Magnavox amp conversion project

[Maxxarcade]

I'm working on a Magnavox AMP-175-55 to standalone conversion project.

When I tested it in all original condition, it sounds great with the exception of not having much bass. Comparing the schematic to other 6V6 push-pull amps, it looks like the coupling caps are quite small. Between the 12AX7 and the 6V6, there is 0.047uf caps, and between the input jack and the 12AX7 is 0.01uf.

On the Bogen DB110 for example, both sets of caps are 0.1uf. I've seen others go as high as 0.47uf between the 12AX7 and 6V6.

Is there any harm to be done by putting larger caps in the Magnavox? I also plan on replacing most or all of the resistors as well.

The other part of the project will be adding a preamp with bass and treble control. There may be enough room to add it to the same chassis, since there is quite a bit of unused space there, and even a spot for another tube socket.

Note that I have already removed unused components in the picture below. Also need to change the feedback network for channel 2 to match channel 1, and remove the balance pot. Afterward, I will install new power and speaker jacks, and a fuse holder.

Any advice is greatly appreciated

 

[avguytx]

Great little amp. I have the 9304-20 EL84 amp which is darn good sounding. There are many great thread in here in regards to the little Maggie amps and many people who are waayyy more knowledgeable than me to chime in.

Enjoy and Merry Christmas!

 

[rmunson]

Hey Maxx --
I was advised by a fellow ak'er to use .1's on that 12ax - 6v6 cap --

some pics:
http://347c.cla.umn.edu/audio/magnavox1_24_08/

Thanks for reminding me on this -- I'm in the process of recapping another 175 right now and forgot about those .01 caps --

are those little .01s just providing a certain dc voltage to the 12ax? why is a .1 or .47 better to use? Do they filter out an unwanted frequency?

 

[Maxxarcade]

Hey Maxx --
I was advised by a fellow ak'er to use .1's on that 12ax - 6v6 cap --

some pics:
http://347c.cla.umn.edu/audio/magnavox1_24_08/

Thanks for reminding me on this -- I'm in the process of recapping another 175 right now and forgot about those .01 caps --

are those little .01s just providing a certain dc voltage to the 12ax? why is a .1 or .47 better to use? Do they filter out an unwanted frequency?

Wow, great pics and info there. I had the same idea about using a second terminal strip at the speaker output/feedback connections.

I already have a set of binding posts, a new power cord, and a fuse holder to install. Once I get the electrical work done, I'm going to see about making it look nice too.

How is the sound quality/frequency response on these with the new caps, and original output transformers? If it turns out nice enough, I might put bigger transformers on it. I spent about $200 buying this amp and some parts, but I couldn't find one cheaper anywhere

 

[Raul]

In mine I replaced small electrolytics and coupling caps (with .047 russian pio's), and it sounds very nice to me. I didn't think about increasing capacitance as I'm a newbie in electronics. Maybe in the future I can try this mod. Here are some pics of mine just for your "cosmetic" inspiration:

 

[markus]

in this thread, GordonW explains some recapping 'upgrades' . . . i.e. using slightly larger values as you're suggesting . . . specific to the 175 / 185 magnavox chassis

http://www.audiokarma.org/forums/showthread.php?t=123337

 

[Maxxarcade]

Thanks guys, more great info and pics!

I did find a schematic here last night that shows some other mods, such as a pot for adjusting bias instead of the fixed resistor network. Gives me some more ideas to try in the future.

Regarding the discussions on removal of the phono stage in the 185, I'm interested in adding a line level preamp to my 175 using that spot on the chassis. Anyone know of a diagram using a 12AX7 or similar, with bass and treble control circuits, that I could build into that space if needed?

I also like the idea of adding the wood sides to the chassis. Definitely something I'm going to try for.

If it is still not up to my liking as far as sound quality once rebuilt, what would be a good set of output transformers to upgrade to?

 

[Maxxarcade]

It's coming along well so far. Got almost all the resistors changed, speaker posts installed, feedback network replaced, power jack and fuse installed.

Once I get the caps in the mail (Orange Drops from Radio Daze). I'll finish the electrical work, clean up the wiring, and start on the cosmetics.

I'm debating where to put the power switch. I don't really want to put it on the back, and I can't put it on the front because I want to make a wood frame.

I played it for a couple hours last night, and I notice an improvement even with the original caps in place. There were some resistors that were off in value quite a bit.

Anyone know how many volts the transformer winding is that went exclusively to the preamp socket?

 

[Glassweights]

Found my maggie peeps!

Anyone know how many volts the transformer winding is that went exclusively to the preamp socket?

Found where my Maggie peeps went! I was missing them in the "another maggie" thread where we are talking about fixing up this circuit to the audiophile circuit

Just kidding, I would not know an audiophile if it bit me unless it dropped some money on it's way past.

