Magnavox 185-23 Schematics

[himm37]

Does any one know where I can find a copy of the Schematics for a Magnavox 185-23 console amp?

Thanks in Advance!

 

[Tom Bavis]

AMP185 is in Sams Photofact folder 449-13 - I don't have it but you might be able to get it from your library, or one of the online sources mentioned in the "where to find schematics..." (sticky message at the top).

 

[Randy Bassham]

Gimme a couple of days, I should have it in my Magnavox lit., If not I should have that Sams.
PM me if you don't hear back from me, I tend to forget things.....

 

[himm37]

Thanks Guys,
I downloaded the Sams for the model 175-XX amp from the yahoo group, it looks like it's the same minus the ceramic phono pre section, I've already eliminated it from mine! I'll look into it further later on.

 

[markus]

hey himm37

huh . . that sounds funny

I have a 185 amp that I need to bypass that first phono-gain tube on. . . how hard is it do do ?

just run the inputs to the grids of the 12ax7 ?? :scratch2:

 

[GordonW]

huh . . that sounds funny

I have a 185 amp that I need to bypass that first phono-gain tube on. . . how hard is it do do ?

just run the inputs to the grids of the 12ax7 ?? :scratch2:

Use the AMP175 input config- from the center pin of the input RCA jack, through a .01uf cap in series, then a 470K resistor to ground, then a 47K resistor in series, to the first-stage grid of the 12AX7.

That'll definitely work... by doing this, you will have converted the amp to an AMP175, for all intents and purposes.

You can "upgrade" to an .022uf cap, if desired... IME, it works fine, and seems to help make the bass more "linear" (ie "smoother"), to a lower cutoff bottom-end frequency. Especially if you upgrade the other coupling caps proportionally (if it's .047uf, use a .068 or .1, for example) in the circuit. The exceptions are the compensation caps (the tiny ceramic caps to ground)... definitely LEAVE them the SAME VALUE, in order to preserve stability in the amp...

Regards,
Gordon.

 

[markus]

Use the AMP175 input config- from the center pin of the input RCA jack, through a .01uf cap in series, then a 470K resistor to ground, then a 47K resistor in series, to the first-stage grid of the 12AX7.

That'll definitely work... by doing this, you will have converted the amp to an AMP175, for all intents and purposes.

You can "upgrade" to an .022uf cap, if desired... IME, it works fine, and seems to help make the bass more "linear" (ie "smoother"), to a lower cutoff bottom-end frequency. Especially if you upgrade the other coupling caps proportionally (if it's .047uf, use a .068 or .1, for example) in the circuit. The exceptions are the compensation caps (the tiny ceramic caps to ground)... definitely LEAVE them the SAME VALUE, in order to preserve stability in the amp...

Regards,
Gordon.

AWESOME ! thank you Gordon - exactly what I needed :thmbsp:

 

[markus]

Use the AMP175 input config- from the center pin of the input RCA jack, through a .01uf cap in series, then a 470K resistor to ground, then a 47K resistor in series, to the first-stage grid of the 12AX7.

That'll definitely work... by doing this, you will have converted the amp to an AMP175, for all intents and purposes.

You can "upgrade" to an .022uf cap, if desired... IME, it works fine, and seems to help make the bass more "linear" (ie "smoother"), to a lower cutoff bottom-end frequency. Especially if you upgrade the other coupling caps proportionally (if it's .047uf, use a .068 or .1, for example) in the circuit. The exceptions are the compensation caps (the tiny ceramic caps to ground)... definitely LEAVE them the SAME VALUE, in order to preserve stability in the amp...

Regards,
Gordon.

gotcha - so you're saying instead of the .01 mF / 10000 mmF input cap use .022 ? and one weird thing is with this 185 amp, one side of the coupling caps uses .047 - the other uses .0033 mF :scratch2:

should I just recap with all .047 or .068 ??

also, the cathode bypass caps are 20 mfd electrolytics - I never replaced these on my amp when I recapped, and it sounds OK ., . . should I bother doing these too ?

thanks Gordon !

 

[GordonW]

Check for other differences between the channels, ESPECIALLY in the feedback circuit. One side may be altered, to work with the balance pot (if equipped).

On the units with the balance pot, I clone the OTHER channel's (the one without the balance pot) layout into the one that had the balance pot, and delete the pot (and the ceramic pot-bypass cap). This makes both channels the same.

WRT the .0033 and .047 uf caps... those are different stages! Both channels should have a .0033uf cap between the first stage of the 12AX7 and the second stage and first output tube (positive phase output tube), and the .047 between the second stage of the 12AX7 and the second output tube (negative phase output tube). There should be a .0033 and a .047 PER CHANNEL (as in, two of each, one on channel 1 and one on channel 2, for each value of cap). If that's the case, I usually replace the .0033 with a .0068, and the .047 with something like .1 or .15uf...

