Pre/Post PI master?

[Tdale]

I saw a post earlier, that a Pre PI master is clearer, but a Post PI makes the amp distort more.

On a Dumble, is the OD section enough to make a nice overdrive tone, even with a Pre PI master?

Tommy

[Normster]

I seem to recall reading that Dumble's distortion comes primarily from the preamp. I don't think I've ever seen an ODS with a post PI master. Just to be clear, the post PI master I'm talking about is this one...

[dehughes]

Very cool. Has anyone here tried this PPIMV on a JTM45ish amp?

[jimipage]

I have an '82 Marshall 2204 that David Bray modded up for me a couple of years ago (plexi/vh brown mod). It uses a PPIMV and I like it -- a lot. It's my understanding that it acts as a master until it is dimed, then it's out of the circuit -- and LOUD! I say if you know how to do it, give it a try.

[krash]

post-PI master vol does not drive the output stage harder unless by "output stage" you mean "phase inverter". I guess it all depends on the amp in question and how much distortion you get at what stage in the amp circuit. If the preamp distortion is working for you, then pre-PI will work. If you require the PI to be slammed to get your tone, then post-PI will be better. If you need the power tubes to be cookin' to get your dream tone, then get an attenuator and forget about MV.

IMHO.

I have not found any MV design I liked when used as an afterthought to an amp that does not have carefully crafted preamp distortion. Any amp that does have carefully crafted preamp distortion probably already has the MV in the right spot.

[dehughes]

...

I have not found any MV design I liked when used as an afterthought to an amp that does not have carefully crafted preamp distortion. Any amp that does have carefully crafted preamp distortion probably already has the MV in the right spot.

Elaborate on this, please.

[krash]

...

I have not found any MV design I liked when used as an afterthought to an amp that does not have carefully crafted preamp distortion. Any amp that does have carefully crafted preamp distortion probably already has the MV in the right spot.

Elaborate on this, please.

Well, for example ... take the ubiquitous Marshall 18W. This is an amp that sounds killer when dimed. But it's really a combination of speaker breakup and compression, power tube distortion, some distortion in the PI, and almost no distortion in the one preamp gain stage. If you put an attenuator on it, then you retain everything except the speaker breakup and compression. If you put a post-PI MV then you will lose all of the power tube distortion as well (which is like most of the overdrive tone) unless you crank the MV to 10. If you put a pre-PI MV then you lose not only the power tube distortion & speaker element, but also the PI distortion ... and so on. Basically any tone contribution that happens after the MV is going to be lost when the MV is turned down. So the challenge is to make an amp that doesn't really rely on post-PI components for their overdrive tone characteristic.

To make an amp that really sounds good with the MV turned down, then it has to be carefully designed so that it gets a great tone without requiring power tube distortion or speaker breakup to help out.

I haven't really experienced a lot of amps that had great tone with the MV turned down. I have heard a bunch that could sound real good as long as everything was turned all the way up but really lose it when you turn it down. I think they rely on tone contribution from elements of the system that happen after the MV, and if so, then they are going to suffer when not cranked.

IMHO and hopefully a qualified amp builder will correct me when I am wrong

[dehughes]

Makes sense to me. Thanks!

[loverocker]

I haven't really experienced a lot of amps that had great tone with the MV turned down.

Find a Guytron; that's one approach that definitely works (not cheap!). B+ control (Power Scaling by London Power and Maven Peal's thingy) is another.

Most people who put PPIMVs on their non-MV 50/100W Marshalls are happy. Unlike the 18Wer, in these amps most of the famed Marshall power valve crunch actually comes from the PI.

[krash]

yes my first round with a 36W had this "feature" ... PI crunch and a master vol. Yuck. It was back to the drawing board for me.

[Darkbluemurder]

There are amps that have both a pre-PI and a post-PI master volume. Sundown is one that comes to mind but this amp relies pretty much on preamp distortion. A more recent one is Bluetron's Blueverb. From the description this amp relies on small amounts of distortion in each stage. The rock sounds on his website are killer, but I was told both volumes were at about 7-8 for that sound which is just a little less than full up.

I haven't played a Guytron but I like the small amp into a large amp concept as well. The clips on that website sound exceptional, too.

I think it is great that there are new builders trying some new things - this is something we all can benefit from.

[Bob-I]

I think it's pretty difficult to actually drive an EL-34 or 6L6 into distortion from a LTP PI. Most of what we think of as power tube distortion is actually coming from the PI.

I've used a PPIMV on a Marshall type and it sounds great. It doesn't come out of the ckt completley by diming it though, you really need to work out some switching to do that. There's extra coupling caps and some signal to ground issues to work out.

[Darkbluemurder]

I've used a PPIMV on a Marshall type and it sounds great. It doesn't come out of the ckt completley by diming it though, you really need to work out some switching to do that. There's extra coupling caps and some signal to ground issues to work out.

Actually I had a PPIMV in one of my modded Marshalls. It was a Trainwreck type 2 which replaced the bias resistors with a ganged 100kA pot. This worked quite well but I removed it. The amp had (and still has) 4 gain stages and is the highest gain amp I have so I thought it really wasn't necessary to have a PPIMV. Plus, it is not the safest PPIMV.

[dreamtone]

Very cool. Has anyone here tried this PPIMV on a JTM45ish amp?

Reports from the LP Forum say that it should work good if not better in non master volume amps like a JTM or Plexi BUT I put one in my Marshall 2205 head and it was out standing! So the amp had a pre-master and the Post-master, all that was needed was a little adjustment. Now I tried this on a 2204 master volume head (home brew) and I did not like it.