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

[Colossal]

Dave - I had seen your video before, too bad there are no sound clips of it.

Brian, I have one clip of the amp here:
http://ppwatt.com/node/16045

Looks fantastic though. The microphonic thing is one of the symptoms. It behaves as if there is a microphonic tube, especially if I tap on the top of the chassis. The 470K mixing resistor is within the area of where it seems to be coming from. This amp did fire up and played. Sounded great and can't wait to get it finished. There is a bit of a run, maybe a couple inches of wire, from the 2470K resistors to the grid input. I also had a dpdt relay at that point for the "JCM800" switch, but have since removed it see if that was the issue. This might be an issue.

Thanks man. Interesting that you are having some similar microphony issues too -- I think Fredaxis ran into some issues with his as well. The mixing resistor region was definitely the source of some mojo-killing oscillation. You could tap tap tap on the cap (I added for the VVR) just after the 470k/470k pair and between the V2 grid and hear a loud "clunk clunk clunk" in reply. I added a piece of shielded cable between the cap and grid and that not only tamed the oscillation but just let loose the beast. The harmonic content really improved. I think Fredaxis may have put the mixing resistors right on the V2 grid instead of the board. I may do that next time, skip the VVR, and use an Airbrake to manage volume instead. The VVR does work great though.

I too have a switch to change the amp from parallel channels to cascade like the JCM800. It does the Def Leppard/ACDC sound very well but definitely increased the noise floor of the amp. It's pretty cool sounding but I opted to disconnect it and just use the amp as a good 'ol '68 sounding plexi. I also think the added wire lengths routing signal from V1, to the switch, to Vol 1 and out to the mixing resistors added some instability (so did away with it). Additionally, I found the silver mica bright cap across Channel 1 to be like an antenna. I had 3 mica caps on a switch (100, 250, 500) but ditched that too as it definitely made the amp more noisy and it was bright enough with 500pF in the tone stack. I may reduce that 500pF to 250pF. There is a lot of mid/highs in the amp but this may be also attributed to the guitar (a gibson style PRS).

I learned a lot from that build and one of the things I think is important with the plexi is giving the components enough room to breathe. I think I packed too much under one hood. I was warned by the Amp Elders (Mark Huss et al) about keeping it simple and while I don't want to say I learned this the hard way, I did learn the value of simplicity! To do it again (which I may do), I would voice the amp one way, no mods, ample space, and call it done

It sounds great when you get it dialed in though! I hope you get yours sorted out. Can you post a gut shot?

[Colossal]

Dave, that was a great video and one heck of a nicely built amp. Got enough switches on that puppy?

Mark

haha yes, there are enough switches! That was my first amp and I did not know what changing this value or that would do so wanted to be able to try different component values and combinations. I also added a 3-way bright switch and a cascade/parallel. Basically the other switches change cathode and coupling cap values. It goes from the '68 values to the '70s values..not much of a difference though sonically...

In the end though, I really don't use any of 'em. I learned what Those Who Have Done This Before know...voice it one way, right, the first time. But hey, it was a great learning experience. I also have a switchable rectifier and that does add some coolness. I should have made that amp more of a test bed rather than a complete head. I'm considering a Plexi 2.0 and that one will be made much more straightforward, no mods or anything.

[Structo]

I built a sort of 6V6 plexi.

It is the Hammond organ amp conversion.
It started out as a Deluxe but I didn't like the way that sounded so it morphed into more of a plexi style circuit.
2 x 6V6 cathode biased.
My amp only has one preamp tube then the PI and power tubes.

I have it dialed in pretty good now, I still have a bit of high freq hash that I'm going to try and deal with by using a high pass treble bleed.

I have my cathode bypass caps on a On-Off-On DPDT switch.

On the down position I have 10uf on both cathodes of V1.
The Off position is no bypass caps for a low gain, clean tone.
The up position is 10uf on V1a and 33uf on V1b, which gives it a deeper boost.

I still have some squeal if I dime the volume, presence and treble.
But that isn't a usable setting anyway.
100K feedback resistor.

My amp just has Bass and treble with a 10k resistor for mids.

On the divider after the PI plates, I have two 220K resistors.

What does lowering those to 100K as the Huss plexi has do?

[Colossal]

I built a sort of 6V6 plexi...morphed into more of a plexi style circuit. 2 x 6V6 cathode biased.

...snip...
On the divider after the PI plates, I have two 220K resistors. What does lowering those to 100K as the Huss plexi has do?

Tom, I found this quote from morcey2 (Matt) on ppwatt.com in the 6V6 Plexi section.

