18W Weber TMB Hum w/ Nothing Plugged In

[fishdogdaddy]

Sorry to start thread #10,536 on this topic. I have searched and read a good bit of threads that seem similar to my issue but nothing exact so I figured I’d post. If you know of a copy thread please point me in that direction and I’ll gladly not waste anymore of your time.

I was gifted a Weber 18W which sounds great but on the TMB channel there is a 60hz hum that increases when you turn the volume up. Even with nothing plugged in it hums. It doesn’t hum when the volume pot is turned all the way down on the amp itself but as you turn the volume up the noise gets louder.

All tubes have been replaced and I have rewired the inputs on that channel. I read in a thread on here if you let it hum and then flip the standby on it will tell if it’s heater or ground. It seems to me like it does take a moment for the hum to fade after flipping the switch. However it seems like it’s a grounding issue since it does even with no cable plugged in. Almost like the inputs aren’t disengaging or something.

Any help is appreciated. If you need pics or anything I’ll gladly post them.

[fishdogdaddy]

Also when I remove V3 which I’m fairly certain is the PI the hum stops

[Daviedawg]

Welcome to 18 watt. Pleased to have you here and happy to help.

I found a layout for your amp but not a schematic. But even so you are right that V3 should be the PI. Removing it interrupts the signal path so confirms that the hum is being created in the preamp or PI stages.

If there is no similar volume responsive hum on the normal channel then we should start by looking at the TMB channel for issues. My suspicion would be a short to ground or a bad joint. You have remade the inputs on the channel affected. So my next move would be to chopstick the signal path through the channel back from the PI looking for a noisy connection, also looking for "whiskers" shorting valve pins to each other and that it is wired in accordance with the schematic. At the same time mark up your schematic with the routes and nodes you have checked so that you can be sure you have inspected everything. If that does not throw anything up my next move would be to rewire the pots.

This kind of troubleshooting can be quick and easy or tedious. If nothing comes from this then come back and post your schematic and voltages from V2 and V3 to see if that throws up anything.

Above all be safe among the dangerous voltages.

Dd

[fishdogdaddy]

Welcome to 18 watt. Pleased to have you here and happy to help.

Thank you very much.

If there is no similar volume responsive hum on the normal channel then we should start by looking at the TMB channel for issues. My suspicion would be a short to ground or a bad joint. You have remade the inputs on the channel affected. So my next move would be to chopstick the signal path through the channel back from the PI looking for a noisy connection, also looking for "whiskers" shorting valve pins to each other and that it is wired in accordance with the schematic. At the same time mark up your schematic with the routes and nodes you have checked so that you can be sure you have inspected everything. If that does not throw anything up my next move would be to rewire the pots.

Thanks for pointing me in a direction. You are correct there is no issue on the normal channel. (Although there is quiteee a bit less gain on that channel and I have nothing to compare it to so I'm not sure if that's normal.) I brought the amp with me to work so I am going to try some of that on my lunch break. Planning to gig it this weekend so fingers crossed it's more on the quick and easy side of things. Thanks again for your help. I will post back with my findings.

[geoff 1965]

found this schematic,good luck sorting the hum

6m18tmb_schem.jpg

[fishdogdaddy]

Since V3 is the PI tube and disconnecting it stops the hum that means it has to be in the outlined section of the TMB preamp correct? Also can someone who is a bit more experience with schematics tell me why one is listed as V3A and one V3B?

[geoff 1965]

hello,
V2 is your phase inverter tube!
12ax7's,12at7's etc are double triode valves,2 triodes in 1 valve,so V3a and V3b are both halves of the same tube.

[fishdogdaddy]

hello,
V2 is your phase inverter tube!
12ax7's,12at7's etc are double triode valves,2 triodes in 1 valve,so V3a and V3b are both halves of the same tube.

Oh ok well I had their purposes swapped but i had the number right. I pulled V3 and the hum went away.

[JMPGuitars]

According to the schematic you posted, geoff is correct, V2 is your PI. That's why it's always a good idea to post a schematic when asking these questions (it's very easy for people to be looking at different diagrams). V3 is your TMB channel tube, and where you drew the outline is half of V1, which is part of your input and preamp.

If you leave in V2, and pull V3 (and you're certain it is indeed V3), then your hum is coming from the preamp section. What about if you leave in V2 and V3 and pull V1? What happens then?

Thanks,
Josh

[geoff 1965]

yes so you're right to be looking at the TMB area, is V3 a 12at7 or 12ax7?

