Lead Dress: OD V2a Grid wire

[ijedouglas]

I apologize if this has been discussed before, I searched feverishly and could not find anything on this specific topic.

We are all familiar with the plate and cathode wires in parallel to tighten up the OD but while going through pics I noticed that Mr. Dumble also tended to move the V2a grid wire away from the V2a plate wire. I am redoing my #124 and spent a bunch of time studying the pics as well as going through pics of others (94/104/124/133/183) for clues on lead dress. There are varying degrees of distance between them but there is definitely a common theme. #124 is by far the most pronounced with the lead actually raised above the board.

Anyone else notice this? Thoughts?

Screenshot 2021-05-04 183133.png

[erwin_ve]

So the closer these 2 wires are, the more negative feedback/tighter tone.
I haven't experienced much on that spot.
What are your preferences Ian?

Erwin

[ijedouglas]

Hi Erwin,

So far I have changed it on 3 amps and I love the results. Increased bottom end without getting muddy, the mids got clearer and more focused and there was a noticeable increase in sustain.

I'd really be interested to hear your results?

[erwin_ve]

Hi Erwin,

So far I have changed it on 3 amps and I love the results. Increased bottom end without getting muddy, the mids got clearer and more focused and there was a noticeable increase in sustain.

I'd really be interested to hear your results?

Ian, to be honest I didnt pay enough attention , I like some experimentation on that spot. So your mentioned results are when grid and plate wires are close to each other?

[ijedouglas]

No, I have separated them. Originally I had V2a running down to the chassis and crossing/touching the V2a plate wire. After studying all the ODS pics I could find, I noticed the there was a clear separation between these wires. Mr Dumble clearly moves these 2 wires away from each other to varying degrees.

[norburybrook]

No, I have separated them. Originally I had V2a running down to the chassis and crossing/touching the V2a plate wire. After studying all the ODS pics I could find, I noticed the there was a clear separation between these wires. Mr Dumble clearly moves these 2 wires away from each other to varying degrees.

what about if they're crossed at right angles does that make a difference? i.e close/on top of each other.


M

[martin manning]

On my LCR meter, 6" parallel leads with insulation touching produces capacitance of about 1pF/inch, calibrated to 0pF with the leads parallel and 1" apart. If the leads are crossed at right angles, it's 0.2pF. As a feedback path in a typical 12AX7 stage, a couple of pF has, in theory, a negligible effect in the audio range, a couple tenths of a dB at 20kHz.

[ijedouglas]

No, I have separated them. Originally I had V2a running down to the chassis and crossing/touching the V2a plate wire. After studying all the ODS pics I could find, I noticed the there was a clear separation between these wires. Mr Dumble clearly moves these 2 wires away from each other to varying degrees.

what about if they're crossed at right angles does that make a difference? i.e close/on top of each other.


M

That is how I had them originally (on chassis, crossing at right angles). I have seen 1 or 2 pics where this is the case although the majority are clearly separated.

The more I look at it, I think Mr. Dumble was using it to shape the OD frequency response.

[ijedouglas]

On my LCR meter, 6" parallel leads with insulation touching produces capacitance of about 1pF/inch, calibrated to 0pF with the leads parallel and 1" apart. If the leads are crossed at right angles, it's 0.2pF. As a feedback path in a typical 12AX7 stage, a couple of pF has, in theory, a negligible effect in the audio range, a couple tenths of a dB at 20kHz.

Interesting. Try it in your amp and see if you notice anything I wonder if there is something else at play? If you look at the pics of this spot, it's pretty obvious that this was intentional

[norburybrook]

if the wire was flown would it be more inclined to vibrate and that's what causes the audible effect?

I suppose my question is: does a wire/component vibrating in a loud amp have a sonic effect?

M

[ijedouglas]

if the wire was flown would it be more inclined to vibrate and that's what causes the audible effect?

I suppose my question is: does a wire/component vibrating in a loud amp have a sonic effect?

M

Maybe... although I think at that length it would be negligible. I suspect it has more to do with a phase relationship with the plate wire.

[Audiodog]

Definitive answer:

Never let the two wires touch, not even at 90 degrees. You will get massive noticeable cancellation artifacts, unpleasant harmonics, and screwed up bass. The vibrations will cause a non linear presentation of all of the above. An air gap of even a mm, will fix this. However, at that point you may have very minor cancellations that reduce gain.

I prefer very separated as in an inch or more.

I have personally troubleshot and fixed two clones where the problem was plate/grid lead dress. The amps were unplayable.

[Audiodog]

This has no bearing on the plate and grid lead dress. You have a huge signal on the plate and the grid is a super sensitive microphone. The grid will induct signal from the plate which is reverse phase. You get tone killing cancellation. Guaranteed.

On my LCR meter, 6" parallel leads with insulation touching produces capacitance of about 1pF/inch, calibrated to 0pF with the leads parallel and 1" apart. If the leads are crossed at right angles, it's 0.2pF. As a feedback path in a typical 12AX7 stage, a couple of pF has, in theory, a negligible effect in the audio range, a couple tenths of a dB at 20kHz.

[martin manning]

This has no bearing on the plate and grid lead dress. You have a huge signal on the plate and the grid is a super sensitive microphone. The grid will induct signal from the plate which is reverse phase. You get tone killing cancellation. Guaranteed.

On my LCR meter, 6" parallel leads with insulation touching produces capacitance of about 1pF/inch, calibrated to 0pF with the leads parallel and 1" apart. If the leads are crossed at right angles, it's 0.2pF. As a feedback path in a typical 12AX7 stage, a couple of pF has, in theory, a negligible effect in the audio range, a couple tenths of a dB at 20kHz.

To check this inductive coupling/cancellation theory out I set up a 12AX7 gain stage on a breadboard. It's a typical 100k Ra, 1k5 Rk bypassed with 22u, and 22k Rg with 1M to ground. B+ is 280V and there is 180V on the plate, so 1mA of quiescent current. Applying 3V p-p signal at the grid, I measured the gain at 50, pretty much as expected. There is about 3.5 cm (1.4") of grid and plate wire length running parallel. Watching the input and output waveforms on a scope, and moving the green (grid) and blue (plate) wires together with a chopstick I observe no effect at either 1kHz or 10kHz.

[alkuz1961]

But what about the heating wiring, about the influence of which everyone knows ?
Low voltages, but tends to penetrate the audio signal in the form of a hum if the wires are not twisted.