Resistance/Voltage checks

[Paul Birkeland]

You can think of this as analogous to the "12V" lights and electronics in your car.  When your car is running, and you have 14.4V coming out of the alternator for the 12V electrical system, this doesn't burn out all the 12V components because there is an allowable tolerance in voltages that they are designed for.

If you look at the datasheet for the 6FJ7, the allowable voltage for the heater supply is specified within, and you are in the acceptable range, so there isn't a lot more to say.

[troplin]

Ah, I should have been a bit clearer.
I totally understand what tolerances are and I'm fine with the measurements being off when they are still in the tolerance.

OTOH, it feels much better to actually know why the measurements are off. I always want to understand everything. I'm not asking for advice, just information.
As far as I understand the circuit, there's only a low pass filter between the rectifier and the heater, so the voltages should be approximately the same, except that they are filtered on one side and unfiltered on the other.
So (assuming I didn't screw up the measurements) my only explanation would be that my meter isn't very reliable if the voltage is not filtered. Which would again imply that I have to take all DC-measurements of my meter that are not known to be constant voltage with a grain of salt.

But I also understand that this isn't support for your products anymore, so don't feel obliged to answer. 

[Paul Birkeland]

But I also understand that this isn't support for your products anymore, so don't feel obliged to answer. 

Nah, we are totally OK going into the technical side.

The heater current draw of a tube like the 6FJ7 won't be completely consistent from tube to tube, and your line voltage is going to not be 100% in the middle of the range of powerline voltages for the power transformer primary winding selection you used, so there will be some variation in this DC voltage from these combined forces.

-PB

[troplin]

The heater current draw of a tube like the 6FJ7 won't be completely consistent from tube to tube [...]

Ah yes, the heater current...
My assumption that the low pass filter doesn't change the overall voltages was wrong. That 0.1Ω resistor in the low pass filter will of course lower the voltage if you draw any current. The heater current is 0.9A per tube, which leads to a voltage drop of 2*0.9A * 0.1Ω = 0.18V, close enough to the 0.21V difference that I measured.

Mystery solved, thanks!

[Adrian]

http://bottlehead.com/smf/index.php?topic=9851.msg91550#msg91550

All of your measurements in the heater supply diode bridge circuit were well with allowable range.
As was said in a previous post "...there isn't a lot more to say."

[troplin]

http://bottlehead.com/smf/index.php?topic=9851.msg91550#msg91550

All of your measurements in the heater supply diode bridge circuit were well with allowable range.
As was said in a previous post "...there isn't a lot more to say."

My understanding was that if the voltage is specified as a range in the manual, it should be within that range and the +/-10% only applies to non-ranged values. I could be wrong though.

Anyway, as I said in my second last post, I just wanted to understand what I'm actually measuring because the values were not aligned with what I thought they should be. But that's now solved.

As was said in a previous post "...there isn't a lot more to say."

There's always more to say
For example my next question could be "Why is the allowable Range at the H-Terminals 2.8-3.2V, which will result in an effective heater voltage range of 5.4V-6.2V (the missing ~0.2V being lost on the 0R1 resistor), which is lower than the heater voltage range specified in the 6FJ7 manual (5.7V-6.9V)? Even the nominal voltage of 6.3V is outside of that range."
But I think I'll refrain from actually asking that question.

[Adrian]

Continue to be curious - continue to learn.