Hum problem with new built Stereomour [solved]

[Kmmm]

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.

 

[Kmmm]

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?

 

[Doc B]

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.

 

[Kmmm]

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?

 

[Kmmm]

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?

 

[Paul Birkeland]

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.

 

[2wo]

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   

 

[Doc B]

Yes, using the 4 ohm tap is a good idea.

 

[Dr. Toobz]

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.

 

[Paul Joppa]

... 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.

 

[jimiclow]

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.

 

[Kmmm]

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

 

[Grainger49]

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.

 

[Paul Birkeland]

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). 

 

[Kmmm]

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