Standby hum?

bluesking
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Standby hum?

« on: December 16, 2011, 01:51:40 AM »

Hi all,
I've got my OTS 100 working beautifully. Really quite amazed at the sound, its a really special (!) amp. Just one observation, when in standby there is quite a loud hum. As soon as I turn the amp fully on the hum goes away completely (in fact the amp is super quiet!). Obviously this doesn't cause me any tone problems, but I wonder whether I have done something wrong in the build.

Has anyone else experienced anything similar or have any ideas as to the cause? It may be a pain for live use as I like to leave the amp on standby between sets, but the hum may make this difficult!

Many thanks,
BK


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plasticvonaband
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Re: Standby hum?

« Reply #1 on: December 21, 2011, 05:16:57 AM »

Mine doesn't hum at allin standby mode, and shouldn't, since the high voltage isn't applied to the tubes so there should be no signal being heard throught the speakers at all

Gregg


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Overdrive is like peanut butter. Some like it crunchy, some like it creamy.
Bluesmaster 50 2x12 combo and some guitars.

ampkits
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Re: Standby hum?

« Reply #2 on: December 30, 2011, 06:19:12 PM »

Hi

No, it shouldnt hum in stdby at all.

Check your wiring again, esp power section?

Thanks!

Nik


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bluesking
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Re: Standby hum?

« Reply #3 on: December 31, 2011, 02:58:33 AM »

Hmmm,
Checked the wiring (again) and I cant see anything wrong. The only thing I can see that may be wrong is that of the various AC taps from the power tranny I might have reversed polarity for some (this shouldn't make a different though, should it). Another thing is that I have twisted my 50v supply line, whereas on the pictures on ceriatone website these seem to be not twisted.....

I measured voltages around the power tubes in standby and the only thing I noticed which didnt seem right was that Grid 1 (Pin 5) on all the tubes measured -50V relative to ground. I cant see any wiring problem though...

Any ideas?

Many thanks!


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bluesking
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Re: Standby hum?

« Reply #4 on: December 31, 2011, 03:07:29 PM »

OK,
I've done some more testing and it seems the problem is nothing to do with the power tube section.

With the power tubes removed the hum still occurs in standby - this makes me think the hum is being induced in the OT itself or the leads to it (probably secondary because the hum volume or pitch do not change as I switch the impedance selector).

When I removed the preamp tubes as well (hence ensuring no heater current flowing through the amp (except the indicator light) nothing changed. Therefore I don't think the signal in the OT is being induced by the heater wires.

Whatever it is causing it I am starting to think that I shouldn't worry about it because the hum stops as soon as I fully engage the amp and its not very loud (about the same volume as my fridge!). I have read elsewhere that a standby hum could be a symptom of a rectifier problem, so I am still worried enough to hear any ideas anyone may have!!!

All the best.


« Last Edit: December 31, 2011, 03:49:17 PM by bluesking »	Logged

plasticvonaband
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Re: Standby hum?

« Reply #5 on: December 31, 2011, 05:08:56 PM »

Sounds like a ground issue, either in the amp or at your location or both, or maybe something in the main power supply caps, filter caps, or one of the nodes. There should be no hum when in standby, and certainly with no tubes in it.

Gregg


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Overdrive is like peanut butter. Some like it crunchy, some like it creamy.
Bluesmaster 50 2x12 combo and some guitars.

bluesking
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Re: Standby hum?

« Reply #6 on: December 31, 2011, 06:21:50 PM »

Gregg,
Thanks for your info. How can it be a grounding issue if turning the amp fully on makes the hum go away? When on fully I cannot see any problems with the amp: no hums, no hiss, no oscillation untill gain and volume levels get ridiculous.


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plasticvonaband
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Re: Standby hum?

« Reply #7 on: December 31, 2011, 10:36:00 PM »

Well, older amps used to have a ground lift switch (the unused one on Nik's). If the ground in the place you were playing in was not behaving, you could lift the ground, and take some of the hum away by "floating" the ground. It sounds more like something is awry somewhere else, though. I'd take it to a tech and see. There's a handful of things it could be, but if you are still getting a hum with the tubes out, I'd say it's something in the ps node feeding the OT. If you put a meter on the OT node , you should get around the same DC voltage, give or take a few volts and your plate voltage. If you switch to AC and see voltage, then you may have a problem with that node of the PS, I would think. The only other thing I can think of, other than a ground issue, a colder solder joint, or a moving short, would be a problem with the standby switch or the way it's wired. The only bad thing about a ground issue is that if that is indeed the problem and it goes away when the HT is put to the tubes, is that you become the ground, which is no fun (I've been the ground before). So just keep that in mind. I'd take it to a tech, just to be sure.

Gregg


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Overdrive is like peanut butter. Some like it crunchy, some like it creamy.
Bluesmaster 50 2x12 combo and some guitars.

arledgesc
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Re: Standby hum?

« Reply #8 on: January 06, 2012, 09:41:05 PM »

In standby my Ceriatone JTM50 hummed quite noticably from the speaker when first built. It was low-level but loud enough to be heard across the room. Drove me nuts for six months. To make a long story shorter the issue is caused by the power transformer coupling magnetically directly to the output transformer. I also experienced slightly less hum when the standby switch was closed and the tubes supplied HV. So was a real head scratcher for a time. I tried everything to remedy the problem but the hum remained (different transformers, grounding, rewiring, mu-metal shielding). I finally fixed the problem for good by physically relocating the output transformer. The output transformer has moved 1/2" further away from the power transformer and, at the same time, aligned the OT to the PT centerline (and still orthogonally of course). Now hum free.

It was a major pain to relocate the OT (drill new holes in the chassis) but gave me a chance to remove all the temporary mods and now have clean wiring again. But it seems the Ceriatone chassis, JTM50 in my case, locates the OT closer to the PT than other amps of similar layout. A little too close in this case. If you are sure grounding/wiring are good, pull all your tubes, and if hum is present in standby then you probably are experiencing cross coupling of the PT magnetic fields into the OT primary. And don't be confused by mechanical hum where the PT vibrates the chassis. In my case the hum would disappear if I pulled the speaker cable.

Really love the amp now but bugged me to no end initially. Best of luck, Scott


« Last Edit: January 07, 2012, 04:44:57 PM by arledgesc »	Logged

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