• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Layout challange: Dual Maggie AMP148 based.

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Having played with the small 9302 Magnavox 6BQ5 PP stereo amp for awhile, I've caught a second case of tube amp syndrome. I heard of another Magnavox tube amp used in Imperial and Concert Grand. Amp model is AMP-148. For those that don't know it's a bi-amp monoblock using six 6v6 PP for the lows (35w) and two 6v6 PP for the highs (15w), two 5u4 rectifiers, and two other 9-pin tubes probably 6EU7s.

I intend to find a pair of these (one already found) and make one large stereo (dual-mono) amplifier chassis out of it.

Any of you guys care to suggest some layouts based on the photo below?

Total chassis mounted parts count:
2 - large power transformers
2 - large output transformers
2 - small output transformers
16 - 6v6 output tubes
4 - 6eu7/12ax7 for pre. and phase inv.
4 - 5U4 rectifiers
6 - capacitors



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Everything on one chassis. It's 2 channels in bi-amp configuration. There are 4 separate power amps and 4 outputs (2 left, 2 right) but only 2 inputs. I may eventually separate them and have the crossover in the preamp but I'm going to try it the way it is first.

Here's the first one I whipped up to start with.



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The larger output transformers will work with a pair of 6L6GB which would save you a lot of space . Most likely not all the 6V6's are good and would have to buy tubes anyway .

I have another 6L6 amp already. This one is partially for show. It has an awe factor of "look at all those tubes!". One thing I've considered is the location of the output transformers pairs (low and high). They should be next to each other. This way it's easier if I wanted to eliminate the bi-amp configuration and do eight 6V6s PP full-range on each channel. 2x6L6 or 8x6V6. Which has more potential?
Careful about eliminating the crossover at the speakers! In most cases doesn't work well. A better approach is a mild crossover at the preamp + its own proper crossover at the speakers.

What I mean about crossovers in the preamp is moving them out of the Magnavox and into a preamp. I assume there's a factory crossover of some sort in the Magnavox set to who knows what. If I relocate that into a preamp, I'll essentially have a 4ch full-range tube power amp having 2x35w (6x 6v6 each ch) and 2x15w (2x 6v6 each ch). I'll keep some caps on my tweets/mids for protection.

I'm interested in seeing (hearing?) how separate low/high output transformers will sound. There are good single op transformers out there but theoretically wouldn't separate opts be more efficient over a wider frequency range?

How does the 6v6 compare to the 6bq5?
How does the 6v6 compare to the 6bq5?

As a power device 6BQ5 is more flexible and efficient. This doesn't mean the 6V6 in not good but, for instance, I found that UL connection really doesn't give any advantage over tetrode connection. On the other end the 6V6 can be a nice powerful cathode follower driver and can be used down to very low plate voltage as a line preamp. I have some very nice 6V6's like JAN-RCA 6V6GTY with true micanol base (not just colored brown...) and gray glass from 1958 and wouldn't swap them with any 6BQ5....:D
Picked up another working EL84 PP amp today. This one is a Motorola SK18M with a HS711A stereo amplifier. It has much larger transformers then the Magnavox amp and uses 12AX7 pre/PI.

Anyone have a schematic for it?




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May I ask what's the motivation to fit everything onto a single chassis? Would it not generate more heat, increase cross-talk between the channels, and break your back when you need to move it? :p


um... I'd rather move 60lbs once then 30lbs twice? :D It just looks impressive. The odds of crosstalk are pretty slim considering it's two complete mono blocks in one chassis including independent power supplies.
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