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Grundig Majestic Console Amp Meets i-Pod?

Maxamillion

Maxamillion

Super Member
I picked up a Grundig Majestic Console MS 50a U/S a couple months ago, and got around to doing something with it today. Since the cabinet was so-so I've rescued the amp and multiplex decoder module, along with the tubes from the rest of the unit and the speakers. Nice Telefunken and Siemens tubes, including three Telefunken ECC83s that test strong. The speakers are three-way on each side with elliptical woofers, round cone midranges and electrostatic tweeters. I even kept the crossovers for reference.

The amp is interesting - it has ELL80 output tubes (test good) and ECC83 driver tubes in a compact chassis. It is hinged so the top section with the circuit board swings up for easy access to the selenium rectifier and the bottom of the board. Pretty neat (see pictures below). I figure it's about 6WPC.

Plan is to make it into an i-Pod dock for my wife to use in the office. I figure a good cleaning and a nice cabinet around the two ends with a cutout in the middle to expose the tubes will look nice. Of course it will need new electrolytics and a silicon rectifier at minimum. I'm not sure the i-Pod will be able to drive it directly, but I can always put some input transformers in there for impedance matching and/or step-up. We'll see. In any case, a nice winter project coming up.


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Yup, a push-pull amp in a bottle, so to speak. I figure with the ECC83 pushing each ELL80 there will be plenty of gain for any line-level input, but we'll see.

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I've completed the re-cap of the Grundig amp. All of the old ERO FOL caps were replaced with PP, and the cathode bypass caps were replaced with new electrolytics. One 600ohm power resistor was burned out so that was replaced with a pair of Mills MRA5 resistors in series that I happened to have on hand.

For the power supply, I replaced the 100-100-8uF can cap with individual 100-100-10uF caps, and replaced the selenium rectifier with a 1A, 600V silicon chip rectifier followed by a 180ohm 10W cement wirewound resistor to drop the voltage to the same level as stock. The new PS is all on the little tan-colored perf board in the picture. I unbolted the AC inlet and fuse holder from the side of the chassis near the PS board and moved it to the back of the unit for later change-over to a standard IEC inlet.

I also removed the socket and associated wiring for the preamp and tuner circuits just to clean things up a bit.

The amp works just fine as is and has TONS of gain, as little as 50mV in produces a loud signal out, so I plan to use the original 2.2M volume pot from the console on the input, and maybe switch out the 12AX7 input tubes for 12AT7 or even lower in the series. Now we go into waiting mode as the wife searches for the "perfect box" to enclose the amp in. The plan is to remove the amp board from the chassis and mount it underneath the top of the box so that the tubes stick out through holes in the top. I'm looking for an i-pod cradle to mount on the box for the input.

Stay tuned, but be forewarned, the search for the perfect box may take a little while...

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So I did some more testing with this amp, and with a 1000Hz sine wave fed to it and an 8 ohm load, it just starts clipping at 1V input. That just happens to be the maximum line output voltage of the i-pod, so I guess it's a very nice match gain-wise.

Listening to my wife's i-pod with Koss M/65 plus mini-speakers the amp is capable of sufficient volume for the room it will reside in (home office) and is able to throw a nice soundstage. More efficient speakers would of course provide a lot more punch, but the wife is happy with the volume levels as-is.

Still looking for the box, by the way.

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Cool. Not hugely common for those German consoles to have stand-alone amp sections. Often its all one chassis that isn't easily converted to a use like yours. Good to hear you got something useful out of it.
 
Yeah, it's like it was made for this purpose, gain-wise. A pair of higher efficiency Klipsch or single driver speakers would be an excellent match; the current speakers are small acoustic suspension units and can still achieve solid listening levels, but no crankage with 8.5WPC. That's just fine with the wife, though.

Since the power supply was meant to feed through to the preamp and tuner sections, I've had to drop the heater voltage from ~6.8V down to ~6.3V using 0.12 ohm resistors in each leg of the supply, due to the decreased current draw. Don't want to burn up those nice Telefunken 12AX7s...
 
Is there a voltage tap for 127 volts by chance? I have a couple German radios and they seem to have 110, 127, 220 and 240 voltage selections on them most of the time. No idea where 127 was used in the 50s but it seems about perfect for modern line voltages in lots of places. If not, if theres an extra heater winding you can wire that in as a bucking coil to drop the voltage without the resistors.
 
