Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Stereomour => Topic started by: Kmmm on November 17, 2013, 11:47:20 AM
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Hi
I have been using my new Stereomour for some days now. It does sound good but I have had trouble getting the hum down.
I have 108db speakers and they are not found of hum.
The amp is rigged for 45 tubes because these are supposed to be a bit more quiet then the 2a3.
I can't seem to get them as quiet as they are supposed to. The tubes in question are a matched pair NOS ST45 Aristocrat (1900Gm)
They read 5,42mV on left and 3,65mV on right channel.
The stock Sovtek 2a3 are only 2,42mV and 2,74mV in the same set up.
If the 2a3 measurements looks OK, then I guess the 45's are no good?
Is there anything else I should check before paying some one large amounts of cash for a new pair?
Please give your opinions on this one ???
Kaare
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I have 108db speakers and they are not found of hum.
Please give your opinions on this one ???
The Paramounts are a way, way, way better match to speakers that efficient. AC heated directly heated triodes will only get so quiet, and ~3mV is a reasonable number to reach.
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Tank you for your reply.
I do belive that an upgrade to paramounts will sett me back even more then a set of new tubes.
I also do believe that after only one week it is a bit early to upgrade. I do think I have red about people getting down to 1mV and that I think would be ok even for these speakers.
Any thoughts about that?
Kaare
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Why not try rewiring for 2A3?
It might work better, and 2A3's are in production.
Then you can get less hum and save up for Paramounts.
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I agree that you should see less that 3mV, and the fact that you do get that low with 2A3s does suggest your 45s make a bit more hum than usual.
Just to be certain - "hum" is not a precise term in ordinary usage. To me it means low pitched 60 or 120Hz fundamentals with little or no harmonic content, as distinct from "buzz" which has substantial harmonic content. Buzz has different causes from hum, so I bring this up to be sure we are taking of the same thing.
Filament hum does vary a little between individual tubes. And the 45 was (I believe) designed to get some portion of hum cancellation by clever manipulation of the filament's magnetic field - though I've never found any details about this. From the simple theory, there is no reason the 45 should be different from the 2A3 since both are 2.5v filaments. For that reason, just trying more tubes improves your odds of finding a somewhat quieter one, but it's no guarantee!
Assuming it is hum (in my meaning above) it is always possible there there are other causes than just the filament power. A common one is imperfect filament power wiring, either unevenly twisted or routed too close to other sensitive components. If you post a picture of that portion of the wiring, we can probably determine if there is any potential for improvement.
Failing that, a DC supply for the filaments is the most likely solution. A simple form of that might be possible with the existing power transformer. It's not the best sounding approach, but if you are interested in it send me a PM.
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Thank you for your up lifting and informative reply :)
I think the best thing will be to start with a picture of the wiring.
I will also contact Brent Jessee from audiotubes.com to see if he can come up with some quiet 45's
Then the last option is to make a DC supply.
I'll post some pictures later tonight.
cheers
Kaare
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Just got an e-mail from Brent Jessee where he tells me that he has not heard that the 45 tubes are any quieter or have less hum than the 2A3. Both tubes, especially the very old ones, are susceptible to noise and hum due to out gassing of the internal components. This may go away with time once the tubes are put into service, and a long burn in period may reduce the hum as the getter absorbs the gas. It may take 200-300 operating hours. He have had hum issues with both types.
Cheers
Kaare
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Here are some pictures of the amp.
There are probably some cables worth moving around?
I have been using the 2a3 tubes for a while and if I get the hum down below 2mV I think I'll be happy.
The Stereomour is only driving the the full range horn(AER MD3). That's also the reason for going with the 45. I really don't need much power.
The bass is 15" BD-design and is driven by two 30w LM3875 (AudioSector).
Kaare
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Have you verified that the hum is only from the output tubes, i.e., have you tried a diferent 12AT7?
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Along the lines of Dan's post above, have you tried shorting plugs to verify that the problem is from the Stereomour forward?
There is a sticky in Technical Topics about Shorting Plugs.