"...using 100k grid-leaks instead of 220k should actually make it distort less. 220k grid leaks are also too big for fixed bias operation. The max grid resistance for fixed is 100k. When you get it working, you'll be very happy. Trust me!"

You are running cathode biased so this may not apply but Mark's design was based on fixed bias.

[jjman]

I find the lack of a dc blocking cap between V2a and V2b to be rather disconcerting, though I am no expert. Maybe try one of those.

Placing a cap to block that DC would drastically "cool" the bias of V2b. (bad) Note that there is no plate resistor on v2b so it' plate is at a higher voltage than v2a's. The circuit encorporates the DC on that grid so you should not add such a cap.

[Structo]

I built a sort of 6V6 plexi...morphed into more of a plexi style circuit. 2 x 6V6 cathode biased.

...snip...
On the divider after the PI plates, I have two 220K resistors. What does lowering those to 100K as the Huss plexi has do?

Tom, I found this quote from morcey2 (Matt) on ppwatt.com in the 6V6 Plexi section.

"...using 100k grid-leaks instead of 220k should actually make it distort less. 220k grid leaks are also too big for fixed bias operation. The max grid resistance for fixed is 100k. When you get it working, you'll be very happy. Trust me!"

You are running cathode biased so this may not apply but Mark's design was based on fixed bias.

Ok, I have revealed some ignorance.
I didn't know those were the grid leaks.
You are not talking about the grid stoppers or swampers right?

I'm talking about the two 220K resistors that go between the PI plates with either a ground in the middle or the tap for the fixed bias.

But I have seen several fixed bias amps with 220K resistors in that spot.
My D'Lite for one and the infamous Dumble ODS for another.

So I'm not sure I agree that 200K is too big for that spot.

[Colossal]

On the divider after the PI plates, I have two 220K resistors. What does lowering those to 100K as the Huss plexi has do?

Tom, I found this quote from morcey2 (Matt) on ppwatt.com in the 6V6 Plexi section.

"...using 100k grid-leaks instead of 220k should actually make it distort less. 220k grid leaks are also too big for fixed bias operation. The max grid resistance for fixed is 100k. When you get it working, you'll be very happy. Trust me!"

You are running cathode biased so this may not apply but Mark's design was based on fixed bias.

Ok, I have revealed some ignorance.
I didn't know those were the grid leaks.
You are not talking about the grid stoppers or swampers right?

I'm talking about the two 220K resistors that go between the PI plates with either a ground in the middle or the tap for the fixed bias.

But I have seen several fixed bias amps with 220K resistors in that spot.
My D'Lite for one and the infamous Dumble ODS for another.

So I'm not sure I agree that 200K is too big for that spot.

Hi Tom, yep, I was referring to the 220k/220k pair spanning the PI plates fed the bias voltage (or going to ground in cathode bias). I too cop the Ignorance Plea though and you're right, numerous other amps have a 220k pair under fixed bias (like the Deluxe Reverb with 6V6s). I just remember reading that same question in a thread a long time ago. The grid stoppers on the 6V6s in the plexi are 5k6 where they are 1k5 on the Deluxe Reverb though.

[Structo]

Tom, I found this quote from morcey2 (Matt) on ppwatt.com in the 6V6 Plexi section.

"...using 100k grid-leaks instead of 220k should actually make it distort less. 220k grid leaks are also too big for fixed bias operation. The max grid resistance for fixed is 100k. When you get it working, you'll be very happy. Trust me!"

You are running cathode biased so this may not apply but Mark's design was based on fixed bias.

Ok, I have revealed some ignorance.
I didn't know those were the grid leaks.
You are not talking about the grid stoppers or swampers right?

I'm talking about the two 220K resistors that go between the PI plates with either a ground in the middle or the tap for the fixed bias.

But I have seen several fixed bias amps with 220K resistors in that spot.
My D'Lite for one and the infamous Dumble ODS for another.

So I'm not sure I agree that 200K is too big for that spot.

Hi Tom, yep, I was referring to the 220k/220k pair spanning the PI plates fed the bias voltage (or going to ground in cathode bias). I too cop the Ignorance Plea though and you're right, numerous other amps have a 220k pair under fixed bias (like the Deluxe Reverb with 6V6s). I just remember reading that same question in a thread a long time ago. The grid stoppers on the 6V6s in the plexi are 5k6 where they are 1k5 on the Deluxe Reverb though.

I wonder what change it would bring to my amp to increase the grid stoppers on the 6V6's.

I'll have to check Aiken's site about that.

