Can’t explain…. I have always heard a slight fattening by paying attention to that area. In a couple cases the amp was so bad that the owners shipped to me or drove it here. The fix was the lead dress both times. Not subtle nor needing a double blind.
Maybe the wire brand and type matters? I have tweaked dozens and dozens of these amps and separating out of phase stages matters in my experience.
Audiodog wrote: ↑Mon May 10, 2021 4:34 amThe issue was the increased bass that caused more of the inductive feedback.
Ok, that makes more sense, i.e. the problem got worse when the bass was turned up. I don't think we know what the root cause is, though, and I will stand by my claim that neither inductive nor capacitive coupling between these two leads is enough to produce the result you are describing.
Audiodog wrote: ↑Mon May 10, 2021 4:34 amPlease try moving your OD1 wires while playing loudly and see if things change.
I had someone come over and play while I poked the grid lead around with a chopstick. I tried both lifting the grid lead and pressing it against the plate lead, and neither of us could hear anything happening. This was done at a variety of control settings, including Drive level, PAB on or off, Mid boost on or off, Bright on or off, Bass up or down, etc. My partner couldn't see what I was doing, and I didn't tell him what I was looking for, other than to listen for some change in the sound.
Just before this topic came up I was playing around with that OD grid wire. I hear a big difference when the wire is elevated all the way from the plate wire and I hear slight differences when it is slightly moved away. When the wire is touching, the OD is shrill, bright, and buzzy sounding. All the way away and the OD is sweeter and nicer on the top end and the mids and bass fill out a bit. I also must have been suffering from the bass control issue. My bass was lose and boomy and the control didn't do much when I rotated it when the wires were touching. After moving the wire, my bass was tighter and clearer and the control works as it should. I will say however, I did find photos of several Dumbles (ie Ry Cooder Borderland) where the grid and plate wires are either touching or close.
CW
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Last edited by Charlie Wilson on Tue May 11, 2021 3:18 am, edited 2 times in total.
Exactly! Touching is super noticeable. Not subtle. Even a tiny gap fixes some of it. Then as you note, you get a fattening and sweetening with more spread.
Charlie Wilson wrote: ↑Tue May 11, 2021 2:51 am
Just before this topic came up I was playing around with that OD grid wire. I hear a big difference when the wire is elevated all the way from the plate wire and I hear slight differences when it is slightly moved away. When the wire is touching, the OD is shrill, bright, and buzzy sounding. All the way away and the OD is sweeter and nicer on the top end and the mids and bass fill out a bit. I also must have been suffering from the bass control issue. My bass was lose and boomy and the control didn't do much when I rotated it when the wires were touching. After moving the wire, my bass was tighter and clearer and the control works as it should. I will say however, I did find photos of several Dumbles (ie Ry Cooder Borderland) where the grid and plate wires are either touching or close.
CW
Scott, I think the trimmer value and plate resistor values may play into how Dumble dressed that wire. I think with an amp like #94 with a 1 meg trimmer and high plates, having those wires closer may have helped tighten up the low end in the OD. Borderland was a 100k plate with a 100k trimmer so I don't know what he was going for there. I do know the amp ended up in Dumbles driveway so..
CW
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Neither of those pics you posted have the wires touching, Touching bad. The gap can be used to tune response, as noted. Touching can cause all kinds of mayhem at high volume.
At this point, I would like to clarify something for my own boneheadedness. Signal goes in grid wire to tube grid allowing more(or less) signal to flow throw plate of tube. Grid wire and plate wire are in phase with each other. Right?
CW
Charlie Wilson wrote: ↑Tue May 11, 2021 5:50 am
At this point, I would like to clarify something for my own boneheadedness. Signal goes in grid wire to tube grid allowing more(or less) signal to flow throw plate of tube. Grid wire and plate wire are in phase with each other. Right?
CW
Rhetoric? Grid and plate wire are OUT of phase with each other.
