The LaMar (LarMar) PPIMV in an Express

[grindomatic]

Rooster,
I'll do my best to answer your questions, from my humble perspective.
First I must apologize to all in advance, -that this home brew is not *exactly* one of the trainwrecks express schematics Ive seen here - but pretty close, I was kind of tied to the existing PT, chassis & skinny little 6X4 rectifier tube (sags SO sweet!) that all came in a homebrew HiFi that I picked on a low budget & gutted to the sockets. It's kind of a test bed, that looks a lot like a bad bowl of spaghetti as of late...

The power stage is almost exactly like Trainwreck Express Version A3, except this is cathode bias with a 150k cathode resistor & 10uf (close to what the bias switch offers on the A5h) 100k on each screen - with +360 on the plate (from tube rectifier) - about half way between the A4 & A5, using a LTP phase inverter with a NFB loop w 100K resistor. If I decide to add a switchable SS rectifier, I may increase the screen resistors, if the PTs plates start glowing.
My preamp stage can go from clean to pushing the PI pretty hard, the local Marshall fanatics love to crank it. I think early Van Halen is a good comparison - not as much "super sonic roar" as a marshall, & more brown.

Ok, for question #1

NFB does clean up the odd order harmonics that are fairly strong in a class AB/AB2 beam pentode. A VOX (at least AC30) is running class A, which has way less odd order harmonics, and EL84 & EL34 are pretty close to a tetrode, and NOT a beam pentode - so they also have less odd ordered harmonics. The other great thing the NFB does is limit the gain based on the amount of NFB used, rather than relying on the particular tubes gain, and its decline as it ages(until its loss gain, over time, becomes less than the NFB loop). Triodes have no odd order harmonics, but are very innefficient, and most people (including me) don't like them for a guitar power amp - but for HiFi, they love it - with no NFB..

question #2

Clean, overdrive & distortion are all subjective & of course depends on what kind of pick ups, how hard we hit it, etc. What I was calling clean was pretty much like the sound of The Eagles "Peaceful Easy Feeling's" studio lead solo, maybe like the mellow part of stairway to heaven..
- and in this mode is when I experience no drop in volume for the 1st 40% of turning down the LARMAR MV - and at that point the presence(which is really a NFB higher freq overide) control doesn't do any thing, and it then it sounds kind of grainy - unless I increase the amount of NFB (which, of course reduces the volume..), by reducing (shunting around) the 100k NF resistor, coming from the speaker side of the OT...

queston #3

what I am doing is a LARMAR MV adapted to cathode bias, by shunting to ground, because there are no bias resistors - also my grid stoppers are around 750k, and its sweep is nice.

Thank You

Craig

- Really trying to find a 3 gang pot to do this all in one knob - hate to many knobs...

[rooster]

Craig - Nice to meet you, BTW, and you seem like a sincere person. And understand that I asked specific questions because basically you are taking this post off subject. We are talking about the Lamar PPIMV in an Express.

But we're all grownups here.

Your cathode amp sounds cool, too, although I am a bit surprised to see that you are using negative feedback with it. The 18/20 watter Marshall does not, of course. So I am thinking you like NF and a pretty clean sounding amp. Sure, whatever you want to do here, its your amp.

This said, and aware that I am now off subject, too, I am still more curious to see you talk often about the alteration of the harmonic content of the audio as it interacts with negative feedback circuit. I don't see this being that case. I hate to quote or link anybody as we talk, but I do wonder if you have ever taken a look at the Aiken's info on the subject of NF? If you have, you will see that nowhere does he comment on a 2nd 3rd - or any - harmonic reduction/generation occuring because of the NF. So where are you getting this?

From my understanding, the NF will dampen certain freqs as it returns these freqs to the PI polarity reversed, but it will not create or change the frequncy signature of the audio. The NF process typically then 'tightens' up the signal when NF is added, but I don't see that it changes the harmonic content of the audio. Do you understand my thought here? OK, so fill me in on your data source if you don't mind.

The reason I am asking is because the Express is a VERY harmonically ladened circuit - and when someone starts talking about altering the harmonic content of such a circuit.... Well, them's fighting words.

[grindomatic]

Thank you Rooster, I don't have time right now to really dig in, but I think I got the "lowering odd order" with NFB, came from O'Conners "Ultimate Tone" section on NFB - I surely could be wrong, all try to verify ASAP. THe NFB with cathode bias(At least in this rig), really has a nice effect on the cleans & very little difference (IMHO) when it is cranked hard, but I am going to take a closer listen both ways when my ears heal up a bit over the next few days.

The power stage in the one I have built is almost identical (except for the cathode bias) to those in the 1st 9 "Trainwreck Express" schematics - which all have NFB ( & fixed bias )

The next 4 schematics that don't have NFB, & use a concertina type PI, called "Blues Express", are cathode bias - mine is really right in between, these with a LTP PI & cathode Bias - so I thought it was ok to post here & I could/ maybe should also try fixed bias - which also would have me end up using the same exact resistance as in the Trainwreck Express schematics.

