MT 100 preamps

[USMC-Bill]

Hi all. New to the forum here. Recently picked up (metaphorically) an MT 100 amp. Upon arrival there was issues with the power tubes. After replacement of a quad of JJ 6L6’s I repurposed from another amp I have, I found that I really like the sound across all channels. Probably the best cleans on any amp I’ve owned. I did send an email to PRS support with accompanying video of the tubes.

As a new owner of a PRS amp, I’ve never known preamp configurations on these amps. A few questions to some seasoned owners if I may.

1. Are there cathode followers? Seems maybe v4 or v4 and/or v5 or v6 perhaps. Only saying cause they stack with one another per channel. Doesn’t mean they are but would be nice to know.

2. What are folks using besides JJ ECC83’s?

In most cases I prefer JJ E83CC vs the ECC83.

3. Long plate in the PI position?
Usually use a balanced JJ 803s in PI spots.

4. Power tubes choice?
Currently using a quad of JJ 6L6 due to issues with the factory quad of TADs. Although when a good quad of those TADs are had, they are pretty good. The last couple of years has bot been good with those TADs. Not sure if anyone else has had that issue. I certainly have.

5. Bias?
This is a big one. With a matched quad the closest I could get all tubes was around 2.8mV while one tube was 4.3 mV above all the rest. Wondering if this is normal for this amp. Never had this issue with any other amps like this. Seems a fair bit off. Overall bias I would average to 33.8 per tube with the lowest being 30.3 and the high being 34.6 mV.

Over all I like the amp and it does seem to operate proper as far as noise level with no oddities in sound or idleness. It’s nice and loud but seems to have a great volume, or should I say master volume taper to it.

Any feedback would be appreciated.

 

[andy474x]

I like your style! The E83CC is one of my favorite new production 12ax7's, too.

I've not changed any of the preamp tubes in my MT100, yet, but I probably will play with at least some positions to see what happens. I did change some in my MT15, in fact all of them now that I think about it, to increase the clarity of the dirty channel. The JJ ecc83s just has SO much gain and can really cause an amp to distort too early in the gain stages and become undefined, especially in the lows and low mids, which with an amp like this is where one really wants that definition.

The tricky thing about the MT100 is that the V1 is common to all three channels - kind of bummed when I found that out, because they specifically advertised all three channels as using their own individual preamp tubes, which I thought was kind of odd that they wouldn't share a V1... oh, fine print, they do! Anyways, I experimented replacing the V1 on the MT15 with anything from a 5751 down to a 12ay7, and in fact I think I still have the 12ay7 in there. I dislike when an amp just slams the gain in the first stage, it stays more defined when it builds gradually through the gain stages. In the MT15 it works well, and the clean channel still sounds excellent and doesn't suffer from low output. My main concern with the MT100 is that I like the amount of gain in the crunch channel and wouldn't want to cut that down at all. So I may end up not liking lower output V1, we'll see. Of course, the later gain stage tubes of the lead channel could be swapped out for lower gain tubes, too, but maybe not to as much effect as V1.

The other most noticeable change I made to the MT15 was to replace the PI with a 12at7, from the stock 12ax7. That also increased the clarity and tightened up the lows, and that might be a more reasonable mod for all three channels of the 100.

On the 100, I haven't changed anything yet, because it seems like they did design the amp to behave a little better with the stock ecc83s tubes. But I'll likely tinker.

What is the problem you've had with the power tubes on your amp? I might be able to give you some more info on that.

 

[USMC-Bill]

What is the problem you've had with the power tubes on your amp? I might be able to give you some more info on that.

First off thanks for the reply. Lots of intel there on the MT15. Almost picked one up and kind of wish I did. Glad to hear another liking the JJ E83cc. Underrated tube. Most folks classify them as JJ’s are dark. Not true. Even the Ecc83s has a brightness to it. It’s just warm mids that give it that reputation. Well that and the high gain factor. Even at 100 it seems more gainy then others. E83cc is a completely different tube vs the Ecc83.

When I powered on the MT100 for the first time there was a loud jet-like airplane swirling sound. With all knobs zeroed out for bias testing. Already had the back cover off so I was able to observe the tubes. 2 were red plating one from each pair (center pair =1 and outside pair =1). Oddly both left tubes. The inside pair right tube had a broken base. Can’t recall all the Vnumbering on the power tubes. Replaced with a quad of JJ 6l6’s and all was well. Was able to play test the amp for a while. Maybe around 3 or 4 hours.

Extremely satisfying amp. Clean is simply the best I’ve ever heard. And can get bone shattering loud and stay clean. Amazing really. Metal of all types in abundance on ch3 and some great crunch on ch2. Ch2 is going to take a bit of time to learn.

Minus the power tubes, I too have left my MT 100 amp factory. Still learning the sounds of the amp at this point. May take several months but there may be some changes. At this time I feel it sounds great as it is. I would like a working quad of the TAD 6L6s. To be clear hear, I’ve gone through soooo many TAD 6l6s and every time they have problems. It’s the version that comes stock; 6L6GCMSTR? I think they are the model name. The STR stands for Special Tube’s going to RedPlate. Lol.

Either red plate, fireworks in the tube and/or structural issues. These tubes, whoever makes them, really need to check their tooling. Cause it’s jacked up. Even went so far as to buy a Mesa set. Big mistake. While they lasted for around 20-30 hours, sure enough fireworks appear in the tube and sonic differences are noticeable. Then… Red plate happens.

I feel when those TADs start to fail, cause they are already hot as heck, they take other things out. They run over 900F. That’s the highest my probe goes so it’s probably hotter. JJ 6L6’s run 680-800 depending on how hard they are pushed; but thats pushed temps. Idle maybe 480 or so . Tads run 900 (plus) at idle after 30-60 minutes. Yes idle. So if they are in your amp I recommend you monitor them closely. You cannot see the initial read plating with the room/studio lights on. You need to have it dark. Well, simply the lights off. Because it starts very slightly but will hold the bias properly. Goes nuclear from there. Then all of a sudden amp failure.

With those Tads tubes I monitor them every time I power up and mid playing. A quad messed one off my amps up really bad and I am still dealing with it and the manufacturer. Causes random failures probably because it cooked the main pcb and other stuff. Although their arrogance won’t let them l, see it. Probably because they do make great amps but should ditch Tad. (So should PRS impo) It’s actually kind of a nightmare to be going through honestly.

They are really good when you finally get a good set. Which is kind of hard to do. Even then they last around 30 hours. Cause they run too hot. I have 18 tubes and all have failed. 10 of them are from another amp builder. The other 8, I purchased. Never again, cause of the MT100, now I have 22 worthless Tad power tubes. All in almost 2 year span.