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ADA Power Amps => Microtube 200 MT-200 => Topic started by: Soloist on March 09, 2016, 09:46:36 AM

Title: MT200
Post by: Soloist on March 09, 2016, 09:46:36 AM
I scored an MT200, will be picking it up in a day or so. New neon bulb already installed with a few extra! Can't wait to hook it up :whoohoo!:
Title: Re: MT200
Post by: MarshallJMP on March 09, 2016, 09:52:57 AM
These are good amps  :thumb-up:  and they are LOUD!!!!
Title: Re: MT200
Post by: Soloist on March 09, 2016, 10:08:13 AM
I figured it would be as loud as my DCM200L but with a little tube warmth. Not happy at all with my Mesa Fifty/Fifty. I completely retubbed it but it's just not doing it for me :dunno:
Title: Re: MT200
Post by: El Chiguete on March 09, 2016, 11:36:29 AM
Not happy at all with my Mesa Fifty/Fifty.

Are you willing to do a donation to someone needing a poweramp :)
Title: Re: MT200
Post by: MarshallJMP on March 09, 2016, 11:42:08 AM
Not happy at all with my Mesa Fifty/Fifty.

Are you willing to do a donation to someone needing a poweramp :)

Haha great thinking !!!  :lol:
Title: Re: MT200
Post by: Kim on March 09, 2016, 12:39:56 PM
I'm happy with my MT200, but I'd love to take a Mesa 50/50 for a test drive.  :)
Title: Re: MT200
Post by: MarshallJMP on March 09, 2016, 12:48:41 PM
I have a MT200 and i did service some 50/50's and they are both good amps.The boogie sounds more heavy and is a bit more mid scooped then the MT200.

Hey Soloist,what's the problem you have with the boogie?Did you get the correct power tubes because these amps don't have a bias pot so you need to buy selected tubes.
Title: Re: MT200
Post by: Soloist on March 09, 2016, 01:28:24 PM
I put Mesa tubes in it. STR440 6L6 matched reds. Driver tubes Mesa STR12AX7's in V2 and V3. Mesa SPA12AX7 in V1.
What I don't like is it's either too quiet or too loud, no happy medium. Other issues it had were resolved when I changed all the tubes, like a light crackling in the background when not playing anything and one volume pot had too be turned up more than the other to get equal sound in stereo. There's nothing wrong with it, just my taste I guess. It doesn't anything like my old 2:90, but then again that one had EL34's in it.
Title: Re: MT200
Post by: MarshallJMP on March 09, 2016, 01:36:55 PM
What do you mean by " it's either too quiet or too loud"?

Well i must admit that i'm also more drawn to the EL34 side.
Title: Re: MT200
Post by: Soloist on March 09, 2016, 02:07:09 PM
I am either over powering everyone else or everyone else is drowning me out. Level 3 is too quiet but if I bump it up a bit it's too loud. Like it jumps from 3 to 8 volume wise. Maybe the volume pots need replacing?
Title: Re: MT200
Post by: rnolan on March 10, 2016, 03:33:33 AM
IIRC boogie guitar amps were a bit like that, really hard to get the level right, starts too soft then a huge jump with very little turn of the knob.  Though with power amp, can't you just turn the preamp output down to adjust ? Also you probably don't really want a high gain input tube (12AX7) as the phase spliter, better to use a 12AT7 or even a 12AY7 as lower gain but much higher current (which is what you want from a phase spliter).
Title: Re: MT200
Post by: MarshallJMP on March 10, 2016, 05:57:44 AM
Like R says use the preamp to control the volume?

Well a 12AT7 will only give a bit less gain.
Title: Re: MT200
Post by: Soloist on March 10, 2016, 08:39:03 PM
Also you probably don't really want a high gain input tube (12AX7) as the phase spliter, better to use a 12AT7 or even a 12AY7 as lower gain but much higher current (which is what you want from a phase spliter).

Those a$$hole's I talked to at Mesa Boogie California told me to put a 12AX7 in there. That's what they ship from the factory with. That's how they are tested and set up for. I figured I would re tube it OEM to get a feel for how it once was. But alas 6L6's don't do it for me :(

Like R says use the preamp to control the volume?
Well, with my monster rig it ain't that simple. The gain staging and signal levels running multiple pre's a mixer and multiple fx units you need to adjust everything...F*ck that! :nono: It took me forever to get it where it all worked. If I turn down 1 pre it kills my tone.
 
