Magnavox 142 - rebuild/modification thread

[Jcricket]

I just reameasured it. I had my meter set on auto scale. DUH!
It is 1.1volt and 3.2mV
The 1.1 is too high, correct?

I am going to go back and check again. I think I still have something wrong.

 

[Kegger]

Yes 1.1 is to high, double check things, try another tube, and do you have
a preamp plugged into it? If so disconnect the preamp and remeasure then.

470k is a fairly high grid resistor, if the tube is suspect and has grid current
you could get a reading like that, you just may want to use something more
conservative like say 200-270k there instead.

 

[battradio]

The filament rectifier winding is rated ate 6 amps so 5U4GB's can be used .On the input of the 6SL7 you can go as low as 47K ohm .

 

[Kegger]

Hey that is great to hear the 6amp rating.

Sounds like 5U4's should be safe then, I'd check the power tranny heat if using them.

-----------

I still would suggest using the 5V4 and or 5AR4 an keep the current down on that pt
plus the fact I do prefer the slow ramp-up of voltage, not necessarily for the cathode
stripping some feel does or doesn't happen, but for the slow voltage rise on the whole
amp, even the VR tubes I feel benefit from having the tubes draw current as that B+
comes up so they don't have to soak up all that current at once.. Just my preference.:thmbsp:

 

[Jcricket]

How will replacing the input resistor help? Will there be an audible difference?
I ask for knowledge. I will do this for sure.

Yes, measurements were taken with a preamp connected. I'll remove and check again.

I did some playing with it yesterday, some tube rolling. The 6sl7 used will clearly affect the sound output quality. Some distorted a bit , others will cary it to higher volume before clipping. Also, I used several different rectifiers in the unit. The PS tranny got slightly warm - just warm enough that you could tell it wasn't cold.

Aslo, on the od3's, they all act differently. Some would not even light up. Some would only illuminate the bottom section, others would burn really bright.:scratch2:

 

[Scuzzer]

The 6sl7 used will clearly affect the sound output quality. Some distorted a bit , others will cary it to higher volume before clipping.

The input 6SL7 section is only biased at -0.17V according to your measurements so it would be quite easy for tube to tube variation to be huge. Once you get the bias to -1 to -1.5 volts the distortion should go away (spec is -1.6v). You could put an input capacitor on this amp to make sure that preamp offset voltage isn't screwing up your bias.

 

[battradio]

How will replacing the input resistor help? Will there be an audible difference?
I ask for knowledge. I will do this for sure.

Yes, measurements were taken with a preamp connected. I'll remove and check again.

I did some playing with it yesterday, some tube rolling. The 6sl7 used will clearly affect the sound output quality. Some distorted a bit , others will cary it to higher volume before clipping. Also, I used several different rectifiers in the unit. The PS tranny got slightly warm - just warm enough that you could tell it wasn't cold.

Aslo, on the od3's, they all act differently. Some would not even light up. Some would only illuminate the bottom section, others would burn really bright.:scratch2:

Hi Mark ,

Lowering the imput impeadance does have an effect , it make the amp quieter , but harder to drive and orverdrive .

Instead of going for higher B+ , you could lower the screen voltage with a devider net work after the shunt regulators .

 

[Jcricket]

Hi Mark ,

Lowering the imput impeadance does have an effect , it make the amp quieter , but harder to drive and orverdrive .

Instead of going for higher B+ , you could lower the screen voltage with a devider net work after the shunt regulators .

If I did this, how would it affect the ability to possibly change over to the 6l6 tubes? I 'll look up the operating voltages on the 6l6's and see too.

I'll drop the input impeadance to about 2.2k.


Scuzzer, to set the bias for the 6sl7, I ned to change teh 8.2k resisor - correct? Any suggestions what to put in there? Myabe a 4.7k?

 

[Scuzzer]

Scuzzer, to set the bias for the 6sl7, I ned to change teh 8.2k resisor - correct?

