6v6 plexi-ish build questions

[donzoid]

Thanks. If I end up doing that tonight, I'll put in the resistor too, as I can use all the hum reduction possible.

[donzoid]

OK....I scrounged around for a good bit and found a 40uF but no 22uF's. So, what I did was swap out the 1st PS filter cap with this 40uF, then, I'll use the 22uF from that position for V1 and V2 using a 2w 1.8K I rounded up.

I am just about out of juice tonight...for a lot of "other" reasons; so tomorrow I will make the rest of the changes. I returned the 100k/100k PI split and the PI plate cap I'll do tomorrow.

I don't see a need to run a 5y3 in this, I didn't think the PT once loaded would drop that far but it's a pleasant surprise since "if I feel like I want" to stiffen up the response I can just remove the 100R sag resistor. Plus I think this will help lower my noise floor a bit more.
So, I will end up with 40/40/20/20/22 which is "close enough for rock n roll"yeah?

Then if this thing still sounds like its got a blanky over it, maybe I'll have to try a different approach on the tone stack.

I really, really appreciate everyone's input on this. It's been a bit "more educational" than I expected.

[JazzGuitarGimp]

I'm really enjoying this thread - lots of good information here!

[matt h]

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[joeboo88]

Hi: i have been following this thread and also leaning alot about this.
donzoid
just to follow along which layout/schematic are you using, i read the whole topic, but couldnt figure out what you are using
I hope you get this up and working the way you wanted

[donzoid]

LOL...I think I had a mis-labeled cap on the PS string. Yes it was 22/40/20/20 (my bad, should have physically looked, like NOW) and now after hunting up the extra axial (which tucked in there quite nicely) it is 40/40/20/20/22.

Yes Matt I know the the choke spec its from Weber its 7H, 200mA I think they advertised it as 10H but actual measurement is 7. It was 200R resistance. It isn't even thinking about getting hot if that's your concern. Physically its the same (or close) in size to the Hammond 158L (though different spec).

I still have to plumb the last cap in there and put on that 30pF balance cap then we'll see if I buggered it.

What...me??? Off the path?

[donzoid]

updated the power string - still not looking right in v2 but I'll get that figured.

EDIT: Already found a mistake, tail resistor on PI is 15K not 22k.

Matt, that cap goes across plates of PI (pins 1 and 6) correct?

[Colossal]

Don and Matt H:

Hang on a minute, this is confusing. Don, the schematic you just posted above shows the 18W Normal channel mixing into TMB channel. This similar to Mark Huss' 6V6 Plexi. I thought this this was how your amp was originally wired; like Mark's amp. However, we later discerned (or thought we did) that the Dockery 18W TMB schematic I found, modified, and posted was actually what you had. This arrangement would have the second PI input being used for the 18W Normal channel input.

EDIT: Already found a mistake, tail resistor on PI is 15K not 22k.

Don, I think you are confused here. I originally suggested changing the PI values from the 18W arrangement to the more traditional Plexi values of 82k/100k plates and hotter biased Rk at 470R and tail at 15k to produce higher drive for the 6V6s and add some soft clipping at the PI, like a Plexi. You said you wanted more output and the amp was set up with EL84s in mind which require much less signal swing to drive, and made the changes accordingly. I also thought that the amp was wired with the Normal channel mixing into the TMB channel with a single input into the PI. So this is why I suggested the a more Plexified PI.

Don's last voltage chart with the Plexi PI values suggest balanced conditions at idle and Don confirmed that the gross current imbalance followed the tube swap, indicating a bad tube. So anyway, I'm wondering which preamp configuration is in play here? A modified Huss Plexi (as shown above) or the Dockery 18W/TMB preamp?

Matt, I am in agreement that added filtering and a dropping resistor is needed to decouple all those shared preamp nodes. Fender does this and I have always thought it's a bad idea and bad design. But it worked for Leo I guess.

[sluckey]

EDIT: Already found a mistake, tail resistor on PI is 15K not 22k.

That tail resistor also must have a path to ground. Probably just a drawing error?

[donzoid]

Thanks Dave, I am constantly confused, but it's all good. I'm glad you made comments here, as I am not sure which divergence to take.



Yes sluckey the tail resistor has a path to ground. learning to use this software, and make schemo's at the same time as building this thing is a challenge for me for sure (and I work with unix all day long). There are a lot of mistakes and I think Dave's is probably way closer on the pre and PI part (or exactly right, and mine way off).

So, Dave, where would it mix into the PI - the normal channel is connected to pin2 and the TMB is on pin7 so yeah, the whole pre/PI part of the drawing I made needs to be re-done. I don't "get" where it mixes, unless I jumper the channels together? But that's just me bein' clueless/cornfused.

I can easily put the offset PI feeds back in, this thing has become a test bed of sorts now. I wasn't hearing oscillation with it, until the very top of the vol/mv range (mv all the way up, vol control last 1.5 to 2 numbers on a ten scale). Tonally it's not really good enough yet to tell whether I liked it or not. But if we are saying the offset PI values are more plexi vs. the balanced values being more like a JCM800, then I would lean toward plexi. LOL, I had an 800 and got rid of it - sounded way too brittle for me compared to the JMP's. Three days ago I was ready to rip out the entire turret and start over - the next build won't be this design, probably the one Fowler suggested where both channels are always jumpered together (like xtian's recent Monkeymatic build).

I do have a question about adding the filter cap. Right now the last 20uF filter is feeding both of V1, V2. By adding the new filter cap, am I dedicating that to V1, leaving the PI and V2 with their own separate 20uF filters? Just want to make sure that's the intent - one filter cap per valve. Seems sensible, but we've already established my cornfusion.

[donzoid]

Oh, and Leo was known for being notoriously cheap. I am too, but not when it comes to a few bucks and some time for tone's sake.

[donzoid]

since its buried back a few pages, I'm re-posting Dave's far more accurate pre diagram.

[matt h]

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[Colossal]

Don,

Rather than answer all of your questions and add more confusion to those trying to help out and those following the thread, I would just suggest building this (see attached).

Notice that it has the Normal channel mixed into the bright channel via 470k resistors (with the added peaker over the bright channel's mixing resistor), a single tone stack, a single input into the phase inverter, the Plexi phase inverter values, the PI fizz cap, and FIVE B+ filter nodes (to reflect Matt's suggestion as well as the standard Plexi filtering scheme). I've also shown an optional NFB loop and presence control, should you want to add it.

Honestly, I would build this and be done with it. It is well tested and sounds great.

[matt h]

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