Voltage question ... measuring?

[bluefoxicy]

Hey guys, in measuring my tube voltages, do I measure with the tubes in or out? With the tubes out there's higher voltages everywhere; the tubes themselves have inter-electrode resistance, so with no tube B+1 can be 370V while with a power tube in B+1 might read 300, and drop lower as you bias the amp colder.

 

Which is the voltage I'm concerned with?

[Eracer_Team]

first time test you may wish to measure the amp with the tubes out, but with no correct load on the amp your readings will be false.

 

Measure with tubes in is the only way to get correct readings

[bluefoxicy]

Alright. That's how I've been measuring but my voltages are always low (mind you, I'm using 6V6 rather than EL84; I'm WELL over the EL84 rating).

 

 

The plate voltage changes as I change the bias due to inter-electrode resistance.

[Eracer_Team]

Well from what i understand that if you are running the 6v6 and not the el84 you'll need to bring the voltage's up a lot by dropping R10 below the stock valve of 220ohms to something more in line with 100ohms.. but you'll have to work the voltages out on your rig.

[bluefoxicy]

Using 22 ohms, and a bigger screen resistor. I'm likely to try pulling B+2 down to get 225V as per RC-30, although I'm considering using a 3 position switch for 225V, 260V (what I have now), and junctioned to the plate (Beam Triode mode, rather than Beam Tetrode mode). This will be accomplished by having On-Off-On SPTT, with On(1) adding a resistor in parallel to bring the voltage up, and On(2) hooking the screen to the plate.

 

Amusingly, the 12AX7 max is 330V, and the B+3 is smaller than B+1; while the EL84 (6BQ5) rates 300V max design center value, and 250V typical in a Class A1 (250V or 300V in a Class AB1 push-pull). I understand that most power tubes handle more power -- an EL34 (6CA7) can run 800V! 6L6V at 500 too... -- but that power supply could have reasonably started with the 12AX7 in this case.