Magnavox stereo amp

[hoff70]

I picked up an old amp today. Best I can figure is that it's a 189-0 or 189-10. It uses 50C5's and 7025's and looks like it's a "hot-chasis" amp.

Pretty sure this is it: https://app.box.com/s/87rlvqtizmauyr3qcgeo

I'm pretty comfortable fooling with shuffling electrons around but most of my experience is with microcontrollers and solid state stuff so this will be my first tube project. It's been a dream of mine to build/modify a tube amp for years so I figured I'd pick up a cheap one to experiment with.

I'll know more when it arrives

One of the first orders of business will be to pick up an isolation transformer and figure out what pots it uses. Then new caps and check out the resistors etc. I'm guessing it used some stacked pots?

Any advise would be most welcome!

My goal is to make it back into a phono amp. I'm not terribly worried about super sound quality this is mostly for the learning experience.

 

[hoff70]

LoL!

I swear it looks like the same amp in this thread:

http://www.audiokarma.org/forums/showthread.php?t=287815

Got it off the auction site dirt cheap and it's from the same town as the OP :scratch2:

 

[hoff70]

Sorry to bomb my own lonely post But maybe it'll glean something useful to others...

Just looking around for some stuff I reckon I'll need:

Looks like the original had "Loudness", "Bass" and "treble" controls. The "Balance" is still with the amp but the rest of the controls are gone :sigh:

I've been looking and I can't find ANY pots that are the right values. 3 and 3.3meg draw blanks although 1megs seem to be common. I'm also unsure about taper.

Thanks

50EH5: http://frank.pocnet.net/sheets/093/5/50EH5.pdf

50C5: http://frank.pocnet.net/sheets/093/5/50C5.pdf

7025: http://frank.pocnet.net/sheets/135/7/7025.pdf

 

[GordonW]

Be aware- if you do this, you will need to use a phonograph with a CERAMIC cartridge. There will not be nearly enough gain, or the right RIAA EQ, for a magnetic cartridge.

It might be simpler, to just build the amp as a straight power amp, then use it with an outboard phono preamp. If you get a phono preamp with decently high gain, you probably can just tag a volume control pot at the input of the amp (a 100K or 250K stereo log pot would probably be fine), to control level.

Regards,
Gordon.

 

[hoff70]

Thanks for the input Gordon! I was hoping that I was doing the right thing by getting a phono amp but I suppose I missed the magnetic cart era by a few years

I actually have an old Garrard AII on the way which will probably need a cart so I may think about putting a ceramic on it. It's my understanding that ceramics track heavy as do old Garrard changers.

Any input on the control pots for the old amp?

 

[hoff70]

I'm still doing some study on this thing before it arrives. I'm not sure about the control pots.

The schematic designates the pots as "A" and "B" but I'm not sure if that means stacked pots or different connections to the same pot?

The reference pic doesn't seem to show stacked pots although I understand that's not the best reference.

I also found this set of pots:

Which is from a 151AA but it's a mono amp. The values are the same but I can't find any part #'s for reference. Only a schematic.

 

[Schmidlapper]

Tone controls appear stacked in b/w photo, R2 being best example. Would expect to see additional terminals on opposite side if using same disk for both channels at least for the point to point era. Agree with Godon this would be a good candidate for a simple power amp sans tone controls. Especially if no pre existing need to go ceramic cartridge. Isolation transformer I have used from the Shack had an extra secondary that made a good bucking winding to get closer to the 117v this was designed around. The heaters will be the most benefited.

 

[hoff70]

Thanks! I believe now I will just go with a simple power amp. Do you have a link to the RS iso transformer?

I'll know more when the amp arrives. Could be a basket case...

 

[Schmidlapper]

Cannot find my drawing or receipt and without seeing my drawing I cannot be sure this wired as a bucking tranny actually does isolate. I know I installed a new chord and hooked it up correctly for hot and neutral, but that won't guarantee the outlet was correct. So I won't in good conches suggest an actual part. I know I originally had thought it isolated when I put it I the radio I needed it for.

 

[hoff70]

Fished this dude out of the bay:

Hopefully it'll keep me from getting 'lec-tro-cuted

Have to scope out the caps and such when it comes in...

 

[Schmidlapper]

That looks like the ticket.

 

[hoff70]

It came in today

Still waiting on the iso trans so I guess I'll look at the cap values and order some replacements. Some of the ceramic caps look a little funky but I'm not sure if it's just goop on them or if they're near failing?

Now that I have it in hand I'll study the schematic and see what's what. This is my first foray into this stuff so any pointers would be great!

I'm going to attempt to build it as a simple power amp as Gordon suggested but I'm unsure how to proceed with doing away with the tone controls?

Guess I'll hit the googles!

OK, please correct me if I'm wrong.... But it appears as if the tone/loudness controls are all before the amp input so I needn't even worry with it? Things look a little clearer with the unit in front of me.

I'm also a little confused about why 1 channel uses 50CS while the other uses 50EH5? I'm also pretty unfamiliar with audio type circuits but the balance pot doesn't seem to function the way I would expect. Is it not a typical R/L balance control?

Sorry for all the goofy questions. I am reading up on these things!

