The LaMar (LarMar) PPIMV in an Express

[rooster]

jerry - Hey there. Well, I loaned the amp out to a friend and he will have it till Wednesday. I just talked to him, he's had it for the weekend, and he is all raves about the Express clone (something he has never played but wanted to) AND the MV. What he said in particular that I haven't really dwelt on is how transparent the MV is throughout its range. In other words, it doesn't get darker as you turn it down but instead stays 'clear'. I did the loan on the spur of the moment because he has helped me out with amp parts and also we talk amps a bit - it just kinda happened, I'd been threatening to do it.

Eh, so I will do the sound bites on Thursday. Sorry about that to anyone who was waiting. I'll do it.

[Roe]

The LarMar PPIMV doesn't sound as good as non-mv even when totally cranked so it's 220.. there is some kind of blurring to the tone that I can't describe probably due to the shielded cables and extra lead length.

I really hear this difference and do not like the LarMar because of it, It's deffinately the most transparent MV I've ever tried though.

...

I don't agree. the larmar can be implemented without any extra lead length. indeed, you can even shorten the leads in a plexi (although it wont matter much). I haven't experienced any problems with a good cable. if you use a cable with much capacitance you need to reduce the 47pf fizz cap a little.

pm if you want some cable

[funkmeblue]

what about it's effects on the negative feedback loop? That's what has always put me off trying this master on an express. Works great on my cathode biased zero negative feedback amps, better than my airbrake.

[PlinytheWelder]

what about it's effects on the negative feedback loop? That's what has always put me off trying this master on an express. Works great on my cathode biased zero negative feedback amps, better than my airbrake.

I actually like the way it reduces negative feedback as the master is turned down. Another variable in the sonic palette.

[Structo]

The only thing I don't like it as you mentioned is if you have a presence control it pretty much defeats that unless the MV is wide open.

But there are workarounds for that, one being the cut control.

[rooster]

OK, I appreciate all your comments, and I am reading what is being written. But the thing is, like I said, this MV is WOW! Whatever you think you are objecting to, you will learn that the objections are not worth the ink here.

The idea of the negative feedback being reduced may actually be valid in science - but in the case of the Express - not the case sonically. Think about it. The output section, from the PI on, is all vintage Marshall, just like the Express. If you go to the Metroamp forum you can read 82 pages of posts and not find one bad reaction to the LaMar PPIMV. Eh? And frankly, when you hear the amp, the very last thing you will need is some added Presence. The thing has this clarity to it that will stun you. I mean really, I was stunned to hear this thing in my Express. .............ESPECIALLY after hearing all the reasons why - in this very forum - I would not like a MV in the Express. That's why I built the airbrake and probably why everybody else here has, too.

But the thing is, this MV circuit will cause most people here to toss their airbrake. I know, all that work and all that money spent on airbrake parts when $5 worth of parts will blow that airbrake out of the water. No, not exaggerating here.

Sound clips tomorrow, and then you will hear what I'm talking about. FWIW, IMO, this MV and the Express circuit were made for each other. If you are looking for quality volume reduction in the Express, look no further, that's what I'm saying.

[Gaz]

If you go to the Metroamp forum you can read 82 pages of posts and not find one bad reaction to the LaMar PPIMV. Eh?

You will find, however, Larry saying that he does not use this master volume himself.

[Big Jim]

I pondered doing a LarMar on my Metro JTM45 for a long time....Then I thought, for a few bucks I'll try it. If I dont't like it, or my amps mojo leaves town or whatever, I'll just take it out.... Well it's still in there. It sounds awesome and works perfectly. It made a really loud amp much more usable. Now is it right for an Express? I surely don't know. I've heard that this circuit does not like master volumes. I also heard it was O.K. to drill for oil in the Gulf. It's a few dollars worth of parts. If you don't like it , you can always take it out.
Jim

[greatwhitemark]

Hi Rooster, just spotted this thread and wanted to add my support.

I've been road testing a LarMar MV in my Express clone for the last few months and it works great for my setup. I think it does reduce the compression slightly in the output stage, but for me that's a good thing as it means I get a slight boost when it comes to solo time. I generally have all settings around 12 which isn't maxed out on the gain and works as a good rhythm sound. For leads I hit an overdrive set for max level and little bit of gain - with the MV not driving the power tubes so hard it allows a slight volume boost. Most of the time my MV is around 11 oclock and I still consider it loud....

It may effect the clean to mean performance but that's never been part of my live routine so it may vary for others.

Certainly made the amp more useable for me.

[rooster]

OK, here you go. Straight in with a Strat, channel volume on 1 o'clock. The MV starts at full on - call it 5 o'clock, then 2 o'clock, then 12 o'clock, then 10 o'clock then 9 o'clock.

No apologies for the playing, just a quick run thru into my handheld Roland R-1 recorder. Note there is overload distortion on the mics, but I did not use a limiter or compressor and this is why. Its just an honest recording. The positions on the MV I chose because they seem the points that come up on the lineal pot. If you ran an audio taper it would be different.

