Question: OK, very nerdy tube amp tech question.

cvogue

cvogue

Yes, that's Oolong. :)
So my amp tech fixed a noise problem in my tube amp. It was a microphonic cable going from the gain control to V2. He said he did an "old hi-fi trick" where he attached the shielding of the cable from the gain control to the plate of the V2 tube (responsible for a lot of the gain on the dirty channel) in a reverse polarity way. Not sure what that means exactly but would tying shielding to the plate of the tube affect the tone/amount of gain?

Gotta be some tube amp tech savvy folks out there who might know about this!

Sorry, it's a pretty obscure question.
 
Hopefully, someone here has an answer for you. But, there are only seem to be a few people who regularly post here.

I suggest posting this same question in the amp section of thegearpage.net. You may get a few cantankerous responses. However, you will likely get thoughtful feedback as well. I’ve had quite a few problems solved by helpful players there.

Good luck.
 
jrockbridge said:
Hopefully, someone here has an answer for you. But, there are only seem to be a few people who regularly post here.

I suggest posting this same question in the amp section of thegearpage.net. You may get a few cantankerous responses. However, you will likely get thoughtful feedback as well. I’ve had quite a few problems solved by helpful players there.

Good luck.
Click to expand...

Done, thanks for the tip. I've lurked on there off and on for quite a while but never signed up. See what snarky responses I get!
 
cvogue said:
Done, thanks for the tip. I've lurked on there off and on for quite a while but never signed up. See what snarky responses I get!
Click to expand...
I just ignore the snark and concentrate on those who actually want to help. There are plenty of the Harmony Central alumni over at TGP. There are even a few players from here that post there.
 
cvogue said:
Done, thanks for the tip. I've lurked on there off and on for quite a while but never signed up. See what snarky res
jrockbridge said:
Hopefully, someone here has an answer for you. But, there are only seem to be a few people who regularly post here.

I suggest posting this same question in the amp section of thegearpage.net. You may get a few cantankerous responses. However, you will likely get thoughtful feedback as well. I’ve had quite a few problems solved by helpful players there.

Good luck.
Click to expand...
Wow, got good responses already, thanks for the tip! I now know what it's called, a "hot shield". The gurus over there don't recommend it and said it could indeed be causing the behavior I'm seeing, loss of gain and high end. The tech said I might have a bit of high end loss and if I didn't like it he could remove it. Thanks again for the tip!
Click to expand...
 
Reverse polarity is like out of phase - perhaps to another gain stage.

So, if you think about a signal having peaks and valleys, and a gain stage amplifying the signal - then another gain stage amplifying in the opposite direction, the overlapping peaks and valleys cancel each other out.

That cancellation will make the overall signal much less in many ways including volume.


Here’s a simple example.


IMG_2826.png



Is that what is happening to your amp?
Maybe.
It really does depend on the rest of the circuit and if it is still all connected as originally designed.

If it is, that third diagram is probably closest visual to what is happening.
There are some parts of the wave that are in phase (louder or more present) and some parts that are out of phase (quieter or less focused).
The net of that third diagram is there are two waves of higher amplitude. When combined, the result is a wave with lower amplitude (less volume).

In the context of gain, if you’re talking about “edge of breakup” clipping kind of tone, yeah, a wave with less amplitude is not going to be “tall” enough to get to that clipping ceiling where the peaks get cut off.

Another simple example of what I’m talking about.

IMG_2827.png


I can’t say for sure this is what is going on in your amp, but this is exactly what you are describing.
And the amp tech’s comment about “reverse polarity “ leads me to exactly this concept.


Oh! Another thought. Phase polarity can be introduced with pedals too.
Does this happen with no pedals connected at all?
If so, then it’s just the amp.
If not, then cycle through all of your usual pedals, adding in one at a time and see if the problem comes back.
If one pedal introduces the problem, it’s reversing the phase.
That’s not a fault of the pedal in and of itself. It’s just a thing that needs to be accounted for. Either by chaining another pedal that reverses the phase again (back to ‘normal’) or swapping out the pedal for a similar pedal that doesn’t reverse the phase.


And I may be rambling now. So, I’ll stop there. LOL!
 
dodgechargerfan said:
Reverse polarity is like out of phase - perhaps to another gain stage.

So, if you think about a signal having peaks and valleys, and a gain stage amplifying the signal - then another gain stage amplifying in the opposite direction, the overlapping peaks and valleys cancel each other out.

That cancellation will make the overall signal much less in many ways including volume.


Here’s a simple example.


View attachment 94737


Is that what is happening to your amp?
Maybe.
It really does depend on the rest of the circuit and if it is still all connected as originally designed.

If it is, that third diagram is probably closest visual to what is happening.
There are some parts of the wave that are in phase (louder or more present) and some parts that are out of phase (quieter or less focused).
The net of that third diagram is there are two waves of higher amplitude. When combined, the result is a wave with lower amplitude (less volume).

In the context of gain, if you’re talking about “edge of breakup” clipping kind of tone, yeah, a wave with less amplitude is not going to be “tall” enough to get to that clipping ceiling where the peaks get cut off.

Another simple example of what I’m talking about.

View attachment 94738

I can’t say for sure this is what is going on in your amp, but this is exactly what you are describing.
And the amp tech’s comment about “reverse polarity “ leads me to exactly this concept.


Oh! Another thought. Phase polarity can be introduced with pedals too.
Does this happen with no pedals connected at all?
If so, then it’s just the amp.
If not, then cycle through all of your usual pedals, adding in one at a time and see if the problem comes back.
If one pedal introduces the problem, it’s reversing the phase.
That’s not a fault of the pedal in and of itself. It’s just a thing that needs to be accounted for. Either by chaining another pedal that reverses the phase again (back to ‘normal’) or swapping out the pedal for a similar pedal that doesn’t reverse the phase.