I do believe that the 108dBW speakers are the culprit, but we must ask these questions first.
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Have you verified that the hum is only from the output tubes, i.e., have you tried a diferent 12AT7?
Hi Doc B.
I have verified that it's not the 12AT7.
I have also tried a NOS Siemens E81cc. It gives more or less the same result.
The 45s perform slightly better with this tube. The 2a3 a bit worse. But it is only very little.
Kaare
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Along the lines of Dan's post above, have you tried shorting plugs to verify that the problem is from the Stereomour forward?
There is a sticky in Technical Topics about Shorting Plugs.
I do believe that the 108dBW speakers are the culprit, but we must ask these questions first.
Hi Grainger49
Yes I have a set of shorting plugs and use them. I have learnt that 108db speakers might be a bit on the sensitive side for the Stereomour.
But I would like to get the hum balance down to below 2mV as mentioned in the manual. 1mV would be even better.
I could listen to the 2a3's with 2,5mV but it would be nice to get the hum a little lower.
Not being able to get the 45's below 3,5mV at best, is a bit annoying.
So
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Looking at your build, it kind of looks like there isn't a lot of clearance between your output transformers and the chassis plate. Are the fiber shoulder washers installed?
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Have you tried moving the amp to a different location and remeasuring the hum level? Sometimes the magnetic field that is creating the hum is from other gear near the amp or at least some of it. This is what makes it so tricky to troubleshoot, there can be more than one cause.
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Looking at your build, it kind of looks like there isn't a lot of clearance between your output transformers and the chassis plate. Are the fiber shoulder washers installed?
Hi Fullheadofnothing.
You are absolutely right. The OT-2's are touching the chassis on the underside. There where 4 fiber shoulder washers missing.
I could try to find a substitute.
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The shoulder washers are there to keep the inductors from coupling hum into the chassis and circuit. You may find some reduction of the hum level with the shoulder washers in place. The output transformers are theoretically the least likely to be an influence (and the power transformer theoretically the most), but since we haven't built a Stereomour without them we can't say for sure just how much or how little influence their insulation from the chassis might have.
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It's worth a shot:-) I'll source some washers tomorrow :D
Any thing else I should look at? Re routing wires or any thing like that?
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It is most likely that your AC meter measures RMS voltage, the same as we use for the hum spec. But if you happen to be measuring peak to peak voltage instead (for example by measuring the hum with a scope) bear in mind that peak to peak voltage is 2.8 times the rms voltage.
One thing you might try is taking a reading at the binding posts with the amp off. Since you have such sensitive speakers they might be picking up stuff that is coming into the amp from other sources thru the signal ground, or thru coupling with the output trans. If you measure some residual level of hum with the amp turned off you know that it is from somewhere outside the amp and you might need to track that down.
Do you notice any buzzy content to the noise, or is it just a soft 120Hz or 60Hz hum?
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The hum in my 45-configured Stereomour is 0.9-1.0mv. I lowered the plate dissipation to 7.6 watts, getting only 1.4-1.5 watts/channel.
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Hi Jimiclow
What was your hum reading before you lowered the plate dissipation?
How sensitive are the speakers you have?
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Hi Doc.B
With the amp off the reading is 0.00
There is no hum from the gain clones (These are on 24/7)
When I turn the amp on there is a loud hum settling after 15 seconds or so.
Most of the hum comes through the bass but some also comes through the full tone.
There is also a slight "hizz" from the full tone if I go really close to the speaker. When the music starts playing I don't notice the hum or hizz much. It does not increases with the volume. It stays the same. It does go down a bit after the tubes get really toasty. But only by a little.
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I have installed the washers. Still the same. I have tried to move the amp to another room and the hum level is still the same. There might be a low pink noise together with the hum. I un hooked the bass and gainclone all together. But it is low.
What's next? I know I can't get zero hum. But below 2mv should be achievable?
