Minor debugging needed for 1987 Plexi

[soundguruman]

Thanks for all the replies.

I'll talk for a second about how my ground scheme is set up and maybe you can comment on likely corrections...

The two grounding busses are grounded through the end that has the screw, the other end is soldered to an eyelet that's only there for mounting purposes. I made two of them to separate the signal grounds and power grounds. The filter cans are grounded with the PT CT's with the mains ground located right next to it. PT cathodes are grounded right at the sockets, and finally the pots are grounded to themselves.

I was considering grounding the pots to the signal buss. And possibly moving the other buss' grounding point to be closer to the PT end. Anyone see any other possible spots?

You should be grounding the power supply filters and bias filter as far away from the audio as possible.
You have it grounded in the middle of the chassis. That's going to cause more hum.
You need to move it away to the PT side, away from the audio ground.

Separate those grounds as far apart as possible.

You should be using a nut bolt ring terminal and washer. Not sheet metal screws.
The grounds should be solid and permanent.

YES separating the PS and audio grounds, far apart, makes a HUGE difference.

[mort]

You should be grounding the power supply filters and bias filter as far away from the audio as possible.
You have it grounded in the middle of the chassis. That's going to cause more hum.
You need to move it away to the PT side, away from the audio ground.

Separate those grounds as far apart as possible.

You should be using a nut bolt ring terminal and washer. Not sheet metal screws.
The grounds should be solid and permanent.

YES separating the PS and audio grounds, far apart, makes a HUGE difference.

I may very well get to that, but it shouldn't have anything to do with a problem that is specific to the bright channel volume..


That 500pF cap on the bright volume pot, when I tap on it I can here the tap through the speakers. No other capacitors or resistors seem to do this. Symptom?

And here are some voltages on V1-3.

V1
1. 152.7
3. .9
6. 196.9
8. 1.7

V2
1. 152.9
3. 1.1
6. 272.5
7. 152.9
8. 154.2

V3
1. 177.0
2. 24.8
3. 46.3
6. 176.6
7. 26.4
8. 46.3


Thoughts?

[soundguruman]

I may very well get to that, but it shouldn't have anything to do with a problem that is specific to the bright channel volume..


That 500pF cap on the bright volume pot, when I tap on it I can here the tap through the speakers. No other capacitors or resistors seem to do this. Symptom?

And here are some voltages on V1-3.

V1
1. 152.7
3. .9
6. 196.9
8. 1.7

V2
1. 152.9
3. 1.1
6. 272.5
7. 152.9
8. 154.2

V3
1. 177.0
2. 24.8
3. 46.3
6. 176.6
7. 26.4
8. 46.3


Thoughts?

There are 2 possibilities:
Bright channel is amplifying heater (filament) AC noise.
Bright channel is going into high frequency oscillation. You hear a 60 cycle buzz.

The volume control grounds should not ground to the chassis, where the pot is mounted.
The control ground should go back to the input jack ground.
AND that input jack ground should be as far away from the power supply ground as possible.

The design of the grounding on this amp - I can see it's defective.
If you want an amp to be as quiet as it can be, you DON'T ground it that way.

You are just allowing the power supply noise to feed right into the audio path.

This is a very common mistake, repeated by many amp designs.

It is normal for the bright cap to be microphonic.
You should change it from ceramic cap to siver mica.
Bright cap (ceramic) has a habit of picking up AC heater buzz, and allowing it into the audio path.
silver mica will have a lot less problem.

[big_teee]

On the bright cap, for shitz and giggles, try putting a regular .022uf mylar in place of what you have.
That will prove or disprove the ceramic bright cap.
I personally don't like them that bright anyway, but YMMV.
T

[soundguruman]

take it out completely. It's too bright anyway.

[mort]

Well I clipped it out to see if it cleared up but removing it actually brought on another unwanted noise, some random crackling type noise at higher volumes. I'm about to place a parts order for something else so I'll tack on some silver micas. Any other values I should get to experiment with the brightness? It is pretty darn bright on that channel...


for the time being I guess I can play around with the grounding scheme ....

[mort]

ok I found a 300pF silver mica and put that in there. the fluttering is still present, but extra noise is gone from removing the cap. I'll get a 500 and a 680 to play with I guess but I don't expect it to cure my symptoms...


