Need assistance addressing hiss

B

bjorgens

New member
Could use some assistance to remedy hiss/ white noise on my paramounts

This is not hum, it is white noise that is present regardless of volume level.  Likewise, it is present even with source/ pre amp disconnected and amp inputs shorted with shorting plugs.

Amp is setup with relatively new EML 300Bs and some NOS WE396A tubes. Coupling caps are Vcap, parafeed caps are mundorf Silver in oil.  Noise was present before these components were installed.

Interestingly enough, the hiss is worse in left channel.  There is also an inrush sound when amp is first powered on.

Plate voltage is right at 175VDC on both, and hum is at 2.0mV on both

Any recommendations on where to first start looking?

Bryan
 
Hi Bryan,

Hiss is often tube related and is not easy to get rid of. Try swapping the tubes to see if the hiss level is linked to your tubes. And try using them for 50 hours to see if it goes away. I had that with my stereomour 12AT7.

I have to say that hiss never completely disappeared from my system and that it is quite common to audio gear. I have a Naim solid state amp with higher hiss level than my stereomour.

Maybe someone at bottlehead has a circuit change that could help you with definitively removing hiss.

Fabien
 
Thanks

Just flipped over my left channel amp again, and noticed that there appears to be oil leaking from one of the 270uF electrolytics on the power supply board.  .. Not a ton, but presumably any is too much.  Not the case on the right channel amp

Aside from me needing to address this for obvious reasons, could this be source of the inrush and louder hiss on this channel?

Doc B
Am I better off replacing the PS board entirely?

B
 
Those caps need replaced if they are bulging or leaking though they should be electrolytic caps and don't have fluid in them.

The constant white noise could be tubes but if you have high sensitivity speakers, you are hearing the regulator on the driver board.  It's normal for this circuit. 
 
bjorgens said:
Thanks

Just flipped over my left channel amp again, and noticed that there appears to be oil leaking from one of the 270uF electrolytics on the power supply board.  .. Not a ton, but presumably any is too much.  Not the case on the right channel amp
Click to expand...

This is likely worth posting a photo of.

"Oil" could just be some solder flux.  The power supply capacitors in the Paramounts will last a very, very long time.
 
This is a bit of a story...

Re: the oil...It was a lot more than I would have ascribed to solder flux... More on that below

Incidentally, this is the same amp that I sent in to have you guys help me get working after initially attempting to build with what we subsequently learned was a mislabeled transformer.

You fixed it, and I put it happily into use.  The inrush noise/ constant hiss was there at that time on that channel only, but I just figured it was what it was.  I should note that I did not check plate voltage after getting the unit back from you... Again... More on that in a sec.

About a month ago, I read that when changing tubes to reset plate voltage to 175VDC.  Since I had changed the 300Bs some time earlier, I checked the voltage and discovered it was at 215VDC  on the left channel (!)  The other amp was at ~179 IIRC... ie big difference between the two, and for all i know, it could have been that way the entire time.  That was reset to 175, and this was when I also first noticed the oil.  At that time, I thought my coupling cap (a Duelund cast PIO) had failed, so I decided to change that to the Vcap now installed.  I now realize that it was oil from the electrolytic leaking down onto and saturating the coupling cap. At same time I also decided to change out the parafeed caps, and swapped to the EML 300Bs and WE396As.  Plate voltage was reset to 175VDC on the dot.

So... What to do?  A picture won't show much at much at this point unfortunately, as I wiped said oil away already.

Could there have been more initial damage elsewhere than we thought from the mislabeled transformer?

Was my failure to check and set plate voltage after getting the amp back from you a potential culprit?

Or is it just a bad cap?

Thanks as always !

Bryan
 
Hello Bryan,

None of this sounds like anything that would damage the amplifier.  The mislabeled transformer would result in lower B+ voltages - so no harm to the caps.

IIRC - I put a new power supply board in there anyway (I think).

The plate voltage of the driver tube should be 175 +/- 20% (tighter for the 2A3).  If you had 300V on the plate of your driver tube, then we'd be rocketing into that as a problem.

Can you swap tubes to see if this might follow one of the tubes?

BTW - the startup noises are normal to the circuit. 

2.0mV of hum is quite high from a Paramount, are you by chance on the 16 Ohm setting?
 
Thanks Paul!

Does not follow the tube on previous tries, but I will double check on that and the impedance setting and advise ASAP

Cheers
Bryan
 
Hello Bryan,

OK, this is helpful information.

In the amplifier, there is a 249K resistor on the terminal strip by the 300B tube.  Can you temporarily solder a piece of wire across this resistor, then listen for hiss again?

This will remove the contribution of the driver stage and isolate the source of the hiss to one half or the other of the amplifier.

-PB
 
to ensure I understand, leave the 249KOhm resistor attached but ALSO add a jumper between the same terminals (7U and 10U)?

b
 
Ok... Soldered in the jumper, powered back up, and amp is dead quiet with shorting plug in input.  No hiss and no inrush sound on startup either.
 
will give it a try Paul...thanks
Acknowledging my own ignorance, could you briefly explain what you think is going on and what the cap will do? 

WRT to the leaking electrolytic on my PS board...any sage wisdom on how to get that thing off so I can replace it without pulling the whole board?  :)

b
 
bjorgens said:
will give it a try Paul...thanks
Acknowledging my own ignorance, could you briefly explain what you think is going on and what the cap will do? 

WRT to the leaking electrolytic on my PS board...any sage wisdom on how to get that thing off so I can replace it without pulling the whole board?  :)
Click to expand...
If the TL431 biasing the tube is being a little noisy, the cap across it will present a low AC impedance that will calm that down.

I'm still suspect about the leaky electrolytic, does it exhibit any signs of a bad cap?  The top of it should bulge out and rupture the plastic wrap if the cap is bad.
 
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