Noise floor questions...

[altec604e]

Hello,

I have some questions regarding my Stereomour 2a3 tube amplifier.

1.  How much noise is usually "accepted" or "normal" with the Stereomour amp:

Mine is not dead quiet - I use a pair of Altec 604E`s that are rated to 16 ohm operation.
And the speakers are rated at 100dB efficiency.

The output transformers are wired for 8 ohm operation right now, I heard a lot of difference from the 16 ohm wiring - the noise floor did go down, but it is still not as quiet as I want it to be.  It does not have to be dead quiet, but a bit more would have been great! 

The 4 ohm operation was even more quiet - but at the same time I lost a lot of information - especially vocals was lost ( I have not done a lot of testing , but this is how I heard it yesterday ).

Previously I have been recommended to lower the ohm setting on the output transformers because of the high efficiency speakers I`m using.  4 ohm is definitely the setting with the lowest noise floor, but I cannot find the sound convincing because of other things that are lost - based on your knowledge of these amps - is this loss of for example vocals just in my head??

One more thing on the subject: is there anything else I can do to lowering the noise floor on my Stereomour amp?

 

[Grainger49]

I'm going to agree that with 100dBW speakers noise will be hiding in the background.

You don't say how much noise it is.  Is it only heard between tracks or is is heard during quiet passages?  How loud do you listen, 60dB, 70dB, 80dB or higher?  How big is your room?  The larger the room the farther away you will probably sit (attenuating the noise).

There may still be a few tweaks to minimize the noise and PJ will probably come up with them.

 

[altec604e]

Hi !

The noise is definitely present during quiet passages.

To answer you questions:
I do not know how loud I listen to when you're talking about dB - normally I use from the Stereomour from the 9 o`clock position to the 12 o`clock position .  I`m guessing the loudness Im listening to is quite low to normal - not loud or extremely loud.

My room is roughly 30 square meters.
I`m sitting roughly 3 meters away from the speakers.

Thanks for your replay !

 

[Paul Birkeland]

The Stereomour is AC heated, so some filament hum is to be expected.

Can you describe the nature of the noise itself?  Is it a hum or a hiss?

It's also worth mentioning that the Paramounts are significantly quieter than the Stereomour.

 

[Paul Joppa]

If it's hiss, could be the bad batch of 431s.

 

[altec604e]

The noise can best be described as a hiss.

What is 431s?


One more questions:

1.  Right now I`m using my Svetlanas - the HUM-balance is set to as low as it gets with my Svetlanas - to 1.8 and 1.9 mV AC.  When using my RCA-s I cannot get it lower than 2.3 or 2.2 mV AC. 

It says in the manual that the HUM-balance varies between which tubes you use. 

So my question is what does this HUM-balance do? - and what difference is it between using these two sets of 2a3 tubes I got ? 


I mean one pair
the Svetlanas is right below 2.0 mV AC and
the RCA´s is right above 2.0 mV AC
- what difference does this make on the noise floor? - if any?

 

[altec604e]

OK - just went through the manual - are you talking about the LM431 adjustable shunt regulators?

Then I got something to share -

1.  When turning the amplifier on there is always a couple of seconds with both hum and hiss.

2.  There were some points when doing the DC check when first firing the amp up that got me a little bit confused - especially the measuring points around the circuit boards...

I will attach all the DC voltage checkpoints in a PDF document - please let me know if you can see if there is a problem related to the items you are talking about....

 

[Grainger49]

That is the 431 (LM431) that they mean.  They have new ones that meet the spec, the bad lot didn't.  Just contact replacementparts (at) bottlehead (dot) com or fastest thing is to call Monday.  They are very quick about getting parts out.

 

[Paul Birkeland]

Regarding the 431's, you can look at the writing on the 431 to be sure.  We received some 431 regulators labeled "WS" that entirely meet spec.  If you have these 431's and sensitive speakers, you ought to replace them.  If you don't have these 431's, then there isn't any need to replace them.

 

[altec604e]

I have the 431´s with WS labeled on them.

I think something is missing in your post replay....

If I understand your post correctly - these WS labeled 431´s did NOT entirely meet the specs - and are from the bad batch of 431´s...

This is the answer to my problem then.  Will write a email to the replacementparts email address.

Thank you !

 

[johnsonad]

Change those WS 431's! I did so and it made all the difference in the world!

 

[altec604e]

Hello!