This one cranked does sound like the clips of the Trainwreck Express's Ive listened too, but haven't had a REAL one (or perfect clone) in front of me (yet)

BTW you're calling the LARMAR MV, - "LAMAR MV", (is that intentional?) I think LARMAR stands for Larry, and another guys name I can't remember, maybe Mark or Marvin?

Thanks again
Craig

[Roe]

yeah, its larry (of larry amps) and mark (rockstah). larry designed this MV and mark drew a layout and did some testing

[grindomatic]

Thank You Rooster,

I see what you mean - so far haven't seen any cathode bias guitar amps with NF(at least here in Trainwreckville) ... So maybe it is helping mine cause I am slamin the PI to hard from the pre.. and the NF lowering the overall input signal to the PI is stopping some, not so obvious (to my ears) clipping...

I think my brain deduced the part I said about "NFB reducing odd order harmonics " from one little paragraph I may have over weighed, from Kevin O'Connors "The Ultimate Tone" at the end of page 6-5 it read (talking about push pull power amps)...

When either tube cuts off, the clipped output of that tube represents a seriously distorted signal. Fourier analysis suggests significant odd-oder distortion, which will sound harsh. Some of these artifacts can be reduced with feedback around the power amplifier. They can also be minimized by never driving the output grids positive.

well, now I really am tempted to set up a negative rail & try fixed bias - but after I determine if I am slamin the PI & if so, how it sounds adjusted, and with no NFB. I also GRATEFULLY have read through Aiken's site (& tried to digest) a few months ago, and constantly refer to it more & more, now often before I get myself in a pickle (causing me to look a bit like my current avatar) .

Thank you for the education., and thought provocation.

Craig

[raiken]

Negative feedback will reduce distortion in the stage that the feedback is wrapped around. Basically, it tries to make the output "look" like the input, so if any harmonic distortion (odd or even) is added by the stage, the feedback will try to remove it.

Negative voltage feedback does the following things:

(1) Reduces distortion created in the stage(s) feedback is applied around. Note that it will not reduce distortion added in previous stages.

(2) Flattens frequency response (removes peaks and dips and extends the bandwidth on both high and low end, resulting in a flatter overall response).

(3) Reduces output impedance - this causes less interaction with the peaks and dips in the speaker's impedance with respect to frequency, causing the amp to react less to the speaker.

Note that feedback only works to do these things if the stage is not clipping and there is enough excess gain for it to work with. Once the stage hits clipping, all bets are off. Basically, feedback trades off excess gain for flatter response and less distortion and a more "ideal" output impedance. This may or may not be a good thing, depending on your perspective and tonal tastes. In general, too much feedback can make an amp sound "solid-state", while too little can make an amp "boomy" or "loose" feeling, especially if the output stage is improperly designed.


Randall Aiken

[grindomatic]

What Randall posted above (& I think is the same as what I read on his website) about NFB sounds true to my ears - and only affecting the distortion made by what is inside a particular loop. NFB used for a power amp loop, (in a moderately push hard amp, with at least half the distortion coming from the pre) will reduce - in the final mix - mainly odd order, since triodes (as in guitar pres) only make even order & they are not in the power stage NFB loop, so all their (even order) distortion will come through, the pentodes / power stage create the odd order, as well as even - and that pentode power stage "blend" of harsh & smooth distortion will be reduced simultaneously by it's NFB loop (unless pushed to clipping, which then blows past the NFB)-, and then combine with the preamp distortion. Maybe that is why it sounds like mostly odd ordered (harsh sounding) harmonics have been reduced to my ears, also love the tighter bass - all of these observations are with the amp in the clean to mild over driven power amp range. When I really push the amp, as Randall says - in relation to NFB "All bets are off!" and it sounds about the same with or without the NFB loop on the power stage. I guess if all a person plays is real heavy over driven distortion - with the power stage pushed in to clipping-, then they probably won't even have the need in that amp for a power stage NFB, cause it won't really be doing much, if anything - and in that mode, the standard PPIMV will work great....

But getting back to my real situation, THANK YOU Rooster again, you inspired me to really try this cathode bias amp (again, but now with the rest of the amp more dialed in) with out the "OT secondary NFB" power amp loop and to increase the "natural" cathode bias local feed back loop between the power tubes, by using a larger cathode resistor, and a smaller bypass cap.(Also added some high freq attenuation, by putting a small cap around the plate resistor on V1a.) Without the power stage NFB loop, the LARMAR PPIMV seems fine. I got my idea of a power stage NFB loop from SINGLE ENDED cathode biased Fender amps - I can't even find any cathode biased PP amps with power stage NFB loop. Thanks for being easy on me Rooster.

OTOH, from what I have read heard - and heard with my own ears - I still believe a 3 gang PPMIV doing as I explained in my earlier post will be very nice for an amp with a power stage NFB, especially in clean to slightly pushed mode. One of the many generous members here sent me a link to a decent looking source for custom pots, with a very slick name
http://www.potentiometers.com I still want to see how this works, but am not good at mathematically figuring out the optimal values, so I will have to set it up in a fixed bias amp and write down the changing values as a manually dial the 2 separate pots to rough it out- or start digging into the math - like I really should - especially with the great write ups on the web about how to do it, at the top of my current list are the explanations that Randall (Thank You) put together.

