Maggie refurb project

[radtech]

Finally getting started on refurbing this magnavox amp I pulled out of a console stereo sitting next to a dumpster about 10 years ago.

Stage 1: Schematic.

I worked up a schematic using Joel Tunnah's component images, came out pretty well I think.

[kevinkr]

Nice looking schematic, what tool did you use to draw it? Very clean looking.

[kevinkr]

Is that .0015uF correct? I know Magnavox did a lot of weird things with their amplifier designs, but I would have expected this to be .015uF at the least..

[radtech]

I drew it with some schematic symbols by Joel Tunnah, just copied and pasted into a drawing program (I used an old one called Picture Publisher, Paint or Photoshop should work) and connected them with lines. I used the program's text tool to add labels and did a little editing to get some symbols that weren't in Joel's library (the power trans., filaments).
Not the easiest way to do it, but I like the results. You can download the symbols here: http://www.joeltunnah.com/
I used the tektronix style set.

The .0015 value is correct, in this article, http://members.tripod.com/~gabevee/maggie.html
they use an even smaller one, .0012, apparently it works.

Another odd thing, the .0015 and 100pF caps in each channel appear to be factory original, but are not identical in both channels, some are tan and some are reddish, they must have just had bins of the same value by different manufacturers and just grabbed a few for each amp without regard for using matching brands in each channel.

[adason]

hi radtech

i am no expert, but i had refurbished two of those Magnavox amps
they had purposly different caps in each channel
each channel had different feedback resistors as well
only one chanel had brilance control

Magnavox never cared about flat frequency response of the amp
rather they tried to have balanced presentation all across
from turntable to speakers

i have many amps, but SE Magnavox from old console i have is the most musical amp i ever heard (not pp)

[radtech]

Here's a pic of what I'm starting with, still have a few decisions to make, such as should I convert it from 6EU7 pinout to 12AX7 pinout.

Also have a few options for the cathode resistor, it has one 4W 100ohm for all 4 6BQ5s, got hot enough to burn up a wire running under it.. I bought some 200 and 400 ohm 4W resistors, figure on either a 400 in each cathode, a 200 in each output pair, or two 200s parallel for all 4 tubes. Open to any suggestions.

[kevinkr]

I had one of those old stereo Magnavox SE amplifiers in high school. Had a pair of 6BQ5 and IIRC a single 12AX7 (not 6EU7) It sounded way better than it ought to have. These days I have 3 dht based SE amps and a pair of PP 300B monoblocks, that little amp was probably the start of it all, although I did not realize it at the time..

I would replace the 6EU7 with 12AX7, they are electrically identical except for filament configuration and pin out which of course is totally different.. LOL The 12AX7 option will give you a lot more choices in terms of tube rolling potential. Good 6EU7 might tend to be a little more costly as well.

I rebuilt a magnavox stereo pp amp with a quartet of 6V6 some years ago, it actually sounded pretty good once completed.

[radtech]

Yeah, I figured the 12ax7 pinout will give me more options and be easier to get.
You can see in the pic that there's also a hole cutout for another socket, could put a preamp in there maybe, although I'm thinking of using it as an amp for my Rockman X-100 guitar amp so I'll probably just need line level.

[adason]

hi radtech
yours look a little rusty, but I am sure you will take care of it
here is a pic of mine, serving me well for some years
all I have done was new tubes and electrolytic capacitors
all other parts are original
and I added side wood bars

[bcrescen]

Hi Radtech,

Thanks for the advice in the other thread. I have been listening to the amp for a few days and it sounds nice. I did notice some low frequency distortion when I cranked it up playing music with a lot of bass. Do you think this is tube related or leaky capacitors? I ordered replacement capacitors and will not use the amp until I install them. Could you provide more detail or suggestions on the following?

Connection details on how you will replace the single 4 watt resistor multiple resistors.

Tube replacement. Should I replace all tubes or just the EL84s? Any suggestions on reasonably priced tubes (Ebay vintage or Partsexpress new)?

I would also like to replace the power cord. Any thoughts on adding a fuse and the fuse value? Should I use 2 or 3 prong power cord?

Thanks
bcrescen