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Magnavox 150aa Monoblocks - best set up

Ntc76

Ntc76

Member
Hello Folks,

I recently bought another 150aa to match the one I have in my 1957 magnificent magnavox. It's the one that is mono with a seperate treble circuit, and it has two 12" speakers and one Jensen R103 metal horn tweeter. What I want to do is use these two as monoblocks with some nice martin logan motion 30xt's or something to that effect. I also would like to use a turntable to play the collection of 1960's records my parents passed down to me, which btw is the reason I have found myself in the wonderful world of tube amps. I would also like to be able to use bluetooth or at least the headphone jack of my iphone to stream music. I already have an Onkyo TX NR838 on my home entertainment system. It does have pre-outs as well as a pure audio setting that I don't quite understand. Would it defeat the purpose of using tube amps with a solid state, the 838, as a pre-amp? What is the basic flow of equipment using a tube pre-amp? Why are there phono pre-amps and regular pre-amps and are these small cheap tube pre-amps such as the little bear a waste of money and sound quality? I'm not a rich man, but I do realize You get what you pay for. I just want that warm tone and clear sound to fill my listening room. And I want to see the glow of those tubes.

Thanks for any information.

-Nate
 
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I'm converting one of these amps, and parts from a second one (AMP150 and AMP179), to a stereo PP 6L6-based integrated amp right now, for 63pontiacgp.

I remove the SE/high amp transformer and all the bi-amp circuitry (including the big transformer box at the front of the chassis), and use the PP (bass) output transformers from both units. I install both of the PP output transformers on one chassis- one in its' original location, one right next to it in a new hole cut into the chassis. I remove and reconfigure the tube sockets so I have four octal sockets (for the four 6L6 output tubes), and I cut out a socket location between the two smaller tube sockets, for another 9-pin tube. I also enlarge the 7-pin tube socket (originally for the SE/high amp) so I can put a third 9-pin tube in there (one for preamp, one to drive the output tubes on one channel, and the third to drive the output tubes on the second channel). I convert the power supply to solid-state rectification, replacing the choke with a larger one, suitable for running two pairs of 6L6s, and put in a 4-input selector switch, volume pot and jacks for four inputs and one pair of speaker outputs.

It's the third one of these conversions I've done, from AMP132s, AP142s, AMP150s and/or AMP179s. It results in a good, solid, 20w/ch amp (even more into 4 ohms), that sounds darn good.

BTW: The left-most image in my signature picture, is one of these. I think that was the first one I built like this, as an integrated amp.

Regards,
Gordon.
 
As for converting to monoblocks- that would be considerably easier than what I describe above. You can delete the bi-amp circuitry and the high-frequency amp, and rewire the rest as a Magnavox AMP142. The AMP150 is essentially the same chassis as the AMP142...

IME and IMHO, a good tube preamp is a very wonderful thing. In fact, I think that most of what we attribute to "tube goodness" in a tube system, comes from the preamp, more than the amp. The preamp is the "nerve center"- get that wrong, and no amp can make it right afterward.

OTOH- tube phono stages can be tricky. Cheaper ones tend to be noisy (hum, hiss, etc), and better ones more expensive.

That said- a modded Dynaco PAS (with the tone controls removed and the line stage converted over to buffered output) is one way to get into a tube preamp with a very good phono stage, for relatively cheaply. It could be a good match up with "monoblocked" AMP150s...

Regards,
Gordon.
 
Thank you Gordon. That is some great information. You've got me thinking now. I'm just learning about these things, but I do have an avionics background from my Navy days just never tubes. So, I've been watching some Uncle Doug on youtube, and picked up some literature. But, no one puts the exact information I'm looking for like you just did. Do you do these mods as a business? While I really want to understand all of it and build my own, the amount of experience required and getting it challenges my patience greatly. I'm wondering about these other maggie stereo tube amps specifically if there is one that will sound as good as the 150aa's. It appears to me by the size difference in the transformers that the 150aa might have more kick but I really don't know. A stereo tube amp might be alot easier for me to begin with but I definatley want some decent power.
 
Just another possible thought, if you have speakers that are bi-amp capable, you could run them as designed. The trick will be using speakers that work out well with the stock crossover point, which I want to say is somewhere around 1300 hz. There is a limited amount of adjustment possible within the crossover box, but honestly I don't know how much it will give you. The adjustments are under that box on the front corner, its an adjustable slug in the middle of the coil.
 
gadget73 said:
Just another possible thought, if you have speakers that are bi-amp capable, you could run them as designed. The trick will be using speakers that work out well with the stock crossover point, which I want to say is somewhere around 1300 hz. There is a limited amount of adjustment possible within the crossover box, but honestly I don't know how much it will give you. The adjustments are under that box on the front corner, its an adjustable slug in the middle of the coil.
Click to expand...

Thanks I didn't know about that. It looks like I have a few decent options which is good. If I went that way I might try to use the RP103. I would just have to get one more.
 
The horns in those did pretty good for mids, but they don't go way up. A tweeter in addition to the horn might round it out somewhat better.
 
GordonW said:
As for converting to monoblocks- that would be considerably easier than what I describe above. You can delete the bi-amp circuitry and the high-frequency amp, and rewire the rest as a Magnavox AMP142. The AMP150 is essentially the same chassis as the AMP142...

