Test tone gets through with volume at zero

[Paul P]

If I hook up a 250mv 1000hz test signal to my amp I can hear it faintly
out of the speaker. If I measure the ac voltage on a dummy load I read
about 30 mvac which drops quickly to about 8 mvac (for some reason)
and then remains steady but this includes the power supply hum so it
must be a pretty weak signal.

Here is a trace on the dummy load without a test tone. I presume the big
humps are 120hz hum from the power supply :



Here is the measurement with the test tone into the input jack with the
volume at zero :



I have hunted around the preamp trying to see how the signal is getting
through and while I've found signal radiating all over the place (on the lowest
setting of my scope) nothing I have moved or changed has had any effect
on the output. The signal is coming out of the preamp and then moving
on through the rest of the amp to the output.

The only thing I can think of is that the signal is getting across from the
first half of the preamp to the second half through the preamp tube itself
through crosstalk. Is this possible ?

What I find odd is that the sound is clearly audible from the speaker when
the signal is what, 5mvac ?

I'm not worried about this, just find it interesting.

Paul P

,

[tubeswell]

If the vol is at zero, then you are grounding everything before that point in the circuit, so it must be something after that point that's causing the oscillation.

Could be a leaky coupling cap letting DC through??

Enzo gave me this tip once and its a great tip.

Hook up a .1uF 600V film cap to a wire with an insulated alligrater clip at each end.

Clip the wire from plate to ground at all the various gain stages in the amp. When the oscillation stops , you have found the stage that's causing the problem (i.e. the one before the grounding clip), then you can quickly work through the coupling cap, and cathode bypass cap (and resistors, solder joints) etc till you cure it. Jack Darr decsribes a similar troubleshooting process in the last part of his book

[Paul P]

Hi tubeswell,

If the vol is at zero, then you are grounding everything before that point in the circuit,
so it must be something after that point that's causing the oscillation.

The V1a triode is always full on because it's before the volume control. The
volume grounds the grid to V1b but there is still signal radiating all over the
place from the V1a plate resistor and lead, and the tone caps. The volume
control is isolated from the V1a part of the circuit because of the capacitors.

My thinking is that the signal is jumping from the insides of V1a into V1b inside
the tube which gets the signal around the volume control. I originally thought
that it was getting across through coupling between the various components
but I put the main ones up on stilts to get them away from each other and
this had no effect at all.

Paul P

[cbarrow7625]

Paul, the most likely cause of this is poor grounding throughout the amp. The pot may be on "zero" but it is likely not making a good, solid ground contact to actually short out the signal like you think it is.

What kind of amp is this and how is the grounding accomplished (star ground? Brass Ground plate? Scatter-ground? Sequential ground?).

Sure up all of your ground connections & this will go away.

It is not radiating from inside the tube to the other tube.

Chris

[tubeswell]

Hi tubeswell,



The V1a triode is always full on because it's before the volume control.

Paul P

Doesn't matter. If the vol pot is on zero it will be grounding all the signal from whatever gain stage is preceding it. Therefore either the grounding isn't right as cbarrow7625 says, or the oscillation is coming from somewhere behind the vol pot and maybe feeding back into V1 via (an NFB loop?). Have you got NFB loop connected to the cathode of V1? That would cause oscillation to be fed back through to V1 from subsequent gain stages where the problem might be occuring. If so, disconnect the NFB and do the isolating trick on each gain stage. (Switch off the amp before connecting and disconnecting each shorting wire.)

[Enzo]

That assumes that the zero position on the pot is actually zero ohms. I won't assume that. We have threads about how controls won't go to zero around here.

I wouldn't worry about crosstalk in the tubes, but crosstalk within the chassis is common.

What amp, what schematic, commercial or homemade?

Is the signal riding on any B+ node?

Ground your scope to the main star point, or to the closest thing to one your amp has. Now probe the GROUNDS through the preamp. In other words scope the bottom end of the cathode resistors, the bottom ends of volume controls, etc. We want to see if the signal is on the ground. If it is, it would be like the floor in your room shaking, everything sitting on it would shake as well.

[Paul P]

This is a homemade amp built from brown and blackface Fender schematics.
I'm using a sort of star grounding system.

I grounded the output of the preamp coupling cap directly to the chassis
bolt and this made no difference to the output. I then hunted around the
amp and still found the signal all over the place.

I then grounded my scope to the chassis bolt and looked at ground at the
first filter stage and got :



This (and all photos here) is with my scope at its lowest setting, 5mv/div,
with 1 div being one of the boxes of the grid.