(Oh, I say at one point on the recording, '3 o'clock', I meant '9', I was just thinking about it backwards at this point. Also, I cut off the back of the track - the lowest volume setting - while editing it to MP3, but I assure you the note continues just like the other samples. Sorry about that. Also, this is just one single recording, no stops or starts. A friend is sitting behind the amp turning the MV down as the recording takes place. I would also offer that while it may be hard to hear the distinct volume differences on the recording, if you were in the room they would be obvious. This must be because the built in mics are being overloaded as things begin. Eh, hard to gauge this. Also, the last passage really drops the volume, yes, but right about here is where the volume starts to really come down and then quickly disappears. Here I probably should have talked over this to give you a bettter idea, but this is total bedroom level - and yet its still fun to play!)

[funkmeblue]

I'm sold....think I'll try Ken's master first then the lar-mar and see if I notice any noise difference or change from the master cranked.

[grindomatic]

Using this type of PPIMV (on my NFB amp RUNNING IT CLEAN) I heard the grainy, but really only severe when running the amp clean - and stumbled on the reason why in Kevin O'Connors "Ultimate tone" and should have figured it out, if I actually thought it through.. Of course it wipes out the NFB in the process - and about the 1st 40% of turning it down has very little reduction in volume(while clean)but increase in graininess , because the NFB is disappearing, allowing unbridled gain = increasing signal level& odd order harmonics=graininess.

This problem is only predominant IMHO when running the amp clean - but with heavy distortion, freakin works GREAT, nice even reduction! Like it almost as much as the Weber MASS.

Back to the CLEAN mode problem... So I completely unhooked the NFB loop & it then the MV lowers the volume evenly and is staying grainier, just like most pentode amps sound to me without a reasonable amount of NFB (when running it clean to mild overdrive).
I am using 100k NFB loop rsistor like on many older marshall 50w amps.

BUT I think I found a pretty good cure - lowering the resistance in the NFB loop(to give more NFB) correspondingly in tandem, as the signal to the power tube is attenuated.

What we need is a 3 gang pot, with the 1st 2 pots used normally like they are, with the 3rd gang having decreasing resistance, as it's turned counterclockwise, paralleled with the NFB resistor.

I faked it with a 3rd 1M pot (turning them both while my friend played the guitar - SOUNDED GOOD!) paralleled with the 100k NFB, which should be figured out mathematically properly, it seems to me like a 500k log for this 3rd gang(along with 250ks in the 1st 2 gangs as suggested in the LARMAR version), with a 80K resistor ought to work out right - but ( embaarrasingly) I have a very weak amount of working knowledge in electronic mathematics.
OTOH I still have way more of my ears than I should considering the warnings (& loss of high end) they gave me, and they say this sounds very good down to about 80% attenution - then I can hear the grainy coming back, cause we are getting beyond the compensation of the resistance being taken out.

Another thing I could see being a problem, is the signal to noise ratio will be getting worse as we open up the resistance, but it didn't seem to be a noticeable problem in my home brew 2 x6V6 Cathode bias amp. I do have pretty good filtering in my PS and have the heater line attached to the DC above the cathode resistor.

So my big problem is where to buy a 3 gang pot like this, probably even one with all 3 gangs @ 250 K audio (then probably a 68K NFB resistor) would be fine.

Thank You Everybody, what an awesome forum!

Craig

[Roe]

there's two problems when turning down the larmar: NFB is reduced and the presence control gets more and more inactive. for me the fix was to install a variable NFB (I increase NFB as I turn down the volume) and to install a aiken/larry/vox style presence circuits. all the details are to be found in the larmar thead.

a third problem is that if you slam the PI it may lead to blocking distortion. in order to minimize this problem I reduce the pi input cap to 2n2 or 3n3

[grindomatic]

Thank You Roe,

That is exactly what I am doing temporarily while testing - using an dedicated pot to increase the NFB, as the master is decreasing the volume.

What I want to do is integrate the larmar MV & varible NFB into 1 pot, thats why I am looking for a 3 gang pot,
as described in my post titled "PPIMV sounds grainy - increasing NFB simultanuously helps!"

So my big problem is where to buy a 3 gang pot like this, probably even one with all 3 gangs @ 250 K audio (then probably a 68K NFB resistor) would be fine.

[rooster]

grindomatic - Well, from what you write, I am now suddenly very confused as to the function of the NF circuit. Fill me in, but do so by answering my questions here please:

1) Doesn't negative feedback CLEAN UP the output signal by sending low level reverse polarity output signal back to the PI? How does a lack of NF make for a 'grainier' sounding amp? An AC30/Rocket comes to mind, no NF. Is this considered a grainy sounding amp?

2) In the Express, given any playing situation, at what volume level is it considered 'clean'?

3) Have you put a Lamar MV in an Express, and are your complaints regarding the LaMar MV based on a personal experience?

My take on things is not based on how the LaMar performs in a Plexi, but the Express. And actually, when I consider that the NF is reduced in signal strength to the PI, I think 'Cool!' because I don't hear any added noise. And noise is the main reasons why the Express needs negative feedback to the PI in the first place. (Take it out and you will understand, of course.) And given a PERFECT amplifier circuit, there is no need for the NF, of course, unless your goal is something more strident. So when the Lamar is engaged, the NF is reduced (your point) - I think this is a pretty good thing to happen in an Express if you ask me!