1959 Magnavox mini Trapezoid

[electroking]

Funny that they put a low-noise 7025 as the input tube. Any old 12ax7 will do
in this application. The factory may have gotten a special surplus lot of 7025s,
or the tube distributor tipped the design engineer so he would specify the higher
grade tube, who knows?

[kbmuri]

Magnavox tended to use the best stuff, even on their low-end products. I worked there for 13 years, a little later than tube amp days, but still back when pride in your work was a factor over raw profit. Doubt it was a surplus buy or greasing a buyer. A phono cartridge needs a lot of gain, and is inherently noisy, so the 7025 was a good design choice there.

It's probably too much gain for your iPod. The iPod will overdrive the 7025 and it'll distort. You might want to try a 12AU7 instead. 12AT7 next. 12AX7 last. 12AU7 probably. If the 7025 tests new, selling it on that ePay place will pay for your isolation transformer (they can be found on that ePay place too).

I have a high-end Magnavox mono phono from the pre-stereo days of circa 1950. 12AX7 input preamp driving a quad of 6L6's (parallel push-pull) into 5 speakers, in a nice hardwood piece of living-room furniture. I don't use it for vinyl but occasionally plug my Fender Strat (guitar) into it. It screams quality. I think your S/E 50EH5's will "suggest" quality and you'll be pleased with the outcome if you go for it.

Your hot-chassis issue won't quite be solved by enclosing the amp in plexiglass and wood. If the chassis is hot, the cable shielding will likely also be hot, meaning the ground-side of your iPod will also be hot. Which may or may not be the case of your iPod. Risk is there for shock handling it, risk of damaging the iPod too.

Safety #1 is the isolation transformer. Lesser but cheaper alternative #2 is .022uFd blocking capacitors in the line leading to your iPod. The cap solution works for guitars, but might be a fidelity issue on your iPod up in the highs or way down in the lows. I haven't gone there, maybe another AKer has experience here.

[electroking]

Not to start a big argument, but the ceramic cartridge used here does not need
a lot of gain, and if a source is noisy, a low noise tube is not helpful. The 7025
is fine as a preamp tube if a magnetic cartridge is used, but I maintain that any
good quality 12aX7 will do perfectly here.

Good day to all.

[Jeffhs]

The hot-chassis issue is not restricted to Magnavox as any series-string radio, TV or phono amplifier will have a hot chassis, depending on which way the plug is inserted in the wall socket. I have a Zenith MJ-1035 stereo FM radio with phonograph inputs, supposedly isolated from the line by a capacitor. However, I would not think of connecting anything to those jacks without replacing that isolation cap first, and even then I'd think at least twice before plugging any kind of modern electronic gear such as an iPod into them. As was mentioned, this is (or could very well be) a disaster waiting to happen. I wouldn't plug anything other than a stereo turntable into those jacks, even after replacing the isolation cap. The risk of shock and/or equipment damage, in that order, is too great.

BTW, the preamplifier stage in my MJ1035 is in fact a 12AX7, the output stages are 50EH5s, and the radio sounds absolutely great, except for some background hum (probably a loose or missing ground connection). Zenith obviously did not skimp on much of anything, IMHO, when they designed this set, the speaker issues (which have been discussed at length in this forum) notwithstanding. I personally cannot tell the difference in sound between the MJ1035 and my C845 (the latter has the same speaker system as the MJ), except that the MJ sounds more mellow than the C845 because of the MJ's larger cabinet. I have found, after doing some research on the MJ1035, that the radio uses the same chassis (11J01) as Zenith's console stereos of 1965 vintage. I'm not surprised. This chassis has "high fidelity" written all over it, not to mention a front end hotter than a firecracker (I was listening to CBC Radio One from London, Ontario, Canada on my MJ1035 this afternoon, and I can get stations from Windsor and other SW Ontario cities, not to mention Detroit, Toledo, Ohio and Erie, Pennsylvania on this radio as well from my apartment in northeastern Ohio, using just a six-foot length of wire as an antenna).

The only thing I wish Zenith had included on the MJ1035 is external stereo speaker jacks. If this set sounds as good as it does through its own speakers, I would think it would sound as good as any hi-fi system if it were driving high-efficiency speakers. The only problem I can see with using any other speakers with an MJ would be that the impedances are non-standard; the main speaker is 4 ohms and the HF driver is 46 ohms. While replacing the main driver with an 8-ohm speaker won't harm the output stages (it will only reduce the output volume a bit), the tweeters are a different story. Putting any type of speaker of less than 46-ohm impedance in this position in an MJ1035 (or any type of audio equipment, for that matter) is asking for trouble, as the use of a speaker of lower impedance than the original will damage the output transformer and possibly other parts of the output stages as well.

[zenithfan1]

You're right Jeff, I think plugging in an iPod directly might be unsafe for it although I've done it with no harm, yet. I listen to my iPod with an FM transmitter {when I actually use the damn thing} I hate using those little headphones. Those C845s are nice, I'm listening to the Drive in Chicago right now, these radios are some of the best Zenith made. That hum issue with your MJ1035 could be a 'lytic going bad, I've had a couple Zeniths do that over the years.

[kbmuri]

Ok, no argument. I just know how those engineers thought, and they wouldn't have designed a 7025 where a 12AX7 would do. Agreed a low-noise tube would not fix cartridge noise. Perhaps in this particular cabinet design, the amp chassis happens to be mounted too close to the speaker's acoustic path and when testing the betas of this thing, the engineers got some ringing somewhere. Or maybe the speaker and amp structurally shared the same "sounding board" element of the cabinet. Whatever it was, they chose a 7025 to address it. My point is, I don't think Emerson or Wards Silvertone or GE went to that level. Magnavox went the extra mile. All their stuff, to this day, is better than the average bear.

[JGlenn]

I know this is a pretty old thread now but I just picked up a maggie suitcase phono that uses this same amp. I'm wondering if you ever recapped yours? I got together the caps to swap out the old can but I cant find the markings on it anywhere. I thought maybe somebody would have some insight on this ? theres a cardboard cover over the can but its just the same marking under that.

[Reece]

Quite typical to have a 20-50 ohm resistor in series with a diode power rectifier.

Reece