So I picked up the micro tube today and slapped it in. Running it wide open with presence at about 10 o'clock it sounds great. I was running my carvin wide open before as well. It's not as deep of bass as I had before but I think I had a bit too much anyways. Nice saturated tone. My Piranha and Voodo Valve huge improvement!! :thumb-up: The JMP has more balls to it as well as my MP1! I also removed the Pro Q from my rack. The MT-200 is like my carvin on a medium dose of steroids with some tube warmth. I think this will be my go to power amp. I will wire in my carvin as a back up. 2 Power amps 1ru each and not much weight between the 2. :whoohoo!:
Title: Re: MT200
Post by: rnolan on March 11, 2016, 04:57:08 AM
But a 12AT7 will give you 10 times the current vs 12AX7 and a bit less gain (70ish vs 100 ish, remember the article SC posted about phase spliters, but they were mostly chasing output tube distortion. All the gain (tone wise) happens in the preamps, this is not a spot in the circuit you want more gain, you want headroom and "grunt" which comes from the much higher current of the 12AT7 to drive the output tubes.

My TS100 also uses 12AX7s as both buffer and phase splitters; and the model/version I bought has GT 6L6s in it, nice amp, and it's not to say 12AX7s are terrible for phase spliter, just lower gain higher current tubes like 12AT7s and 12AY7s are a better fit for that job.

Hey glad the MT200 is gong well  :thumb-up: hybrid land, tubes warming up some transistors and nice and compact. Also well matched to the ADA preamps
Title: Re: MT200
Post by: El Chiguete on March 11, 2016, 05:04:35 AM
Again all I can say is give it to me!!! :)
Title: Re: MT200
Post by: MarshallJMP on March 11, 2016, 12:02:13 PM
12AT7 will not draw much more current unless you start changing the anode resistors.Also tubes have a high input impedance so current isn't so important,voltage is.

A 12at7 and 12au7 are different tubes then a 12ax7 and they need to be biased different.So a good option for a lower µ preamp tube as replacement for the 12ax7 is the 5751.These are a sort of low gain 12ax7.(µ=70 vs 100 for 12ax7) and they sound good.I use these in my mp-2.
Title: Re: MT200
Post by: rnolan on March 11, 2016, 02:43:21 PM
Yeah but for a phase spliter current is king!! not gain, did you read the article SC posted ??, if you want poweramp distortion (not that we do...), you need current in your phase spliter, not gain, and a 12AT7 has 10 times the current of a 12AX7 (albeit less gain). Basically (well from what I've researched and discussed with tube nuts), is a 12AX7 is the worst phase spliter tube as it's got lots of gain and bugger all current. Not that it's terrible just the wrong way to go for a phase spliter ... and gain structure wise it make sense to me. All that said, my Carvin TS100 has 12X7s' (buffer and phase spliters) and 6L6s' and it works just fine.. (well better than fine, it's shit hot as a poweramp!! :thumb-up:
Title: Re: MT200
Post by: MarshallJMP on March 11, 2016, 02:55:13 PM
Where is that article?
Title: Re: MT200
Post by: rnolan on March 12, 2016, 01:39:09 AM
This one IIRC http://adadepot.com/index.php?topic=543.15 (http://adadepot.com/index.php?topic=543.15), Reply #15 on: Fri 26Sep 2014 04:22 PM
Title: Re: MT200
Post by: Systematic Chaos on March 12, 2016, 04:48:37 AM
http://adadepot.com/index.php?topic=543.msg5269#msg5269 (http://adadepot.com/index.php?topic=543.msg5269#msg5269)
Title: Re: MT200
Post by: DorsetRatt on March 16, 2016, 09:47:58 AM
Yeah but for a phase spliter current is king!! not gain, did you read the article SC posted ??, if you want poweramp distortion (not that we do...), you need current in your phase spliter, not gain, and a 12AT7 has 10 times the current of a 12AX7 (albeit less gain). Basically (well from what I've researched and discussed with tube nuts), is a 12AX7 is the worst phase spliter tube as it's got lots of gain and bugger all current. Not that it's terrible just the wrong way to go for a phase spliter ... and gain structure wise it make sense to me. All that said, my Carvin TS100 has 12X7s' (buffer and phase spliters) and 6L6s' and it works just fine.. (well better than fine, it's shit hot as a poweramp!! :thumb-up:

