Determining plate current from schematic

[Brainojack]

Am i making proper assumptions here?

Viewing the schematic of the HeathKit EA-3, the voltages are labelled. In order to find gain of the stage i need the plate resistance and the load resistance...

Plate Resistance:
Is finding the plate resistance just a matter of calculating all the current through R9 (as seen in figure) ?
Using this current to find plate resistance (from graph) [i've gotten 69 kohm]

Load Resistance:
Is the load resistance of this stage the volume pot in parallel with the plate resistor? (any Grid resistance is bypassed by C11?)

If i take the full 1Mohm of R12 I get a gain of 56.85.

Regards,
Drew

 

[thorpej]

0.75V drop over 1.2K resistor (the cathode resistor) means 0.625mA of cathode current, and this being a triode means 0.625mA of plate current. That would be a 62.5V drop across a 100K resistor, which would give you 132.5V at the plate... which doesn't quite match the schematic, but is also within the typical 20% tolerance of the components in the day. So, your 0.7mA calculation based on just the plate circuit voltages is plausible, and thus your reading of the characteristics graph is correct. Your calculated loaded gain of 56.85 is right in line with my cheat-sheet triode gain stage calculator (a spreadsheet that has a bunch of fixed "typical" plate resistances for a bunch of tubes hard-coded in it) that gave me 59.2.

 

[GordonW]

Yep, you both agree within about a third of a dB or so. That's definitely close enough for analysis.

Regards,
Gordon.

 

[primosounds]

Load Resistance:
Is the load resistance of this stage the volume pot in parallel with the plate resistor? (any Grid resistance is bypassed by C11?)

If i take the full 1Mohm of R12 I get a gain of 56.85.

Regards,
Drew

The load resistance refers to output transformer requirements of the primary.

 

[cozido512]

He is working on a preamp stage - nothing to do with the OPT.

 

[primosounds]

The load resistance refers to output transformer requirements of the primary.

He is working on a preamp stage - nothing to do with the OPT.

It doesn't matter what he is working on the "Load Resistance" refers to the impedance of the primary of the output transformer.

 

[GordonW]

It doesn't matter what he is working on the "Load Resistance" refers to the impedance of the primary of the output transformer.

It doesn't matter if it's a direct resistance/impedance, or an impedance reflected through a transformer. Transformers don't themselves provide significant load. Transformers only transform a load impedance, proportionally to the square of their turns ratio, from the output back to the input- given the same numerical impedance, the AC load on the stage is the same whether the impedance is from a transformed load, or a direct load, and the resultant gain is also the same.

Regards,
Gordon.

 

[cozido512]

It doesn't matter what he is working on the "Load Resistance" refers to the impedance of the primary of the output transformer.

There is no transformer in the circuit he posted, the 12AX7 is driving the 6AU6, or are you referring to some other circuit?

 

[primosounds]

There is no transformer in the circuit he posted, the 12AX7 is driving the 6AU6, or are you referring to some other circuit?

The OP is using the wrong terminology as it pertains to tube electronics. Where the load resistance refers specifically to the output transformer primary impedance. Maybe he is talking about circuit resistance or something else, i am not sure.

 

[cozido512]

Wow... I hope you are joking, btw, what is “tube electronics”?

 

[Brainojack]

thorpej, thanks for explanation. Perhaps your method is better suited for design, and mine was only available since the voltages were on the schematic?

Yes, 'Load Resistance' as seen by that stage. Looks like I cropped out the bypass cap of the next stage so that R13 would not included in Rload.

Regards,
Drew