First post, my “new” 18W

[Skydog6653]

On the TMB circuit shown, (Valvestorm) why aren't the treble and bass pots grounded? download/file.php?id=11528

[geoff 1965]

that's how all the TMB tonestacks are wired,notice your treble & bass pots are wired the same except you don't have the master volume.how's things going with the amp?

[JMPGuitars]

On the TMB circuit shown, (Valvestorm) why aren't the treble and bass pots grounded? download/file.php?id=11528

A better question is why would they be grounded? Always look to a schematic for more details (never rely only on a layout): storage/18watt_tmb_JAschematic.pdf

Every part has a function within a circuit. Find the tone circuit you're talking talking about on the schematic, and see if it makes more sense from the schematic perspective.

[Skydog6653]

Your question is a valid one, but beyond my ability to answer. I was studying the layout and noticed that some, not all, of the pots are grounded. I was curious as to whether it was an oversight or not. I replaced the 2/1M resistors on the input jacks and the 100KR on the V-T normal and the 82K on the TMB side. Also, the 1K5R/25uf cap. I ran the 12 awg square bus wire for the new ground and tied everything from the pots etc. into it with the 22awg stranded. I left the ground near the trannies intact. I grounded the power cap etc. to its own ground. After a 10 minute warmup I took the amp off standby and it sounds good. Not great, not yet! It still has a hum, albeit a lot less. The hum is accentuated by the volume. It is slightly more noticeable on the TMB channel. Just how quiet can I reasonably expect this amp to be? Right now I'd say it's giggable, but not recordable or miced, if that helps. I'll take some more pics tomorrow with better light.

[JMPGuitars]

My question was posed for you to learn more about the circuit and improve your schematic reading skills. Pots are variable resistors, that's all. If you can find the part of the tone circuit on the schematic, you can see how those resistors interact with the tone circuit. You can study further and see why only some of them connect to ground.



Remove the PI ground from the preamp ground. Connect that to the power amp ground.

After that if your hum is still annoying, tap things with a chopstick, and try chopsticking wires and see what affects what.

Thanks,
Josh

[Skydog6653]

My question was posed for you to learn more about the circuit and improve your schematic reading skills. Pots are variable resistors, that's all. If you can find the part of the tone circuit on the schematic, you can see how those resistors interact with the tone circuit. You can study further and see why only some of them connect to ground.



Remove the PI ground from the preamp ground. Connect that to the power amp ground.

After that if your hum is still annoying, tap things with a chopstick, and try chopsticking wires and see what affects what.

Thanks,
Josh

Oh, I know that and very much appreciate all the invaluable help from you, Geoff, and this forum. I hope you understand this comparison: me starting with this amp, reading the schematics, layouts, etc. is akin to a raw beginning guitarist starting with Richie Blackmore's solo in Highway Star! But I have learned a lot already. The “move” you're saying is from the 2/470K resistor junction? Move the other end off the bus wire and connect it to the ground with the 500V 22uf cap?

[geoff 1965]

hello skydog,
i've circled the phase inverter ground Josh is referring to, also don't forget to replace that B+ dropper "circled"
has the initial "squeal" problem stopped now?

02E45CE4-7C6E-4215-8BB9-ADD63D480336.jpeg

[Skydog6653]

Will do! Yes, the squeal and excess crackling has stopped.

[JMPGuitars]

No, skydog was right, I was referring to the other one with the 470K pair at the junction between the power tubes and PI.

The one you circled is part of the tone stack and should remain on the preamp ground.

Follow the signal path.

Thanks,
Josh

[JMPGuitars]

Regarding the schematic:

If you look at the layouts in our downloads section, they match the schematic I linked. If you compare to the ValveStorm layout, you'll see that ValveStorm reversed the numbers for V2 and V3. It's arbitrary, but you need to be aware of it when comparing it to the schematic. ValveStorm V3 = PI, our downloads secition V2 is usually the PI for the TMBs. But always check and make sure before assuming.

18watt_tmb_tonestack.jpg

Since we know that V2 is the PI, and pin 2 of the PI is the input, we can see that V3 is feeding the tonestack that goes into the PI. So V3's the starting point of following the signal path through the tone circuit. There's a lot more going on here that would take some reading to really understand, BUT keep it basic for now and consider what happens at each point.

The arrow pointing at the resistor symbol both tells us that the resistor we're looking at is a potentiometer, and represents the middle lug (the sweep). Following the path, you can see that the circuit functionally connects to ground where it needs to.

Thanks,
Josh

[geoff 1965]

"Remove the PI ground from the preamp ground. Connect that to the power amp ground."
that's what i followed for the "circled" phase inverter ground Josh,you meant the power amp grid leaks ground.
good to see skydog is getting there and the squeal & crackling has gone!

[JMPGuitars]

"Remove the PI ground from the preamp ground. Connect that to the power amp ground."
that's what i followed for the "circled" phase inverter ground Josh,you meant the power amp grid leaks ground.
good to see skydog is getting there and the squeal & crackling has gone!

LOL! That's really funny. I can see how that could be confused. Yes, the grid leaks, but not everybody knows what that means.

[Skydog6653]

I moved the ground and replaced the B+ resistor this morning. Still a low hum. I chopsticked around and the only bad spot I could find was at the other end of the wire running from V1, Pin 6. I pulled V1, but the hum continued and the scratching noise when probed also continued. This same noise also occurs at Pin 6 when probed with the chopstick. I reflowed those connections, but no change.

B34E002F-7837-4BB2-B5AB-0617C578F7D4.jpeg

[colossal]

That yellow capacitor just behind your chopstick...it looks like it got nicked with the soldering iron. With your multimeter, check to see if DC voltage is leaking across it. Scratching noises on pots is very typically due to a leaking (often mica) coupling cap.

[Skydog6653]

No leaking; it tested slightly under the rated 0.022 at 0.0197.

[JMPGuitars]

Make sure no part of your ground bus bar contacts the chassis anywhere except at the single point it's supposed to.

[colossal]

No leaking; it tested slightly under the rated 0.022 at 0.0197.

Ok, that's good. Carry on!

[geoff 1965]

you're getting there with the grounding and i've just been through the whole post from page one and one component we've overlooked is the dual can capacitor.if that is original then it's 16 years old and could be leaky which will cause hum so it might be worth considering?

[Skydog6653]

I found the dual can cap with no problem; looks like an exact match https://www.tubesandmore.com/products/c ... ectrolytic I can't find the 5W 1K4 resistor.

[geoff 1965]

the dual can looks good,in the same link click on resistors,power, then 5W wirewound, they have 1.5K which will be okay,
check the width of the can will fit your clamp.
what voltage rating are those 2 red/orange capacitors in the preamp skydog?