5e7 voltage question

[camnicklaus]

After a week of trouble shooting, trying to figure out why the voltages on V2a were not right according to the original fender schematic, I finally noticed that my voltages don't really seem all that wrong? I get about 190V on the plate/grid of v2a/b and about 1.4 on the cathode of v2a. those seem like pretty standard numbers, but on the fender schematic I've been referencing it's 140 and 2.2 Plate and cathode respectively...

http://www.kilback.net/homebrewtweak..._5e7_schem.gif

anybody understand how they arrived at those numbers or is it just a misprint?

[Amp Kat]

You have to realize those are figures from the early 60's and the transformers were different and the line voltage was only 110 and now it's 122 to 126 which explains why your at 190 and 1.4. It's not a misprint and these voltages are at quiescent points so any signal of anything going thru the tube will effect it to a small degree but your close enough. If you have a problem and it's not working post what it is and I'm sure lots on here can point you in the right direction.

[camnicklaus]

Thanks for the reply! I should mention that I get really close (within 10V) everywhere else and before I added a bias pot and screen resistors I was dead on (voltage wise) at the 6l6 plates (my B+ is right on as well). Would higher wall current account for just that one stage being different?
I started down this road because I'm getting a very "crackly" nasty sounding distortion when the amp is in transition from clean volume to overdrive. I've heard the same kind of distortion on other old fenders but not nearly as bad as mine. somebody suggested a poorly biased preamp stage and here I am.

[Enzo]

ALso note that on at least many old Fender schematics there was a note telling you the values were approximate and could vary 20%. COnsider tham ball park estimates, not precise voltages to expect.

[camnicklaus]

I get that there is mojo in every built thats gonna affect it, and the old fender voltages are rough guides from a time when wall outputs were different but still, they got 140V on the first valve stage just like mine, as well as all the same B+ readings. Seems like if wall voltage played a part it would show up in the B+, and to be fair 190V is more like 35% off...
It just doesn't feel like a tolerance thing, it really seems like I'm not understanding something about the way that stage works. I'm beginning to grasp a straight ahead triode gain stage, but I get a little lost when half the tube is a gain stage and the other half is a CF.

[Amp Kat]

So do you have 280 volts at the 8uf cap ? You also should check the 100k resistor at the cathode is grounded good. If you have 280 there you should have 140 at the plate and 140 above that 100k cathode resistor. The CF is a low impedance source driving the controls and doesn't give that stage any gain so there is no amplification there so to speak. In the builds I've seen most of these things or bad connections or something going somewhere it's not supposed to. Double check your wiring real good and that pot connection on the treble pot.

[Enzo]

OK, 190 is not the B+ voltage, it is the plate voltage.

If the mains is 120v and the B+ is 360v, then for every volt of change on the mains, we should see three volts change in the same direction on the B+. But what it is to start with can vary a lot as well as the mains varying.

Your voltage is a function of current through the tube and the resistance of the plate load. And the voltage at each B+ node is a function of the current through the stages adding up and flowing through the dropping resistors.

With empty filter caps, what do the plate load resistors and cathode resistors actually measure in resistance?