18 watt TMB squeal & hum

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Explorerman
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Re: 18 watt TMB squeal & hum

Post by Explorerman »

Yes please, let us know how you go! I’ll be very interested as my parts have only just arrived, so I plan to put this together over the next couple of days.
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

MikeyV wrote:
Thu 01/16/20 9:06 pm
I'd love to hear anyone's opinions on that ground layout before I dig in!
It's not bad, much better than the original. I hate that layout in general though, it's very sloppy.

If you use shielded wire, make sure to only connect one end of the shielding, and heatshrink the other side so it doesn't contact anything.
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Re: 18 watt TMB squeal & hum

Post by MikeyV »

OK, so I re-did the grounding scheme as shown in the diagram I posted.

Good news first. The Normal channel still works fantastically.

Sorta good news, with the gain on CH2 turned to 10, I can turn up the master to 6-7 now before all hell breaks loose. Now that I can get it higher, I'm hearing new noises.

-Lots of hum, like when you touch the tip of your guitar cable with your finger.
-wha-wah-wah sound only at higher volumes, motorboating? It comes on and then gets stronger until you kill it with the volume knob.
-Less squealing and more moaning/feedback.
-The TMB knobs affect these noises, but not much.

Something more. If I dime the master with the gain all the way down, I get what I'd call a normal hiss of a dimed tube amp. As I bring in the gain knob, then I start getting the loud hum, feedback, oscillation squeal, etc.

Doing the opposite, If I dime the gain knob, then bring in the master, it just gets super loud really quick.
It seems like the gain knob and what it's connected to bring in the trouble.

Now read this: I think I may have a bad component.
If I chopstick the whole thing, it's all very quiet, until I get to the 470K resistor that comes from the gain pot and feeds the grid of the second stage of CH2. See the schematic I posted with highlight. The wire coming off the gain pot, going to the 470K reistor mounted on the board, and the wire going from that to the grid of the tube are all very noisy. If I tap the actual body of the resistor, I get a loud "ploink!" sound thru the speakers. Only this resistor does this. Is that puppy bad?

I was going to re-flow the solder joints there last night...but Football..then the baby was asleep.

I plan on cleaning out the old solder, checking the component, re-soldering. If it's still bad, I'll order a new one.
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

Chopsticking isn't just hitting things. You can also use the chopstick to reposition wires and see if that effects your parasitic oscillation.

It could be the resistor, but I would check the gain pot wires first. You might need shielded wire from that pot. But it could also be adjacent wires causing trouble. Good luck and be safe.
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Re: 18 watt TMB squeal & hum

Post by MikeyV »

I've also moved wires around with no real difference. I focused on the CH2 stuff mostly.

It's interesting to me that the first stage input wire is only mildly sensitive to mechanical rustling, but the input wire to the second stage is super sensitive.

It's my first "high gain" amp so I've never really run into this issue with the Tweed and Blackface stuff I've built.

Kinda lame that the Mojo layout doesn't really work. But like I said, re-grounding it didn't help that much. There's still something very wrong with this channel.

Using my brain hurts, but thinking about it, if that grid resistor was bad, or not providing the resistance it's supposed to, I guess it COULD give the problems I'm seeing.

I dunno. I'll change that that resistor out and fiddle more with the gain pot wires and see what's up.
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Re: 18 watt TMB squeal & hum

Post by MikeyV »

Made some progress.

Fiddled around with a few things, put most back. I wired the input jack differently, like shown down at the bottom .

I also added a shielded input wire from the jack to the grid, and put the 68K resistor right on the tube.
This helped a lot. I can turn it up much more, but there's still feedback and squealing as the volumes go higher.

Next step is to shield the wire from the gain pot to the 470K resistor on the input to stage 2 grid, and move that same 470K resistor to the tube socket and off of the board. The *ploink* sound is gone, must have been a bad joint.

A few questions:
1) I can only measure 500K resistance across the Master Volume Pot. I de-soldered it and conformed it's 1 Meg, but in the circuit, it only shows 500K max, if the other TMB controls are in a certain position (on/off). Is this normal?
I'm thinking it the Master is only providing half the resistance, it may account for the jumpy action of the master know ( nothing to face melt in a short range, then squealing beyond that). I've checked the wiring here a lot, but not I don't trust the damn Mojo schematic or layout.

2) Is there something wonky in the wiring of the Bass pot to the treble and mid pots? It matches the CeiraTone TMB layout, but it doesn't seem to jive with the schematic provided by Mojo. There's an error in the bass pot wiper wiring on the schematic and it's throwing me for a loop. Again, checked many times, don't trust the Mojo drawings.

