into a thread reply.
-
ok, so bias VDC, what is it with PA tubes pulled and measuring at the "tube-side" grid1?
IS it stable?
is it close to schematic value?
IF SO;
put tubes back in n measure at "circled" point, is it stable?
Is it close to schematic value?
:l2: as I forgot to attach pic!
-
Whether by accident or design, Marshall developed a bias adjustment arrangement that was resilient to the trimmer failing. ie if it went open circuit, the magnitude of the bias voltage applied to the control grids increased to its greatest level.
Foolproof?
Never underestimate the ingenuity of fools!
A doofus at Mojo rearranged the bias adjustment arrangement, such that if the trimmer failed and became open circuit, the bias voltage magnitude would reduce to its minimum, perhaps even fail completely.
Leaving the amp to cook itself.
Great job Mojo :laugh:
-
To determine what frequency the hum is one can go to youTube where there are videos with 60hz and 120hz sound. Play one of the videos and compare to the sound your amp is making. There are numerous ways to make an audio/video file for upload to a website. Here are some ideas
https://support.lesley.edu/support/solutions/articles/4000034124-how-to-record-audio-on-your-computer-or-phone
-
I’m re-reading the OPs questions, and I think the OP needs to do a little more background reading about how amps work.
Rob Robinette’s site has a good primer, and Uncle Doug’s videos are also great for visual and auditory learning.
In particular, the OP needs to understand how filtering works at each stage, how push-pull power amps cancel out noise, how power tube bias works, and how bias balance affects that common mode noise cancellation.
I don’t think we have enough information to definitively say that the noise is 60Hz hum.
Fill out a full voltage chart for each tube, and make sure you understand what each measurement means to the health of the amp. Then you can start troubleshooting. Otherwise it’s just chasing phantoms.
-
For those that find this thread in their search for fixes to amp noise on a Mojotone JTM45. Here's what seems to be the problems I had. The amp is now as quiet as any store bought unit or better. (Adjusting the bias wasn't relevant.)
1. One heater wire had slipped out of original position at a tube socket and was closer to signal path. I rerouted some of the V1 V2 heater wires to come in vertically, placing them further away from the signal path.
2. The Mojotone ground scheme uses the original method used by Marshall. Other than to just claim historical accuracy, not sure why they put that in their directions. With the wires soldered to the back of pots, several ground loops are created. I've attached a picture of the ground layout I used in the repair. It places half of the amps grounds on one place on the amp chassis and the signal path sensitive grounds on the opposite side of the amp. Picture comes from another post on this forum. The other diagram is the original MojoTone layout that seems to be more of a problem for ground noise.
While I wish I had time learn and understand more amp operations, that's going to have to what for retirement or when a lot less people are demanding my time.