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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Magnavox AMP-185

mackinthebox

Member
2010-10-19 11:03 am
I may have found myself a decent tube amplifier to start myself out with here locally on the cheap!
I would like to get some info on the amp but so far I havent been able to find much

Does anyone have a schematic for a Magnavox AMP-185?
I havent got the amp yet so i dont have much more info than that and some tiny pictures:
[IMGDEAD]http://images.craigslist.org/3n03m43p55Q15P25S3aabe5439868f5ef1394.jpg[/IMGDEAD]
[IMGDEAD]http://images.craigslist.org/3k53md3lb5T15X35R5aab24b6a184b1361829.jpg[/IMGDEAD]

Does anyone have any experience with these amps that can recommend from good mods for it, if it needs any?

Thanks!
Mack
 

petemoore

Member
2010-01-31 7:44 pm
I've had really good luck with amps like that.
...once the can-caps acted up enough that I changed them, these days I use old caps like that only for pre-test, any ffffttt or gkk sound and out of the power supply section they're escorted off the platform if they're not gone already! Generally all the electrolytics have to leave at that time.
I had 1 bad OT not too long ago, the old tubes are in my amps.
Play 'em, they sound great !
When the sockets got loose, gut out time, especially when the solid core wires were knotted and then soldered to the lugs. Large [8pin] sockets seem to last longer.
Worth the trouble ?...Especially if you don't have one !
I pick a platform and play it these days. I used to repair and rebuild anything that got under the bright light...then decided pick the platform, build it strong like when it was new, then use it. Crusty brick resistors that drifted might matter, bigger=more heat dispersion, so the next Watt size up or double-watt if will fit.
I'd play it through the speakers before and after considering mods [generally the voicing of these amps is just 'wide frequency range'] unless there was some major difficulty in the performance...such as when used for guitar, tightening up the bass at the HP filters, usually the preamp provides sufficient control over this though.
Take care to have fun !
 
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20to20

Member
2010-06-23 9:25 pm
I may have found myself a decent tube amplifier to start myself out with here locally on the cheap!
I would like to get some info on the amp but so far I havent been able to find much

Does anyone have a schematic for a Magnavox AMP-185?
I havent got the amp yet so i dont have much more info than that and some tiny pictures:
[IMGDEAD]http://images.craigslist.org/3n03m43p55Q15P25S3aabe5439868f5ef1394.jpg[/IMGDEAD]
[IMGDEAD]http://images.craigslist.org/3k53md3lb5T15X35R5aab24b6a184b1361829.jpg[/IMGDEAD]

Does anyone have any experience with these amps that can recommend from good mods for it, if it needs any?

Thanks!
Mack

One important thing to check on the schematic with this model is the location of the feedback components. I believe they were placed on the radio chassis instead of the amp chassis and the feedback signal was routed back to amp via cable. If so, you will want to plan on reinstalling them on the amp if you don't want the high distortion numbers (5% or more) you could get without the feedback.
 

mackinthebox

Member
2010-10-19 11:03 am
I finally got the amp :)
it is a 185AA
I added 1/4" Jacks to the back
Installed a PC Power connector, the D shaped connector
Installed an inline fuse
Installed a power switch

Here are the pics :)

[IMGDEAD]http://sphotos.ak.fbcdn.net/hphotos-ak-snc4/hs886.snc4/71984_157561354280080_100000786499990_249646_5342549_n.jpg[/IMGDEAD]
[IMGDEAD]http://sphotos.ak.fbcdn.net/hphotos-ak-snc4/hs925.snc4/73856_157561377613411_100000786499990_249647_2321235_n.jpg[/IMGDEAD]


Added power connector:
[IMGDEAD]http://sphotos.ak.fbcdn.net/hphotos-ak-ash2/hs475.ash2/74860_157561434280072_100000786499990_249648_4215207_n.jpg[/IMGDEAD]

Added 1/4" Connectors:
(I also made my own speaker cables with 1/4" ends and Banana Plugs)
[IMGDEAD]http://sphotos.ak.fbcdn.net/hphotos-ak-ash2/hs471.ash2/74419_157561454280070_100000786499990_249649_1763665_n.jpg[/IMGDEAD]

Added power switch:
[IMGDEAD]http://sphotos.ak.fbcdn.net/hphotos-ak-ash2/hs449.ash2/72256_157561467613402_100000786499990_249650_5853975_n.jpg[/IMGDEAD]
 
Next up im going to replace the RCA connectors, the ones on it now are rather worn and Ive got some gold replacements
I am also going to add 2 POTs, 1 for each side, to control input levels of each channel

Right now it sounds good but the highs are a little low, the low end is incredible though
Is there something I should check to get better highs out of it?

