Magnavox 196 measurement results

Sumner2020

New Member
Magnavox 196-00 that I’ve bypassed the balance pot, added power switch and inline fuse. Hum balance pot still in circuit. Also still have a single 56 ohm cathode resistor for both 6BQ5’s. First reading is what the schematic shows second is what my amp reads.

5U4GB
Pins 4 & 6: 260VAC — 275VAC
Pins 2 & 8: 285V — 300V

6BQ5
Pin 3: 7V — 7.7V
Pin 7: 270V — 273V

56 ohm resistor measures 56.7 and 7.88V

Something is not right here.
 
Without the 5U4 in place, pins 2 & 8 are the heater and should and will measure 5 volts AC between them. Pins 4 & 6 will have over 500 volts AC between the two. It will be half of that when measuring each leg to center tap or chassis ground.

With the 5U4 in place, you measure the rectified DC high voltage at either pins #2 or 8 along with chassis ground or center tap.......same thing.

Those voltages are well within the design specs.
 
You're well within tolerances here. Most of these things had at least a 10% margin. Also, consider that line voltage plays into this. Check the heater voltage at the 6BQ5 pins 4 and 5. If its more than 6.3 volts, your line voltage is higher than what was used when these were measured 50 years ago. If you've got a variac, back it down until the heater string is at 6.3 volts and see if the rest doesn't fall in line.

Frankly though, I would not worry about this at all.
 
My concern was the plate dissipation I calculated.
Voltage drop 7.88 divided by 56.7 = .138977 or 138mA
Plate voltage 273 times .138 = 37.6 watts plate dissipation

Is that correct or is that divided by 2 since they both utilize the same 56ohm resistor?

I'm thinking maybe I should split the tubes cathode resistance. I read where Dave did this with the DGSE1. I copied and pasted below how he split.

"Each output tube cathode terminal (pin #3) should have a 120 ohm 1/2 resistor connected between it and the ground lug. There should also be a 100 uF 16V cap connected between each pin #3 and the ground lug as well. The negative terminal of these caps should connect to the ground lug."
 
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yeah, divide that in half. Also subtract out the cathode voltage if you want it to be as accurate as possible. The tube only sees voltage from cathode to plate. And keep in mind the number you get is plate + screen dissipation, actual plate dissipation will be slightly lower but not enough to really worry over. Basically it adds a bit of a fudge factor into the mix.

Functionally, a single 56 ohm resistor vs two 120 ohm resistors is pretty nearly the same. A common resistor for 2 tubes is 1/2 the value that it would be for a single tube. Dave's setup using one resistor would be 60 ohms vs your 56. Effectively the same if you figure in tolerances and standard values.

Screen voltage is a bit on the high side. If this was meant to power a tuner, you may need to bump the resistance between wherever the output transformer feeds from and where the screen feeds from to drop it. That will drop the current through the tube.
 
Sounds like I'm good to go then. Thanks men.

I have a Zenith 6BQ5 PP that is actually still in the console that I bypass the internal speakers to my 2 channel speakers. It sounds amazing. I'm anxious to side by side this SE amp with it.

Thanks again
 
Magnavox 196-00 that I’ve bypassed the balance pot, added power switch and inline fuse. Hum balance pot still in circuit. Also still have a single 56 ohm cathode resistor for both 6BQ5’s. First reading is what the schematic shows second is what my amp reads.

56 ohm resistor measures 56.7 and 7.88V

Something is not right here.
According to the Mullard tube data for the EL84 Rk or Cathode resistor is 130 ohm for a single tube or single ended. That is why you current consumption is so high.
 
Without the 5U4 in place, pins 2 & 8 are the heater and should and will measure 5 volts AC between them. Pins 4 & 6 will have over 500 volts AC between the two. It will be half of that when measuring each leg to center tap or chassis ground.

With the 5U4 in place, you measure the rectified DC high voltage at either pins #2 or 8 along with chassis ground or center tap.......same thing.

Those voltages are well within the design specs.

Did you settle your concern on plate dissipation?

With Cademans reply and Gadget not voicing a concern I assumed my plate dissipation was fine as measured. No?
 
According to the Mullard tube data for the EL84 Rk or Cathode resistor is 130 ohm for a single tube or single ended. That is why you current consumption is so high.
Not that far off the 112 ohms he has in the shared 56 ohm resistor...
 
Whats the screen voltage at 250v though? I suspect the OP's is on the high side and thats why its reading like that. It does math out too high, roughly 18 watts on a tube rated for 12. I admit to not having done the math initially.

Dropping the screen voltage will knock down the current considerably, and that will drop the Pdiss with it.
 
It's pretty far off with the data sheet calling for 250v and 6v of bias for 130R.

So he has 265V and 7V. Not very far off and tons of amps are run a good distance from manufacturer's specs.

Edit: ok, depending on his measurements 16-18W dissipation is well beyond
the rating and not something that's often fudged.
 
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I’ve been putting this 196 through the paces for a few weeks now and really enjoying it. These are truly great sounding “flea power” amps. A wide genre of tunes has been fed to it and it hasn’t stumbled. I have a couple Blackburn rx1 EL84’s and an early Blackburn 12AX7 doing work at the moment. Love it’s mobility, too. Took it from my listening room to the family room and blasted Christmas music for 3 days straight.

And in the process I found what I would consider a lost gem in speakers. I bought a rack system for $50 a couple years ago because I wanted the turntable. A LAB-420. Yes a Realistic. You may chuckle and that’s fine it keeps the market price low. It also had a decent sounding STA-80 receiver. Long story longer it also came with the speakers. Realistic T-100 towers. I hooked these up briefly 2 years ago just to see if they worked. Haven’t touched them since. They have a pot to accentuate or diminish the treble and both needed cleaned and lubed. So I did just that last night and plugged them into the 196.... as I said a lost gem in my book. Really hide themselves well and have great control. I like to use Tool, Forty six & 2 at the 4:44 to 5:10 mark for woofer control. These behaved extremely well. Extremely. Yeah I was surprised.

Anyway thanks for all the help with my flea!
 
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