 

[hoff70]

Now to try and run some tunes through it

 

[gadget73]

The ceramic caps are probably dipped in wax as a moisture preventative measure. Likely they are electrically OK. I just happen to think they sound lousy. I'd leave them be for the moment. The cap in the cardboard tube is the one I'd be replacing. Rectifier looks like it may be a silicon type, so its probably functionally fine. The Good-All cap and those ElMenCo also would probably go if they are wax ends. If its the hard epoxy, they may be OK. Some of those were mylar caps, which lasted virtually forever.

 

[hoff70]

Thanks! I'll examine the cap ends. They look like wax but I'll investigate further.

I'm studying up on "Stuffing" the multi cap. From what I understand, e'lytics are fine for that spot? I'll probably leave it in place and make room underneath for new ones.

I did hook it up to a super-cheap record player with aux. speaker out and some cheezy 4 ohm speakers and it does seem to work just sounds like poo... At least both channels work so I have an operational base line!

I'm a little confused about proper speaker impedance. The schematic has 3 speakers of varying resistance in parallel (3-4 ohms,5-6 ohms and 7-8 ohms). I ran the #'s through an on-line calculator and came up with about 2 ohms which doesn't sound right...

I'm looking to add a volume pot on the front-end and find some decent speakers once I figure out impedance.

Thanks again for the help!

 

[gadget73]

The multicap is just 3 or 4 electrolytics stuck in one tube. Modern individual electrolytics work fine as a sub. You can either cut the original open and put new ones inside, or put them underneath. I've cheated with cardboard ones and just tossed the aluminum can part that I cut off, and used the cardboard sleeve to cover the new caps inside. Otherwise, its usually underneath. I hate re-stuffing cans unless I have no choice about it.


I'd guess the output impedance is probably around 4 ohms. The crossover makes it sort of difficult to work out directly. The math for that stuff baffles me a bit, but its not quite as straightforward as adding the 3 in parallel since it changes with frequency.

Try a CD player or something as your source. That would probably be a closer match. Some have variable output level controls.

 

[GordonW]

I'm also a little confused about why 1 channel uses 50CS while the other uses 50EH5? I'm also pretty unfamiliar with audio type circuits but the balance pot doesn't seem to function the way I would expect. Is it not a typical R/L balance control?

Sorry for all the goofy questions. I am reading up on these things!

I THINK (I don't have documentation for this, though), that the unit can use either 50C5 or 50EH5 They are the same pin-out. I would want to have all four be the same thing (either four 50C5s or four 50EH5s), though, so that they all bias up the same. The 50C5 is a little heavier built (7 watt plate, whereas the 50EH5 has 5.5 watt plates), but if a set of 50EH5s isn't red-plating anywhere, I would give them a thumbs-up, too...

The balance control is just a level trimmer for one channel, IIRC. One channel is unaffected- you just use that to make the one channel match the other.

Regards,
Gordon.

 

[hoff70]

Yea Gordon, Kind of weird that the specs call for 50C5's on one side and 50EH5's on the other :scratch2: That's what's in it now. Gonna pick through the specs of each tube...

The balance control is also kind of weird, it seems to effect both channels and sounds almost like a "presence" control but it's kind of hard to explain exactly what it does...

I did rig it up to my phone and while it doesn't sound great, it doesn't sound bad either

https://www.youtube.com/watch?v=HZux2pEBsok

Could someone comment on the "click", "click" sound at the end of the vid? it doesn't do it all the time. I need to stop fooling with it until I square things away a little more so I don't damage it!

It also gets mighty hot! Even the output transformers get pretty warm.

I'm feeling really encouraged by the sounds coming from this little amp :yes: I was afraid it was going to be a big let down but I can hear a LOT of potential! It's also VERY quiet :thmbsp: I was scared it was going to be a hopeless hum box but it was a nice surprise to hear how quiet it is.

Now to figure out some good speakers to pair it with. The vid is with some 8 ohm A40's which seem like a poor match.

I suspect that getting the right speakers is gonna be the key to really enjoying this amp. I'm thinking some really efficient 4 or 6 ohm jobs?

And thanks gadget! I'm going to look through everything and probably replace all the caps except the discs. As far as I can tell, only 1 cap has epoxy. The rest feel a lot softer.

Thanks!

 

[hoff70]

Another thread bomb

Had it up and running pretty well but I noticed one channel was really weak, especially the vocals. It sounded like one of those deals that's supposed to mute the vocals on a song... I traced it back to the channel that has the "balance" pot on it so I just bypassed it and the amp really woke up!

I'm going to order some new caps and a couple of 100K and 250K stereo log pots to put on the input as suggested. I'll just get one of each to try out. Probably some different type of speaker hookups for it as well. Now it has flat-head screw deals.

Any other advise on some components to order would be very welcome! I fool with electronics and have lots of discreet stuff just nothing rated high enough for this type of stuff

Still researching appropriate speakers. I'm leaning towards DIY.

 

[hoff70]

Bottom of the dreaded multicap:

PITA drilling down to it:

Poor amp:

I think I'm just gonna cover up the chassis:

Tried engine-turning the aluminum but it didn't turn out that great...



I ordered some new film caps (Russian oil jobbies) and electrolytics. Any advise on good replacements for the ceramics?

I also noticed that on the schematic the 2 channels are slightly different. The caps are different values from one side to the other as are the the tube pairings as well as a couple of resistors. Then there is the balance pot which I just removed from the circuit :scratch2: I have ordered enough parts to make both channels identical and use 50EH5 tubes. Good or bad idea?