Fresh Off the Workbench: Magnamutt II Integrated Tube Amplifier

GordonW

Speakerfixer
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Just finished this.

magnamutt_ii_front.jpg

It's another in my series of stereo amps built from the Magnavox AMP150/AMP179 chassis. Originally mono PPP 6V6, I have converted many of them to PP 6L6 stereo integrated amps. This requires a matched pair of output transformers, and basically starting over wiring everything.

In this case- the original output transformer from this one was taken away to build an earlier stereo integrated amp on an identical chassis (which 63pontiacGP has now), and the power transformer was taken to assist someone in resurrecting a Magnavox Concert Grande amp (an extremely worthy reason to rob it from this amp!)- so I was left to my own resources, to find a suitable transformer set.

magnamutt_ii_rear.jpg

I discovered, that with a little trimming, I could install the output transformers from a Heathkit AA100 amp (which I have acquired from that auction site a while back), and a power transformer from a Harman-Kardon A700 amp (which I had acquired from an antique radio guy, many years ago). These turn out to be perfect, to build my favorite 6L6 PP integrated amp circuit- the right voltages and impedance to work just like I wanted.

The replacement transformers also led to the name of this amp- Magnamutt II. I came up with the "Magnamutt" name a while back, to describe an 6V6 PP integrated amp I created on a Magnavox AMP175 chassis, but with a set of Thomas organ output transformers and a color TV power transformer. Since it wasn't all Magnavox iron anymore, it was a "mutt" of an amp... and hence, it needed a new moniker...

magnamutt_ii_opt.jpg

So, I used my circuit (substantially modified Pilot AA902 layout) that I've used many times- this time with fixed bias (the first time in one of these Magnavox chassis). This uses a four-input source selector (with three sets of inputs on the rear, and one on the front for convenient access for portable music players and such), my "stupid simple" preamp (12AU7), and the Pilot amp section (12AU7 again), but with the circuit values heavily tweaked to decrease gain and increase drive (higher plate current) in order to reduce distortion. It uses about 13dB of feedback... a good balance between good clipping behavior and good bass damping.

magnamutt_ii_under.jpg

Since it was fixed bias- I had to construct a bias supply. Unfortunately, the HK power transformer did not have the correct voltage on it bias winding for this (not enough AC voltage to make the needed -40V DC that I needed, after rectification)- so I used a 30VAC transformer, that originally ran the motor in an old turntable that I had scrapped years ago. The bias supply uses a bridge rectifier diode setup, and three stages of filtering, feeding individual bias pots for each output tube.

magnamutt_ii_bias_supply.jpg

By modelling, I was able to determine that with the B+ voltage (about 375v), and the OPT impedance (about 7.5KCT), that a screen voltage of about 325v and an idle plate current of about 44ma per tube, would give great results- in theory, about 32 watts output, and including normal losses, a good 22-25w/ch or so. Exactly what is needed, for these output transformers to perform at their best.

magnamutt_ii_above.jpg

To finish it out, I used some ABS trim to cover over the area around the preamp tubes, and made up some oak trim to cover the front and sides, like is my normal practice on these amps. The only big cosmetic difference on this one, compared to my earlier one, comes from the fact that the fixed bias supply occupies the area where I normally put the power supply caps underneath- so, on this one, I used small can caps and clamps, mounted in what were previous tube socket holes in the original Magnavox amp design. I think it came out very well- I may just do this on all of these amps, from now on...

magnamutt_ii_lf.jpg

In all, it came out well- the sound is pretty incredible (the bass is outstanding- these Heathkit transformers do have that over the original Magnavox transformers)- very dynamic, very smooth, great imaging. I am quite happy with the way it turned out!!

Regards,
Gordon.
 
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Looks good. I like the red/black output connections too, makes a nice match with the inputs for indicating which is what.
 
Yep- the output connections are, from left to right, 16 ohm +, 8 ohm +, 4 ohm +, and common -. The top row is the right speaker, the bottom row, the left.

I always try to make it as intuitive as possible...

Regards,
Gordon.
 
I appreciate that, if for no better reason than the factory amps aren't always intuitive. I have stuff with mixed channel and side markings which can be annoying. One amp I tossed together out of parts bin crap I simply color coded the inputs and outputs with a sharpie. The red input jack mates with the red output terminals.
 
Beautiful. Do you have a schematic that you could post, with the fixed bias arrangement?
 
Update on this- I decided that I both needed a little more gain- and that the amp might benefit from a little more feedback.

So, I converted the amp gain stage to a mostly-bypassed cathode, instead of a totally unbypassed cathode. A cap across a "bias"/cathode resistor, and two resistors for the feedback voltage divider (from the output transformer secondary to ground), with the bias/cathode resistor connected at the junction of the two divider resistors. By doing this, I was able to get a bit over 4dB more open-loop gain- which allowed increasing the input sensitivity by close to 3dB, while increasing the feedback to just over 14dB, simultaneously.

Man, THAT made a difference. Louder, a little better tonal balance, and the bass actually made my CD player skip, once. Even more solid bottom end. Not quite like the "Dual Quad 84" amp did, but close. And out of one OPT and a pair of 6L6s per channel (whereas the Dual Quad 84 amp required two OPTs per channel to do what it did). Got to be close to at least 25w/ch of clean power, here, now. And, its clipping behavior is pretty exemplary, still- it's very difficult to hear it clipping, even when you see the waveforms start to "flatten out" on their tops and bottoms on the scope. You definitely can use every bit of the volume control on this one. :D

Regards,
Gordon.
 
Very cool Gordon! I used those same output transformers with my Fisher SA-100 Clone several years ago, and they are indeed superb performers. I have no doubt that your repurposing effort with them in your Magnamutt II produces stellar results! Congrats on a very neat project!

Dave
 
I've gotten to listen to this amp quite a bit more now.

Been playing it through a Creek Evo CD player and a pair of Tannoy Definition DC8Ti floor-standing speakers- it's quite remarkable how well this thing will image, and how well it does with dynamic scale. Must have hit some sort of sweet spot with the combination of output tube, output transformer, operating point and circuit tuning... in fact, other than being less power- I think it's every bit the equal to the Eico "Hot Rod 6550" amp (based on Eico HF70 output transformers) that I just finished (in fact, I played them back to back- the Eico-based amp with a VAC Renaissance V preamp, and the "Magnamutt" all by itself).

It's going to be hard to top this one, with these Magnavox-chassis integrated builds, after this...

Regards,
Gordon.
 
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