Help With Hiss

[Daver]

I just fired up a new build and it has a fairly loud hiss. It's essentially like a tweed Bassman with a See Saw paraphase inverter and no Middle control. The hiss seems to be coming from the second/cathode follower stage. It stops if I remove that tube but not if I remove the first tube. The tone controls, especially the Treble, control the hiss. Turning down the Treble all the way just about gets rid of it. Grounding the grid on the second tube reduces it a little. So far I have.....

Tried several new and NOS tubes.
Replaced every resistor and capacitor around and for that tube. I'm using 1 watt carbon film. I did try a metal film for the plate.
Bypassed the cathode follower and ran the tone stack from the plate.
Pushed wires around.
Checked voltages. Nothing seems odd or suspicious.
Disconnected power amp negative feedback in case there was some weird oscillation.
Swapped out power supply string dropping resistor. It was a 1 watt carbon film, now it's a 2 watt metal oxide.

Any ideas? thanks!

Dave

[Steve Conner]

Maybe try adding a grid stopper resistor to the cathode follower?

[soundguruman]

I just fired up a new build and it has a fairly loud hiss. It's essentially like a tweed Bassman with a See Saw paraphase inverter and no Middle control. The hiss seems to be coming from the second/cathode follower stage. It stops if I remove that tube but not if I remove the first tube. The tone controls, especially the Treble, control the hiss. Turning down the Treble all the way just about gets rid of it. Grounding the grid on the second tube reduces it a little. So far I have.....

Tried several new and NOS tubes.
Replaced every resistor and capacitor around and for that tube. I'm using 1 watt carbon film. I did try a metal film for the plate.
Bypassed the cathode follower and ran the tone stack from the plate.
Pushed wires around.
Checked voltages. Nothing seems odd or suspicious.
Disconnected power amp negative feedback in case there was some weird oscillation.
Swapped out power supply string dropping resistor. It was a 1 watt carbon film, now it's a 2 watt metal oxide.

Any ideas? thanks!



Dave

Hiss has almost always originated in the first stage of the pre-amplifier, then is re-amplified over and over by the following stages.
install a 39 pf cap to ground on the first stage grid of the amplifier, V1A (grid to ground) (silver mica if possible)
install a 7 pf 1000V cap between the grid and plate of V1A, put it on the socket pins. (silver mica if possible)
use low noise parts for the first one or two stages of the preamp. metal film resistors, especially the grid input resistors and plate resistors.

[Chuck H]

Hiss has almost always originated in the first stage of the pre-amplifier, then is re-amplified over and over by the following stages.
install a 39 pf cap to ground on the first stage grid of the amplifier, V1A (grid to ground) (silver mica if possible)
install a 7 pf 1000V cap between the grid and plate of V1A, put it on the socket pins. (silver mica if possible)
use low noise parts for the first one or two stages of the preamp. metal film resistors, especially the grid input resistors and plate resistors.

The OP clearly stated:

The hiss seems to be coming from the second/cathode follower stage. It stops if I remove that tube but not if I remove the first tube.

The OP also mentioned idealizing with low noise components in the offending stage with no change.

This is what makes the problem interesting. Hiss is usually a byproduct of gain and series resistance. That's why pulling the first tube should cut the hiss. This may be something else. It would be best if we had a schematic and a sound clip. Trying to work the diagnosis as if this were normal hiss is clearly not the answer.

[Daver]

Chuck is correct. The hiss still exists without the first tube in and with the grid of the second, offending, stage grounded. I don't really have a way to make a clip of the hiss. It sounds very much like regular amp hiss but louder. The amp is very much a tweed Bassman with a see saw inverter. No high gain stages, no high frequncy enhancements other than an 82pf bright cap on one volume. It's pretty strange. Thanks!
Dave

[Chuck H]

If grounding the grid of the triode that feeds the cathode follower doesn't stop the hiss, but pulling that tube does, your problem is between the plate of the stage feeding the CF (possibly the cathode?) and the PI input. I don't know what sort of bench tools you have, but here's an idea that doesn't require any. Take a .01uf/600V capacitor and solder one end to a two foot flexible lead. solder an alligator clip to the other end of the flexible lead. Tape the cap and part of the lead to a chopstick (covering any bare wire) with the unsoldered cap end poking out a little at the tip. Now clip to the chassis and poke along the signal chain to try and isolate the earliest point that has a significant effect on the hiss level. That should narrow the problem further. To a point between the plate of the triode feeding the CF and the next point in the signal chain where you get results with the probe.

Obviously, poking the probe into the signal path beyond the problem will also reduce hiss. Following the signal chain from the triode feeding the CF forward, your looking for the first spot where the probe makes a significant difference in the hiss level.

[J M Fahey]

It might be oscillation, which often sounds like some weird kind of hiss (we do not hear the actual oscillation but its artifacts).
Please post the schematic which by your description is not a Bassman any more.

[Chuck H]

+1

A layout might also prove helpful. A clear gut shot pic would be best.

[Daver]

Chuck, I have such a device! Touching the cathode follower cathode reduces the hiss a little. Touching the plate of the stage driving it pretty much eliminates it! Changing the tube to a 12AU7 reduces the hiss. So it's in that stage somewhere!
J M Fahey, I'll have to draw something up that's more readable. Thanks.
Dave

[Daver]

Here's my lousy, quickly drawn schematics. Hopefully they will good enough. The second stage plate voltage is 155, its cathode voltage is 1.1 and the cathode follower voltage is 155. Thanks!
Dave

[Chuck H]

OK... Well... NOT a f56a. But it hardly matters. Having narrowed the hiss to somewhere in between the second stage grid and the second stage plate means that's where the trouble is. So check any circuity there.?. Oh, you can't. There's NOTHING but vacuum!!!

I don't have any answers. But I might try ungrounding the cathode on that triode to see if that kills the hiss. And if it does I would try grounding it somewhere else.

Perhaps other preamp grounds?

Got a scope?

[soundguruman]

Chuck, I have such a device! Touching the cathode follower cathode reduces the hiss a little. Touching the plate of the stage driving it pretty much eliminates it! Changing the tube to a 12AU7 reduces the hiss. So it's in that stage somewhere!
J M Fahey, I'll have to draw something up that's more readable. Thanks.
Dave

Shock-bait.

[Chuck H]

Shock-bait.

I'm listening.

[Daver]

Chuck,
If I disconnect the second stage cathode resistor from ground I still get a little hiss. If I disconnect the CF cathode resistor from ground I get full volume hiss for about two seconds when flipping the standby to on, then almost nothing. Weird. I do have a scope. For what and where should I be looking? Thanks for all the help!
Dave

[Chuck H]

It sounds like AC is somehow coupling through the power supply until the caps charge up. Which is odd because I've only ever heard that with hum. I suppose if your caps have a high esr and two in phase stages are sharing a filter ground this MAY happen, or something like it. Check filter grounds and undo any daisy chained grounds. Run grounds individually to the buss or star or whatever. It may even help to add another filter node to isolate V1 from the CF tube. Something may be wired cattywhompus around the CF and the stage feeding it. Any other symptoms or oddities?