I also replaced the 20uf caps (on the cathodes of the 12AX7 first stage) with 20uf (or 2 10uf caps in parallel) 100 or 150 volt mylar or poly caps. Does seem to make it sound a little better...

Regards,
Gordon.

 

[markus]

WRT the .0033 and .047 uf caps... those are different stages!

oh yeeeaaaaahh. . . I'm still wrapping my head around PP circuits - a little more going on than SET's :yes:

looks like I need to put together a capacitor order :thmbsp:

got this one, birddog's sylvania, biglou's funky Kelton monoblocks, and my Ampex's

thanks for all the help Gordon !

 

[himm37]

Thanks owed from me too! You beat me to it and answered the next question I had as to differences in the two channels. I was looking at r9/c10 and r20/c17 and wondering why they did that.

 

[markus]

hey himm37 -

here's some inspiration to make your 185 amp nice and purdy :

http://www.audiokarma.org/forums/showthread.php?p=1332860#post1332860

haven't finished it yet, but it's gonna be nice :thmbsp:

 

[GordonW]

Or this one (AMP175, but the same chassis, just an earlier year model):

Regards,
Gordon.

 

[himm37]

Markus,
Just wondering how far you've gotten, I just finished cleaning up the input circuit as per Gordon and the 175 schematic, huge improvement. When I first got it I did a quick bypass, but there was still some of the phono "stuff" in the way. I basically gutted and rewired everything from the input jacks to the 12ax7's.

Gordon, I used .02uf ceramic disc caps at the inputs, would a different type make an improvement, AND if I used a polarized cap, where does the + go!

Thanks Again

 

[markus]

Markus,
Just wondering how far you've gotten, I just finished cleaning up the input circuit as per Gordon and the 175 schematic, huge improvement.

purely cosmetic stuff at this point - I need to order some caps before I go diving into the circuit . . . glad your's is singing ! :thmbsp:

I'm going to finish the wood base this week, and hopefully have the caps by next week sometime . . .

 

[GordonW]

Markus,
Just wondering how far you've gotten, I just finished cleaning up the input circuit as per Gordon and the 175 schematic, huge improvement. When I first got it I did a quick bypass, but there was still some of the phono "stuff" in the way. I basically gutted and rewired everything from the input jacks to the 12ax7's.

Gordon, I used .02uf ceramic disc caps at the inputs, would a different type make an improvement, AND if I used a polarized cap, where does the + go!

Thanks Again

I prefer not to use ceramic caps... they're not that good for fidelity.

.02 or .022 mylar or polypropylene caps... those would be best.

Don't use polarized caps, as coupling caps... they're OK in the power supply, but not very good in an AC signal location.

Regards,
Gordon.

 

[markus]

Thank You Gordon !

finally got to Sixcats! 185 amp today - bypassed the first stage 7025, and deleted the balance pot, cloning the other channel. Also took your recomendations on cap 'upgrade' values. Listenign to it right now on a little pair of Minimus 7's, and it does sound nice . . .

no I gotta get off my butt and finish the wood base . . .

 

[SixCats!]

What ? NO ? Your not going to get off your Butt and finish the Base ? LOL!
Glad to hear your making progress with the Magnavox 185 Mark. I look forward to hearing it when finished.

Regards,
SixCats!

 

[markus]

What ? NO ? Your not going to get off your Butt and finish the Base ? LOL!
Glad to hear your making progress with the Magnavox 185 Mark. I look forward to hearing it when finished.

Regards,
SixCats!

huh huh . . .

yeeeaaaaahhhh. . . . I'm thinking of going a little crazier with the base, since I DO have some real mahogany veneer kickign around in the garage - it'd look better than just the stained birch . . . . that and I want to put on the beefier binding posts so your big ham-fists can manage them :lmao:

You'll also do well to snag an actual matched set of 6V6's and 12ax7's, just for consistency's sake . . . I'm running it with some nice telefunken ECC83's and those cool RCA 6V6G coke bottles I have . .

with those cap upgrades, per Gordon, the bass should be alot better - it'll be interesting to hear 'em on those big EV / University refrigerators of your's :thmbsp:

 

[SixCats!]

Sounds good to me my friend. Heck, my big paws can use all the help they can get lol! Oooh, I was in the Garage loft today but...I didn't play with the EV's/UNIVERSITY's. I am still playing with that single PILOT Speaker/MONO Tube Amp Console. Since I already removed the PILOT Tube Amp, I connected my ol' MONO HK SOLO Tube Receiver and (just for fun) my famous 99 cent DYNA FM 3 Tube Tuner. Heck, it sounds pretty darn good on the local Classical station. Yea, we gotta try the EV with the Magnavox!

SixCats!