[fishdogdaddy]

yes so you're right to be looking at the TMB area, is V3 a 12at7 or 12ax7?

V3 is a 12AX7

[fishdogdaddy]

According to the schematic you posted, geoff is correct, V2 is your PI. That's why it's always a good idea to post a schematic when asking these questions (it's very easy for people to be looking at different diagrams). V3 is your TMB channel tube, and where you drew the outline is half of V1, which is part of your input and preamp.

If you leave in V2, and pull V3 (and you're certain it is indeed V3), then your hum is coming from the preamp section. What about if you leave in V2 and V3 and pull V1? What happens then?

Thanks,
Josh

It’s def V3 i have checked the schematic and layout and have them labeled to limit my own confusion. I will pull V1 this evening when I get home from work and let you know the results.

[fishdogdaddy]

If you leave in V2, and pull V3 (and you're certain it is indeed V3), then your hum is coming from the preamp section. What about if you leave in V2 and V3 and pull V1? What happens then?

I pulled V1 and it also got more quiet. Basically same result as pulling V3.

[JMPGuitars]

I pulled V1 and it also got more quiet. Basically same result as pulling V3.

What does that tell you?

[fishdogdaddy]

I pulled V1 and it also got more quiet. Basically same result as pulling V3.

What does that tell you?

It's before V1b in the TMP pre-amp section. So it's either something with the input jacks or one of the resistors before V1b. I am not an expert though lol.

Edit: Theoretically couldn't i swap the wire over connecting the normal channel to V1A to V1B and know immediately if it's in the TMB's input section?

Also there is no capacitor so it's either a bad input jack, bad resistor(can they actually cause hum), or most likely a ground problem.

[JMPGuitars]

It's before V1b in the TMP pre-amp section. So it's either something with the input jacks or one of the resistors before V1b. I am not an expert though lol.

Edit: Theoretically couldn't i swap the wire over connecting the normal channel to V1A to V1B and know immediately if it's in the TMB's input section?

Also there is no capacitor so it's either a bad input jack, bad resistor(can they actually cause hum), or most likely a ground problem.

The issue could be anywhere in the V1 section, but it also might not be the only issue. It's the starting point that we'll look at.

This is a good time to post photos and a copy of the actual layout you used.

Hum could be a lot of things, but grounding issues is one of the most common reasons.

[fishdogdaddy]

It's before V1b in the TMP pre-amp section. So it's either something with the input jacks or one of the resistors before V1b. I am not an expert though lol.

Edit: Theoretically couldn't i swap the wire over connecting the normal channel to V1A to V1B and know immediately if it's in the TMB's input section?

Also there is no capacitor so it's either a bad input jack, bad resistor(can they actually cause hum), or most likely a ground problem.

The issue could be anywhere in the V1 section, but it also might not be the only issue. It's the starting point that we'll look at.

This is a good time to post photos and a copy of the actual layout you used.

Hum could be a lot of things, but grounding issues is one of the most common reasons.

The normal channel also shares V1 and doesn't hum. Doesn't pulling the tube show that its before the tube? I might be struggling to wrap my mind around what certain actions are actually telling me.

[JMPGuitars]

The normal channel also shares V1 and doesn't hum. Doesn't pulling the tube show that its before the tube? I might be struggling to wrap my mind around what certain actions are actually telling me.

Could be, TBH, I didn't read everything.

I would start by posting the build photos and layout, and check the input and preamp grounds.

[geoff 1965]

hello,
yeah don't forget you said pulling V3 also stopped the hum and the TMB channel cascades from V1b to V3a so it could be anywhere from the input to past the tonestack! i would start at the jack and work through doing what Josh & DD suggested checking grounds and solder joints.
i found this layout which is the same date as the schematic but you will have to confirm if it's the same as yours,good luck chopsticking!

6m18tmb_layout.jpg

[fishdogdaddy]

So I used the wire that connected the normal channel input to V1 and moved it to the pin on V1 where the TMB channel connects and it stopped the humming totally. I thought I was totally out of the water and tested it numerous times and everything seemed fine and the TMB channel was working.

Now though it’s changed. The hum is gone but a hiss has developed that is on all the time. I pulled V3 and it stops hissing. Pulled V1 and it doesn’t. I disconnected the wire going into pin 2 on V3 and it kept hissing. Disconnected pin 3, 1, 7, and 6 and it keeps hissing. The only way I can make it not hiss is to disconnect the wire coming out of the valve or the Filament connection on pin 4/5.

I am totally stumped as of right now.