According to the schematic I have (and the wires I see) there are only 115V and 220V windings. 115V is close enough, the little resistors were on hand, and it all works very neatly. I thought about using the extra heater winding in that way but instead went for the simplicity of resistors.
 
So we've found the box (dovetailed pine) and I've got a nice .062" copper plate for the top. I'm adding a GlassAudio 3-position selector switch so that we can attach other sources as we desire.

In terms of charging the ipod, I'm planning to use the unused 6.3V, 2A heater winding on the PT to charge the ipod when it's in the cradle (was used for the tuner and preamp sections of the console unit). I'm thinking I can rectify that, smooth with a cap, pass through a 7805 regulator chip, and run it directly to the +5V USB pin in the dock, which is what the ipod touch 2nd gen and higher uses. That way there will be no need for an AC adapter either. Of course this depends on whether I can disassemble the dock and get to the appropriate pin without destroying it. Stay tuned.
 
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So, chargiing the ipod was not as straightforward as it seemed. First off, Apple has changed its charging scheme to make it more difficult for third party cradles to charge the newer ipods, so I went out and bought an Apple Universal Dock, which also has a nice little remote for convenience. I did not want to use the USB wall charger, though, since it's bulky, throws off heat and noise, and just seemed like a lazy solution, so I was still determined to use the extra heater winding on the PT for charging.

It turns out that to run the dock and charge the ipod with the USB connector you must have the proper voltages on the USB data +/- pins as well as +5V on the + pin. After doing some digging I found out that +2V on each of the data pins will tell the ipod to charge at 500mA, so I built a little board to rectify and filter the AC from the unused heater winding and also supply the +2V needed to the data pins using a voltage divider from the +5V (see pictures, two resistors are on the back of the board). A divider of 75kohms in series and 49.9kohms in shunt works perfectly.

I used a bridge rectifier I scavenged from an old piece of lab equipment at work, a 3900uF, 16V smoothing cap, a LM2940CT-5.0 low dropout voltage regulator, and a 100uF, 10V cap after the regulator.
 

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Of course I needed to install the volume control and input selector in the box; here is a picture of how that came out. As you can see, I also placed a grounded piece of TI-shield between the power supply and input selector to limit noise pickup from the PS.
 

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To mount the ipod and give the box more strength I cut a piece of 3/8" plywood with a "tongue" in front to support the ipod, and bolted that through the bottom of the box and the amp chassis. Some copper tape around the edge of the plywood gave it a nice finished look. Feet are copper-finished drawer pulls.

The knobs used were metal and from an old Zenith radio - the wife loved them as is, even with some wear on them. Also, she didn't want me to fix up the box at all, insisting I leave the screw holes from the lid hinges and clasp and any little dings that were there to give it a "used, retro" look; I did get her to agree to a couple coats of wipe-on poly to brighten up the finish and give a little protection from scratches.

The finishing touch was to move the tubes up to the top plate, which is a solid piece of .062" copper. The copper is finished with 800 grit sandpaper in a brushed look with no lacquer coating - the intention is to let it develop an aged patina to match the antique copper paint on the ipod cradle and screws. The pictures don't do the top justice, in the right light it sort of glows.

I made socket extenders with socket savers on the bottom end and new bottom-mount sockets on the top, connected with 18 gauge rubber-coated wire, mounted the sockets to the top plate and just plugged the socket savers into the existing sockets on the amp board.

Here are some pictures of the finished product. It sounds very good with the ipod input, and charges the iPod Touch Gen 4 with no issues. It sounds even better with a CD player plugged in, but I'll need some in-line attenuators to get some more range on the volume control with the higher output from the CD player, if we ever decide to hook one up.
 

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Very nice project. Its great to see this stuff re-purposed into something useful rather than being tossed in the landfill. Also a big hats off for figuring how to use the existing power supply to charge the ipod without cheating and using the wall wart.
 
I gotta give you some props here, really nice work. I am local to you, live in Asbury, NJ. Saw you live in B-Burg in another thread.
 
Nice!!!!
iPod dock is excellent idea. Can the iPod be charged while inserted?
Thanks, P.Z.
 
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