I tried removing the grounding point on the bias buss bar and tied it in with the mains ground. No difference at all.

I just noticed a non-spec cap in the V1 cathode for the normal channel. Spec is a 320uF and there's a 250uF in there. Is it possible that this could be causing me issue?

edit: I also bypassed a pair of diodes (I had doubled up) and plate voltage dropped about 5-8v. The problem seems a little quieter but is still mostly present.

[Chuck H]

If removing the bright cap caused crackling noises at higher volumes, but putting it back stopped it, I'll wager you have HF oscillations happening and the effect shifted with the bright cap removed but shifted back to full ultrasonic with another cap back in there. Check lead dress and grounds. Make sure the presence lead is far away from the preamp leads. Make sure grid leads are short. Make sure any OT leads are twisted and flat against the chassis away from any preamp leads. Be sure to add the control grid resistors we discussed.

EDIT: This might explain any biasing troubles. If the amp is in ultrasonic oscillation it will be conducting and drawing current even though you can't hear anything. If you do find and fix the oscillation you'll want to re-bias the amp.

[Chuck H]

I just looked over your layout in post #8. Unusual construction. You have all your tube leads connected to the under side of the board so I can't tell how much is stuffed under there. You're using ground buss but I can't see much grounded to them other than two preamp filters being grounded with the bias supply (no no). And you have what looks to be a black, paper fiber circuit board. I hate those things because of the problems they cause but I'm going to ignore it as causative for now. The way you have the amp built it's going to be hard to police the layout and grounding, but I think that's what needs to happen. Consider moving the tube pin leads to the top side of the board. This makes for much easier service and modification later anyway.

[big_teee]

I agree, I built one amp with the leads underneath, and It was hard to work on.
Since then, I put the leads on top. I personally like turrets, and pay the extra for them over eyelets.
I put all straps on top also, black for ground, and red or orange for B+ Straps.
After having a conductive board on one of my builds, I only now use the finest G10 Yellowish green Board material.
I don't even use black G10 because of my conductive paranoia.
T

[mort]

I've used these boards twice before without issue, but I guess anything's possible. They are a grey "vulcanized" fiber board (whatever that means).

While it's not "OCD neat", the layout and routing is fairly short and clean. Presence is far from preamp, twisted leads, for the most part flat against the chassis. I poked around and tightened things and what not. Got about a 50% reduction in noise by tightening the inputs down real tight. The pots are all good and tight. And yeah those control grid resistors got installed shortly after it was brought up.

Still have same fluttering noise but it's calmed down quite a bit.

[mort]

Found another silver mica, looks like a 600pF, and put that in. No difference.

Routed the two big filters on the board to ground on the signal buss. Also no difference.


I suppose I could try another pot. I'm not sure what else to try at this point...

[Chuck H]

At any range on the controls do you notice any sudden changes in performance? Anything like a pop as the treble passes three o clock? That sort of thing? What happens if you bring the volume up slowly with all the other controls at five? Any consistent noises that occur at a certain setting? Is your presence control working correctly?

[mort]

The controls all seem to do their job like they're supposed to. Increase in noise is gradual as the bright volume is increased, a little bit of hiss as the mid and presence controls get increased, and if all the controls are dimed, then as I near the top of the swing of the bright channel volume, there is a sudden low pitched hum that starts.

Here's an odd thing... so I found I could greater or lessen the oscillation by moving around the leads that go to pins 2 and 3 of V2. Oddly, it seemed to get quieter the closer I get them to the heater leads. So here's the weird part.. the oscillation is still slightly audible but if I place my hands at certain places near [not even touching] the chassis the oscillation will completely go away. I move my hands to the left or right and the oscillation comes back. Move them back to the certain spots and the noise will completely vanish.

[Chuck H]

Since the layout looks pretty neat and tidy I'll speculate that grounding may have something to do with it AND/OR some stuff pressed under the board is too close to OT leads under there.

So short of tearing it up to get to the ground connections and lead dress your other option is to add top end bleeders to the circuit. This CAN be done with values that won't affect the audio spectrum of the amp. But they still carry a bad rep and most builders don't like to include them in vintage designs.