Superlite TMB Schematic Voltages

[kinmike]

Personally I would try the resistor method first. Measure your current (carefully), then do the math.

If you're going with a dropping resistor, you apply R = V / I (resistance = volts to drop divided by current).

The wattage of that resistor is determined by P = I² R (wattage = current squared times resistance).

If you want to be lazy, you can use this calculator: http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html

Thanks,
Josh

Josh,

The calcs don't bother me, I'm just not sure about the current. The other thread mentioned to expect around 80 ma for two EL84's and two 12ax7s. What exactly is the current measurement I'm supposed to take?

Thanks,

Mike

[geoff 1965]

just play it safe Mike and use a 10 watt,this is what a senior amp tech Curtiss told me when i was working out my sag.
"The voltage drop is determined by the current draw; greater current draw gives greater voltage drop across the rectifier, ergo lower voltage at the B+ terminals. Two EL84 in P-P plus two 12AX7A will draw something less than 120 mA at our usual B+ voltage (I-squared R gives about 2.16W) so a 5W resistor will do fine. Install it so there's a little room for air to circulate around it to help dissipate heat"
i upped mine to 8 watts as mentioned earlier

[JMPGuitars]

Josh,

The calcs don't bother me, I'm just not sure about the current. The other thread mentioned to expect around 80 ma for two EL84's and two 12ax7s. What exactly is the current measurement I'm supposed to take?

Thanks,

Mike

Let's assume that's correct. Or maybe "under 120mA" is correct. You can average it out at 100mA. Or measure your actual current draw.

You're only dropping 20V, and anywhere in that range doesn't give you a significant wattage requirement. That said, you always want overkill. 3W or 5W is probably fine. I'd go with 5W, you can go higher if you feel like it. Doing real measuring + math would be preferred over guessing. Assuming any of these numbers are close enough, you'll likely need a 150 to 250 ohm resistor depending on your actual current draw.

Thanks,
Josh

[kinmike]

At the risk of causing thought overload, I feel bound to mention the other approach of fitting "protecting resistors" to each anode of the rectifier.
You can see this mentioned in the Philips EZ81 data sheet, and you can find explanations from Merlin's site (bottom of this page, and see others) -
http://www.valvewizard.co.uk/bridge.html

Zuceno,

Thought overload is good as long as I can skulk back to my lair and ponder everything for as much time as I need.

I checked out the link. It doesn't explicitly state that it can be used as a technique to lower voltage but did include affects that lower voltage.

Can you share your thoughts on using this technique?

Thanks,

Mike

[kinmike]

Let's assume that's correct. Or maybe "under 120mA" is correct. You can average it out at 100mA. Or measure your actual current draw.

For actual current draw I measure idle current from center tap to plate for each tube then add them up?

Thanks again,

Mike

[zuceno]

For actual current draw I measure idle current from center tap to plate for each tube then add them up?

Not quite - that will just give you the current draw when idling.

Can you share your thoughts on using this technique?

My first thought is that you are not getting the hang of this, so I will try to explain the points you are not grasping.
Whichever method you use - whether you use a big resistor in the line, or two anode protecting resistors - you are introducing resistance to reduce the voltage and consequently limit the anode current.

You really want to know the voltage and current values at high output to make an informed decision, but actually measuring these is not practical for most people. You need something like a sine wave generator to provide a signal, then measure the current in the B+ line at full volume - in a good workshop the amp would be running into a dummy load to save your ears. Add to that the fact that the voltage is not constant - the rectifier's voltage drop comes into play when more than idle current is drawn, the drop increases with output - you really need three meters on this job, and setting it up is a lot to think about.

What JMPGuitars suggests is to guess the current at 100mA-120mA and base calculations on that, which is a valid way to approach the problem.

The suggestion of using protecting resistors allows you to select the resistor values by interpolating between the voltages/resistances/current values given on the Philips EZ81 data sheet. What I like about doing things this way is that it is definitely a recommended method (which most amp manufacturers from Leo Fender onwards have ignored to save pennies).

