6v6 plexi, close, but no cigar (OK, now I can light one up)

[jjman]

OK, take it easy on me because I am still learning this stuff.

Why would you use the 1 meg pots there instead of a dual 100K?

It seems when I have seen that type of PPMV that the pots are the same value as one of the fixed resistors.

Like if there were two 220K you would use a 250K dual or if they are 100K you use a 100K dual pot.

Also should the dual gang be audio or linear taper?

For instance, this 6V6 Plexi uses a 100K dual gang pot.

The "bootstrapper" is on the inputs to the phase inverter tube and the normal resistors that would be there are 1meg each. The plexi MV posted is on the output tubes and also correctly replaces resistor values that would normally be present instead of the pot(s.)

[Structo]

I'm an idiot!

For some reason I didn't look at the picture you posted carefully enough.

I just glanced at it and assumed that it was post inverter.......

So how well does this bootstrap master work?
As opposed to the standard post inverter master?

I have a master on my 6v6 plexi that I borrowed from my D'Lite, single
1 meg pot just as a variable resistor feeding the PI.

Is this bootstrap master like what Kevin O'Connor talks about?
Doesn't work well at all.

I wired it like this:

[Colossal]

So how well does this bootstrap master work?
As opposed to the standard post inverter master?

I have a master on my 6v6 plexi that I borrowed from my D'Lite, single
1 meg pot just as a variable resistor feeding the PI.

Tom, I used KF's Master Volume Type 1 on my Plexi. I also have a VVR in the amp so wanted to include a PPIMV just to mess around with it in conjunction with the VVR. I normally just keep the MV maxed and use the VVR to control sound pressure. The MV is useful though when the VVR is turned down to about 9 o'clock for bedroom/after hours playing. I use the MV at 9 at this setting and it keeps the distortion sounding pretty decent.

Anyway, I'm interested in the D-style Master Volume that you did. How do you like it?? Do you find that it keeps the distortion character of the amp but just lowers the volume or does it change the character since there is less signal going into the PI.

I'm trying to contrast the various MVs out there and their relative value vs. just building the amp without a MV and VVR and just using an Airbrake to control sound pressure.

Thanks,
Dave

[gldtp99]

Last yr i built a 6V6x2 2204-type head using the mhuss 6V6 Plexi schematic as a general base---- Heyboer 18 watter PT/OT, AES 125C1A choke, Sozo caps (in initial build ---some swapped out later for different values----used .015uF metalized films in some .022uF slots), JJ 6V6S-- fixed bias, Blackburn Mullard ecc83 in V1, GE 12ax7 in V2, RFT ecc83 in PI.
Modded 2204 preamp and a variable NFB pot----no big problems but i did some component tweaking and tube swapping along the way.
It sounds like this: Click on "Mule 25 JCM 800"-- http://www.soundclick.com/bands/default ... dID=741589

[Structo]

Colossal,
No it doesn't really work that well.

I don't know if it is the taper of my pot or what but it isn't a gradual turn up on it.

I mean it's ok just not that great.

I ordered some stuff from Hoffman today and I'm going to try that bootstrap MV.

[Colossal]

Colossal,
No it doesn't really work that well.

I don't know if it is the taper of my pot or what but it isn't a gradual turn up on it.

I mean it's ok just not that great.

I ordered some stuff from Hoffman today and I'm going to try that bootstrap MV.

Cool, thanks Tom, I was wondering what you thought of the D-style MV in that position in your Plexi. Reason I ask is that I'm contemplating rebuilding my 6V6 Plexi (v2.0) without a VVR or MV of any kind and just using an Airbrake to control sound pressure. I did a lot to complicate my circuit in this build and while it works well, there's a lot of functionality I'd like to strip out to make it very simple the next time around. The VVR works very well to take the (painful) edge off of the volume in close quarters so its utility is excellent. But I'm trying to understand if I could get the same result, quality wise, just running the amp into an Airbrake with no VVR or MV setup. Also, FWIW, there is a video of Gregor Hilden running his Bludotone ODS on 1 on the MV (just barely opened, not 1 o'clock on the knob) and it sounds awesome. So that's why I was wondering if the MV in the position that you have it (the D-style location) is good or not in a Plexi. I suppose I could just try it and see. A log/audio pot would give more span before it just gets loud as opposed to a linear pot.

Dave

[Structo]

Yes the JJ 6V6 are reportedly 6L6 in disguise.

Stanz, I don't believe you are even biased at 50% at that setting.

I think you could increase your headroom by increasing the bias current.

How about 20 ma?

The bias chart I have doesn't even go beyond 400v on the plates!!

You may want to consider dropping that voltage on your power tubes.

What kind of voltage are you getting on the preamp tubes?

You are right, I was more concerned about just getting it up and running. I think they high plate voltages had me spooked. As for the JJ 6v6 being a 6L6 in disguise, that may be closer to the truth, at least if size is your main determining factor. They are so much bigger than any other 6v6 tubes I have seen. The just physically look like a tube that can handle more than its share. The voltages on the 12ax7 are pretty typical, below 200v on the first stage, about 240v on the cathode follower side of the second stage.

What kind of master volume are you going to put in?

Lately I have been using a bootstrapped master which uses a dual ganged 1MA pot in place of the two 1M resistors before the PI. Usually I power scale only the power tubes, but I want to try the power tubes and PI. In theory, since the MV is right before the PI, hopefully it keeps most of its gain structure through out the amp.



Brian

Brian, I'm getting ready to wire a bootstrapped MV into my amp.

On the schematic you posted it has the pots labeled with X and 0.

Do you know what those designate?

I haven't seen that type of labeling before.

[Stanz]

Brian, I'm getting ready to wire a bootstrapped MV into my amp.

On the schematic you posted it has the pots labeled with X and 0.

Do you know what those designate?

I haven't seen that type of labeling before.

0 is the lug that the wiper touches if you have the pot tuned to zero. I do not know if this is what it meant to designate, but that is how I remember it. So for typical volume pot type set ups, that usually means the lug that will go to ground.

In trying to figure out what was going here I see two things happening, you still have 1M from the 0 lug to the X lug, same as when the 1M resistor was there, plus, you are adding resistance before the grid. In fact, the 470R is the only thing between both grids and both cathodes.

If the pot is at max, it is exactly the same as with the fixed resistors.

[Structo]

Thanks.
Have you tried this MV yet?

[Structo]

Here is another MV that is Post PI using the dual gang 1M pot.