Magnavox 142 - rebuild/modification thread

[Jcricket]

Okay folks, I am starting on a set of magnavox 142 monoblock amps to rebuild. These are 6v6 PPP pentode amps.
I have had some input from a few folks and I am going to completely strip/rebuild these. Hopefully I will end up with a silk purse from the sows ear.

A few notable mods right off.
1. I am going to change the original choke to a much larger choke. Obvious reasons on this.
2. I am going to add a voltage regulation circuit for the screens on the 6v6's. This is done as an experiment in sound, but also because it is a convienent way to fill a few holes on the deck with some cool glowing glass.
3. I intend to change the 12ax7 to a 6sl7 - then everything is octal and I think it would look better.

Does anyone have any suggestions for mods to help this amp? I know GordonW has made suggestions on other amps as to what capacitors to replace(up their value) to improve the sound. Anyone have any other mods to recomend.

Here are some pics of the first unit. I have started stripping it now. Also, in the pic, is the choke that will be used on the top of the deck.

Notice underneath the previous flameout. It took the old choke and toasted a socket, but somehow the PS survived just fine.

I imagine this will tkae me the better part of a month to completel both. I'll update as I make progress.

So now I am off.

 

[Kegger]

I've been a little busy lately with not much time to spend on AK, but I'll follow
along an see if I have any glaring suggestions to what your doing, keep it up..

The upgrade I do most to these old amps is get some decent capacitance in the
supply, like say 120-400uf at the point where the output transformers get power.
(that will go a long way to improving the amps)

 

[Jcricket]

HEy Kegger - muchly apreciated.

In the schematic it has 30uf cap now at that point. So maybe put in a couple hundred uf to give it more avialbe current? What about idle current - think it will be a problem??

Mark

 

[Kegger]

Not sure what your alluding to with the idle current..

But yes adding more supply capacitance aids in the filtering, and putting it where the
most current will be drawn(the output tubes) helps the supply keep up with the amps
demands an helping to not starve the amp at any other section "after it" as well there.

 

[Jcricket]

here is the schematic.

I am probably using wrong terms. If you increase the capacitance in the circuit, doesn't the PS have to produce more current continually to charge it. I kind of figured that would increase the current at all time, at high volumes even more so. The end result would be the PS would run hotter - correct?

Make sense? Not an issue? Wrong terms?

 

[Jcricket]

Here is a scheamtic of what I think I am going to do to the b+ supply for pin 4 of the 6v6's.
So, disconnect the b+ shown on pin four, and connect this supply to that.

Battradio helped me with this - I hope I got it right.

 

[battradio]

Hi Mark ,

40 uf before the choke and 250 to 500 uf after the choke .Those chokes look real nice glad you got a good deal on them .

Mark

 

[battradio]

Here is a scheamtic of what I think I am going to do to the b+ supply for pin 4 of the 6v6's.
So, disconnect the b+ shown on pin four, and connect this supply to that.

Battradio helped me with this - I hope I got it right.

The drawing is wrong , the regulator tubes go to ground. the 100 ohm resistor goes to the top of the OD3's not the bottom .

Mark

 

[Jcricket]

to be sure i undestand,
change the two 30uf on the original schematic to the 40 and 250-500.
I think I have it.

I have four 220uf/300v caps. I would like to use them, but I suspect the voltage rating is just too low for this application.
What do you think?

 

[golana]

I tore one of those up last summer. I can't remember, was there only a 4ohm tap on the OPT? The outputs actually look pretty decent. As do that choke, hope it serves you well.

 

[Scuzzer]

What do you think?

I think 300v is too low for this application. No use completely rebuilding something and using an iffy part.

Do you have enough B+ to strike the voltage regulators? The datasheet says 185v minimum B+ for reliable starting but they might light at 160v. With two in series you probably need at least 320v to get them lit. I could be wrong though because I've never done anything with them other than draw up the schematic for your Bogens.

 

[battradio]

to be sure i undestand,
change the two 30uf on the original schematic to the 40 and 250-500.
I think I have it.

I have four 220uf/300v caps. I would like to use them, but I suspect the voltage rating is just too low for this application.
What do you think?

Wire the in series , so two of the make a 110 Uf /600 volt cap .
You can alaways add more capacitance later .

Mark

 

[Scuzzer]

In looking at it a bit more, you could use a 0A3 and 0D3 and regulate the screens to 225v which would be similar to the RCA manual typical operating parameters. If you did that then you can calculate the dropping resistor using: [supply voltage-regulated voltage]/[regulator tube median current+ load current]

For this case that would be (305-225)/[0.02+(0.0022*4)]= 2777 ohms. So somewhere around 2700 to 2800 ohms would be a reasonable choice if I've done the math correctly.

edit: Perhaps it would be better to use the median screen draw instead of the idle draw to calculate the dropping resistor. In that case it would be (305-225)/[0.02+(.0041*4)]=2200 ohms. The .0041 is the midpoint between zero signal (2.2 ma) and max signal (6 ma) screen current draw. That way when the screens are pulling 6 ma at max signal there's still enough current flowing through the VR tubes to keep them lit. Only if I've done the math correctly of course.

 

[Jcricket]

HEy Scuzzer, thanks for the input.
Question:
The 142 schematic shows 305v at pin 4 of the 6v6gt's. With your example, what would the supply voltage be at pin 4? 305 or 225?? If it is 225, will that work? Sorry, but as usual, I am several watts shy on this stuff.

 

[Jcricket]

Scuzzer,
If I take the "supply" voltage for the 0d3 right off the 5y3 rectifiers, not after the choke, it should be high enough to strike the VR tubes. Thus my output for the regulated supply to the 6v6gt's would then be 300v. I think that is what battradio and I discussed.

Make sense?

Mark

 

[Jcricket]

I tore one of those up last summer. I can't remember, was there only a 4ohm tap on the OPT? The outputs actually look pretty decent. As do that choke, hope it serves you well.

according to the sams photofact, the output is a 4.3ohm. The output tranny has a 3.7k impedance. Interestingly enough, the output has two output taps. one was for an auxillary speaker. It is not labled as to what output impedance should be used.

ANyone here know? Anyone here know how to measure it?

 

[Jcricket]

If anyone wanted to scratch build one of these 142 magnavox units, here are some trannies from edcor that would work very nicely'
edcor output
http://www.edcorusa.com/products/433-cxpp30-ms-3_8k.aspx
These would also give you 4,8,16 ouputs too.

edcor PS tranny
http://www.edcorusa.com/products/678-xpwr081_120.aspx

 

[Jcricket]

Here is revision 1 of the power supply circuit. This look okay???

 

[Scuzzer]

Here is revision 1 of the power supply circuit. This look okay???

Ahhh, now I understand why you had diodes in the last drawing and yep the initial voltage rise should be enough to light them.

I thought you were drawing this on your own so that's why I was offering suggestions. I will step out of the way if you're working off of Battradio's knowledge. Just one last suggestion though, run the plates of the 12AX7 off the regulated voltage as well.

 

[Jcricket]

Ahhh, now I understand why you had diodes in the last drawing and yep the initial voltage rise should be enough to light them.

I though you were drawing this on your own so that's why I was offering suggestions. I step out of the way if you're working off of Battradio's knowledge. Just one last suggestion though, run the plates of the 12AX7 off the regulated voltage as well.

Gary,
please don't bow out. I would like your opinions too.
I hadn't thought of running the 12ax7 plates off the same ps. any benefits to that you think?