[kinmike]

What JMPGuitars suggests is to guess the current at 100mA-120mA and base calculations on that, which is a valid way to approach the problem.

Zuceno,

Thanks for you reply. I will use this approach.

The suggestion of using protecting resistors allows you to select the resistor values by interpolating between the voltages/resistances given on the Philips EZ81 data sheet. What I like about doing things this way is that it is definitely a recommended method (which most amp manufacturers from Leo Fender onwards have ignored to save pennies).

Sounds like you have used this technique but don't see much discussion on the use of it here at 18Watt.com

[zuceno]

Sounds like you have used this technique but don't see much discussion on the use of it here at 18Watt.com

That is because you have to deduce what you want from what you are given as information.

The table in the data sheet shows what protecting resistors you would use to get the DC voltage to approximate to the AC input voltage at given current values - and in this case, you want a higher DC voltage and a lower current than those illustrated. And, once again, you have to guess at the "dimed" output current. It takes thought - and a read of the next page of Merlin's site.

Because you have to guess, whatever way you choose will get you into the right area, and then you have to change values around to get what you want.
This is called a pragmatic approach!
Science is true, do not be misled by facts.

[kinmike]

At the risk of causing thought overload, I feel bound to mention the other approach of fitting "protecting resistors" to each anode of the rectifier.
You can see this mentioned in the Philips EZ81 data sheet, and you can find explanations from Merlin's site (bottom of this page, and see others) -
http://www.valvewizard.co.uk/bridge.html

Further down in the paragraph Merlin’s discussion of this also says that alternatively you can place one resistor in series with the rectifier tube cathode. That’s the same as what Josh (JMPguitars) is suggesting, correct?

[zuceno]

You need to consider the intent of placing the resistors in the circuit to decide what is best for you.

Resistors on the anodes are to protect the rectifier from excess current draw, i.e. an overload.
A single in line resistor, as suggested by Merlin, can also offer protection and will promote sag, which is why most people put a resistor there.

JMPGuitars' suggestion is a response to your desire to drop the working voltages.

Any of these techniques will drop off some voltage, but when the amp is idling the current is low, and the voltage may not settle at a value you expected.

The important point is that the working voltages at idle will not necessarily drop as you might think. You can only experience the full drop when the amp is working hard, "dimed", and the B+ current output is largest.

[JMPGuitars]

Josh,

The calcs don't bother me, I'm just not sure about the current. The other thread mentioned to expect around 80 ma for two EL84's and two 12ax7s. What exactly is the current measurement I'm supposed to take?

Thanks,

Mike

Hey Mike,

Did you work this out yet? I just built an amp and went with the resistor method. For my build (less drop than yours) it only required 100 ohms/5 watts...but I'll probably throw in a higher wattage at some point. It sounds killer.

Thanks,
Josh

[kinmike]

Ola amigo...

I’m on vacation in Cozumel, snorkeling, etc. Looking forward to working on the build when I get home.

Mike

[kinmike]

Gents,

I had some 150 ohm, 6.5 watt resistors handy from long ago so I tried one. The B+ voltage went down to 320, too much. So I put two in parallel for 75 ohms and B+ is right at 338 volts. Success!

In parallel I can combine the power rating of the two resistors as well, correct?

I will post all the other voltages in the morning.

Thanks,

Mike

[kinmike]

Guys,

Here are my voltages after using the dropping resistor to get B+ to around 340. JMP's original target voltages are shown in parentheses after mine:

A: 337 (340) B: 309 (310) C: 283 (285) D: 233 (210)

V1 (Preamp 12AX7):
Pin1: 154 (135) high
Pin3: 1.15 (1.2)
Pin6: 128 (105) high
Pin8: .71 (.75)

V2 (PI 12AX7):
Pin1,6: 212 (175) high
Pin2,7: 43 (25) high
Pin3,8: 62 (35)

V3, V4 (EL84):
Pin2: 10mV (very low mV - usually around 10mV)
Pin3: 10.55 (12)
Pin7: V3=323, V4=325 (340) (low)
Pin9: 300 (320) (low)

V5 (EZ81):
Pin1: 290 (290)
Pin3: 340 (340)
Pin7: 290 (290)

Something is still off. My pre tube voltages are high and my power tube voltages are low. Let me know what you think.