3) Why is this thing so loud? The normal channel seems OK to me on full blast, loud but not sounding like it's gonna blow out the windows. The TMB channel is as loud as a full stack with 50W JMP50 on top.

I tried a lower gain tube (5751, with an Au = 70, I think) in the second gain stage and not much change there.

Thanks for bearing with me, I'll keep going. I think that shielding that volume pot wire will help.

Mike
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MikeyV
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Re: 18 watt TMB squeal & hum

Post by MikeyV »

OK, looks like I fixed it.

Like I said before, changing the grounding helped a little, then adding a shielded wire from the input jack to the first gain stage on CH2 (along with moving the 86K resistor off of the cliff jack and right on to the tube) helped a lot.

What really fixed it was adding a second shielded wire from the gain pot to the second stage of CH2, along with moving that 470K ohm resistor right onto the tube.

Only real issue left is a good amount of hiss, but I realize that's mostly normal with everything dimed. I reckon I'll work on that a bit, probably can't do much.

Thanks for the help. I hope this helps some others that get the Mojo kit.


Cheers, Mike

PS:I'll post a few current photos soon.
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Re: 18 watt TMB squeal & hum

Post by Explorerman »

Great to hear (gives me a little more hope with my current build)! Yes please list some pics when you get a chance.
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MikeyV
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Re: 18 watt TMB squeal & hum

Post by MikeyV »

I forgot to add.

It's still LOUD as HELL. If you dime the gain, it's pretty loud at 3-4 on the master, and then it seems to get a bit louder, and adds more power section distortion as you crank the master volume up.

Pretty rippin' for 18 watts. Guess I'll look into an attenuator. lol.
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

MikeyV wrote:
Sat 01/25/20 11:56 pm
I forgot to add.

It's still LOUD as HELL. If you dime the gain, it's pretty loud at 3-4 on the master, and then it seems to get a bit louder, and adds more power section distortion as you crank the master volume up.

Pretty rippin' for 18 watts. Guess I'll look into an attenuator. lol.

Congrats on making all that progress! Yes, these amps are LOUD. People think wattage = volume, but it doesn't. a 10W amp is 50% the volume of a 100W amp. Then you figure in other things like speaker efficiency and cabinet, and it's crazy.

I'm assuming your kit had carbon comp resistors. If so, they hiss. The only place that theoretically benefits from carbon comps is the plates, and usually only on the PI. Carbon Film is much quieter, and Metal Film quieter than that. Some argue that Metal Film is sterile by comparison, but nobody has really proven that scientifically.

One easy place that makes a difference, and I would use MF (metal film) for is the input grid resistors. Those are the most prone to noise.
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Re: 18 watt TMB squeal & hum

Post by DFJB »

MikeyV,

It appears you and I are on parallel paths of discovery concerning the mojotone TMB. I am also having squeal and hum issues. I would love to see some photos of your layout, especially your implementation of moving the 68K and 470K resistors to the tube sockets.

And, I absolutely agree with you. This amp is pretty darned loud. Probably too loud for my needs.

DFJB
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Re: 18 watt TMB squeal & hum

Post by MikeyV »

Yeah, I'm beginning to get the wattage/loudness thing thru my thick skull. I have amps from 7 watts, or a single 6V6 in a Tweed Princeton to 135W '79 Fender Vibrasonic Reverb, and they're all too loud, really. LOL.

Next purchase should be an attenuator.

JMP, Thanks for your help, and that further info on the MF resistors.
I think I'll order some and re-do that part. Easy enough.

I'll post pics, cell phone is dead at the moment.

Mike
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Re: 18 watt TMB squeal & hum

Post by MikeyV »

This thing is still pretty angry.

Most problems gone except for nasty loud feedback when turning up the master volume past maybe 6. It goes from silent up to about 2, then to full blast at about 3, then gets crunchier up to about 5 or 6, then feedback starts.
If I turn the guitar volume off, that feedback stops. Then the hisssss remains.

Is shielding the cable to/from the master volume pot a thing?
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Re: 18 watt TMB squeal & hum

Post by Gradysmusic »

MikeyV wrote:
Thu 01/30/20 12:04 pm


Is shielding the cable to/from the master volume pot a thing?
Yes its a thing on most high gain amps
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Re: 18 watt TMB squeal & hum

Post by zaphod_phil »

Actually all long wires carrying signal (ie not power rails) should be shielded.
If adjusting the guitar volume affects the squeal, you probably need potted pickups. Also, a loud thin hiss is what ultrasonic oscillation usually sounds like to our human ears.
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Re: 18 watt TMB squeal & hum

Post by gheorge77 »

Gradysmusic wrote:
Thu 12/23/21 10:22 am
MikeyV wrote:
Thu 01/30/20 12:04 pm


Is shielding the cable to/from the master volume pot a thing?
Yes its a thing on most higher gain amps
I know this is an old topic but another question to consider. Using shielded wire and moving the 68k and 470k to the sockets helped with noise but I still get some oscillation. My 1987 build is far quieter without any oscillations at all. The thought dawned on me why does this need a master volume if the 1987 doesn't (at least for me). It seems like the Mojotone mv is the big source of problems here.

