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Bridging Magnavox Stereo Amps?

MarkNJ

MarkNJ

Active Member
As I think is the case with many AKers, I troll craigslist and freecycle for Magnavox consoles. As a result, I now have one 8802 series and one 9302 series amp.

I've done some mods to the 9302 and it's a pretty good stand alone amp at this point. I've wondered if it's possible to take two matching series maggie tube amps and convert them to bridged as if they were a pair of monoblocks. This would turn the 15-20 WPC channel amps into a 30-40 WPC stereo setup, right?

Anyone done this before? I'm assuming it's important to get the same series, as they would be mis-matched in terms of power. (Both of the ones I have are PP, though, right?)
 
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You could by using a phase splitter stage right before the amp and connect ohe output to one amp channel with the reversed phase output connected to the other channel. Then you will need to tie both grounds from thw output transformers together and then connect your speaker to the positive speaker output terminals. The positive wire going to the in phase side and the negative speaker wire going to the reversed phase side. Keep in mind the output impedance of the amp will be double what it was before.

I don't know how well it would work or sound though.
 
Tube Radio said:
You could by using a phase splitter stage right before the amp and connect ohe output to one amp channel with the reversed phase output connected to the other channel. Then you will need to tie both grounds from thw output transformers together and then connect your speaker to the positive speaker output terminals. The positive wire going to the in phase side and the negative speaker wire going to the reversed phase side. Keep in mind the output impedance of the amp will be double what it was before.

I don't know how well it would work or sound though.
Click to expand...

I don't know that I followed that totally but is that essentially making it a push-pull circuit out of the two and combining the two SE opts as if a PP opt?

EDIT: Oh wait, the referenced amps above are already PP.... Nevermind.
 
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you'd still end up with something that effectively made the amp pairs into push-pull. I'm not totally sure what the topology would be called though. Results may be interesting, though its hard to say how it would actually sound. Even if you don't have two identical amps right now, you could use both channels in one amp to come up with a bridged mono amp that would work the same.
 
Anyone care chime in with to the work/effort to just rewire as parallel push pull (PPP). Would that be a realistic option with the parts at hand or is it not worth the effort compared to above suggestion? Prolly need a beefier OPT though eh?
 
Yes, I was wondering if this is something that would be done permanently or is there a way of switching back and forth with relative ease. I think I remember reading that there are some brands (NAD, maybe?) that go into bridged mode with the flip of a switch. I'd be happy enough doing whatever rewiring was needed.
 
Most Magnavox stereo tube amplifiers which use four output tubes (2 for each channel) are about 10 watts-per-channel. The one using the four 6BQ5 output tubes are about 12-15 WPC. There is a Magnavox (142 I think) amplifier which uses four 6V6 tubes in push/pull/parallel for 20 watts total mono.

Offhand, I don't have a schematic for one (maybe Tom Bavis has one) but you could try to wire a Mag stereo amp to push/pull/parallel, and remove both output transformers, and add a bigger one.

I never owned a Dynaco ST-70, but I have read that they can be bridged by connecting both +'s and -'s at the speaker output terminals.

There is also a Bogen MO-200 tube amplifier which uses eight 8047's, and two output transformers in parallel for an output of two hundred watts. I have read where people seperate these into a single 100 WPC using four 8047's and one output transformer for one channel, and the same for the other channel!

EDIT: Seeing batts post, I forgot to say that you do need up the input ohms on a replacement output transformer, but the general jist is to either wire up PPP, or wire the existing output transformers together. Many options to these tube amplifiers!

Otherwise, IMHO, leave 'em alone and enjoy them as they are!
 
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You would need different output transfomers for ppp , the originals would current saturate using four tubes on them with out much gain in power .The impeadance of the output transformer would need to be around 3.8K ct to 4.2K ct .

http://www.edcorusa.com/Products/ShowProduct.aspx?ID=558 .

You would need the transformers and eight small inductors to make them into stable push pull parallel amps and would remove one 12AX7 or 6EU7 .

http://dir.groups.yahoo.com/group/M...navox tube amps schematics/magnavox_amp142ab/
magnavox_amp142ab1.jpg


If you want to bridge them a bridging transformer would be the simplest .

http://en.wikipedia.org/wiki/File:SingleTransformerHybrid.jpg
 
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depending on what speakers you have, you also may want to consider bi-amping, using the larger amp on the bottom and the smaller one on the top.

Not the same as bridging from a wattage perspective, but lot of people swear by bi-amped setups.

IIRC one of the larger Magnavox consoles with set up to bi-amp the speakers...
 
The Concert Grand and at least one other ran bi-amps within one chassis. There were two different amps, one with an SE 6v6 for highs and a PPP 4x 6v6 setup for lows, and another with a PP 6v6 for highs and a PPP 6x 6v6 low frequency section.

The 9300 would make a rather lovely mid-high freq amp. Its a bit lacking on the low end but the mids and highs are very smooth. I don't know anything about the 8800 series, but if they have comparable sound they'd probably also be well suited.
 
kirk57 said:
depending on what speakers you have, you also may want to consider bi-amping, using the larger amp on the bottom and the smaller one on the top.

Not the same as bridging from a wattage perspective, but lot of people swear by bi-amped setups.

IIRC one of the larger Magnavox consoles with set up to bi-amp the speakers...
Click to expand...

What I would do. Not really dual monoblocks, but kinda.... Seems easier too.
 
Ive got a 142 amp that has the PPP setup, 20 watts mono with 4 6v6's and a fat piece of iron, and my imperial, has one 81-01 amp, stereo bi amp. PP 6v6's for the lows and a SE 6v6 for the hi, 2 channels worth with the crossover built in

Mark
 
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