Interesting article ... just out of curiosity has anyone on the forum done a before/after test when replacing a 12AX7 with a 12AT7 for the phase inverter? More power tube distortion? More warmth? More sag/compression?
Title: Re: MT200
Post by: rnolan on March 16, 2016, 06:08:48 PM
I'll probably give it a try when I retube the TS100 but that's a way off.  A consideration though is that while all the 12A? series tubes are plug/socket compatible, the 12AT7 delivers 10 times the current of 12AX7 so (while I don't think it would be a big deal for most poweramps), the power supply needs to be able to cope.
Title: Re: MT200
Post by: MarshallJMP on March 17, 2016, 08:14:28 AM
 Gain wise a 12at7 will not differ too much from a 12ax7 and this also depends on what type of phase splitter is used.
For instance the ADA T100S tube power amp uses a cathodyne phase splitter which has a gain of 0.97.

The most used one is the long-tailed-pair phase inverter or to be more exact the schmidt phase inverter.Let's take the marshall inverter values for a 12ax7 and a 12at7 (ECC83 and ECC81).Now these are datasheet values,in real world they can differ a bit and i presume both triodes in the tube are the same.

12ax7 inverted gain 27.8 non inverted gain 29.1
12at7  inverted gain 26.0 non inverted gain 26.9

As you see a small difference which you won't able to hear (difference is less then 1 dB)

Current wise yes a 12at7 can deliver more current but this depends on the anode resistor so replacing a 12ax7 with 12at7 which has a Ra of 100k won't give 10 times more current at all.But what you will get is a lower output impedance.

Let's do some calculations

A circuit with an Ra of 100k,Rk 2.7k bypassed by a Ck cap.No load.

12ax7   gain=61.5 output impedance of 38.5k
12at7    gain=54.0 output impedance of 9.8 k

So the 12at7 is better for driving lower impedance loads.

Note;12AT7's should have lower Ra's (and Rk) more between 20 and 50k to get it biased correct.
Title: Re: MT200
Post by: MikeB on March 17, 2016, 12:59:22 PM
Yeah. That's what thought.
Title: Re: MT200
Post by: Systematic Chaos on March 17, 2016, 01:18:53 PM
Bottom line: individual subjective preferences ;-)

I personally like Sovtek 12AX7LPS best in the PI slots of my 2:90. I find them boring in any other preamp or input slots but in the PI they´re the best sounding (subjective taste) I used so far. Also, I like them in there best when NOT balanced.

The input 12AX7 in my 2:90 (currently a Tube Town E83CC) is balanced...cuz each of the 2 triodes works one of the 2 channels of the poweramp....

...roll some glassware and stick to the setup that sounds best for you...
Title: Re: MT200
Post by: MarshallJMP on March 17, 2016, 01:35:30 PM
Actually yes,a 12AT7 has some better drive capability's so maybe the myth about it has some truth in it as that it sound cleaner.

So it really comes down to individual subjective preferences like SC says.But now you guys now the "real" differences and it's up to the person if he likes what he hears or not.
Title: Re: MT200
Post by: DorsetRatt on March 17, 2016, 04:07:23 PM
Mmmm .... I was reading the article whilst at the same time trying to diagnose a Mesa 20/20 I bought several years ago. I re-tubed it with a full set of Harma's when I first got it, but it has always sounded a bit too cold/sterile (lacks warmth). The article mentioned touch dynamics with regard to the PI tube, hence my interest. Could it cure my Mesa?
Title: Re: MT200
Post by: rnolan on March 17, 2016, 04:59:49 PM
Hey DR, Possibly, as MJMP says, let your ears decide. I've been discussing this with my tube nut audiophile friend, he'd never use a 12AX7 as the PI/PS, his go to tube for this is the 12AT7. If you decide to try them, let us know how you go.
Title: Re: MT200
Post by: MarshallJMP on March 17, 2016, 11:44:00 PM
Mesa amps are most of the time underbiased (power tubes),i would check this first.
Title: Re: MT200
Post by: rnolan on March 17, 2016, 11:50:45 PM
Mesa amps are most of the time underbiased (power tubes),i would check this first.
+1  :thumb-up: , good advice
Title: Re: MT200
Post by: Systematic Chaos on March 18, 2016, 01:38:21 AM
Mesa 20/20 Deep Mod:
http://forum.grailtone.com/viewtopic.php?f=5&t=31723 (http://forum.grailtone.com/viewtopic.php?f=5&t=31723)
Title: Re: MT200
Post by: rnolan on March 18, 2016, 05:26:04 AM
Hey SC, just wanted to say I really appreciate you finding all this stuff for us all  :thumb-up:
Title: Re: MT200
Post by: Soloist on March 18, 2016, 10:57:44 AM
I put a pair of Tung Sol reissues in the MT200.. and WoW!! Nice warm tone! :thumb-up:

