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Magnavox 142 - rebuild/modification thread

Scuzzer said:
I forgot to ask, what's the power output when the distortion/0D3 issues start happening?



Yep.
Click to expand...

Don't know. Today is the first time I have broken out the O-scope and frequency generator. I am on a pretty steep curve trying to lear those and understand what is happening wit hthe amp.
I know it is a bit high, I am not quite sure it is at clipping though. Bass beats put the light out.
 
So using ohms law we can figure out that 87/2000=43.5 ma of current is flowing through that resistor. Since the screens are at 284v the 100 ohm resistor is dropping 4 volts so your total screen draw is 4/100= 40ma. OK, something already doesn't make sense here.

Is it wired the same as your initial power supply drawing? According to the tube manual the screens should not be drawing 10ma each at idle.
 
Jcricket said:
I am on a pretty steep curve trying to lear those and understand what is happening wit hthe amp.
Click to expand...

Steep but fun part of the curve. I only asked because once you hit clipping (20w or so) I'm not sure what to expect for screen current draw. To figure the power output you hook up an 8 ohm non-inductive power resistor to the 8 ohm output tap and measure the AC voltage across it using a sine wave on the amp input. It's a bonus if you have a DMM that can measure AC at different frequencies, if your unsure use something like 100hz. Output watts= voltage squared/8
 
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What is the AC voltage in with the signal generator when the output clips on the scope with a 8 ohm dummy load , Second what is the AC voltge across the 8 ohm load just before cliping . Third what is the DC voltage on the screens at cliping .
 
Ah yes good point Dave, I completely forgot the screens were from a different rectifier,
in this case solid state instant on diodes, an yah I wouldn't want the plate and screen
to have voltage at quite a different time, so either directly heated rectifiers or delay the
screen voltage, could use a single damper diode after the SS rectification if one wanted. :D

dcgillespie said:
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
Click to expand...
 
You can disconect the voltage from the screens and use the ultralinear taps if need be till you get it working right .
 
battradio said:
You can disconect the voltage from the screens and use the ultralinear taps if need be till you get it working right .
Click to expand...

If he's got UL taps then he should at least put a switch in so he can choose which way to listen to his amps. Might be interesting comparing UL to regulated screen pentode mode. If you do put in a switch you should probably only switch it with the power off though.
 
here is a redraw of the current PS supply circuit. This is as it is.
 

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Hey Guys, between the beer, the long day, and fat tummy, my head just went to sleep. I'll get to this in the morning. I really, really appreciate the guidance and help.

Mark
 
Most likely what is going on is the B+ is droping lower than the screen voltage at higher power out and drawing more screen current , we can make a voltage devider for the screen voltage regulators to get to about 260 with a 18 K ohm resiator and an 82 K ohm resistor to ground and the junction of the two resistors to the screen .

Or sub two 1N4007 diodes for the 5U4's

Mark
 
battradio said:
Most likely what is going on is the B+ is droping lower than the screen voltage at higher power out and drawing more screen current , we can make a voltage devider for the screen voltage regulators to get to about 260 with a 18 K ohm resiator and an 82 K ohm resistor to ground and the junction of the two resistors to the screen .

Or sub two 1N4007 diodes for the 5U4's

Mark
Click to expand...

Hey MArk,
With the 5u4's here are the new screen and plate voltages
3. 308v plate
4. 284v screen

Do you still think tha the voltage is dropping too low. If so, I'll goi ahead nad put in the divider network.

I'll get those measurements in just abit.
 
Jcricket said:
Hey MArk,
With the 5u4's here are the new screen and plate voltages
3. 308v plate
4. 284v screen

Do you still think tha the voltage is dropping too low. If so, I'll goi ahead nad put in the divider network.

I'll get those measurements in just abit.
Click to expand...

Hi Mark ,

The screen voltage will stay constant , the plate voltage will drop when the amp clips whant to know the plate voltage at clipping .
 
Hey Mark,
Here is the measurements

Signal generator is 1.37 volts ouput
Ac before clipping across the 8 ohm dummy load is 12.24volts
Dc on the screens right before clipping is 268 volts.
dc on the plates right before clipping(the anode correct?) is 151 volts

Also, I took the measurements at 40hz and at 2khz - measuremetns were nearly identical

EDIT - when I took the measurements on hte screens, as I increased power to the amp, the screen voltage dropped. When I decreased the power it, the voltage increased
 
Scuzzer said:
Steep but fun part of the curve. I only asked because once you hit clipping (20w or so) I'm not sure what to expect for screen current draw. To figure the power output you hook up an 8 ohm non-inductive power resistor to the 8 ohm output tap and measure the AC voltage across it using a sine wave on the amp input. It's a bonus if you have a DMM that can measure AC at different frequencies, if your unsure use something like 100hz. Output watts= voltage squared/8
Click to expand...

So I am currently getting about 18.5 watts before clipping. That is at 40hz too. Cool!!!
 
Jcricket said:
Hey Mark,
Here is the measurements

Signal generator is 1.37 volts ouput
Ac before clipping across the 8 ohm dummy load is 12.24volts
Dc on the screens right before clipping is 268 volts.
dc on the plates right before clipping(the anode correct?) is 151 volts

Also, I took the measurements at 40hz and at 2khz - measuremetns were nearly identical

EDIT - when I took the measurements on hte screens, as I increased power to the amp, the screen voltage dropped. When I decreased the power it, the voltage increased
Click to expand...

We need to find out why the plate voltage is droping , is that the power transfomer that was over heated if so try the other power transformer you got from me .But first do you have a 100 ohm 5 watt resistor ,if so paralell it with the choke .The run the same test again .
 
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