Thanks,

Mike

[crgfrench]

Maybe your second resistor In the B÷ line is a little low, ohms. (R5 and or R6 in the Stinger schematic, I mean.)

[JMPGuitars]

V3, V4 (EL84):
Pin2: 10mV (very low mV - usually around 10mV)
Pin3: 10.55 (12)
Pin7: V3=323, V4=325 (340) (low)
Pin9: 300 (320) (low)

V5 (EZ81):
Pin1: 290 (290)
Pin3: 340 (340)
Pin7: 290 (290)

Something is still off. My pre tube voltages are high and my power tube voltages are low. Let me know what you think.

Thanks,

Mike

Actually, I think your preamp looks great. V1 is cascaded, and the idea appears to be to have the first anode slightly higher than the second anode, and you have that.

You PI looks exactly like a typical 18 watter PI: 225, 50, 70 targets respectively. You're very close there, and that's all you need to be. Keep in mind that for both preamp tubes, a tube swap can potentially change those numbers a lot. You're in the ballpark, so you're good.

On the power tubes, Pin 7 is supposed to be higher than pin 9, and it is. So that's okay for now.

Pin 3 you need to get closer to 12V. Start there, and then retest your power tubes.

Question: Is pin 3 of the EZ81 really 340? If you placed resistors at the rectifier anodes, then that would make sense. If you placed your resistor after the rectifier cathode, then I would expect to see your old voltage at Pin 3, and the correct B+ after the dropping resistor at "A."

Thanks,
Josh

[kinmike]

V3, V4 (EL84):
Pin2: 10mV (very low mV - usually around 10mV)
Pin3: 10.55 (12)
Pin7: V3=323, V4=325 (340) (low)
Pin9: 300 (320) (low)

V5 (EZ81):
Pin1: 290 (290)
Pin3: 340 (340)
Pin7: 290 (290)

Something is still off. My pre tube voltages are high and my power tube voltages are low. Let me know what you think.

Thanks,

Mike

Actually, I think your preamp looks great. V1 is cascaded, and the idea appears to be to have the first anode slightly higher than the second anode, and you have that.

You PI looks exactly like a typical 18 watter PI: 225, 50, 70 targets respectively. You're very close there, and that's all you need to be. Keep in mind that for both preamp tubes, a tube swap can potentially change those numbers a lot. You're in the ballpark, so you're good.

On the power tubes, Pin 7 is supposed to be higher than pin 9, and it is. So that's okay for now.

Pin 3 you need to get closer to 12V. Start there, and then retest your power tubes.

Question: Is pin 3 of the EZ81 really 340? If you placed resistors at the rectifier anodes, then that would make sense. If you placed your resistor after the rectifier cathode, then I would expect to see your old voltage at Pin 3, and the correct B+ after the dropping resistor at "A."

Thanks,
Josh

Pin 3 of EZ81 is 348. I think I might have switched rectifier tubes after my first measurement.

[JMPGuitars]

Pin 3 of EZ81 is 348. I think I might have switched rectifier tubes after my first measurement.

That makes more sense. Raise the voltage on Pin 3 of the power tubes, and I think you should be good. What value is the resistor there?

[kinmike]

That makes more sense. Raise the voltage on Pin 3 of the power tubes, and I think you should be good. What value is the resistor there?

Josh,

It's 150 ohms. Maybe try 180 ohms?

Mike

[JMPGuitars]

Josh,

It's 150 ohms. Maybe try 180 ohms?

Mike

doit.gif