First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by zaphod_phil »

Yamariv wrote:
Sun 03/29/20 10:43 am
So for the grounding scheme, you guys are saying the whole amp should ideally only have one ground point to chassis all coming from the buss?
No, that buss wire is only for the more sensitive parts of the amp - ie the preamp and PI. The power amp and power supply should be grounded to a single "star ground" connection to the chassis, at the far opposite end from the input jacks and preamp, labeled as the "power grounding point" in the Mojotone layout. It should also be kept separate from the AC Safety Grounding point.

There should be no connection between the power grounding point and the preamp/PI ground buss wire, except through the chassis.

The filter caps supplying the preamp and PI stages should be grounded to the buss wire, with each stage's filter cap grounded immediately adjacent to where the stage's cathode resistor+cap are grounded, to help prevent ground currents crossing between stages.

I hope that made some sense...
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by divali »

Thank you for all this helpful info. I am also building an 18 watt Tubedepot amp, so this is all handy stuff.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Thanks Phil makes sense! Just a little confused about the last paragraph but all good I get the gist of what you are saying.

Has anyone had a chance to listen to the video I posted on Youtube showing the sound the amp makes? Any recommendations upon hearing it? Amp sounds great otherwise..

On a side note, I'm building a 5E3 right now, would you guys recommend a separate buss ground for the filter caps and a separate buss for the preamp or put them all together? I hear theres debate on that one..
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by zaphod_phil »

Depends which filter caps you mean. If you mean the one or two big ones that feed the power stage, then they must absolutely *never* share the preamp ground buss. Only filter caps feeding the preamp or PI stages can be grounded there. This is not something that can be debated, unless you’ve now developed a liking for hum and buzz. :)
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Lol! Makes sense, I'll make sure to separate them for sure then! Thanks :D
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by zaphod_phil »

Another thing I'd recommend separating is that bundle of wires shown in the middle of the picture you posted. They will all radiate into each other like that. You should replace them with solid core wire that you can bend tidily to shape, and run separate lengths between the points you need to connect. Shielded wire is instead recommended for any longer stretches, with the braid connected to the ground buss at one end only.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Good point, that bundle of wires is for the master volume. As soon as I finish the 5E3 build in a couple of days, I'm going to bring the 18 Watt to the bench to mod a few things and redo a few others.

I'm going to redo the entire heater wire (not looking forward to that :roll: ) and I'm going to move the master to the front of the amp. That will be a win win as the wires will be much shorter and having the master underneath in the cab is a pain in the butt.

What's the consensus on add a choke to the 18 watt? I have a spare one out of an EL84 Organ Chassis I was thinking of trying
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by colossal »

Yamariv wrote:
Wed 04/01/20 9:20 pm
What's the consensus on add a choke to the 18 watt? I have a spare one out of an EL84 Organ Chassis I was thinking of trying
You can try it and it will likely change the feel of the amp (for better or worse) but I wouldn't do so as a means to fix your noise problem by brute force. Your issue is most certainly lead dress related. 18W amps should be very quiet at idle. Matchless amps, specifically the Spitfire, is closest to an 18W topology and uses a choke in the power supply, FWIW.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

For sure, makes sense about the lead dress. I was more just curious about a choke on its own, what will it do exactly, tighten up the sag? Not sure if I want to loose any sag so maybe I should wait to try a choke and do it down the road just for an experiment.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by colossal »

Yamariv wrote:
Thu 04/02/20 11:46 am
For sure, makes sense about the lead dress. I was more just curious about a choke on its own, what will it do exactly, tighten up the sag? Not sure if I want to loose any sag so maybe I should wait to try a choke and do it down the road just for an experiment.
Depends on the inductance and DC resistance of the choke. Chokes resist changes in current. Think of them as frequency dependent resistors where a resistor provides the same voltage drop at all frequencies. A choke will provide much better DC regulation with a greater reduction in AC ripple. You could certainly try it. The magic of the 18W is in its touch sensitivity which, in part, is contributed to by the 2k2 resistor feeding the screens. EL84s draw a fair amount of screen current on large transients and the voltage drop across that resistor will cause some compression. Matchless amps have low DCR chokes (screen voltage is very close to plate voltage) and are biased very hotly, so will sag less, giving them a stiffer feel and more zingy tone. This may or may not float your boat, but it won't be an 18W anymore.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Makes sense, thanks for the explanation! :D Given that, I don't think I will put it in just yet, I'm going to focus on the hum fixing first then maybe down the road I'll pop the choke in for curiosity sake since it's easy to install and remove. I definitely don't want to loose the 18 watt feel.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by zaphod_phil »

Yamariv wrote:
Wed 04/01/20 9:20 pm
that bundle of wires is for the master volume.
If that's a Post-PI MV, I trust you realize that with a classic 18W, it will just delay the onset of breakup,rather than work as you would probably expect an MV to work. An attenuator or VVR is the best MV for an 18W.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Yeah, it's a PPMV.. I'm not sure I know what you mean, it's not the most ideal master for that amp?

I spent the afternoon moving a few things around, chopsticking and completely redoing the heater wires on the amp and am frustrated as the noise is still there and just as strong. :( I've put the amp in the corner for a couple days to rethink my plan. I'm also not very happy with the sound of the amp..maybe it's the Tube Depot version or just the 18 Watt characteristics but I find it very Middy with very little clarity / clean overdrive. I'm not expecting it to be a high gain amp but as it is right now the most it will do is a weak crunch sound on the normal and the Tremolo barely breaks up at all.

Just starting to research other versions and need some advice. Are the TMB models usually designed with more gain on tap? If that's the case, then maybe a TMB model would be better, which is the most proven layout you guys would recommend?
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Bieworm »

Yamariv wrote:
Sun 04/05/20 9:51 pm
Yeah, it's a PPMV.. I'm not sure I know what you mean, it's not the most ideal master for that amp?

I spent the afternoon moving a few things around, chopsticking and completely redoing the heater wires on the amp and am frustrated as the noise is still there and just as strong. :( I've put the amp in the corner for a couple days to rethink my plan. I'm also not very happy with the sound of the amp..maybe it's the Tube Depot version or just the 18 Watt characteristics but I find it very Middy with very little clarity / clean overdrive. I'm not expecting it to be a high gain amp but as it is right now the most it will do is a weak crunch sound on the normal and the Tremolo barely breaks up at all.

Just starting to research other versions and need some advice. Are the TMB models usually designed with more gain on tap? If that's the case, then maybe a TMB model would be better, which is the most proven layout you guys would recommend?
I had a very hummy TMB tremolo build. Turned out to be the power tubes. It is hum free after the swap.

Of you want gain, just build the TMB tremolo. It's endless gain on that one..
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Daviedawg »

Your description of the sound from the 18 watt is far from what I would expect. It is possible (likely even) that the hum and poor performance are caused by the same issue. A break from it is a good idea. But don't forget or give up on it.

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