Has anyone removed it? Seems like all I'd need to do is move the wire from the board going to the mv to the treble pot, no?
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Re: 18 watt TMB squeal & hum

Post by Bieworm »

gheorge77 wrote:
Tue 02/15/22 2:48 pm
Gradysmusic wrote:
Thu 12/23/21 10:22 am
MikeyV wrote:
Thu 01/30/20 12:04 pm


Is shielding the cable to/from the master volume pot a thing?
Yes its a thing on most high gain amps
I know this is an old topic but another question to consider. Using shielded wire and moving the 68k and 470k to the sockets helped with noise but I still get some oscillation. My 1987 build is far quieter without any oscillations at all. The thought dawned on me why does this need a master volume if the 1987 doesn't (at least for me). It seems like the Mojotone mv is the big source of problems here.

Has anyone removed it? Seems like all I'd need to do is move the wire from the board going to the mv to the treble pot, no?
Maybe you could post detailed pics of your build and the according schematic?
And you clued us by mentioning Mojotone.. that's a synonym for trouble.
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Re: 18 watt TMB squeal & hum

Post by gheorge77 »

Bieworm wrote:
Tue 02/15/22 3:54 pm
gheorge77 wrote:
Tue 02/15/22 2:48 pm
Gradysmusic wrote:
Thu 12/23/21 10:22 am


Yes its a thing on most high gain amps
I know this is an old topic but another question to consider. Using shielded wire and moving the 68k and 470k to the sockets helped with noise but I still get some oscillation. My 1987 build is far quieter without any oscillations at all. The thought dawned on me why does this need a master volume if the 1987 doesn't (at least for me). It seems like the Mojotone mv is the big source of problems here.

Has anyone removed it? Seems like all I'd need to do is move the wire from the board going to the mv to the treble pot, no?
Maybe you could post detailed pics of your build and the according schematic?
And you clued us by mentioning Mojotone.. that's a synonym for trouble.
Not really looking for ways to decrease the oscillation. I've added shielded wire to the TMB side and honestly don't see the point of the master volume on this amp. It appears to really function to slightly reduce gain until it's dimed and just adds noise. The normal channel is dead silent even without shielded cable.

I was more curious if anyone had tried removing the master volume on this.

-Ian
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Re: 18 watt TMB squeal & hum

Post by Bieworm »

gheorge77 wrote:
Tue 02/15/22 7:41 pm
Bieworm wrote:
Tue 02/15/22 3:54 pm
gheorge77 wrote:
Tue 02/15/22 2:48 pm


I know this is an old topic but another question to consider. Using shielded wire and moving the 68k and 470k to the sockets helped with noise but I still get some oscillation. My 1987 build is far quieter without any oscillations at all. The thought dawned on me why does this need a master volume if the 1987 doesn't (at least for me). It seems like the Mojotone mv is the big source of problems here.

Has anyone removed it? Seems like all I'd need to do is move the wire from the board going to the mv to the treble pot, no?
Maybe you could post detailed pics of your build and the according schematic?
And you clued us by mentioning Mojotone.. that's a synonym for trouble.
Not really looking for ways to decrease the oscillation. I've added shielded wire to the TMB side and honestly don't see the point of the master volume on this amp. It appears to really function to slightly reduce gain until it's dimed and just adds noise. The normal channel is dead silent even without shielded cable.

I was more curious if anyone had tried removing the master volume on this.

-Ian
The general consensus is that an 18W will not do what it does best with the addition of a MV. The famous touch sensitive complex overdrive is mainly due to the saturation of the EL84 tubes. There are versions where you can get superb preamp overdrive though. The Tremolo TMB, for instance, has a pre PI mastervolume AKA 'volume' ;)
You're only compromising the gain with a regular TMB MV, and it doesn't do much on volume level either
A PPIMV is obsolete.. still no power tube overdrive then
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

In other words, he's saying you're fine to remove the master volume. If you're not sure about the wiring without the MV, compare to a schematic in our downloads section.

Thanks,
Josh
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