IME and IMHO, a good tube preamp is a very wonderful thing. In fact, I think that most of what we attribute to "tube goodness" in a tube system, comes from the preamp, more than the amp. The preamp is the "nerve center"- get that wrong, and no amp can make it right afterward.

OTOH- tube phono stages can be tricky. Cheaper ones tend to be noisy (hum, hiss, etc), and better ones more expensive.

That said- a modded Dynaco PAS (with the tone controls removed and the line stage converted over to buffered output) is one way to get into a tube preamp with a very good phono stage, for relatively cheaply. It could be a good match up with "monoblocked" AMP150s...

Regards,
Gordon.
Click to expand...

How much would you charge me to trick out my 150's with your custom mod?
 
I'm not really a speaker guy but I'm sure there is one. I have a set of small, inexpensive ones I put into a console to get some treble, but I picked them up secondhand for small coin. They were originally from I think Parts Express also for smallish money. Vifa maybe? Vila? Maybe Bob is making speaker components now.
 
Ntc76 said:
Good to know thanks. Any tweeter in particular that works well without too much $$$$$?
Click to expand...

I used the "1005" piezo horn (only about $5 or so each) in the Hatu speakers I designed. It used the Magnavox horn for mids. Here's one that's like the one I used:

http://www.parts-express.com/grs-pz1005-3-1-4-piezo-horn-tweeter-similar-to-ksn1005a--292-442

Crossover was easy- a .47uf cap, a 20 ohm resistor to ground, and a 2 ohm resistor in series with the tweeter. If there's too much "sizzle", you can substitute a .33uf cap for the .47uf. If it's too 'dead', you can use a larger cap, such as a .68uf or 1uf. Just connect the crossover/piezo in parallel with the horn.

Regards,
Gordon.
 
A next-step upgrade above the piezo would be something like the Eminence APT150 supertweeter.

http://www.sweetwater.com/store/det...e=&network=g&gclid=CLuQvd-8qc8CFYc6gQodyosGlw

It's larger, so it would take some packaging to fit in some locations. A 1.5uf cap (1uf if there's too much overlap with the existing horn), and a 15 watt L-pad (to adjust level- it will likely be louder than the Maggie horn) should do it, for mating with the existing speaker. Again, just wire in parallel with the existing horn.

Regards,
Gordon.
 
I have several amp 150s and 182s. The crossover is approximately 1200hz. I used these to biamp a pair of ESS AMT1As at the office and Klipsch KG2.5 MTMs for lows with a Heil Tweeter for the highs in my "lab". I need to add some EQ to the low end on the Klipsch setup to flatten the response but otherwise it is flat within a few db from 50hz to the limit of my test equipment.

I have been reading about schade feedback using a 6AU6 to drive the output stage. The Amp150 uses a single 7 pin base 6AT6 to drive the 6V6 SE stage. I would like to rewire one of my mono blocks to use a 6AU6 in place of the 6AT6 and employ schade style feedback in the SE stage.

I don't know how to design this driver stage. If I can get some help I will do the modifications.

Regards,

Steve
 
The 12AT6 would probably be better.

The 12AU6 has less gain than the 12AT6. Given that additional feedback (the addition of plate-to-plate feedback) will further reduce the overall gain- starting out with more, is a good idea.

Regards,
Gordon.
 
The 6AT6 is a 7 pin socket so I only need to rewire to use a 6AU6. I would have to drill a new hole and add a 9 pin socket to use the 12AT7.

I use tube preamps (not buffers) between the source and power amps. So my input to the power amps can be much more than the 2V RMS from a CD Player. How much gain do I need?


Thanks for the comments,

Steve
 
Sorry, I misread above. 6AU6- PENTODE. That will have LOADS of gain, depending on how you run the screens.

Using a pentode there- it'd be interesting to try and do some sort of "ultralinear driver"- running feedback from the output plates to the driver screens. I haven't personally seen that done- but theoretically, it should be possible. I have no idea if it'd be beneficial...

Regards,
Gordon.
 
I pulled two of the amp-150s I have from storage and put them on the bench. I will modify the high frequency amplifier on one to use a 6AU6 driver and add feedback from the 6V6 plate to the 6au6 plate and compare it to the other. If the results are worth sharing I will start a thread on it.

Steve
 
Will that make the sensitivity of the two amp sections considerably different? If so, how are you planning to deal with that?
 
Ok folks, between my own desire to learn by doing and my poor man's budget, I have elected to run the two AMP150's as bi-amp monoblocks. This leaves me doing some beginner level work, replacing caps, simple wiring changes to install channels outs and a power switch. Still not sure about the preamp.

Here's where you're gonna slam me...I think. I went on several websites (probably all the wrong ones) and filled my cart with orange drops and other expensive caps. Only getting enough for two amps, my cart was $160 plus whatever shipping was. Because I can't afford that, I went back on ebay. I found almost every cap I needed either from Austria, Japan, or good ole China, and in a quantity that I would have many extras, all for $60.

I'm just wondering, how much does it really matter?
 
Oh boy, this question. Put on your flame suit :)

Honestly I use a lot of basic yellow caps for coupling. The Illinois ones are perfectly serviceable and they're cheap. 716P orange drops also aren't stupid expensive, and from memory there just aren't a whole lot of parts in these. Probably what, 10 caps plus the electrolytics per unit? For electrolytics it ends up being any of the big names from Mouser or Digikey. I'm not super picky, it just has to be from some company someone has heard of, not One Hung Lo capacitor company.
 
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