The trace is not a line but a sort of a cloud. I got identical traces at all
power supply node grounds.

I then looked at B+ and found signal everywhere. I was pretty sure I'd done
this yesterday and got only the cloud-like line of ground but I guess not.

Here is B+ at the preamp (the trace jumped around a bit so it looks wider
than it was) :



I added a 20uf cap to the preamp node and this cut things in half or more
at the output :



I added another 20uf cap (total 60uf) and this reduced things still further
but not by as much as the previous one :



The signal is down to around 5mv p-p but is still faintly audible (this is with
a 250 mvac 1khz input signal which is a pretty strong signal).

So how may uf's are worth the bother ? Is there anything negative about
adding a pile more ?

I'd be interested in knowing what B+ looks like in other people's amps under
similar conditions.

Paul P

,

[Enzo]

ANy chance of a schematic so we can see the B+ circuit, dropping/decoupling resistors etc. ANd just how your sort of star ground is laid out.

[Paul P]

The circuit is made up of preamps and tremolo from the Fender Bandmaster 6G7-A :

http://www.schematicheaven.com/fende...6g7a_schem.pdf

with reverb from the AB763 Deluxe Reverb :

http://www.schematicheaven.com/fende...b763_schem.pdf

and the output stage is parallel 6v6's with individual adjustable/fixed and cathode bias.

I added a gain stage to the output of the tremolo using the unused triode.

I've followed the schematics except for some extra bits to tie the circuits
together (like a coupling cap on the output of the tremolo) and unsharing
the cathode resistors and caps that were shared. I've put the reverb on
the normal channel and the tremolo on the vibrato channel.

I did my own layout.

My grounding scheme is as follows :

- all jacks are isolated from the chassis
- each jack is grounded to the circuit it serves
- each circuit is grounded to its decoupling cap
- decoupling cap grounds are collected and grounded
at the first filter stage
- PT, power tubes, OT, first filter and mains are grounded
at the chassis bolt
- the chassis is not used for any ground

Here is a schematic. The voltages are correct (and need to be adjusted a bit)
and the resistor values have been changed slightly. The chassis bolt grounds are
the yellow triangles :



Paul P

[Enzo]

I know it would be clumsy to draw, but in your drawing, are there really separate ground wires from each block back to the first filter? As drawn it looks like a ground bus with each block hopping on as it drives by. I suspect that was shorthand, yes?

You showed a photo of the ground at the filter with scope grounded to the chassis, but did you look at the ground at each of the preamp stages with scope grounded to the first filter? If the signal is bouncing the grounds somewhere, it could show up in other stages.

It might be instructive to go down the signal path and ground it here and there - either with a wire or a cap and wire where there is DC. Say at each grid. We might gain insight as to where ther signal is coming in.

[cbarrow7625]

- all jacks are isolated from the chassis
- each jack is grounded to the circuit it serves

While this seems like a good idea, it is not the best way to handle inpiut jacks. Any input jack (be it the guitar input, power amp in, effects return, reverb return, etc.) should be connected directly to the chassis.

Why? Connecting the ground on input jacks directly to the chassis sends any interference, RF, noise, etc. picked up on the shield of the cable external to the amp directly to chassis ground and isolates it from being injected into your amp circuit via the ground wires (Yes, your ground connections do have some impedance that can allow interference, especiallt RF, get into your amp). When you connect the "ground" wire from input jacks directly to circuit ground insead of chassis ground, you invite all kinds of noise, interference, etc to get into your nice, clean circuit ground inside your shielded chassis. Why use a metal shileded chassis in the first place? To keep all of this outside noise outside. With the cable shield wire connected directly to signal ground, you have just given a direct path for outside noise, etc. to be directly injected into your circuit!

Whether or not this will solve your particular problem, I do not know but I thought it was worthwhile to mention as this is a common problem with cheap gear (PC Board Jacks) and a common mistake that otherwise good-intentioned builders, likeyourself, make thinking they are doing the right thing.

Can your way work OK? Yes, in some cases. However, it really is a high class design no-no.

It is easy to re-wire your jacks & see if any of your problem is alleviated. Good luck.

Chris

[Paul P]

I know it would be clumsy to draw, but in your drawing, are there really separate ground wires from each block back to the first filter? As drawn it looks like a ground bus with each block hopping on as it drives by. I suspect that was shorthand, yes?

The grounds are actually wired as shown. The ground and B+ wires are always
paired together and are twisted together to form the smallest loop size.