Now, on the Mesa 50/50 I was reading that V2 and V3 are both phase inverters :facepalm:. V1 splits the signal to each side. Can someone confirm this? MJMP? SC? rnolan? Anyone?
Title: Re: MT200
Post by: Systematic Chaos on March 18, 2016, 11:00:02 AM
Correct! Is that the old 50/50 or the new one?
Title: Re: MT200
Post by: Soloist on March 18, 2016, 11:09:47 AM
I think it's a newer one. Blue power light on it.
Title: Re: MT200
Post by: Systematic Chaos on March 18, 2016, 11:19:22 AM
I'll mail you some MB docs
Title: Re: MT200
Post by: Soloist on March 18, 2016, 01:00:59 PM
Thanks SC!
Title: Re: MT200
Post by: MarshallJMP on March 18, 2016, 02:57:28 PM
Hey Soloist,do you got what you need from SC?
Title: Re: MT200
Post by: rnolan on March 18, 2016, 06:07:35 PM
Well that's an interesting way to do it, so V1 is the buffer tube for both channels (one triode each) and V2 and V3 are the PIs, one per channel to drive the output tubes (which I assume there are 2 per channel in push/pull)
Title: Re: MT200
Post by: Soloist on March 18, 2016, 08:47:44 PM
Hey Soloist,do you got what you need from SC?
Yes I do! Thanks Guys :thumb-up:
Title: Re: MT200
Post by: Systematic Chaos on March 19, 2016, 01:35:51 AM
Hey SC, just wanted to say I really appreciate you finding all this stuff for us all  :thumb-up:

All cool bros!
I did some mods and maintenance on some Boogies ;)
Title: Re: MT200
Post by: DorsetRatt on March 19, 2016, 04:12:31 PM
Mesa 20/20 Deep Mod:
http://forum.grailtone.com/viewtopic.php?f=5&t=31723 (http://forum.grailtone.com/viewtopic.php?f=5&t=31723)

Thanks for the link SC. I've got the deep mod on the 20/20, it was done by the previous owner ... I have no idea what the 20/20 sounds like as standard, but consensus of opinion seems to favour the modded version.

I'll try some different tubes for the PI positions, probably the 12AX7LPS that you've mentioned previously. If it doesn't work out for me I'll try the 12AT7, I can reuse the 12AX7LPS and install them in my SP-77 (I seem to remember Soldano recommending the Sovteks).

MJMP, under-biased power tubes are a concern ... it wouldn't surprise me if they were, I think perhaps it's a Mesa tube strategy. It's a little frustrating that the bias is not easily adjustable (trimpots, etc), so I'd be really out of my comfort zone.
Title: Re: MT200
Post by: Systematic Chaos on March 20, 2016, 02:00:29 AM
I don´t have the schematics of the 20/20 DynaWatt, but there´s a 33k resistor in the fixed bias circuit... Replace that with a 15k resistor and a weber WP503L 50k bias pot in series  :thumb-up:
Title: Re: MT200
Post by: Dante on April 02, 2016, 08:35:52 PM
Hey SC, just wanted to say I really appreciate you finding all this stuff for us all  :thumb-up:

All cool bros!
I did some mods and maintenance on some Boogies ;)

I can't thank you enough for the advice on re-tubing my DC-5 combo  :bow: