Asking for advice an B+ and bias

[billstets]

There have been a few threads knocking around, including the Udo mess, addressing B+. Using Flame1950's advice below, I took a look at my build with the stock Metro PT. So if I back the bias pot way down, the B+ is 375 and the formula yields a bias of 50.4. I brought up the bias near that and my B+ dropped to 357, meaning the bias should be 53. I iterated again and it didn't change much. I'm concerned to run it there, so I backed it down to 51 and 46, and the B+ is now 360. So two questions:

1. Is my B+ too low? And if so why? Wall voltage is fine (120). The readings at the rectifier tube socket are correct (320ish). The rectifier is the stock JJ, so I have ordered an NOS. I wonder if the rectifier tube is the culprit? Would 400-410 sound a lot different than my 360?

2. If my B+ is that low, is it ok to run the bias at around 50 with these Valve Arts? There was another recent thread on this where someone posted that they have had no problem. Mine aren't red-plating and it sounds really good.

You have to start with the bias voltage as big in a negative direction as you can set it (cold) and run the equation. Set it to the amount of current the equation gives you, and then you'll find that the plate voltage has gone down because of the increased cathode current. So you run it again at your new plate voltage, set the current again, and go back and forth, until you settle in to where changing anything is too small a difference to be of any use. And that's the figures you quote as your B+ and bias.
You can't just set it to 50+ mA because somebody else did, every set of tubes and each amp may be different. And the two figures interact every time you change the bias setting -- plate voltage goes down as your bias voltage comes less negative.

[Kevin]

Well... At 50mA with 360V on the plates, you're only at 72% or so, which really isn't high. As long as you're not redplating, don't worry about it.

What kinda voltages are you getting on the plates of your preamp tubes? You might try swapping your voltage dropping resistors for something like 4k7. See if that nudges up your preamp voltages a bit. You might gain a little bit of headroom and punch.

I imagine there's a bit of variance with the PTs. Maybe you just ended up with one on the low side of things.

[billstets]

Well... At 50mA with 360V on the plates, you're only at 72% or so, which really isn't high. As long as you're not redplating, don't worry about it.

What kinda voltages are you getting on the plates of your preamp tubes? You might try swapping your voltage dropping resistors for something like 4k7. See if that nudges up your preamp voltages a bit. You might gain a little bit of headroom and punch.

I imagine there's a bit of variance with the PTs. Maybe you just ended up with one on the low side of things.

Thanks for the response. I really appreciate it. Sorry, I'm one of those electronically challenged guitarists! Which resistors are the voltage dropping resistors?

[Roe]

the big 8k2

[billstets]

the big 8k2

Ok thanks. Over next to the bias caps right?

[Flames1950]

I'd wait for the NOS rectifier before changing anything else, I've been less than impressed with many of the current GZ34 rectifiers and the vintage one may be your ticket to getting the B+ back up in the 400 range that most of George's amps settle in at.

[billstets]

I put the NOS rectifier in and no change. I still end up at 360 on the plates and about 50 bias.

I'll check my voltages at the pres and think about changing that resistor. BUtu at the end of the day, is my Metro Heyboer PT defective? How much of an effect on the sound is there having only 360 instead of 400-410. Should I consider a new PT?

[Kevin]

I don't imagine it's defective...

I'm curious about your voltages down the line. Post your preamp plate voltages. There are two voltage dropping resistors which can be changed - the big 8.2k and 10k resistors. It won't be a huge difference, though...

The big question is whether you want more headroom, balls, etc., or whether you're happy with the way the amp sounds.

[billstets]

I don't imagine it's defective...

I'm curious about your voltages down the line. Post your preamp plate voltages. There are two voltage dropping resistors which can be changed - the big 8.2k and 10k resistors. It won't be a huge difference, though...

The big question is whether you want more headroom, balls, etc., or whether you're happy with the way the amp sounds.

I built it two years ago and the voltages were fine then, but I am going to check them again, hopefully this weekend. I'll post them. If the wall voltages are fine and every rectifier tube I put in (incl. NOS) is delivering the same voltage, then it seems that the PT is not delivering enough voltage right? It would seem to me that it's not working right then. I like the amp the way it is but wouldn't mind a tad more punch and headroom, but I don't want to turn it into a BF Fender either!