Info and opinions on capacitors and their application please

[vibratoking]

That's not good enough. Martin wants you to change it now!

[Reeltarded]

HAH!

[donzoid]

Yeahhhh.... I got that too. But that would seem like tweaking for the sake of it.

[martin manning]

Exhibit A...

[donzoid]

That's cheating! Alas, true...

[martin manning]

You can certainly leave it as-is, but if you are going to get under the board at some point I'd go ahead and put the Lar-Mar in. You'll have to pull the two 220k resistors going from the PI coupling caps to the bias supply, and solder some leads in their place.

[Firestorm]

Martin makes a good point. You don't want to be lifting the board half a dozen times.

[donzoid]

I am pulling the old MV to install the LarMar and I "am pretty sure" I need to remove this resistor and just send the brown wire straight to the ground bus. Correct?

The resistors I am replacing with the LarMar are replacements themselves...but instead of the expected 220k for el34 they are 150k's...I guess that point is moot since they are coming out.

While I'm at it, and waiting for the new transformer, I am thinking I should move the NFB over to the 8ohm tap as it's on the 4ohm now. I'm a bit surprised at the value of the NFB resistor on the PCB...it looks stock but isn't that a bit high at 100k? Seems like it should be 47k. That's the second box in the pic (if I have the right resistor.)

[martin manning]

That resistor needs to come out. I don't know where the brown wire is comming from, so I can't say if it should be grounded.

The basics of the Lar-Mar are this:

Remove the 150k's (that value goes with the USA variant with 6550's).

The elements of the dual pot replace the 150k's, with the CW terminals connected to the coupling caps and the CCW terminals connected to the bias supply. Twist the leads in pairs CW and CCW.

The wipers are connected to the power tube grid resistors. Twist those leads too.

The 100k FB resistor is the stock value, from the 4R tap, but of course you can try the 8R.

[donzoid]

Thanks Martin, yes I am using shielded triax belden for the signal wires, and twisting the bias voltage feed.

I have Colossal's excellent pdf from the MV thread here on TAG:
https://tubeamparchive.com/viewtopic.php?t=16140

Here is where that brown wire is going. I think its pin 1 of V1 through a .022 cap....if that helps.

[martin manning]

Bias supply is only one wire, connecting to both CCW lugs of the MV.

That brown wire was the input to the original volume pot, I think. Are you moving the MV to the back panel and restoring the preamp volume to the front?

[donzoid]

I was thinking about it...for a moment. But I hate having to reach around the back to make an adjustment at gigs, especially when the amp is sitting up on the drum platform level. I guess what I need to know is, with this config (MV in the front, and that wire to ground) are both channels tied together (as with a jumper) or not as I think I would rather jumper the two channels with this head. If not, I would be willing to move the MV to the back and put the other vol. back in. Or internally connect the wires so they are "always jumpered". Your thoughts?

Yes I saw that the two holes in the PCB were jumpered on the back side for the bias v.

[martin manning]

So originally there were two volumes on the front panel, one for each channel? One way to do it would be to replace the other original volume with a dual 1M pot and hook both channels up to that. It's possible that the unused channel would add some noise to the one in use if the pre volume was up and the PPIMV was down low, but that would put everything on the front panel, and keep the option of jumping channels. The only thing you couldn't do is vary the mix. I suspect that the set-up there now was a fixed pre volume for one channel with the 100k going to ground.

[donzoid]

Well, odd coincidence...the old MV pot is a 1 meg dual.
Yes there was two separate vol. controls for channels 1 and 2.

[martin manning]

I think it's worth a try.