I have not measured it, but those Maggie Power transformers normally have a 5 volt winding and qty=2 6.3 volt windings. So that should be 6.3 volt. Should measure a little high if you stick your volt meter on it.

Nice looking maggie! Are the can capacitors running nice and cool? I typically replace them underneath and go larger on the second cap after the rectifier tube, just so I can pump out a little more bass. On the ones I don't replace I feel them to see if they are getting hot from time to time, and they are really on my 'to do' list.

You know, it is funny, Thumping bass normally irritates me, but once you put some tubes in the mix it is all good.

Dan

 

[Maxxarcade]

The original caps are running cool, but I'm going to be replacing them soon anyhow.

If that is another 6.3v winding, maybe I can use it and get away with putting in a quad of 6L6G's and a pair of Hammond output transformers

Right now it is sounding pretty good. I really need to make a good preamp with tone controls for it, since I'm one of the types that likes to boost the bass and treble a bit. I've only been able to test it by plugging it into the headphone jack on a stereo system.

By the way, I'm only getting 8 volts RMS into an 8 ohm load with this amp before clipping. Shouldn't it be more than that? 8 watts seems small for a push pull 6V6 amp. I'm wondering how much of a difference the new caps will make.

 

[Maxxarcade]

Well, it's 6 AM and I can't resist playing with it some more :screwy:

I decided to bring it up and connect it to the preamp output on my Marantz 1030 that I use on my PC, and connected the Zenith Allegro 1000 speakers...

This thing sounds amazing for having the stock caps and transformers on it. I really want to make a preamp for it now :yes:

I haven't had a chance to turn it up too much while everyone else is sleeping, but judging how loud it drove my Pioneer Project 60's, it will make these Allegros honk

 

[Chazb11]

Excellent job with that cabinet Raul. Simple and elegant.

 

[markus]

nice job maxxarcade ! :thmbsp:

concerning the (eventual) wood base . . . .

when I did my 185 XX amp, I made all my 'add-ons' to the top of the chassis (IEC plug socket, speaker binding posts, fuse, switch) so that way I can make a one piece wood 'sleeve' that slides down from above, and that I can secure with screw on feet from below . .. kinda like my (as yet unfinished) Sylvania amp with a Teak Base:

http://www.audiokarma.org/forums/showthread.php?t=93832&highlight=teak

 

[MarkNJ]

For those of you that added a three prong power cord, how did you avoid any ground issues? I thought remembered reading that you shouldn't upgrade to a newer style cord...

Also, I have a 9200 amp, what would be the appropriate fuse to install? 3A slow blow? 5A fast?

 

[markus]

For those of you that added a three prong power cord, how did you avoid any ground issues? I thought remembered reading that you shouldn't upgrade to a newer style cord...

Also, I have a 9200 amp, what would be the appropriate fuse to install? 3A slow blow? 5A fast?

when I can, I upgrade to a modern, but 2 pronged 'polarized' plug. I figure that keeps potential ground issues at bay, and always makes sure the chassis isn't hot. I did a 185 magnavox amp where I installed a 3 prong computer style cord, but just did not connect the center (green) wire to the amp chassis.

for fuses I always go slow blow, anywhere from 2 to 4 amps . . if its any frame of reference, my Sherwood 8000 Mk IV came from the factory with a 3 A fuse, and it's got about 12 tubes on the chassis (P/P 7868)

 

[Rex Everything]

Not sure if you've solved your switch problem yet but you could put it on the top side like the one on my amp

 

[gogofast]

wow, rex,
that thing looks like totally different amp. great! :thmbsp:

i have yet another mag amp, model 185-33 waiting for restoration. and from the info i've gathered by reading these threads, i'll have to:

bypass phono section by connecting the rca input directly to 12ax7's, bypass balance pot and match the channel to the other by replacing with same value resistors/caps. plus cap value upgrade.

am i missing anything regarding converting 185?

 

[Rex Everything]

wow, rex,
that thing looks like totally different amp. great! :thmbsp:

i have yet another mag amp, model 185-33 waiting for restoration. and from the info i've gathered by reading these threads, i'll have to:

bypass phono section by connecting the rca input directly to 12ax7's, bypass balance pot and match the channel to the other by replacing with same value resistors/caps. plus cap value upgrade.

am i missing anything regarding converting 185?

I did not do this AMP185. Jaymanaa did this amp and completely rewired and changed the circuit from what I know.

Sorry I don't have any answers for ya on this. I can tell you this amp is absolutely amazing sounding.

 

[gogofast]

that is really a good looking amp.

did he change the output transformers or are those just covers?

 

[Rex Everything]

that is really a good looking amp.

did he change the output transformers or are those just covers?

They are just nifty covers he made from pvc end caps and painted.

Not to hijack this thread but I posted a thread about this amp the other day

http://audiokarma.org/forums/showthread.php?t=200850