Help With Hiss

[Chuck H]

Shock-bait.

Still waiting... Don't lose face... Step up.

[J M Fahey]

Considering where it comes from, I suspect a grounding or *layout* problem involving both volume pots and the tone control driving stage.
For clarity please redraw schematic (which is basically right) adding tube designators (V1, V2, etc.) and tube elements (plate/grid/cathode).
They can be guessed by external connection, of course, but clarity helps, specially in not-so-clear problems.
Probably adding a small cap plate to grid in which I suspect is the oscillating stage might help, but it will kill sparkle, not the *real* solution.

[Daver]

Thanks guys. I've attached my now messier drawing indicating how it's grounded. The power tube grounds, filters, PT centertaps and bias are all grounded at the output jacks. Essentially there are two ground points for the local stars. There are no in phase stages sharing a filter. The only other symptom I've found is that when the volume(s) are dimed with the treble and presence dimed I do get a little high frequency squeal. It sure seems like an oscillation. I added a 100pf cap from plate to grid on the second stage. It reduces the hiss, but it's still there. Putting one on the 1st stage too really knocks it down, but it sounds dull as hell.
I've built amps with more gain in tweed chassis before using similar grounding and layout and never had a problem. Sometimes these things take on a life of their own.
Dave

Preamp W Grounds.PDF

Could the problem be caused by the wires going from the PI coupling caps to the power tubes being near the filter cap "+" for the preamp?

[Chuck H]

If it's wired as shown, you do have in phase signals sharing grounds. You also have loops. I can't mod PDF files or I'd doctor your drawing and re post it. If you can post it as any other file type I could. Otherwise I'll need to use some online service to swap it into a jpeg or something.

You still haven't indicated which tube is which, at JM's request. I assume the first preamp tube is both inout triodes, the second is the second gain stage and CF.?.

[Daver]

Chuck,
Yes the first preamp tube is the input stages. The second is the second stage/CF and the third is the phase inverter. The first two are in parallel and out of phase with the second stage, so where do you see in phase stages sharing a ground? Where are the loops you mentioned? I know the drawing sucks, but the stages have their grounds connected to their respective filter cap ground and one wire from that "bundle" goes to chassis ground at the input jack. The board is layed out in order without wires from previous stages crossing over each other. The leads are as short as possible. If you see in phase grounds being shared and/or loops, please let me know. Thanks!
Dave

[Chuck H]

Obviously this sort of thing needs to be planned from the start. You probably have ground bundled points on the board. And the truth is that some bundling is ok. Or at least the problems are so tiny that it can be ignored. Like two out of phase cathodes sharing an eyelet or turret and then one lead to the star. Or two parallel stages that aren't on the same channel sharing an eyelet or turret and then one lead to the star. There are other examples, but these are the obvious ones. I don't do it, but many do and it seems to work fine. But if your scheme is as drawn then it is flawed.

EDIT: All the above assuming that you have your filters grounded and then sent to the input ground *(I assume this because your drawing says "filter cap ground, and is then connected to the input ground). And in the "idealized" drawing, the filters are grounded via the red leads only.

[g1]

Chuck: Here's a jpeg version of the pdf:
For quick & dirty pdf conversion, I view the pdf at a scale so I can see it all on my screen, then I hit the print screen (prt scrn) key, then paste into my image program (edit, paste in paint) for cropping and saving as jpeg or whatever. I use MS paint because it is simple and I can add lines, text etc. but you can use whatever program you like. You will have to crop out some of the other stuff which is on your screen as the screen shot contains everything on your screen aside from the drawing.
Once you do it a few times, the print screen button becomes very handy.

Edit: I posted that without looking at your drawings Chuck, so I guess you already knew this, sorry .

[Chuck H]

I actually used a "free service". I'll be expecting my email about Viagra any day.

[Daver]

Thanks for your insight Chuck. The way I've done my grounding, mostly, is to attach the local "-" to the filter cap "-" and then to chassis ground. Anything after a coupling cap gets bundles with the next stage "-" That's why the tone controls are connected to the phase inverter rather than the CF. It's essentially how Kevin O'Connor suggests in his books. Maybe my sloppy drawing doesn't show that exactly. The input is chassis ground. Any time I've had grounding issues it's showed up as hum, not hiss.
I put my scope on the output cap of the CF. The hiss is uneffected if the first tube or phase inverter are in the circuit. It's always there and doesn't increase in size. If the phase inverter is in then the hiss is obviously amplified going into the power section, but the amount of hiss from the CF is constant. I guess I'll try to rewire the grounds and see what I get. Thanks!
Dave

[Daver]

Well, I'm back working on this thing after building a couple of other amps (that work fine) and doing a bunch of repairs. Still no joy. The hiss remains. I've moved grounds nine ways from Sunday on all the stages and power amp. All I get is more hum, the hiss is not affected. I've replaced filter caps and added an additional stage of filtering for the cathode follower. I've tried reducing gain of the stage feeding the CF by removing the cathode bypass cap, reducing the plate resistor value and using a 12AY7. No change. I'm stumped. I can reduce the hum to almost nothing if I put a .047uf cap across the plate resistor of the stage feeding the CF, but all the highs go away too. If I ground the grid it stops. If there is just a 100K resistor from grid to ground and the first stage, volumes, etc. preceeding it are removed, it still hisses. Is it at all possible a conductive eyelet board could cause this? Everything is new, but..... Thanks.
Dave

PS, I also changed the phase inverter to a regular long tail type with the usual presence control. That didn't change anything hiss-wise, although I think the amp sounds better.....

[Daver]

I've now tried a new V2 tube socket. I tried swapping V2 A+B sections with each other. I can't stop the hiss or even make it worse. All I know is that it seems to be coming from V2. This is the most maddening problem I've ever encountered. Any ideas before I throw it out the window? thanks.
Dave

[soundguruman]

I've now tried a new V2 tube socket. I tried swapping V2 A+B sections with each other. I can't stop the hiss or even make it worse. All I know is that it seems to be coming from V2. This is the most maddening problem I've ever encountered. Any ideas before I throw it out the window? thanks.
Dave

Well, I will dehiss it for $35 an hour...
but you better pack it good.

[Chuck H]

Because of the odd symptom of having the second stage cathode resistor lifted and full hiss, then nothing after two seconds when flipping the standby switch, I'm going out on a limb and guessing that you may have a cruddy or failing filter cap or caps. The cap that is feeding the V2 plate and/ or the cap that is sharing a ground with V2 circuits. If you have another cap on hand go ahead and swap out the caps one at a time. If you don't have another cap, but do have a .1uf 400V+ film cap, try adding the film cap parallel to the each filter stage and see if it makes any difference. It may not and it may still be a bad filter cap.

[Daver]

Chuck,
Thanks, but I did swap out that filter cap as well as create another filter node for V2 so it wasn't sharing V1's filter node. Any chance the PI filter cap could influence V2? I'll try replacing it.
Dave

[Chuck H]

Well, you did say that each triode shared a ground with the following stages filter. I'm just intrigued by the odd symptom. It implies a slow time constant circuit. Slow like a filter charging up. Have you tried replacing the bypass cap for the offending stage?