Troubleshooting hiss

[CitizenCain]

Up until now I've always been more concerned with battling hum in an amp than anything else. On my latest project, the Heathkit conversion, I'm not battling hum, but hiss instead.

Where do you start troubleshooting hiss? The amp has a hiss like white noise that's always present, gets louder as you turn up the vol. I added a variable feedback control and a midrange tone pot and when I back those off, the hiss gets quieter, but is still pretty loud.

Push me in the right direction boys!

[Steve Conner]

A certain amount of hiss is to be expected in a cranked amp. Try swapping tubes around, in case you have an abnormally noisy one somewhere.

[CitizenCain]

What about if you take the tube out and you still have all the hiss? I mean, there's a lot of hiss! It's controllable by the vol pot, so I assume it's in the circuit before the vol? Could it be bad plate resistors, caps, etc? In an attempt to keep some of the "mojo" of the original Heathkit, I reused some components. Probably shouldn't have.

Also, I paralleled the two halves of V1, isn't that supposed to make it a little quieter?

[Enzo]

If you pull a tube and the hiss - or any other sound for that matter - remains, then the source of the hiss is after that tube in the circuit.

[CitizenCain]

Thanks, Enzo. That gives me more options to look at The hiss remains wiht no tube in place, plus the vol control still affects it.

I replaced some plate resistors that I had reused and that didn't help. What other things can i look at that could be hissy?

[TheTinMan]

IMHO using a listening amp is great for finding the source of hiss/hum/etc. There's a basic description at the bottom of this page in the Hoffman Library. In addition to a few simple parts & cables, you need another little amp and an attenuator or dummy load so that you don't hear the amp you're working on.

Here is a post on the one I'm using. Make the ground lead longer than I did. Also, make sure you put your amp on standby before moving the probe and set the listening box volume to zero too. BE CAREFUL!

Chip

[CitizenCain]

That listening amp is pretty cool.

TinMan, did you mean to link to a post? I don't see anything.

[TheTinMan]

CC - sorry about that. Here's a post with pics of the box and probe I put together. Nothing to brag about but it works: missing link

Chip

[Enzo]

That listening amp idea is a good one. They are generally referred to as signal tracers. In fact there are numerous commercial products that do this. Old Heathkits and Eico tracers are still around. I like that they sometimes include a "magic ete" tube as a sort of VU meter.

Any tube you pull that has no effect is before the noise source, and any control that DOES affect it is after the source.

[TheTinMan]

Any tube you pull that has no effect is before the noise source, and any control that DOES affect it is after the source.

Correct about pulling a tube, but not necessarily about controls. You have to be looking at a schematic and remember that a signal can be grounded either before or after the point you are listening to. For example, you may be able to hear a bad cap in the tone stack at the grid of the next triode and the noise could be affected by the related tone control.

Also, you can pull your power tubes completely for listening to the preamp. It's much better to listen at points isolated from the power rail by a coupling cap. Oh, and remember to turn the listening amp/box volume down every time before you move the probe. I was reminded of that again this morning moving from the preamp to the power tube grids!

HTH

Chip

[Enzo]

???

The amp is a system with signal flow from one end to the other. If you are listening to a point in the system and you hear some noise, and the tone control affects the noise, then that noise had to come through the tone control. That pretty much means the noise came from an earlier point in the circuit, doesn't it? Whether we listen through the amps own speaker or use a signal traceer, that still is the case.

remember that a signal can be grounded either before or after the point you are listening to.

Exactly, if you ground the signal path after the point you are listening to there should be no effect on what you are hearing. If you ground before the listening point, then you will cut off any earlier signal/noise and remaining will be any signal/noise from that point up to your listening point.

[txstrat]

I would be interrested in your opinion about how much of a hiss is normal.
My recent built (35 watt vibroverb clone) is silent as a church mouse until I turn the volume up to, say 7 or 8. Then the hiss is quite loud.
If I plug in a guitar at this state the sound is deafening loud. Is the hiss to be expected?

[TheTinMan]

Enzo - FWIW the build I was troubleshooting when typing my earlier posts had a stupid mistake. 100K instead of 100 ohm resistors on the virtual center tap for the heaters. That would explain why I was hearing some affect from the volume pot later in the circuit even after eliminating V1 as a source of noise.

That amp is now extremely quiet except when the reverb is turned up a fair amount.

Grounding issues can drive me crazy!

txstrat - hard to say how much hiss is "normal". Have you tried the amp in other rooms and on other power circuits? Dimmers, flourescent lights, soldering irons, etc. have all frustrated me before Did you use shielded wire from the input jacks to the first triode? How are the input jacks grounded? Where is the preamp grounded in relation to the input jacks? Do humbuckers or two cancelling single coils reduce the hiss a lot?

I'm asking all these questions because the hiss shouldn't get that much louder with a guitar plugged in, at least in my limited experience.

Chip

[txstrat]

Chip

I have used and tried all your recommendations before. Thanks anyway.
I'm ashamed to say but after I posted my reply I went to the amp and fired it up again, just to make shure how bad the hiss really is. This time I had no cable in the input and the hiss was barely audible even with the amp turned fully up. The hiss came from my effects board.

Matt