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If the hum is soft hum that has no buzzy content, that parts are all stock and the wiring layout closely matches the photos in the manual then you are probably at a point where you may simply have to try different tubes until you find something quiet enough. 108dB sensitivity quite is a challenge for any amp, moreso for an zero global feedback amp with AC powered filaments. For what it's worth the Stereomour we currently have in our demo room has a hum level of 1.6mV rms with the stock tubes. But 2.5mV rms is certainly within the range of typical levels that 2A3s can exhibit with AC heating. By the way that 1.6mV measurement is made with the speakers disconnected. Loaded with speakers the number will be a little lower - in our case about 1.1mV rms.
Are your hum pots all the way to one end of their travel when you hit the lowest hum level? Or is there some adjustment range left in both directions? This is not too likely, but I'm trying to be as thorough as possible.
By the way our measurments are made on the 120V AC mains version. Is this a 120V Stereomour or a 240V one? That could have some influence on the hum level, as I think the radiated field may be a wee bit higher from a 240V PT-6. PJ will correct me on that if I am wrong.
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Hi doc
It's a 240v version. The hum pots have more room for adjustments.
The amp is weired for 45 tubes. But the lowest hum I can get this far is the 2a3. But I have only tried this in the 45 config. Do you think there will be any difference if I wire it for 2a3?
Since most 45 tubes are quite old it might've harder to find quiet ones.
Also will lowering the plate dissipation effect the hum?
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By the way. As I mentioned. There might be a low pink noise coming from the full tone speakers. As I mentioned I have to be quite close to notice. It does not increase with the volume.
Is this related or something totally different?
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Low pink noise could just be tube rush, most likely from the 12AT7.
Adjusting the operating point should have a negligible impact on the hum level.
Changing tubes will nearly always have an impact on the hum level.
Rewiring the output transformers for 4 Ohms will also reduce the hum a bit.
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I get about 2mv with my SR-45 amp, fine with my ~95 db speakers. The hiss is probably tube rush and is normal. If you have some 2A3s on hand, you can try them see if they run lower. May not be a definitive test as the operating will be low but won't hurt anything...John
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Yes, using the 4 ohm tap is a good idea.
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I had the same issue with pink noise and ended up pulling the 431 regulators on the driver tube C4S board and replacing them with cathode resistors. The result is zero hiss, even with my ears up against 99dB/W speakers.
I'm getting just below 1mV of hum (at last measure) with NOS 45 tubes from the 1940's. Not even discernible from more than an inch away. That's on the 8 ohm taps, too.
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... PJ will correct me on that if I am wrong.
Always glad to help out, Dan! :^)
The primary is the innermost winding, so it should not affect the radiated voltage field.
However, it did get me thinking. The 2.5v windings are adjacent to the high voltage winding, and the power supply is arranged so that end of the high voltage winding is at AC ground. Make sure the correct tabs are used for the high voltage supply - red wire to transformer terminal 17 and "Start" on the board, black to 18 and "Finish".
Just a thought.
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Hi Jimiclow
What was your hum reading before you lowered the plate dissipation?
How sensitive are the speakers you have?
IIRC, I think it's only 0.2-0.3mv higher. I forgot to mention that I also lowered the 45 filaments to 2.39 volts. Plate was dissipating 11.25watts before mod.
I have a quad of 97db Hoyt bedfords 8" full range.
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Yes, using the 4 ohm tap is a good idea.
At the moment the amp is wired for 16 ohm speakers
The AER's are 16 ohm. Should I use the 4 ohm taps?
Kaare
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The rule of thumb is to use the one that sounds best. The differences between the taps will not matter with 109dBW speakers. Try 4 ohms and see if the hum diminishes. If so then try the 8 ohm tap too. See which sounds best to you.
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At the moment the amp is wired for 16 ohm speakers
The AER's are 16 ohm. Should I use the 4 ohm taps?
Oh heck yeah! The big disadvantage will be a loss of output power (at 108dB sensitivity, this is unimportant).
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WOW!
Just wiered it for 4 ohm and guess what!
We are down to 1,2mV hum on the 2a3 ;D
That I can totally live with!
Thank you so much for your time, all of you;)
Cheers
Kaare