Its got 12ax7s for the preamp and Magnavox 6V6s for the power tubes
 

petemoore

Member
2010-01-31 7:44 pm
Is there something I should check to get better highs out of it?
What do the capacitors look like inside ?
Its got 12ax7s for the preamp and Magnavox 6V6s for the power tubes
These are easy to get hold of, replacements might not be needed but for the ability to have a compare with the existing tubes/spares.
Even swapping channel tubes, but assuming both channels have near identical response...
It would stand to reason it's something that is common to both channels such as power supply capacitors [which if they haven't been changed on this one is a good measure to consider] or the voicing of the amplifier.
Perhaps the input expects a low impedance source and there's an impedance mismatch.
 
I replaced the RCA connectors and removed most of the phono stage from the amp, or at least disconnected it...
Im going to remove most of the components either in the morning or this week sometime
After replacing the RCA connectors the sound is much cleaner and the highs are coming in alot better

one thing i noticed was that each channel has a 47k ohm resistor in place but that the physical size of the resistors on each channel is different
Ch 1 has a larger resistor and Ch 2 has a smaller resistor, both rated at 47k ohms
Ch2 is louder than Ch1 by a very small amount and Ch2 has cleaner highs
Should I replace these resistors with equal resistors? would that improve my sound quality and even the sound out?
 

petemoore

Member
2010-01-31 7:44 pm
The circuit only sees a value of resistance a from resistor, this can be measured with DMM. Knowing where it is and what is it doing gives a better indication how strictly the "generally +/- 10% tolerance is ok" rule should be followed, how much power it will be expected to handle, how much surface area the body should have in order to reliably dissipate/regulate heat.
About the only thing a resistor sees [that we care about other than it's R rating] is power, only thing it feels is internal + external heat. They generally don't show notices of any stress until P is in the form of large currents, then, runaway thermal conditions and tolerance drift tend to be associated with body discoloration and burnt dust smell, the circuit doesn't notice these side effects until/unless heat causes R value drift.
A 1/8'' bleached white, lightweight softwood dowel [long stick-heat test] clearly shows discoloration when heated, if the resistor looks like it'd make a half decent woodburner element, it is probably a too hot.
The circuit Can feel the heat, and generally withstands it and performs it's duties until catastophe hits in the form of burnt wire/resistor/drycap/socketfry, magic smoke outgassing, or some other ailment.
 

20to20

Member
2010-06-23 9:25 pm
I replaced the RCA connectors and removed most of the phono stage from the amp, or at least disconnected it...
Im going to remove most of the components either in the morning or this week sometime
After replacing the RCA connectors the sound is much cleaner and the highs are coming in alot better

one thing i noticed was that each channel has a 47k ohm resistor in place but that the physical size of the resistors on each channel is different
Ch 1 has a larger resistor and Ch 2 has a smaller resistor, both rated at 47k ohms
Ch2 is louder than Ch1 by a very small amount and Ch2 has cleaner highs
Should I replace these resistors with equal resistors? would that improve my sound quality and even the sound out?

Do you have the diagram for this?

Have you cleaned and adjusted the balance control yet?

There are two different pairs of 47k R for each channel. One pair for the input to 7025 and one for the 12AX7 input. Is that the reason for the different size.

Do you have a meter?
 
Do you have the diagram for this?

Have you cleaned and adjusted the balance control yet?

There are two different pairs of 47k R for each channel. One pair for the input to 7025 and one for the 12AX7 input. Is that the reason for the different size.

Do you have a meter?

I actually deleted the POT for the Ch2 Balance, it actually made it more balanced by doing so
the original POT was in pretty bad shape

The sound is great and is so much better than when I got it after everything ive but
The balance seems to be close to center but still a tiny bit louder on the right hand side or Channel 2
Which is the channel that has the smaller (only in size) resistor

Im going to go ahead and replace both the resistors and hope it evens things out, will also be replacing alot of the electrolytic caps as well

I do have the schematic and followed it to disconnect the phono stage as well as delete the balance POT
according to the schematic the second POT is a hum control and hooks up between the voltage lines for the heaters of the 6v6 tubes
 
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