Sentura II Quiescent Noise

[patlaw]

When my Sunn Sentura II is sitting idle with the volume turned all the way down, there is a certain amount of hiss in the speakers. When playing loud, it's not a problem. When recording in a studio, it would be nice to have it quieter. The amp has been completely recapped. The tubes are in good shape. Everything is working as it should - except for the noise. Some people say the problem is resistors. I'm wondering where you think I should look. The hiss level does not change with the volume control, so the problem is not in the preamp section.

[nosaj]

When my Sunn Sentura II is sitting idle with the volume turned all the way down, there is a certain amount of hiss in the speakers. When playing loud, it's not a problem. When recording in a studio, it would be nice to have it quieter. The amp has been completely recapped. The tubes are in good shape. Everything is working as it should - except for the noise. Some people say the problem is resistors. I'm wondering where you think I should look. The hiss level does not change with the volume control, so the problem is not in the preamp section.

Noisy plate resistors?
http://www.geofex.com/ampdbug/hiss.htm
nosaj

[patlaw]

Noisy plate resistors?
http://www.geofex.com/ampdbug/hiss.htm
nosaj

The article mentions "noisy resistors in the B+ decoupling string, often around 10K in value." Which resistors are those?

[loudthud]

Does turning the Treble Bass and Contour controls to zero affect the level of hiss? How about turning the Reverb control down?

[patlaw]

Does turning the Treble Bass and Contour controls to zero affect the level of hiss? How about turning the Reverb control down?

Yes and no. The hiss I'm talking about is with everything turned down. If I turn up the volume or the reverb, a different hiss comes in, but that's normal and to be expected.

Let me say that this hiss is not a major problem. Since I play the amp at home at low volume and do some recording with it, I'd like to reduce it as much as possible. That's why I wonder if the source is as esoteric as the plate resistors, which are easy enough to replace. I'd still like to know which components comprise the B+ decoupling string.

[Helmholtz]

The PS resistors adressed in the link would be the ones between points A,B,C,D in the schematic. These resistors may produce some noise, but if the filter caps are good, most any noise is shorted to ground.
If they are carbon compound types, replacing them with wirewounds might help a little. I wouldn't expect a significant difference.
But things are different with bad/aged filter caps.

[patlaw]

Thanks for the info. I just noticed something strange on the schematic. The plate on the pentode of the 6AN8 shows 70 volts while the cathode shows 75 volts. Please confirm that's an error. There are other errors on the schematic, but this is the first time I've seen this one.

[g1]

And whatever that voltage is, it isn't coming through the 12pF either.

[g1]

Here's one that shows the plate resistor. Working from that 270K value and the supply node & 70V at the plate I think that cathode voltage should read .75Volts. 75V with that cathode resistance is pretty much impossible.

[patlaw]

The PS resistors addressed in the link would be the ones between points A,B,C,D in the schematic. These resistors may produce some noise, but if the filter caps are good, most any noise is shorted to ground.
If they are carbon compound types, replacing them with wirewounds might help a little. I wouldn't expect a significant difference.
But things are different with bad/aged filter caps.

The filter caps are new.

[nosaj]

The filter caps are new.

New , does not mean good.

nosaj

[loudthud]

I would be suspicious of those two high value (220K and 620K) resistors at the input (pin 8) of the 6AN8. Replace with metal film if you can find them.

[gui_tarzan]

This is interesting as I have one of these to restore. I didn't notice the cathode voltage listed, that needs more investigation. I also have a Spectrum II that I'm almost done with and that schematic shows .75v, not 75v on the cathode of the 6AN8.

[gui_tarzan]

And whatever that voltage is, it isn't coming through the 12pF either.

Nice catch! It does show a proper path on my Spectrum schematic.

[loudthud]

Some early Sunn schematics contain errors. Since many Sunn amps are nearly identical, compare it to a similar amp as @g1 did in post #9 above.,