There are three ground wires I didn't show :

1) The bias supply grounds : there's two of them, one for the supply itself
which is grounded to the chassis bolt and the other for when the power tubes
are set to cathode bias which grounds the 220k grid lead resistors. This I
grounded to the bottom of the PI node cap (at this point I'm not using
cathode bias so this wire shouldn't be doing anything).

2) The heater bias voltage ground : I hooked this up to the first filter ground
since it was about an inch away figuring I could always go to the chassis
bolt if it caused problems (which I don't believe it does). There's maybe five
inches of 18ga stranded wire between first filter ground and the chassis
bolt.

I looked at the other grounds from the chassis bolt and they were all identical
except for maybe a slight widening of the 'cloud' the further away I got
from the chassis bolt. Certainly no signs of a signal. I will try the same
grounded at the first filter.

I will continue hunting down the source of the signal. I've been thinking
that it was coming from the preamp onto B+ and then being sent to all
the other circuits on the same node (preamp 2, reverb, tremelo gain stage)
but maybe not. I will also try grounding different points and see what
happens.

Paul P

[Paul P]

While this seems like a good idea, it is not the best way to handle inpiut jacks. Any input jack (be it the guitar input, power amp in, effects return, reverb return, etc.) should be connected directly to the chassis.

cbarrow7625, I'm going to try and explain why I didn't do this but please keep
in mind that I am a newbie to all of this and have been working so far almost
entirely on the opinions of others like yourself who have far more experience
than I do. So I'm not being argumentative in any way in case it looks that way
in this virtual medium.

The grounding of the input jacks has caused me a lot of trouble since there
is no concensus on the best way, as there might be for most other parts of
an amp. There are at least two camps :

1) those who recommend grounding them to the chassis at the input and
also grounding the preamp circuit grounds to this point as well, with the
chassis/bolt ground (or at least the other end of the chassis) taking the
heavy current components like the transformers and power tubes.

2) the other camp, if I understand things correctly, recommends not using
the chassis to ground the input jacks or the low power circuits, but to ground
them to the root of the star.

In my opinion, the main thing that the second method has going for it is
that there are no ground loops whereas this cannot be said for the first
method. I'm in no position to say if the possible ground loops could cause
problems or not but I've always considered ground loops to be something
to be avoided at all costs.

I understand your point about the interfence getting into the amp from
outside by way of the cable shield. I have seriously considered going to
a balanced cable to solve this (and rewiring my guitar accordingly). This
way the shield would be complete and independent of signal ground.

I'm not convinced that the reason for the signal appearing at the output and
on B+ can be explained by how I've grounded my inputs. I've considered
the input issue to be one of hum and interference pickup (which maybe is
your point of view as well). So far I'm satisfied with how my amp behaves
in this respect. It is currently sitting insides up towards 14 fluorencent tubes
(about 4 feet away at their closest) and there is very little audible hum or
interference coming from my speaker even at full volume.

Paul P

[cbarrow7625]

That's all rational. The good thing is that you understand there are alternatives. I would strongly encourage you to just try different grounding schemes. Just intellectualizing on what seems to you may work the best won't get you toward a solution. Moving wires and trying different ways to ground it will.

There are no absolutes. How you run your grounds and how well they work depend on a lot of other things in the amp & layout (that you may or may not recognize). You'll be surprised how often what you think is a bad idea actually works out to be the optimal solution if you can just get past your intellectual misgivings and give it a try.

What you need is something that works for your amp, not somebody else's.

Try a few different grounding schemes (with all of your ground points, not just the input jacks) and learn for yourself what the different ground schemes do to your sound & signal. DON"T BE AFRAID! This is how you learn.

[Enzo]

If the grounds are wired like the drawing, then ther is a bus running from preamp to first filter, then along that bus, the PI joins in, then the reverb. That means to me that for at least part of its trip back to the filter, the preamp ground shares copper with both the PI and the reverb. That is not the same as running a ground wire from the preamp to the filter, running one from the PI to the filter, and tunning one from the reverb to the filter. Three separate wires would be a real star.

Twisting B+ and ground won't achieve anything. Might look nice. We twist AC power wires, like the heaters, to prevent radiating AC into the circuits. Ground and B+ shouldn't be radiating anything, plus the point of twisting is the two wires would carry the same thing at opposite polarity and hopefully cancel. B+ and ground do not carry the same thing and there would be no cancellation even if it was a radiating situation. WOn't hurt anything, but shouldn't be involved in the problem.