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

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

Post by Yamariv »

Hey Guys,

I finally finished up my 18 Watt Tube Depot build with a vintage Hammond transformer and it sounds awesome! This is the first 18 watt I've played and I can definitely understand why they are so loved!

One minor problem I noticed right away was a low constant hum, to me it's like a transformer hum, constant, sounds like 120 cycle. The amp is dead quiet on the gain hum, even when I turn up the master..

I originally thought it was the transformer but the hum does not start until the tubes warm up and the sound starts to get produced. If I turn the standby switch on, the hum stops. Does this point you guys to anything I can look at? Bad filter cap or something?

Just to clarify, it's not a gain hum, no volume or gain setting affects it, it's just a constant low droning hum. Please help!

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

Post by JMPGuitars »

Sounds like you need to grab a chopstick and chopstick the amp. Also it would help if you could post a sample of the sound.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

I tried to take a video but the sound didn't come across well on the camera.. Spent a couple hours this aft rerouting any high voltage wires that were close to the board and making sure all the wires I could find were twisted together. Got in with high hopes but it's not much better :cry:

Is the 18 Watt design a little noisier in general? I have the amp on now and it's a low hum but bearable. The sound on the other hand when I play is frigging awesome!! I can see why these little amps are so loved! :twisted:
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Daviedawg »

The 18 watt is very bit as quiet as any commercial amp. Have you considered an outside source such as a light or other electrical gear? Switch stuff off to listen for a change.

If it is mains related look hardest at the heater circuit. Then for a poor joint in the power supply to the board connections. Tap all the nodes for noise with your chopstick tool.

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

Post by zaphod_phil »

If you can hear a loud hum through the speaker, it's most probably a grounding issue inside the amp. Can you post the layout you're using and maybe one or two gut shots for the experts here to inspect?

Like Dd said, it could also be from your heaters. Make sure your heater wires are carefully twisted together. Then you could also try "elevating" the heater supply" - see http://www.valvewizard.co.uk/heater.html
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Interesting, thanks for the suggestions to try and narrow this down. Yes, the low hum is coming from the speaker, so that would mean its a ground most likely? It sounds like a low amplified transformer hum drone type of sound. Only is present when the tubes warm up, not when the amp is first turned on.

On a side note, while playing the amp on and off throughout the day I noticed the hum almost disappeared at time and was barely audible, only to find it come back all of a sudden and stay :roll: It doesn't seem to be external at all as it's plugged in to the same spot as all my other amps which don't have this issue.

I'll post some pics, I'm using the Dube Depot version for this build and did the modern grounding bus. My lead dress is probably going to be suspect too. I read the attached paper and mines definitely loosely wound :? I have an excuse why it's loose but excuses won't help the hum! Lol. If it's not a ground issue, then looks like I'll have to redo the heater wiring..

So for heater wiring, which is the quieter way to do it..Tight along the chassis edges like Marshall or hovering above and dropping down right over the tubes like Fender used to do?

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

Post by Yamariv »

1818Watt.jpg
20200109_233132_resized.jpg
20200109_233230_resized.jpg
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Daviedawg »

Yes grounding is a highly possible cause. But I was starting with simple to check stuff. As for heater layout, over the top and drop is the means that is recommended by most knowledgeable folk. I have never gone that way myself but I can see how it is the most reliable..
Grounding is a big topic and may need serious changes to the wiring layout. Josh has plenty advice posted recently to read through.

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

Post by JMPGuitars »

I think he already did our modern ground scheme?

But the thing that jumps at me from the photos is the lead dress. I think chopsticking is probably needed.

At the same time, from his description, I dunno if he really has a problem or not. Is the hum a noise problem hum, or is it just the low noise you expect from a tube amp? If it's too quiet to record...
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Hey Guys, definitely used the modern grounding rail style for this amp. So if I chopstick the amp in the next few days, I just look for loud pops or weird sounds as I move things around?

For the heater wires, If I was just to redo them completely should I just use the above then down to the tube version instead as an ultimate fix it issue?

Also, I was able to get a good recording of the low droning hum this morning but couldn't attach it to this website as it says the MP4 format that my phone used isn't supported :cry:
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by JMPGuitars »

Sometimes repositioning wires can reduce noise. For example, plate wires shouldn't be too close, or parallel, to grid wires.

Post the mp4 on youtube, and share the link with us.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by zaphod_phil »

The layout picture was too small to see or read the details clearly, but I have my suspicions about how many places your preamp ground buss wire appears to be connected to the chassis. There should be one and only one connection, ideally to a lug right close by the input jacks. The input jacks themselves should also be the isolating type, so they don't connect directly to the chassis. So your ground buss wire should end directly at the ground lugs on the input jacks and then to the one ground tag bolted to the chassis very close by.
JMPGuitars wrote:
Sat 03/28/20 12:29 pm
Sometimes repositioning wires can reduce noise. For example, plate wires shouldn't be too close, or parallel, to grid wires.
That will usually help deal with squeal and other noise issues, but not 60Hz hum (50Hz in Europe)
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by JMPGuitars »

ZP makes a good point here. It's very hard to see what's going on, and what's connected where. The pot grounds should be connected to the bus wire only (it's okay to daisy chain the pot grounds, as long as the final ground from the pots goes only to the ground bus). Only one of the jack grounds should connect the preamp ground bus to the chassis.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by zaphod_phil »

I've attached a snip from the layout of one of my commercial amp designs, to show how the preamp/PI ground buss needs to be terminated. Note, the input jacks are all insulated from the chassis, and only grounded to the buss wire.
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Re: First Test of 18 Watt build - Sounds great but has a steady loud hum - Help

Post by Yamariv »

Ok boys, I uploaded a quick video on Youtube..Here's the link:

https://www.youtube.com/watch?v=E9zwMDi ... e=youtu.be

Also, the low hum seems to sometimes drop to tolerable levels sometimes and other times is so loud I can hear it in my kitchen..not sure if that helps solve it but thought I'd mention it.

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? I'll repost a clearer look at the layout, there are tonnes of grounding points although it does have the grounding rail as well. Please let me know what you think..
MojoToneWiringDiagram.pdf
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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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