Superlite TMB Schematic Voltages

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JMPGuitars
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Re: Superlite TMB Schematic Voltages

Postby JMPGuitars » Tue 09/10/19 5:14 am

kinmike wrote:
Mon 09/09/19 8:45 pm
Ended up using 220 ohm 5 watt for the dropping resistor and 200 ohm for the cathode resistor. B+ is 337 and pin 3 of power tubes is 11.6.

I could probably try 220 for the cathode to get a little closer but that is another Mouser order.

Mike
Drop the B+ to 200ohm. 337 is too low. You want 340 to 345. Then test cathode again. If those are the only resistors in the ballpark, put the 220 on the cathode, and the 200 on B+ and test.

Thanks,
Josh
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Re: Superlite TMB Schematic Voltages

Postby kinmike » Tue 09/10/19 12:41 pm

Josh,

Just from my experience lately of the interaction between the two resistors I was thinking just changing to a 220 ohm cathode resistor would increase both the B+ and the power tube pin 3 voltage. Do you think that would be worth a try?

Thanks,

Mike
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Re: Superlite TMB Schematic Voltages

Postby JMPGuitars » Tue 09/10/19 12:44 pm

kinmike wrote:
Tue 09/10/19 12:41 pm
Josh,

Just from my experience lately of the interaction between the two resistors I was thinking just changing to a 220 ohm cathode resistor would increase both the B+ and the power tube pin 3 voltage. Do you think that would be worth a try?

Thanks,

Mike
Sure, why not? Small changes aren't going to hurt anything.

Thanks,
Josh
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Re: Superlite TMB Schematic Voltages

Postby kinmike » Thu 09/12/19 8:44 pm

220 ohm cathode resistor got me a B+ of 346 and power tube pin 3 voltage of 12.2. I guess I can call it good. I'll post all voltages tomorrow.

Thanks for all the help. I'm just slow in every sense of the word.

Mike
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Re: Superlite TMB Schematic Voltages

Postby JMPGuitars » Fri 09/13/19 6:42 am

kinmike wrote:
Thu 09/12/19 8:44 pm
220 ohm cathode resistor got me a B+ of 346 and power tube pin 3 voltage of 12.2. I guess I can call it good. I'll post all voltages tomorrow.

Thanks for all the help. I'm just slow in every sense of the word.

Mike
That is good! How does it sound??

Thanks,
Josh
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Re: Superlite TMB Schematic Voltages

Postby kinmike » Fri 09/13/19 7:49 am

Didn't get a chance to put it back together and play it, just soldered in the 220 and checked the voltages. I'll play it tonight.

Thanks again,

Mike
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Re: Superlite TMB Schematic Voltages

Postby kinmike » Sat 09/14/19 9:50 pm

Another question: I was checking plate dissipation. I calculated plate current by measuring the resistance from plate to center tap for each tube and voltage drop for same. R was around 140, voltage drop around 7. With plate voltage of 336 times .05 you have plate dissipation of 16.8 watts, well over the max for an EL84 of 12 watts.

What gives? I prolly measured wrong....
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Re: Superlite TMB Schematic Voltages

Postby JMPGuitars » Sun 09/15/19 6:17 am

kinmike wrote:
Sat 09/14/19 9:50 pm
What gives? I prolly measured wrong....
Use a calculator: https://robrobinette.com/Tube_Bias_Calculator.htm

Pick EL84 in the list, then subtract your cathode voltage from the plate voltage and enter it in the form. Pin 7 is the plate. If Pin 7 reads 340, then 340 - 12.2 = 327.8. Press the first calculate button.

Then on the second section you'll use the cathode resistor to determine dissipation. 220 ohm resistor, shared by 2 tubes, with 12.2 volts across the cathode resistor. This will tell you your cathode biased dissipation. Press the second calculate button.

Assuming the numbers I wrote above are correct for your plate voltage, then your dissipation is 9.1 Watts per tube.

Thanks,
Josh
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Re: Superlite TMB Schematic Voltages

Postby kinmike » Sun 09/15/19 9:39 am

Josh,

As always, thanks for the quick response. The Robinette site is very convenient. My plate dissipation is around 9 watts. I was using the method I described above to allow evaluating each tube individually. Saw it on Uncle Doug on YouTube. Still not sure what I did wrong but I didn't really spend much time on it.

These tubes are about 12 years old, EI's from Yugoslavia. They're supposed to be matched so I should be good.

Mike
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Re: Superlite TMB Schematic Voltages

Postby JMPGuitars » Sun 09/15/19 9:54 am

kinmike wrote:
Sun 09/15/19 9:39 am
Josh,

As always, thanks for the quick response. The Robinette site is very convenient. My plate dissipation is around 9 watts. I was using the method I described above to allow evaluating each tube individually. Saw it on Uncle Doug on YouTube. Still not sure what I did wrong but I didn't really spend much time on it.

These tubes are about 12 years old, EI's from Yugoslavia. They're supposed to be matched so I should be good.

Mike
No problem! Personally, I'm not a fan of UD. ;)

Whether or not they're actually matched is a question you can answer by putting them in a tube tracer. But "matched" is a relative term too. Different people argue matching is anywhere from 5% to 20% difference, and others argue matching is a waste of time. That's a whole other topic though.

Now plug it in and see how it sounds! Post a demo too if you can.

Thanks,
Josh
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