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Old EL84 transformers

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3/8/1998 12:01 AM

Old EL84 transformers

I just recieved the innards of a 62 magnavox console radio. This was the sweetest sounding radio I have ever heard. Why my friend demolished it I will never know, but his loss is my gain, as it was powered by 4 EL84's and a 5U4GB rectifier. I havent measured anything yet, but it worked when he butchered it, so am I correct in assuming I could make one heck of a nice amp out of its guts?

It has 2 identical output trannys each pushing four ohms. Its choke is Marshall size and its power tranny seems to have all the necessary supplys for a guitar amp.

I've never built an amp before but I've done some mods and built some effects, as I've got basic knowledge of electronics. I'm just wondering if it seems I've got a good starting point for my first project here. Maybe a Vox or Matchless, or idealy a Marshall 18 watt (could I possibly get away with using the power tranny for only two EL84's?). any insight or tips would be appreciated.

3/8/1998 12:14 AM
JR	oh, about using the tranny for an 18w marshall. i guess the right thing to ask would be if a ez81 retifier tube (as in the 18w) would be compatable with atranny for a 5u4? jr

3/8/1998 10:04 AM
R.G.	I think you can use the EZ81 in there, but it has a 6V filament, so you'll have to run it from the same 6VAC filament supply as the other tubes, not the separate 5V filament that a 5U4 uses.

3/9/1998 3:05 AM

I have too much respect for RG's many years' of good advice, to make a public disagreement here... but I'd be very cautious about running a tube rectifier heater off the same winding as the amp's other heaters. I once requested this from my local one-off winder (semi-retired from a lifetime making trannys) and he just refused to do it! He delivered the ordered tranny with _two_ 6.3V windings, explaining that he'd had to fix too many disasters where the heater-cathode insulation had failed in an EZ80 or EZ81 rect tube on a shared 6V heater supply. (With a separate winding, it hardly matters.)
I don't know an exact 5V equivalent for an EZ81, and my US friend who builds such amps hasn't found one either. The closest in sound was two 5Y3's, but you probably wouldn't have enough space or heater current. A single 5V4 will run an EL84 p-p amp nicely. ( That's Five-VEE-Four, not a typo)
Alternatively, if there's room between the windings and the window in the iron, your tranny probably runs on something like 7 turns per volt, so somewhere between 5 and 10 turns of _WELL_ insulated wire around the windings and connected in series with your existing 5V winding, would give you around 6.3 volts for an EZ81. Check voltage on load and add or take a turn or so. If you get _less_ than 5 volts, reverse the ends of the added turns.

- Stephen

3/9/1998 5:40 PM
JR	NO NEED TO WORRY ABOUT HEATING THE EZ81 OFF THE SAME LINES AS THE PREAMP TUBES. I FINALLY TESTED THE POWER TRANSFORMER TODAY. THE LINES TO POWER THE TUNER(WHICH I HAD PLANNED TO IGNORE) MEASURED OUT 6.3V. SO, NOW I HAVE TWO 6.3V LINES TO POWER THE HEATER FILAMENTS. JR

3/9/1998 4:46 AM

Why not just leave the power supply as it is for now, and work on building the rest of the amp first? That way you can see how it sounds with the 5U4, or some other 5 volt rectifier, before you decide whether you want to change it.

In fact, if the amplifier chassis (I'm assuming this is separate from the tuner chassis) is actually complete enough to be functional at this point, I'd leave the whole thing as it is to start with, and just add a suitable input stage for guitar. After that you can tweek it and add things bit by bit until the sound you want is achieved.

3/8/1998 10:02 AM

It is indeed the beginnings of a fine amp.

Good starting point. You might make both a Vox 15Watt and a Marshall 18W in the same case.

The power transformer is just fine at lower losds if you want to use it for only one channel.

By the way, the key on the extreme left side of your keyboard, probably just above the "Shift" key, the one marked "Caps Lock" will KEEP YOUR COMPUTER FROM DOING THIS IF YOU PRESS IT SO THE CAPS LOCK KEY GOES OUT.

It's a neat feature, just hard to find...

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