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Magnavox 8303 SET Tube Amp

scorron

scorron

Member
I wanted to share a few photos of the 8303-20 Magnavox tube amp I just finished converting into a stand-alone and thank Audiokarma for some posts that helped me decide to cut out the tone controls ( which freed up a lot of space under the tiny chassis, and made it possible to fit the switch and pilot on the front panel). This came out of a suitcase model record player (circa 1960) with a Collaro changer, two 8" Heppner woofers and two 5" Magnavox tweeters built-in.

I replaced the power supply capacitors with new Nichicons by using a jeweler's saw to cut through the can and drilling holes for the leads. Then JB Welded the can back in place.

The sides are Patagonian Rosewood, not my favorite wood, but the grain on this was gorgeous.

The faceplate is polished black acrylic. I used a 12v incandescent lamp driven off the heater, purposefully to make the lamp very dim. I don't want a blinding pilot while I'm trying to watch tubes glow.

Blued the power transformer bell and painted the output transformers. Polished and clear lacquered the chassis. 5-way binding posts, fuse holder, and a new, reproduction cloth cord on the back.

Tubes are original Magnavox, except for one 6BQ5 which is a Sylvania (one of the Magnavox tubes tested bad).

Last night was the test run. At first, I had no sound from the left channel, but wiggling the preamp tube (after deafening me with pops) brought it in. Obviously I need to clean the sockets. It drove my Polk T15 bookshelf speakers with surprising ease, plenty loud enough for my little apartment at about 1/4 volume. I think I need to replace the volume pot, however, as it is not static free, even after a few applications of Deoxit, and the resistance on one channel is much higher than the other at everything but full volume.

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Nice build for sure, these small amps will really surprise you. Mine has 8ohm Opt's and a choke. Dave Gillespie redid mine and one of my favorite amps.
 
Thanks, all! Here are a couple more photos I like, to show off some of the detail (also every speck of dust). I need to replace the volume pot. I have a brand new Alpha 1meg pot ready to go in, but the threaded bushing is too short and I'll have to figure out how to make it work. I enjoy the fabrication, but sometimes you just get impatient and want to hear it play!

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I'm running mine stock to 4 ohm ADS 200's and am amazed every time I turn it on. I replaced the multi can cap with a Hayseed Hamfest one and put in new resistors, not really stock I guess, but nothing changed in the circuit. I would like to figure out how to add a output for a powered sub though. IMAG0907.jpg DSC01432.jpg IMAG0897.jpg
 
Thanks! After I'm satisfied that the electronics are up to snuff, I'll have to focus on speakers. I was amazed that it drove the Polk T15s with ease, so maybe efficiency is not as important as I feared. I notice the original speakers are paralleled 4ohm speakers, so I guess impendence matching isn't overly important either.

craig25 said:
I replaced the multi can cap with a Hayseed Hamfest one
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That would have been a lot easier. I just assumed I'd never find a replacement multi-cap can. I've done a few antique radios where I wanted to keep the stock can for looks, but this one was printed, not stamped, so I could have used any aluminum can.
 
Well, I hooked it up to my U-Turn Orbit last night after replacing the volume pot and using Deoxit on the sockets. It works great, noise free and only noticeable hum after about 3/4 volume. I think I can live with that.

Listened to Jack White's Blunderbuss and Peggy Lee's Black Coffee. This amp really brings vocals alive. Unfortunately, my Polk T15 bookshelf speakers are not going to cut it. Not just because they're pushing the amp too hard, but they have no bass. That's actually always been my complaint with these speakers.

I listened to some Klipsch bookshelf speakers last night which I quite liked. Wondering if I should go that route, or vintage.
 
I wound up buying Klipsch KG 3.5 speakers off of Craigslist for $120, and the seller threw in an Apt Holman Preamp and an Apt Power Amplifier 1. The Power Amp needs repairs, but the Preamp is currently running my Magnavox 8303, allowing me to switch between my U-Turn and an AUX line for my iPad. The Klipsch speakers are true to form, very articulate in the highs, but still punchy in the bass, and they are PLENTY sensitive enough for this amp to play loudly.

The poor bass response was not wholly the fault of the Polk T-15s. I started tinkering with values in the coupling capacitors and in the Negative FeedBack loop. HUGE improvement once I replaced the coupling capacitors between the preamp and output tubes with .022uf orange drops. I've attached the current schematic for posterity (though I haven't tested voltages yet).

I've used only my ear as a test instrument so far, but with .03uf coupling capacitors, it sounded like the output was muffled during periods of deep bass, leading me to believe the output transformers were saturating. So I dialed it back to .022uf which still seems to have the same audible bass extension (and should, according to calculations).

Increasing the NFB got the volume pot into a more comfortable range, and presumably reduced distortion, which I never perceived anyway, at normal listening levels. I briefly experimented with adding .1uf bypass capacitors back into the NFB and discovered that I hated the sound. It was definitely a "vintage" sound: boomy bass and muddled in the highs.

I don't knew whether the capacitor change or the tweak in the NFB is responsible, but there is now no hum whatsoever, at any volume level.

I'm pretty happy with the outcome and will probably use this amp as my main system for a long time. Classical and jazz sound amazing, even though I'm not a fan of either genre. Piano notes really sound like they're in the room with you. I'm still not enthralled with the bass response, but the vocals, strings, and brass instruments sound so clear and pleasant that I can live with it.

I do perceive a different in sound and volume from the two old 6BQ5 tubes I'm using, one Sylvania, one Magnavox. Can anybody recommend a new production tube that sounds good in a hi-fi application?

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Yeah, I've heard Sovtek EL84M too. Don't get the non "M" version though.

I'm happy with my quad of ElectroHarmonix 6BQ5.
I have some Reflektor 6P14P-EV on the way too. I've heard they are the cat's meow.
So far, I've tried Magnavox, GE, EH and honestly, they all sound about the same and all very good. So, it will be difficult to go wrong with any reasonably priced EL84.

BTW, well done on the amp. Looks fantastic. The paint on the transformer looks brand new, the finish on the wood is beautiful and the multi-section cap is super shiny.

Enjoy

Getting parts together for a DG-SE1 myself so I'll be joining the club soon.:beerchug:
 
jdsalinger said:
BTW, well done on the amp. Looks fantastic. The paint on the transformer looks brand new, the finish on the wood is beautiful and the multi-section cap is super shiny.
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Thanks! The paint on the transformer is new. I faced it briefly on a belt sander then repainted with black lacquer. I'm pretty happy with the way the amp came out. The only thing that bugs me a bit is that the lacquer on the chassis dulled the shine and left a bit of 'orange peel.' Next time, I'd probably desolder everything and strip/replate the chassis. Caswell plating makes some electroless nickle kits which would work great for this.

After seeing how simple this amp can be, I'm tempted to build one from scratch using Dave's improvements.
 
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