B+ VDC. How close to spec?

[Garydean]

Hi, irrelevant to what I'm working on now. I once had a Music Man 2275-130 chassis that had a slightly high B+ of 729VDC instead of 725VDC as called for in the schematic. The output tube cathodes read .564V not exactly .5V, as called for as well. I could not adjust the bias pot any further to bring down the high voltage. The amp played well and did not show signs of over heating. How much tolerance or how "exact" do these numbers need to be? What could I have looked for to bring these #'s down? The cathode resistors off the EL34's measured exactly 10.5 ohms (lifted). not 10 ohms as called for in the schemo. The control grid resistors were well within spec.

thank you,

Gary

[cbarrow7625]

It doesn't need to be extremely close. The fact that the voltages measured as close to the schematic as they did is surprising! Since tube amplifiers run from an unregulated power supply, all of the voltages within the amp are going to swing up and down based upon the voltage coming out of the wall. Depending on where you are, and even what time of day it is, the wall voltage can vary by 5-10 volts easy (120Volts nominal).

What matters is that the relationship between the plate & bias voltages remain fairly constant (and typically they should since they are subject to the same AC input variations). I wouldn't even think about worrying about it unless the voltages were about 20% off (or more). The problem in some very old amps is that if the input voltages are on the high side (125 VAC+), then the DC B+ inside the amp may actually start to exceed the voltage ratings of the capacitors. Some of these amplifiers were built with caps with voltages way too colse to the limits anyway back when nominal line voltage was closer to 115VAC. Now that 125VAC is common in many areas (mine being one of them), the B+ inside the amp can be way over what the amp designers ever expected the amp to see. In a very high voltage amp like yours, the DC voltage swings should be even more apparent than in a typical Fender or Mrshall that has a 300-420V B+.

By the way, the exact voltage on the cathode resistors does not matter, what matters is the current running through the tube. The current and plate voltage will tell you how much power the tube is set up to dissapate. For a cathode biased amp like yours you use the voltage across the cathode resistor & the reisitance of the resistor to calculate the current. The fact that you measured the actual cathode resistor is a bonus because you will ge a more accurate current calculation.

Anyway, yours are spot on. You'll be luck to find anything that close ever again.

Chris

[Garydean]

Awesome Chris! I did not think to take into consideration the VAC coming onto my bench, and I usually do.....doh. I range from 120V to 123V depending on the day. I just worry about the modern tubes at such a high voltage and how long they will tolerate the heat.

thanks again,

Gary

[cbarrow7625]

The nice thing about a class AB amplifier is that the heat is proportional to how loud you play it, unlike a class A where you are at full dissapation all of the time. If you are just concerned about the heat through the tubes you have 2 pretty simple choices:

1.) Don't play as loud for as long. That will keep the hea down.
2.) If #1 is not a viable option, increase the bias voltage and/or the value of the cathode resistor to bias the tubes a little colder for tube lifespan preservation.

If all you are worried about is the heat then you are probably in pretty good shape. Now, why they would design an amp with 700+ Volts on the tubes is another thing.....

Chris

[Enzo]

With 120v mains, and a 720v B+, there is about a 6 to 1 ratio there. And that means for every 1 volt change in the mains voltage, there will be a corresponding 6v change in that B+. SO if the mains is 3v higher today, that B+ will be 18v higher. None of this is rocket surgery, get close and you are OK. 10.5 instead of 10 is only a 5% variation.