And I may be rambling now. So, I’ll stop there. LOL!
Click to expand...

Wow! Thanks dodgechargerfan! BTW my daughter's boyfriend has a charger, loves it. :cool:

Yup, I think this is what's happening. He described what this shielding would do and I said "It's like a humbucking pickup" and he said "Exactly, it'll cut down on the hum/noise"

It's hard to tell from listening to it if it jives with the waveforms you gave but it could well be. I have not tried it without my pedalboard but I'll do that at practice on Thursday (don't have the amp here, it's at the house where the band practices).

The gain explanation is good too, yup it's the "when does it break up" kind of gain. Again I used to have it at about 1:30 and now I gotta have it almost dimed. It also doesn't sound quite as good.

No worries about the rambling, I *really* appreciate all the info!

Oh I have an email in to Scott Splawn, see if he can weigh in as well. Nobody knows the circuit like him. :cool:
 
I should have added to the pedal conversation for clarity.
If a pedal is reversing signal phase and the result goes into the amp, that is then reversing it again, you get the cancellation effect I describe above.
So, if you do find that taking a pedal out of the chain helps, then that pedal is reversing things - likely by feeding the signal out of an op-amp back into itself, which is a very common thing with op-amp circuits (and transistor circuits that op-amps mimic).
 
The output of tube in common-cathode config (which is what guitar amps use) is 180 degrees out of phase from the input. So by connecting the shield of the wire to the output, you are introducing a voltage that is 180 degrees out of phase with the noise that causing the hum. So, like humbucking pickups, it tends to cancel the noise out.

While I suppose that would work, it also introduces a high voltage on the shield of the wire. Most people would assume shields are connected to ground, so that could lead to a nasty surprise to an unsuspecting tech. Although the current would be very limited - so no real danger, just an unpleasant surprise.
 
Interesting....

Normally microphonic noise from the gain channel is a tube running away.

How did the amp sound when it was in the clean channel?
 
cvogue said:
Heard from Scott! Great guy, basically said "Don't do that it kills the tone and gain". Taking it in later this week to have it undone.

Clean channel is fine
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Agree with the killing the tone and gain. That is what phase cancellation does. Probably why I never heard of someone doing that.

I suspect a noisy or microphonic 12AX7m tube in the lead channel. There also could be some component that is leaking over in some strange feedback way. He'll probably check lead dressing and the feedback circuit besides swapping and tube. Might be as simple as a bad solder joint.
 
I just saw the tag, so a week late to the game.

cvogue said:
It was a microphonic cable going from the gain control to V2.
Click to expand...

I'm a little confused. I take this to mean the amp used a shielded, coax cable from the Gain pot to V2 grid (tube input)?

cvogue said:
He said he did an "old hi-fi trick" where he attached the shielding of the cable from the gain control to the plate of the V2 tube
Click to expand...

This confuses me....if it's coax cable, the shield gets grounded and only one end goes the ground (connecting both ends could create a ground loop). If one end is connected to the plate, what is the other end connected to? Can't be ground, that would short the guitar signal.
 
Wyatt said:
I just saw the tag, so a week late to the game.



I'm a little confused. I take this to mean the amp used a shielded, coax cable from the Gain pot to V2 grid (tube input)?



This confuses me....if it's coax cable, the shield gets grounded and only one end goes the ground (connecting both ends could create a ground loop). If one end is connected to the plate, what is the other end connected to? Can't be ground, that would short the guitar signal.
Click to expand...
The shield connects to the plate on one end and nothing on the other. In essence, the shield become an antenna that injects a signal that is identical to the incoming noise except 180 degrees out of phase thereby canceling the noise.
 
GomezAddams said:
The shield connects to the plate on one end and nothing on the other. In essence, the shield become an antenna that injects a signal that is identical to the incoming noise except 180 degrees out of phase thereby canceling the noise.
Click to expand...
Exactly, he described the process to me and I said "It's acting like a humbucking pickup"

Unfortunately it really did kill the gain and the tone was flubbier on the bottom end. Probably due to extra capacitance added to that gain stage, someone on The Gear Page gave a really good explanation:

-----------------------------
The output of most tube stages is opposite-polarity compared to the input. So any signal-path from output to input would result in negative feedback at that stage, which will reduce stage-gain somewhat.

The small length of wire between tube & pot means this length of cable is a very small capacitance. However, the effect of this capacitance is amplified by the tube's gain such that it acts as though it were a bigger capacitance. Depending on exactly how much it is, there is a gain-reduction either at radio-frequency, or (for larger capacitance) the high audio frequencies.
 
Wyatt said:
This confuses me....if it's coax cable, the shield gets grounded and only one end goes the ground (connecting both ends could create a ground loop). If one end is connected to the plate, what is the other end connected to? Can't be ground, that would short the guitar signal.
Click to expand...
Right? Tying a shield to the plate with it also tied to ground would basically shunt all signal to ground. Tying the shield to the plate without it being grounded forms a noise antenna for the plate. Doesn't make sense to me.

The only other thing I can think of is that he added a negative feedback loop and described it poorly?

What amp is this? Is it the Splawn Street Rod from your other threads?

This was the only schematic I could find and it's low res.

1721500063381.png
 
GilmourD said:
Right? Tying a shield to the plate with it also tied to ground would basically shunt all signal to ground. Tying the shield to the plate without it being grounded forms a noise antenna for the plate. Doesn't make sense to me.

The only other thing I can think of is that he added a negative feedback loop and described it poorly?

What amp is this? Is it the Splawn Street Rod from your other threads?

This was the only schematic I could find and it's low res.

View attachment 95223
Click to expand...

Yup it's the Street Rod.
 
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