Hiss in new build

[überfuzz]

I just fired up a new build. Every tubes glows at ON. When I turn off the stand by there's a hiss. This build has a negative feed back that is hooked up to a pot. The hiss seems to ease off if I get the NFB real high, lowering the resistance given by the pot. I'm not 100% that I'm barking up the right tree here, but if I am. How do I opt a nfb? Now I soldered in a 180kOhm resistor to a 500kOhm pot. Should I just aim from the hip and go with something like 100kOhm + pot..?

Could it be something else hissing?

Hiss = real high pitched noise

[tubeswell]

How is your NFB pot wired? (Fender presence pots decrease NFB as you 'lower' the pot, and if it was a 'fender type' NFB, then you could in fact be lowering the NFB and decreasing oscillation, which sounds more likely than increasing NFB and decreasing osscillation, because oscillation is typically caused by too much NFB).

Hiss could be a lame pre-amp tube. I bought a NOS RCA 12AX7 a few months back thinking 'this ought to sound good', but it hissed like crazy. :-(

Can you post a schematic?

[überfuzz]

Ok... I'm not sure it likely to be a preamp tube. The hiss is there even with the volume on each channel at zero. I'll try different tubes thought, just to rule it out.

[tubeswell]

You could also try lifting the NFB and seeing whether that stops it. (Then you would have a pretty good idea whether it was caused by that or not)

[überfuzz]

I sorta rushed off when the amp was done, just had a brief power up. I turned it on, waited a minute or two and turned off the stand by. I think I mentioned the nfb pot seems to alter the high pitched note, the hiss. I'll check every thing mentioned here asap, when I get back into the tinker den.

The amp is based on a carvin 33 vintage. The normal channel is more or less unchanged. Channel 2 is altered to a 5F6A circuit. The nfb circuit calls for a 220k resistor, that's where a played around with 180k + a 500k pot. (The idea was to find a sweet spot and use a resister of choice + a 25k pot.) I hooked up the nfb to pin 2 and 3 on the pot. With the pot at 0 the hiss was considerably lowered, not gone.

Thanks for the input!!!

[tubeswell]

Ok I see now. If you replaced that 220k with a pot wired as a variable resistor and turned the pot 'down' (lowering the resistance), you would be increasing the NFB supplied to the LTP tail. At this stage I am thinking 'disconnect the NFB and see what that does'. That 220k resistor is bypassed by a cap in the carvin cicuit. What sized bypass cap are you using in your circuit?

(Edit: Or build the NFB 'stock' and see what that does. That amp is quite gainy, and probably prone to instability. I am thinking that the reason they put those particular circuit values in the NFB would be to eliminate some of the likely instability. Modding it may be to invite havoc. Another option for eliminating oscillation from errant NFB is to put grid load resistor bypass caps in - (say 100pF to 270pF))

[überfuzz]

The N/U stands for not in use, I think. All components marked with N/U in the scheme is, or was, not in the circuit. I did however add a big cap in series with the pot/resistor.
Where should the grid load resistor bypass caps you suggested go..? Should I try a by pass off the resistor to maybe..?

Hmm, it's getting confusing. This is the to do list I've got so far.

1. Lift the nfb
2. Try different nfb resistor. Smaller right?
3. By pass the resistor I go for, if the hiss is still there.

[überfuzz]

Oh, I didn't wire the LINE OUT circuit... Is that likely to mess things up?

[R.G.]

Hiss comes from a lot of places. It could be:
- bum tube
- bad contact, especially a cold solder joint or cracked component - it does happen
- poor grounding
- RF or ultrasonic oscillation; high frequency oscillation frequently causes an angry sounding hiss that's a different sound quality from ordinary thermal noise.

Since you're mucking with NFB, there is a fair chance it's the ultrasonic oscillation. Put an oscilloscsope on it and see. Or if you have no scope. remove NFB entirely and see if it stops. If it stops when NFB is open, then it's probably oscillation. If it doesn't it *may* be oscillation from other places or one of the more prosaic causes.

[Jazz P Bass]

As R.G. has stated, the hiss may be oscillation.
And as Tubeswell has stated, this can be alleviated by placing a very small (220pf) capacitor (C34)across the feedback resistor (R33)
C34 being marked NU tells me that it was not inserted unless needed.

[Chuck H]

I'll wager the hiss is from high gain and the reason it lessens with more NFB is because gain lessens with more NFB. Less gain=less hiss.

[R.G.]

I'll wager the hiss is from high gain and the reason it lessens with more NFB is because gain lessens with more NFB. Less gain=less hiss.

Could well be.

[tubeswell]

Could well be.

And if the OP has put a bypass cap on that 220k that could be where the hiss is coming from.

@uberfuzz, the bypass cap suggestion I was talking about to snub high freq oscillation, is for the output stage grid resistors (R40 and R41 in that circuit). But try disconnecting the NFB first, or try taking that bypass cap on the 220k out.

[R.G.]

If our house were burning and I had to choose whether to grab clothes or my oscilloscope as I escaped, I'd probably go for the clothes, but it would NOT be an obvious decision.

8-)

[tubeswell]

I wouldn't know what to grab. Too much stuff to carry (dammit).