So I installed the new 431´s - with great improvement !
Thank you for all help with this issue.

One last question from me:

I´ve set the secondary windings on my output transformers to a 8 ohm impedance.
My speakers has a 16 ohm impedance.

So before making any new changes; will the speakers perform better ( sound more prominent in the room - raised from the background ...) if I set the secondary windings to a 16 ohm imperdance?
I have a sense that the sound has a veil over it...

I mean since I´ve now got a much more quiet Stereomour 2a3 amplifier and the noise floor question I started this thread with is now gone...

The popping when sometimes occurred when turning of my Stereomour is also gone...

GREAT!

Best,
Altec604E

 

[altec604e]

Hi,

Got some more questions....

I do hope this is OK.

Here we go:

    The secondary windings on the output transformers : if set to 8 or 16ohm operation should not have a great impact on sound since my speakers are rated to 100dB sensitivity-  correct?
    Noise: The 100K potentiometer in the 12 o`clock position is a source for noise.  I usually use the volume knob set between the 10 o`clock and 2 o`clock position when listening. 
    Is there any high DC current at the input?  I´m asking since I have been told that there are no input capacitors on my Stereomour.
    If there is no high DC current at the input, then I can use any normal pre amp to control the volume of my Stereomour ( as long as the 100K pot is set to fully clockwise position ) - correct?

    What output current should I choose on the Pre amp I now own.  The 5mV, the 1.6V or the 5V output to use it as a volume controller for my Stereomour?

    So my two 2a3 RCA tubes died yesterday.  They did not last long, did I do anything wrong?  They were supposedly to be NOS.  Now they make this up and down hiss noise...



   
   

   



   

   

 

[Grainger49]

The secondary windings on the output transformers : if set to 8 or 16ohm operation should not have a great impact on sound since my speakers are rated to 100dB sensitivity-  correct?

Nope, the damping, how well the amp holds your woofer, changes if you mismatch the load and OPT.  But try it, you might like it.

Noise: The 100K potentiometer in the 12 o`clock position is a source for noise.  I usually use the volume knob set between the 10 o`clock and 2 o`clock position when listening.

I'm not sure, are you saying 12:00 and above or only at 12:00?

Is there any high DC current at the input?  I´m asking since I have been told that there are no input capacitors on my Stereomour.

Input DC would be from your source or preamp.  There is no DC at the Stereomour input/12AT7 grid.

What output current (voltage) should I choose on the Pre amp I now own.  The 5mV, the 1.6V or the 5V output to use it as a volume controller for my Stereomour?

Again, this is a try and find the best balance.  The output of your preamp depends on the input voltage.  So you don't always have those voltages available at the output of your preamp.

So my two 2a3 RCA tubes died yesterday.  They did not last long, did I do anything wrong?  They were supposedly to be NOS.  Now they make this up and down hiss noise...

How old are they?  Were they new?  I'm thinking they may have been old, old stock if they failed this soon.

 

[Paul Birkeland]

    So my two 2a3 RCA tubes died yesterday.  They did not last long, did I do anything wrong?  They were supposedly to be NOS.  Now they make this up and down hiss noise...

What do the voltages look like with those tubes installed?
   
   

   



   

 
[/quote]

 

[altec604e]

Thank you for giving a replay for my last post !

I will try to give the best answers as possible.
Remember I´m a newbie- willing to learn!

OK.

So I think I got you regarding your first answer - will try to configure the OPT to 16ohm winding - will give a feedback when seeing this.

I think I remember the woofer going all crazy ( vibrating back and forth very fast) on high volumes when the OPT were set to 4ohm.  ONLY when I was playing loud- stopped playing right away when seeing this - this was also when put it back to 8ohm configuration.  So in real world terms I get what you are saying about how well the amp holds the woofer - If I am getting your point; that is.

I´ve been told that when the 100K pot is set to the 12 o`clock position the source resistance is 50K ; it is in series with the input.
This is not the best thing is it?  So controlling the volume with a pre amp is a better option.

There I got my answer regarding no high DC voltages at the input.  Good.  Will try my Quad44 pre with my Stereomour shortly; will give a feedback to you on how this goes too.

To answer both of your questions regarding my 2a3 tubes.  They were supposedly to be NOS, but I also have a feeling that I got old stock tubes since they run out so fast.
The voltage readings were the same as before they started hissing; around 2.6 down to 2.0 ...