No. You need to get your grid voltage (pin 1) down to zero. Once that's done you can figure out if you need to change the cathode resistors.

I'll drop the input impeadance to about 2.2k.

I think you mean 220k.

 

[Jcricket]

WIsh Icould say 220k is what I meant. I was thinking about increasing current(don't ask me why) rather than voltage. Too busy trying to man the smoker, answer the phone, and think clearly here too. Do you think 220k is right, that seems awfully high. Maybe 20k I would have thought.

 

[Scuzzer]

Do you think 220k is right, that seems awfully high. Maybe 20k I would have thought.

470k is a fairly high grid resistor, you just may want to use something more conservative like say 200-270k there instead.

We're talking about the resistor that connects between pin1 and ground on the input 6SL7 section. If your 1v measurement on that pin was due to your preamp you could also put a cap between the input RCA jack and pin 1 as well.

 

[Jcricket]

The input 6SL7 section is only biased at -0.17V according to your measurements so it would be quite easy for tube to tube variation to be huge. Once you get the bias to -1 to -1.5 volts the distortion should go away (spec is -1.6v). You could put an input capacitor on this amp to make sure that preamp offset voltage isn't screwing up your bias.

This is waht I was trying to refer to.
What pin on the schematic/waht resistor are we talking about here?
Mark

 

[Scuzzer]

This is waht I was trying to refer to.
What pin on the schematic/waht resistor are we talking about here?
Mark

Right next to the input jack on the schematic above. The schematic shows a 12AX7 so on it it would be pin2 but on your amp the input grid is pin 1. Perhaps someone needs to redraw the schematic so that it shows what's actually in your amp.

edit: I'll be right back.

I deleted the one above and changed the pinouts for the 6SL7. I might have it correct. If you jot down what is different on your amp I'll make the changes to the schematic so we all know what we're talking about when make suggestions.

 

[Jcricket]

You've got some mislabeling here.

---------------

"The measurements were consistent from tube to tube, so I am only going to list the values of one of each.

TUBE pin schematic Measurement
6v6gt 2/7 heaters , 17.5 volts??? What
6v6gt 3 plate 295v , 286
6v6gt 4 screen 305 , 288
6v6gt 5 screen 0, .2 to .3v .............. (THIS IS THE GRID/INPUT)
6v6gt 8 cathode 18.5 , 17.5

5y3 4/6 290 , 315

6sl7 1 screen -.1 , 1 .................. (THIS IS THE GRID/INPUT)still working on this
6sl7 2 plate 95v , 78.6
6sl7 3 cathode 1.5v , 1.17
6sl7 4 screen 0 , 3.2mv .................. (THIS IS THE GRID/INPUT)
6sl7 5 plate(b) 140 , 123
6sl7 6 cathode(b) 1.7 , 1.92
6sl7 7/8 heaters , 17.5"

--------------

And any of the grids should have basically no voltage on them, I usually suspect
a bad or at least "leaky" coupling cap when they have a few volts on them there.
(anything under .5v here I'd call just a meter sense thing)

label corrections listed above in talics and underlined

 

[Jcricket]

I replaced the 470k with a 270(it is what I had). I also disconnected the preamp and measured.
I have 0.000 volts at pin one now.

Now to get me new preamp.:sigh:

So waht is next - remeasure the voltages while using the 5u4's? See if that helps?

I'll likely not get back to the computer until tomorrow.
Happy thanskgiving to all, and thank you to all who helped me with this.
Mark

 

[Scuzzer]

Now to get me new preamp.:sigh:

So waht is next - remeasure the voltages while using the 5u4's? See if that helps?

For the time being just put a cap on the input like I put in the schematic. It will block any DC offset voltage from the preamp.

Next thing would be to put a shorting plug on the input and measure all the voltages again and then figure out how to raise your B+ above the screen voltage. Perhaps you could try Kegger or battradio suggestions there.