[rooster]

Well, been away and see that Randall A. stopped by for a visit. That was nice, thanks! And, raiken, I wanted to say that I appreciate your site and the archival database you have created. It is a big help on certain occasions.

To grindomatic and roe - yes, I lost an 'r' there. Hm, I like saying 'la-mar' rather than 'lar-mar' so I made a spelling based on personal phonetics. So you guys are correct, I spelled it wrong, but I will keep saying and writing 'LaMar' if you don't mind. Eh, they should have checked with me for a name that rolls off you tongue..... And I mean no dis to either party, Larry still gets top billing even if Mark gets an extra letter.

As to trying the cathode biased circuit without the negative feedback, grindomatic, I think this will help your perspective. No worries on the help, and thank you for clearing up the reasons why and where the harmonic content and negative feedback ideas might have come from. Eh, it happens.

OK, last thought here: Has anybody else tried the LaMar MV in their Express?

[PlinytheWelder]

OK, last thought here: Has anybody else tried the LaMar MV in their Express?

I'm going to put one in mine next weekend. I even have the $$$ PEC dual pot ready to go. I need another knob, though....

VVR and a Lar-Mar PPIMV in one amp!

[raiken]

Well, been away and see that Randall A. stopped by for a visit. That was nice, thanks! And, raiken, I wanted to say that I appreciate your site and the archival database you have created. It is a big help on certain occasions.

Why, thank you! I'm glad the info was of some use to you.

RA

[rooster]

OK, here's something that has come up using the LaMar MV. I made an EV12L cab, like the Dumble cats use, dimensionally, ports, etc. and I didn't like it with the Express finally. Sold it. So I installed the LaMar into an Express I built for a friend and he loves it - I mentioned this before. But he loves Larry Carlton and he has a bunch of EV12Ls that he made the same type of cab for, like mine. But finally he stopped using them because the EV12L was too loud.

....................But now he went back to the EV12L cabs - now using the MV - and he tells me he is in heaven with the sound and volume. ?? So it occurs to me that, with the Dumble and its users, the use of the MV is just a given, and here the Dumble sounds pretty great with the EV12L, of course. So, conclusion maybe, the Express sounds better with the EV12L Dumble cab WHEN the MV is installed and used. ??

He reports that where the bass from the amp into the EV12L is too much without the MV, when the MV is used the bass now actually fills out the sound spectrum, and of course he can now control the volume, a win/win situation.

Anyway, I am going to borrow my cab back and try this. As I think about it, I can easily see where my friend's conclusion is going to be my conclusion as well. Which is kinda good to realize because, afterall, the Express is a good sounding amp and the EV12L is a good sounding speaker. It could turn out that they just need the the MV to bond.

[PlinytheWelder]

Well, inspired by Rooster, I finally installed the LarMar PPIMV in my Express. My initial impression is that when dimed it sounds just like it wasn't there, agreeing with theory. IMO, it even seems to work better than the VVR to tame the amp when trying to go dirty. The VVR sounds better when trying to stay clean or or to keep the touch sensitivity and wide dynamic range that is the hallmark of these amps.

What's really cool is that both together results in _wide_ range of options to tame the amp. I only tried it quickly because I want to get back to watching the F1 race, but it's really versatile now!

Here's how it looks now...
[img574]http://verosurfcam.com/TW17.jpg[/img]

[img977]http://verosurfcam.com/TW18.jpg[/img]

[rooster]

Gary - Well you are one of the few it seems. Good for you. Did you use an Audio taper pot or a Lineal taper? My first go round was with the Lineal and I just now changed to the Audio version Alpha pot. WOW-WOW-WOW! Now I see why it is recommended to use an Audio taper pot!! So this is WAY better than before because it is much more intuitive to dial in. (And I really liked it with the lineal!! Doh!)

What I cannot figure is why all the cats who built the Airbrakes don't toss those boat anchors and install the MV? Hey, I'm just saying. This MV was made for the Express. If you have an Express and made an Airbrake, and like me, were quickly disappointed with the results, this MV will blow you away. It REALLY delivers everything the Airbrake promised and failed to deliver. BUT.... USE AN AUDIO TAPER POT!!!! In fact, if you were like me and used a lineal taper pot, stop! Get the Audio taper pot and do this MV right.

OK, last joyous rant about the LaMar (LarMar) MV from me.

[PlinytheWelder]

I went all out and used a dual PEC pot with audio/log taper. I wanted nothing but the best for this amp. I measured it and used 2 meg resistors in parallel and wound up really close to the stock value.

I used mine for 2 weeks without the MV to evaluate the sound and IMO it really does sound the same as stock when it's dimed. There's a reason for that 80+ thread over at the Metroamp forum. These master volumes really do work as advertised.

I couldn't be more happy with it!

[Markusv]

Quick question

Would this circuit apply to a Liverpool even though the amp is cathode biased?

Thanks,
Markus V