[Structo]

Yeah I knew Randall had addressed this:

Most output stages use relatively small grid resistors, such as the 1.5K seen on the grids of 6L6 tubes in most Fenders, and the 5.6K seen on the grids of EL34 tubes in most Marshalls. In general, the grid resistor at the grid of the power tubes can be as high as 56K to 100K before any noticeable loss of high frequencies occurs. Higher values can help in reducing "blocking" distortion as noted above, and can also take some of the "edge" off of an overly brittle sounding output stage. If the resistor value is made too low, it may not be enough to prevent parasitic oscillations, and the amplifier may exhibit instability in the higher frequency range. This may or may not be audible. Symptoms of oscillations include: high-pitched "squeal", glowing plates at "safe" bias currents, harsh treble response, lack of power, undesirable overtones, and unusual frequency response which makes the amp sound funny. Note that power tubes have a specification for maximum resistance that can be in series with the grid terminal before the tube becomes unstable due to grid current. The maximum allowable resistance is larger in cathode biased circuits than it is in fixed bias circuits because the cathode bias provides some "self-limiting" protection against bias runaway. The total resistance is the sum of the series grid resistor and the bias feed or "grid-to-ground" resistances, so if the max spec is 300K, for example, and there is a 220K bias feed resistor, the largest grid resistance that can safely be used is 80K. Of course, in practice, tubes should not be run that close to the edge of their specifications, to insure reliability.

[Stanz]

Well, I have it sorted out, but not sure that I got the exact cause as I have been trying numerous solutions. Int he end, I took out the "JCM800" switch, which cascaded the first two stages instead of mixing them, (it has enough gain already), and moved the 470K mixing resistors to right on the tube socket. It still has some hiss, but not the scratchy, roaring static it had before. The microphonic characteristic is gone.

I had three sets of 6v6 tubes I was using to test it and the Electro Harmonix pair are toast. 465v on the plates, wow. The JJ 6v6s are working great and a pair of RCA 6v6GT seemed to be able to handle the high voltages, although I did bias them fairly cold at around 14ma. Lots of gain, that is for sure. With both channels on, 9 0'clock on both volumes is fairly distorted. Can't say I am in love with the cleans though. It currently has an OT with a 5.2K primary. I have one with a 6.6k on the way. Now I can put in the master and power scaling. This is for a friend who has a touchy landlord.

I think you nailed the main issue Dave, thanks.

[Colossal]

Well, I have it sorted out, but not sure that I got the exact cause as I have been trying numerous solutions. Int he end, I took out the "JCM800" switch, which cascaded the first two stages instead of mixing them, (it has enough gain already), and moved the 470K mixing resistors to right on the tube socket. It still has some hiss, but not the scratchy, roaring static it had before. The microphonic characteristic is gone.

I had three sets of 6v6 tubes I was using to test it and the Electro Harmonix pair are toast. 465v on the plates, wow. The JJ 6v6s are working great and a pair of RCA 6v6GT seemed to be able to handle the high voltages, although I did bias them fairly cold at around 14ma. Lots of gain, that is for sure. With both channels on, 9 0'clock on both volumes is fairly distorted. Can't say I am in love with the cleans though. It currently has an OT with a 5.2K primary. I have one with a 6.6k on the way. Now I can put in the master and power scaling. This is for a friend who has a touchy landlord.

I think you nailed the main issue Dave, thanks.

Stanz, glad you got it sorted out. For what it's worth, I'm using a Marshall 20w ToneClone OT with a 7.3k primary. You are right in that there isn't a lot of clean headroom in the amp although by backing off on the guitar volume I can get mine to clean up quite nicely even with both Vols up pretty high. The clean is warm and articulate and can even be very jazzy sounding with the neck pickup (I'm using humbuckers), but if you dig in, there is always some bite and grit just underneath. Opening the volume all the way up and you get very warm and loud distortion. I think this is the nature of this amp. I have 440V on the plates and am using NOS Jan/CBS 6V6s which sound pretty nice. I've biased them about 25mA which is right around 70% dissipation. Sounds like your PT is bigger. A few guys have said the best sounding 6V6 plexis have been run with voltage like yours. The JJs seems to sound great and hold up well under these conditions. I used a pair for testing but later switched to the NOSs. Lastly, the amp will easily achieve singing feedback and is very punchy through a 2x12 with Greenbacks. Very classic sounding. You can get a very plausible Kashmir or Van Halen tone out of it.

Maybe you can post some clips when you get further along?

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

[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?

[dobbhill]

I'm with Tom: To increase your clean headroom and improve clean tone, you may want to "heat" up your bias a bit. You weren't clear where you had your JJ's 6V6's biased. I run mine at 80% which at 465v on the plates is about 21-23ma.
Just my opinion, based on my limited experience with running JJ's in my JTM45.
Hope this helps.
D

[Stanz]

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

[Structo]

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.