Questions from me:

1.  What tubes should I consider now if I want to do a tube roll?  Is till got my Svetlanas, what about the 12AT7 tube- what impact does this tube has on the sound?  Should I consider doing a tube roll on this tube too?


Thank you again for your replay!
Really do appreciate it.





 

 

[Grainger49]

In theory the 16 Ohm tap should sound best.  I would suggest trying the 8 Ohm tap as well.  It may lower noise and have enough damping to control the woofer.

The source will always see a 100k Ohm input for the Stereomour.  The variable resistance is to the grid of the 12AT7.

If you want to use your preamp just set the Stereomour volume control to 100%.  That effectively takes the pot out of the audio path.

PB/CB was asking for the plate, grid & cathode DC voltages measured to ground.  You can measure the voltage on both of the small tube pins and one of the large ones.  The large ones are the cathode/heater.

I have used a pair of NOS RCA, the stock Sovtek and the Shuguang 2A3C.  The old RCA are very nice.  The 2A3Cs replaced very worn Sovteks and were a big step up from the worn tubes.  So I can't really compare those.

I believe that the 2A3C sound better than fresh Sovteks.  They are ~$110/matched pair delivered from China.  The NOS RCA beat out the 2A3Cs in detail and soundstage.  But I only drag them out for special occasions.

 

[altec604e]

Hi 

OK- understand.

Will do the plate, grid & cathode DC voltages measured to ground quite soon with the RCA's installed.

Found these 2a3 tubes at tubedepot:
https://tubedepot.com/products/2a3-us-made
Should I consider buying these?  If so which brand?

Any point of investing in a tube tester?  Quite an expensive instrument.... Even if you buy a portable or low budget one ( compared to the really professional ones )
Maybe a good idea to find a firm with a quality tester instead?

What is the purpose of the 12AT7 tube in the Stereomour?

 

[Grainger49]

Last question first:

the 12AT7 has all the voltage gain of the amplifier.  The 2A3 is the current into the OPT.  (I think that is right, pretty sure)

Moving back, tube testers come in all kinds of "flavors."  Not all of them test the same things.  I have one that I have never used.  I just trust in the tried and true sellers I have used. 

Tube Depot is one of the tried and true sellers.  I am partial to Sylvania tubes.  I have used them in my Foreplay, Seduction, Eros and as drivers in my Paramours.  I have never had Sylvania output tubes.  I can only guess they would appeal to me.

BTW, I sold the Thorens TD 124 MKI and the SME 2009 MKII back in the 70s.  Great combination!

 

[Paul Joppa]

I am late to this conversation, but here are my remarks, from input to output:

The potentiometer is usually best set at 100% if you are using an external level control. For the preamp with a choice of outputs, I suggest you try the 1.6v and 5v outputs and see which works best - the biggest difference will be where you set the control. Only in certain circumstances will it be necessary to change the balance between preamp gain and amp gain to obtain the best performance. I do not think you have this problem, I just mention it for completeness.

The 12AT7 is the main voltage gain stage. Your voltage measurements indicate that you have a weak tube (the plate voltage is higher than ideal). It is within an acceptable range for performance, but a fresh tube may make an audible difference. (If it does not, then at least you will have a spare on hand!)

The 2A3 is the power output stage, it provides some voltage gain but all of the output current. Your voltage measurments show that it is operating correctly, so there is no circuit reason that I can see for a short life. Most likely the tubes were worn when you got them. (In the unlikely event that this happens again, post back and we'll explore other possibilities.) The filaments of this tube are powered with AC in the Stereomour, and the hum balance pot allows you to cancel the 50- or 60-Hz hum; the exact setting will be different for each individual tube. There is a residual hum at twice the frequency, which usually amounts to about 1mV RMS at the 8-ohm output (1.4mV at 16 ohms, 0.7mV at 4 ohms). This will vary with different tubes. Remember, few meters will be very accurate at this low voltage.

The matched impedance of the output transformer (16 ohm setting with 16 ohm load) produces the most power. With 100dB speakers at moderate loudness, you probably don't need the maximum power even in your fairly large room, but keep that in mind as you listen - rooms and speakers vary greatly. The higher ourpur impedance also has the lowest damping factor, which will affect the frequency response of the speaker - not a lot, but it may be audible. This effect is strongest in the bass, which is also most affected by room placement, so the only way to judge is careful listening to each setting.