When grid is making a top half cycle, the plate is doing a bottom half cycle and vice versa
OK the phase gets flipped at the plate resistor but isn't the current coming from the tube plate to the plate resistor going in the same direction as the current coming into the grid. The reason I ask is in playing around with this grid wire, it seems more like a positive feedback interaction than a negative one. And no not rhetoric, a question.
CW
Grid is positive charged when transporting a signal. This allows the electron flow from cathode to grid and then electrons get attracted to the plate, as the current increases from cathode to plate, the voltage across the plate resistor will drop. So they are out of phase. Grid and cathode are in phase.
So the grid is controlling the electron flow going from the cathode to plate.
It's been Eons since I revisited this so went back over some older notes. Again!.. I hope this helps!
Grid wires are considered sensitive input wires
Plate wires are both emitters and inputs.
1. Wires are sensitive in proportion to how high the impedance's are that they connect. A wire's impedance may be thought of as the larger of the wire's resistance itself, or the smallest impedance to which it connects at either of its ends. So a wire to ground may be thought of as only the wire resistance, a wire connecting a plate to a grid is the plate's impedance. An open input wire is the input grid resistor's impedance. A wire to an otherwise open grid is an very high impedance indeed.
2. Wires are sensitive to disturbance in proportion to how much gain is available between the grid wire and the output of the amp. For example, the input (pre amp tube) grids are hypersensitive, the output tube grids are only mildly sensitive. Screen grids are mildly sensitive, plates only slightly sensitive.
Crosstalk is picked up three ways: capacitively, inductively, and by shared resistance. Capactive pickup is maximized by wires close together and parallel, with high voltages on one wire and high sensitivity on the other (plate and grid wires). It is minimized by wires far apart, nonparallel or crossing at right angles, and by electrically grounded shielding between the wires.
Inductive pickup is maximized by large loops of wire sharing a lot of common loop area, one of which carries high currents and high frequencies and one of which is sensitive. It is minimized by making current loops small (twisted pair!), by making loops NOT share loop area, and by distance. Notice that shielding signal wires in a loop will not shield the wires from inductive pickup if the loop shares a lot of common area with a high current interfering loop. Soft iron between the two loops helps, but is not as good as minimal loop area and not sharing loop area.
Both capacitive and inductive pickup are inverse square law emitters. That is, if you double the distance, the crosstalk goes down by a factor of four. This is why there is such a premium on getting sensitive wires (Grid) far away from emitting wires (Plates)
Resistive crosstalk happens whenever two signal currents share the same wire. The resistance of the wire converts both currents to voltages, and if one of the signals happens to be an input signal, then the other signal gets inserted into the input.
Those are the basics.
Tony
Last edited by talbany on Tue May 11, 2021 11:20 am, edited 1 time in total.
" The psychics on my bench is the same as Dumble'"
I was wondering if PVC vs. Teflon insulated wire could contribute to the effect. Since PVC is usually much thicker than Teflon (relatively), it would by nature provide more separation even if the wires were touching. When using Teflon wires that are touching they are physically closer.
I played with this a bit more last night. What I am hearing is the closer the wires, the more bass and mids are attenuated until they are touching and then its shrill top end. With my amp, if I get a sound I like at a low volume with a singing top end and then turn the amp up the OD gets hard and bright and looses the sing on the top end. If I move the wire away from each other, the OD sounds a bit dark and dull at a low volume but as I turn the amp up it fattens ups the sound and the top end sing comes in. This is with a rather bright Strat. I believe Dumble flew the wire as he did in #124 to fatten up the sound of a Strat which is what the amp was tweeked for. I will note that I have the same wire in my amp as Martin. There have been many "My amp OD is too bright" posts over the years and I can't help but wonder how many amps could have been made to sound better by moving a wire instead of HF tapers, higher capacitance coax, etc.
CW
The most likely mechanism for coupling into the grid is capacitive, I think, and then you have to ask how could a pF or two plate-to-grid make any difference. That would potentially roll off high frequencies through NFB, but there is a snubber on the socket plate-to-cathode which has a much larger effect.
Last edited by martin manning on Tue May 11, 2021 6:26 pm, edited 2 times in total.
