Troubleshooting 60-cycle hum in the TMB input of my 18w head

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,751
Location
Salt Lake City
More hiss musings. We talked about how you'd gone all in on shielded cables. But can you remind us which end / how you grounded your shielded input cable, and where you hung your input grid-stopper and connected it to the input cable?

And... we talked about how you'd already replaced your 68K input grid stopper with a 10K, a well-known (and significant) hiss reducer. I mentioned that Merlin adds a small cap grid-to-ground when he does this. http://www.valvewizard.co.uk/gridstopper.html

Looking at an RC corner-frequency calculator, if we take his ~200pF capacitance for a 12AX7, adding a 470-500pF cap would move the corner frequency from ~80kHz to ~20kHz. I'm no pro at high-pass filters, but would that quiet hiss?

Also haven't seen a layout to wire up Merlin's schematic, but I'm sure all our smart friends will know.

1661785420777.png
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
466
Location
Utah
More hiss musings. We talked about how you'd gone all in on shielded cables. But can you remind us which end / how you grounded your shielded input cable, and where you hung your input grid-stopper and connected it to the input cable?

This is the Cliff input jack for the TMB channel. The shielded cable runs to a terminal tab (grounded there) and from the ungrounded tab to V1B with a 10K resistor.

IMG_1496.jpeg


IMG_1497.jpeg

And... we talked about how you'd already replaced your 68K input grid stopper with a 10K, a well-known (and significant) hiss reducer. I mentioned that Merlin adds a small cap grid-to-ground when he does this. http://www.valvewizard.co.uk/gridstopper.html
Right. Thank you for that reminder. Would it make any difference if there wasn't a cable plugged in? The hiss doesn't get any louder with or without a guitar cable in and switched on. For kicks I have tried putting a 470pF and 820pF with no noticeable difference. If I'm thinking straight on this, the idea would be to create a low-pass filter so that the high end hiss is filtered out.

FWIW, I compared the hiss against my Princeton Reverb and my JTM45 micro. Both of those amps also have some hiss (seems normal) and if you play at respectable / gigging volumes that won't make your ears bleed, this 18w TMB channel seems to have reasonable amounts of hiss, considering that it's a higher-gain circuit.

It also helps to plug it into the Greyhound speaker instead of the BV-30H since the Greyhound has a little bit less high-end response. I think at this point, it's probably behaving like it's designed.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
466
Location
Utah
I don't know, this saga is a bit confusing and TLDR it all, but that loudest sound to me is hiss. The "hum" is relatively minimal. Perhaps a bad tube, cap or resistor(s)? It doesn't sound like primarily a ground problem. What happens chopsticking/tapping cathode and plate resistors?
I've chopsticked everywhere. The only things that seem to make any noise — and really it's just an amplified tap, are:

1. V2A's 470K grid stopper from the gain pot, which is mounted directly to V2, pin 7. In fact, V2A's plate, grid, and cathode pins you can hear the tap through the speaker when tapped (pins 6 through 8). No crackle, just an amplified tap.

2. The 22nF capacitor connected in between the normal channel's volume potentiometer and the phase inverter's screen (V3A). Again, no crackle or anything, you can just hear the tap amplified a bit in the speaker.

3. The 10nF capacitor connected in between the TMB's MV potentiometer and the phase inverter's screen (V3B). No crackle, but tap is amplified in the speaker.

No other components make any noise/reaction.

EDIT: I'm gonna have to take that back. I swapped the V1 and V2 tubes and it made a difference. I'll make a video later.
 
Last edited:

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
466
Location
Utah
I don't know, this saga is a bit confusing and TLDR it all, but that loudest sound to me is hiss. The "hum" is relatively minimal. Perhaps a bad tube, cap or resistor(s)? It doesn't sound like primarily a ground problem. What happens chopsticking/tapping cathode and plate resistors?
@mountainhick Here's a chopstick video:
 
Last edited:

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
466
Location
Utah
For kicks I have tried putting a 470pF and 820pF with no noticeable difference. If I'm thinking straight on this, the idea would be to create a low-pass filter so that the high end hiss is filtered out.
@King Fan I got a little more scientific about it after I noticed buzzing (I'm sure from my lights overhead) when I had my guitar plugged in. I hooked up a microphone to it and compared with no capacitor vs. 820pF, 470pF, 100pF. Each of those values cut down on the buzz from my guitar pickups/cable. But, the 820 & 470 definitely dropped too much top/treble from the actual tone of the guitar. The 100pF still cut out almost the same amount of buzz but without losing definition in the sound. So, there you go. It was worth it to do that.

Note: Ignore that I didn't run a ground wire all the way back to the preamp ground (didn't seem to create any more hum than was already there). I just ran them to the ground tabs which are grounded right next to the tubes. 🤷🏻‍♂️ This is turning out to be a bit of a Frankenstein amp, but still sounds good. Hehe.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
466
Location
Utah
At some point I think it'd be interesting to track down some non-microphonic tubes or something better than JJ 12AX7 preamp tubes and give them a shot in this amp, but it does work pretty well at volumes you'd actually want to play it at (I think):

 

wavytech

TDPRI Member
Joined
Dec 5, 2020
Posts
24
Location
94043
What kind of tubes are you using in your preamp sockets? Anything in particular? I’m using plain ol’ JJ tubes and I wonder if a less hissy tube would help.
I have the JJs (12AX7 and EL84) that were supplied by Mojotone. It sounds like we're playing through the same amp. This is a bright amp, no doubt about it. There are many ways to tame this, but I find that in a band setting the brightness allows the amp to cut through the mix, especially for leads. I have Fender amps for big clean sounds and classic rawk, but I if I want a more 80s high gain distortion at club volume the Mojotone satisfies. I have many amps but keep coming back to this one.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
466
Location
Utah
I have the JJs (12AX7 and EL84) that were supplied by Mojotone. It sounds like we're playing through the same amp. This is a bright amp, no doubt about it. There are many ways to tame this, but I find that in a band setting the brightness allows the amp to cut through the mix, especially for leads.
Totally makes sense! Thank you for taking the time to answer that. It definitely seems like it’ll have the ability to cut through with no problems.

I have many amps but keep coming back to this one.
Awesome. Well said! Thanks.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
466
Location
Utah
Another discovery in the saga of the 18w hum. Yesterday I was playing on both of my Marshall-style amps (this 18w and the JTM45 micro) and the hum on the 18w was crazy bad. I thought I had figured it out.

Then, I realized that if I unplugged my JTM45 Micro to the wall (and my 18w was on with a guitar cable plugged in) the bad 60-cycle hum went nearly away. What in the world!? So I tried plugging in my Princeton Reverb to the wall. No additional hum.

What could be going on!?

Well, I learned something new … I was using a DIY ABY box to switch between the JTM45 Micro and 18w. Ground loop issue through the guitar cables and the ABY box. Duh!

Simple fix:
Use a socket adapter on one of the amps so that when you plug the power cord in, it omits the ground wire. That way, the amps aren’t trying to reference two ground points and there are no ground loops. Of course I will only use this when they are both physically connected via guitar cables to the ABY box.

Nice and mostly hum-free playing again. They are fun to play in A+B mode for a more stereo sound.
 




Top