I'm getting ready to head out for Thanksgiving as well. Hope everyone has a great day.

 

[dcgillespie]

A very neat project here, with a lot of great advice! I have no doubt this 142 will be superb when finished.

If I might offer two cents to the mix, I would be very cautious about trying other rectifier tubes to make up for the voltage loss of the higher resistance main choke, with the new screen supply in place. In particular, any substitutions should be limited to filament type tubes only.

While I completely agree with Keg's comments on the advantages of slow warm up (cathode) type rectifier tubes, in this case, their use could possibly cause significant damage to the output tubes. That's because slow warm up tubes such as the 5V4 or 5AR4 significantly delay their output -- often until after the output tubes are capable of conducting a significant amount of current. After all, that's how they eliminate the B+ surge to begin with. This is especially true with smaller cathode output tubes like the 6V6.

In the modified amplifier, the screens are now supplied by a SS power supply. This provides instant voltage to the screen grids, while with the original 5Y3 rectifier tubes, the plate voltage is not very far behind. In any event, both are present well before the output tube cathodes start to conduct. This is fine, as when the output tubes do start to conduct, the lion share of the current will be attracted to the plates as it should be.

However, if the plate supply is delayed significantly while the screen supply is not -- as would be the case in the modified amplifier with a substitution to a cathode type rectifier tube -- the screens would start conducting the full current draw of the output stage before the plate supply came on line, likely damaging them permanently. Therefore, I would strongly suggest limiting rectifier tube substitutions to only quick heating types to eliminate this concern.

Good luck with your project!

Dave

 

[Jcricket]

Hey Dave, thanks for the input, it is appreciated.

Here is what I have done so far. I replaced teh 470k with a 270k. I also added the .1uf cap to the input.

I substituted the 5y3's with 5u4gb's. I then added a shorting plug to the input and took measurements.

Here is what I got

6v6gt's
pin
2 17.18v heaters
3. 308v plate
4. 284v screen
5. 220mV - input grid
7. 17.3v heaters
8. 17.26v cathode schematic says 18.5v

All mesurements referenced to ground.

6SL7
pin
1 .8mv input grid
2 78.8v plate schematic says 95v
3. 1.285v cathode schematic says 1.5v
4. 1.6mv input grid
5. 125v plate schematic says 140v
6. 2.13v cathode 1.7v
7 17.3v heater
8 17.3v heater

I also hooked the unit up to an ocilliscope and a fregency generator. If I am doing it right, and read it right it looks superb. I got perfect sign waves from 100 to 20k. I t was pretty tough to look much below the 100, my o-scope won't freeze the sine wave.

One thing I think migh be happening. When I turn up the volume, it is fine to a point, then it distorts very badly and the 0D3's flicker and can even go out. Almost like they are incapable of handling the current needed for the amp.

Any thoughts or comments on the above information??

Thanks!
Mark

 

[Scuzzer]

One thing I think migh be happening. When I turn up the volume, it is fine to a point, then it distorts very badly and the 0D3's flicker and can even go out. Almost like they are incapable of handling the current needed for the amp.

If they're going out that means they don't have the necessary 5ma going through them to keep them lit, they aren't actually supplying current to screens they're just providing enough current to keep a stable voltage on the screen tap. When the amp is pushed hard the screens are at their maximum current draw and may be pulling enough current on their own to push the voltage down too low across the 2k resistor.

Measure the voltage before and after the 2k resistor and we should be able to tell if we need to change that resistor value.

edit: BTW, your numbers look pretty good from here. You should measure the heaters on AC though.

 

[Jcricket]

If they're going out that means they don't have the necessary 5ma going through them to keep them lit. When the amp is pushed hard the screens are at their maximum current draw and may be pulling enough to push the voltage down too low across the 2k resistor.

Measure the voltage before and after the 2k resistor and we should be able to tell if we need to change that resistor value.

Measure it at idle, shorting plug in?