6G2 plate voltage confusion

A 7025 is just a low noise 12AX7. Samo-samo

Did you 0 all the controls on the amp?

If the amp sounds OK you're probably good. If the tremolo works especially.

 

Both plates on V1 read 180vdc? I would suggest a full voltage chart just out of curiosity...and as a baseline if you have never done that. .

 

.. and a schematic to begin with.

Your bias resistor might be a bit off. But since I don't know what it is and should be, that is a wild guess.

 

Doesn't Ohm's law have something to say about this? If you know there's 280V on one side, and 190V on the other side of a 100K resistor, there is 0.9mA flowing through there.

If there is 280V on one side and 135V on the other side of the same 100K resistor, there is 1.45mA flowing.

Could it be a difference in test conidtions? Are your measurements at idle while the specs are with a signal flowing through the amp? Or, have you tried changing out the tube?

 

I have original '62 6G2 and 6G3 amplifiers, and yes the schematics are not accurate. I have no idea why people think all Fender schematics are the yardstick by which all measurement should be compared. It would make sense, it would be nice, but isn't a reality.

If it sounds good, you're happy with your B+, you're not red-plating, you're using schematic component values, then I don't know what there is to worry about. Cloning is a hole that can get deep and wide for no good reason.

 

Yes, Ohm's law has something to say. Maybe the 100k is not what is should be or, after looking at the tube curves, the bias resistor is different...

 

Heh, @FenderLover helped me with a very similar answer to a similar question a few years back: like he said, on a good day it’s just that the preamp plate and cathode voltages, in particular, don’t exactly match Fender vintage values, for several reasons. On a bad day there are other causes. So a complete voltage table is a good thing.

And the 6G2 is a good example of "Were the schematic voltages even right?" You can find debate among owners of originals about B+; some say it's nowhere near as low as 315V even on vintage-correct wall volts. Others say it is. Most folks agree though that to get closer to 315 makes the amp 'more brownface' -- tubey-er, bluesier, earlier drive, etc.

BTW, you note you used the 275-0-275 taps (good idea if you're chasing vintage B+). Did you make any changes to schematic values? Sovtek 5Y3? Different resistors? Etc? And as noted above your bias setup and setting is relevant.

 

Many actual pre tube voltages are way off it seems.

 

If it works out proper....

It is proper

 

Back when those schematic voltages were measured 10% tolerance resistors were usually more than 5% off the value because more accurate were painted more accurate tolerances and I recall readind Fender did use 10% resistors.

You need to bias tubes changing cathode resistors if you want to see other voltages. Optimum operation is not necessarily good on guitar amps when 2nd harminic distortion sound good and it increase when bias is not perfect.

Or change different tubes.

 

Good idea -- get some voltages before you start doing too much work to change anything. We don't have any reason (so far) to think it isn't right. Here's the 'King Fan corollary' of Murphy's Law: If something *isn't* wrong, trying to fix it will change that....

 

Before fretting about another rectifier, what is your B+? I am thinking that if you have that 280vdc at the last power rail mode, your B+ must be close to the schematic, correct? If you get really involved with wanting to see if you can make those V1 voltages look more like the schematic, you could change the cathode resistance since according to your post the voltage there is a bit low. A lower voltage there will cool the bias and raise the plate voltage a bit. Decreasing the value of the cathode resistor will increase current and decrease plate voltage. One could also change the plate resistor.
This thread makes me wish I had a voltage chart from one or all of the 6G3s that I have owned.