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Author Topic: Noise floor questions...  (Read 5785 times)

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Offline altec604e

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Re: Noise floor questions...
« Reply #15 on: June 22, 2014, 10:37:52 PM »
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.





 
Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs

Offline Grainger49

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Re: Noise floor questions...
« Reply #16 on: June 24, 2014, 12:47:16 AM »
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.
« Last Edit: June 25, 2014, 06:03:50 AM by Grainger49 »

Offline altec604e

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Re: Noise floor questions...
« Reply #17 on: June 25, 2014, 04:16:16 AM »
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?







Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs

Offline Grainger49

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Re: Noise floor questions...
« Reply #18 on: June 25, 2014, 06:10:42 AM »
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!

Offline Paul Joppa

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Re: Noise floor questions...
« Reply #19 on: June 25, 2014, 07:23:26 AM »
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.
Paul Joppa

Offline altec604e

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Voltage measurements...
« Reply #20 on: June 26, 2014, 06:01:47 AM »
Again, thanks for the replies!

Will definitely check out the Sylvania tubes from tubedepot.
Will never ever buy tubes from a not trusted or really well known seller again.
Will not buy myself a tube tester.

Cool that you also used to have thorens td124 with a SME tonearm Grainger49 ;)

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!)


Paul Joppa; are you talking about the sheet with DC measurements I attached earlier in one of my posts?

Info:
These DC measurements are from the first time I fired up my Stereomour.  I had the stock 2A3 Svetlanas tubes installed when measuring these checkpoints.

Should I do a second round with these DC measurements?

Do you think the bad 431's had a great effect on these DC measurements?



Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs

Offline Paul Joppa

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Re: Noise floor questions...
« Reply #21 on: June 26, 2014, 07:20:48 AM »
Ooohhh - good catch!

Yes, re-do the measurements, specifically the OA and OB voltage of the C4S board, which go to the plates of the driver.

It is not uncommon for a fresh tube to have a high plate voltage, and then drift down to a lower, stable value as the cathode finishes forming - usually in 50 to 200 hours, depending on the particular tube. Your original measurements showed a slightly high voltage. If you have run the amp for 50 to 200 hours and that voltage has come down, then the tube is fine.

The 431 will not affect that measurement.
Paul Joppa

Offline altec604e

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Redone voltage checklist
« Reply #22 on: July 04, 2014, 02:20:59 AM »
Hi,

I have redone the checklist.

You`ll find the checklist attached.

Checkpoint 4 is really off from the manual.
I was to replace the resistor that goes from A3 to T4. I have a already a spare on hand.

First time installing this resistor it a little bit of the resistor fell off from the body- used a tweezer too hard I guess...  Do you think this is causing the off measurement?

Anything else I should look into further based on the numbers attached?

PS:  Just ordered the Sylvania tubes from tubedepot !

Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs

Offline Grainger49

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Re: Noise floor questions...
« Reply #23 on: July 04, 2014, 03:22:48 AM »
I'm not sure where in the circuit checkpoint 4 is.  It is likely a grounded terminal.  If so 14mV indicates that it needs the screw tightened. 

Give that a try and see what happens.

Offline altec604e

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Re: Noise floor questions...
« Reply #24 on: July 04, 2014, 04:16:42 AM »
Hi again ,

So I tightened the screws on all three terminal strips ( each with 5 terminals) in the front of my Stereomour.

It is Terminal 4 and Terminal 12 that do not have a 0 V DC reading. These two terminals mirror each other-  Both are connected to same terminal to the 2a3 sockets on each side with a resistor - A3 and C3.

I also resoldered all four of the points without any new/good results with my multimeter.

Any idea on what to do next?






 
Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs

Offline Paul Birkeland

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Re: Redone voltage checklist
« Reply #25 on: July 04, 2014, 05:45:22 AM »
Checkpoint 4 is really off from the manual.

0.014V is not really off from 0V.  Terminal 4 is not an issue. 

A3 can't be 24V with the rest of those conditions, I'm guessing 24mV?

Your 12AT7 is reading a bit high, and may not be the quietest of tubes.  Trying a spare isn't a bad idea. 

Wiring the output transformers for 4 Ohms will yield the lowest noise floor, highest damping, least distortion, and least power into 16 Ohm speakers.  If you have ~100dB efficient speakers, this is the way to go.

-PB
« Last Edit: July 04, 2014, 05:52:00 AM by Caucasian Blackplate »
Paul "PB" Birkeland

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Offline altec604e

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Re: Noise floor questions...
« Reply #26 on: July 04, 2014, 05:49:51 AM »
oh man...   ;)

my fault...  got a bit carried away there.

24mV is correct.

I just ordered a new 12AT7 tube when I ordered new 2a3 tubes ... 

Thanks!
Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs

Offline altec604e

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mechanical noise? only on one channel...
« Reply #27 on: August 11, 2014, 09:59:09 AM »
hi,

I´m having trouble with my stereomour...

so the issue is that one channel ( the left channel ) in my stereomour is making a mechanical noise- it vibrates.
in addition: when adjusting the volume knob the noise I´m making when moving the dial is reinforced in the left speaker.

the mechanical background noise that is always present has a much lower noise than when turning the volume knob or touching the chassis of my stereomour amp...

anyone got a clue what`s causing this background noise?

Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs

Offline Paul Birkeland

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Re: Noise floor questions...
« Reply #28 on: August 11, 2014, 10:03:25 AM »
How are your voltages?

What do you mean that the left channel vibrates?  A vibration would suggest that the amplifier itself is vibrating, and this would generally be tough to localize to just one side.

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline altec604e

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Re: Noise floor questions...
« Reply #29 on: August 11, 2014, 10:17:27 AM »
I checket the hum balance last week - they were almost the same.  low.

I have not done any voltage checks since last time I posted my second voltage list.

As far as I see it my left speaker is reinforcing a vibration sound from my stereomour.

I cannot feel that my stereomour is vibrating, but for example when turning the volume knob the sound from turning the volume knob is reinforced through my left speaker....


HOLD ON !   - I may have not put back the lock washer on one of the two screws ( holding the left output transformer in place ) - is this causing the issue?

If it is ... I´m so sorry for bugging you... 

Signal Source : Thorens TD 124 MkI / SME 3012 / Denon DL103

Setup : Quad 44 / Bottlehead Stereomour / Altec 604E in DIY 612C cabs