fogwolf wrote: ↑Tue Jun 19, 2018 10:02 pm
Ok, the voltage coming out of my wall is almost 128V AC so doing the math that does put it around 550V DC (if I did it right).
Your math is correct, and your wall voltage is scorching high. Wow. Bear in mind that cold bias will yield high B+ voltage. With that kind of input voltage, I can see how you might get up in the 560's, as you indicated previously.
fogwolf wrote: ↑Tue Jun 19, 2018 10:02 pm
Also I measure 5.83V AC at the heaters. So it’s close but a little low. I don’t know enough about tube amps to know if that’s acceptable or what the impact could be or if it would effect the sound, performance or burden of the amp at all. I’m thinking it’s probably preferable to keep that a little low and keep the voltage at the plates to 450V since these are still the original caps and that’s what they’re rated at. Would you agree?
5.83VAC is quite low. As far as I know, the only real danger there is that if it's low enough, the filaments will not light and you could do damage to the tubes. There's a small potential to do damage to the amp if some of the power tube filaments light and others don't. At under 6V, I'd recommend visually verifying that when you power the amp up.
fogwolf wrote: ↑Tue Jun 19, 2018 10:02 pm
I’m thinking it’s probably preferable to keep that a little low and keep the voltage at the plates to 450V since these are still the original caps and that’s what they’re rated at. Would you agree?
Not necessarily. 450V is well under what the amp was designed to run at. Van Halen fans might tell you that this sounds good, but I'm personally not a fan of under running Oranges. As I mentioned earlier, if you want the amp to run at spec, set your variac to the point where the heaters are running at 6.3VAC.
Think of it like this;
The B+ voltage shifts a bit with bias, different tube sets, component values, ect. The dropping resistors and plate resistors drift over time. There are a lot of factors that can contribute to B+ being off spec. And even then, you're talking about a 3.3 PT multiplier, than a 1.3 rectifier multiplier. Small changes turn into big ones.
However, you know the heater voltage. They're all designed to run on 6.3VAC. So if you're using a variac to hunt for how the amp is supposed to be run, that's your best indicator. If it were me, I'd use that as my calibration point and bias the amp accordingly from there.
That being said, I don't particularly enjoy carrying around a Variac. They're heavy, and they also carry risk with them. If that knob gets bumped in the wrong direction, you can have an expensive problem on your hands.
FYI, there are other options if you want to lower the input voltage. The Brown Box has less adjustability, but it removes the possibility of over running the amp.
https://www.amprx.net/product.html
And all that being said, I run my '71 head right from the wall. It's being over run. But I'm vigilant with tube replacements, I bias it somewhat conservatively, and I know what's going on under the hood. I prefer this to carrying around a complicated variac setup. I already to have to lug an attenuator around with it. That's enough!
fogwolf wrote: ↑Tue Jun 19, 2018 10:02 pm
Thanks again so much for the info. I know there’s the serious danger aspect with poking around tube amps but it’s so much better to own vintage gear in general if you know your way around them, since chances are good issues will arise, and being qualified enough to address them yourself safely is rewarding and time and cost effective. I definitely feel more informed now which is exactly what I was hoping for with this post. Really appreciate it.
No problem. Be safe. As someone who has been bit by an OR-120, I can tell you it's not a pleasant experience and I was lucky my heart got through it. You make a mistake like that exactly once. Either you're dead or you never ever do it again.
-Nick