Mesa Boogie MK 1 wire from output to input co-ax?

[Mark]

I thought I would ask the brains trust on this one. As shown in the pictures, there is a 220K resistor from pin 3 of one of the 6L6’s which then goes to a wire which has a single turn around the co-ax cables. In the first picture it’s a blue wire (off valve three) and in the third picture it’s a red wire. Looking online it seems to be fairly common in MK 1 amps. Though I don’t believe it’s in the reissue MK 1amps.

I get it that it’s a capacitive couple feedback network, but it’s a single wrap around the earth wire in the co-ax. I would think this would have very little effect on the signal.

I had heard decades ago that this was to make the amp stable though there are amps without it. I don’t recall my Boogie MK 1 having this wire in it.

ezomkn3tzx3bwkbdtcag.jpg

IMG_0059.jpg

13948EF2-7660-4292-A07A-B2CCB24A8755.png

[LOUDthud]

If that isn't strange enough, you slide the wire back and forth along the input coax to tune the feedback.

[Stevem]

I can't see this wire having much off a effect on these amps oscillation issues that some had since it's rapped around a sheilded cable.

[sluckey]

I can't see this wire having much off a effect on these amps oscillation issues that some had since it's rapped around a sheilded cable.

The shield is connected to ground, so just consider this equivalent to putting a small cap between the output tube and ground.

[Mark]

I can't see this wire having much off a effect on these amps oscillation issues that some had since it's rapped around a sheilded cable.

Thanks for your reply. This is what makes me wonder, why they chose the coax cable for an earth, when the cathode is at earth and small caps are abundant.

[maxkracht]

This is what makes me wonder, why they chose the coax cable for an earth, when the cathode is at earth and small caps are abundant.

Style points. No one is impressed by a cap to ground, plus it's cheap.

[Mark]

I can't see this wire having much off a effect on these amps oscillation issues that some had since it's rapped around a sheilded cable.

The shield is connected to ground, so just consider this equivalent to putting a small cap between the output tube and ground.

Thanks for your reply. As stated previously, there are many earth connections in the output stage, I wonder why the earth connection of the co-ax was used.