How to design the straight load line for choke loaded tube ?

[Antonio Tucci]

We are building a 6C45P choke anodic load (40 Hy – 600 DCR) – direct coupled to a 300B.
We’d like to simulate the behavior of the 6C45P on the V-A characteristics curves…but we are not able to design the straight load line. How to design it ?

Which type of input signal we have to consider ? …the p-p or the RMS Volt value ?

Which is the output signal (Volt RMS) of the actual DVD- or CD-Players ?

[Sch3mat1c]

Load line depends on load... if you have no load... you have no loadline (well okay, you always do... it'll be flat, and somewhat oval depending on frequency).

Think CCS.

Tim

[audiousername]

Most CD/DVD players output 2Vrms.

It isn't hard to plot a straight loadline. You get the anode characteristics and draw a horizontal line across it, through the idle operating point. The inductive load will allow the anode voltage to swing above the voltage rail, so Va will be B+ minus the voltage drop across the DC resistance of the anode choke.

The horizontal loadline is a close enough approximation if the grid leak resistor of the next stage is large. At low frequencies and high frequencies it becomes elliptical, but hopefully shouldn't affect anything within the normal audio band.

You would aim to bias the valve such that it will not draw grid current under normal input voltages, which will also depend on the overall input sensitivity of the amplifier.

[Antonio Tucci]

sdfsdf

[Antonio Tucci]

Something is wrong in what I have understood.

If I trace an orizzontal line accross the operating point (-2.4V, 170V, 15 mA) ... and considering an input voltage of 2 VRMS, I have an anodic voltage variation of 200V (275-75) with a null anodic curent variation...
What is wrong ?

[audiousername]

Originally posted by Antonio Tucci
What is wrong ?

That seems about right... I don't see anything wrong.

[Antonio Tucci]

Jason,
...first of all ...Thank you for your replay.

I'd like to ask some specific question about that. In particular:

1) Does "2 VRMS input" mean that we have a variation of +2V on the left and -2V on the right of the operating point ? .. or do we have to consider the V p-p value to design this variation (... +/- 2.828 V) ?
In this last case, we could reach the positivity of the grid (+2.828-2.4=+0.428 V)!

2) How is it possible the 200 V variation without current variation ?

3) Have you had a look at the schematic of the amplifier ? ... what do you think about it ?

[audiousername]

1) 2Vrms is equivalent to 2.828Vpk or 5.657Vp-p. With -2.4V bias, a 2Vrms input signal will cause input clipping by causing grid current. However, the 300B needs only 70Vpk or so to be driven to full power, and the gain proviced by the 6C45P is around 50, so we only need 1.4Vpk, or 0.99Vrms to drive the output stage into clipping. The output stage clips before the input, so all is well. This will mean that some attenuation will always be needed at the input such as a volume pot, but that is pretty common anyway.

2) There is some miniscule current variation, it's just that the impedance presented by the 300B's grid and the anode choke is so large that it is represented as infinite. At low frequencies, and at high frequencies, there will be much current swing, due to the reducing impedance of the choke at low frequencies, and Miller capacitance at high frequencies.

3) A proper Free Lunch/Monkey/DRD design may be simpler, yet still offering the advantages of direct coupling and a choke loaded driver while using far fewer parts (and only needs one power supply). Have a look at this offering from Thorsten:

[Antonio Tucci]

...Some points I do not like about that schematic:

1) Parafeed.
2) Capacitors (directely or indirectely) in the signal path.
3) Single power supply.

As to the straight load line ...

I have now two versions about the straight load lines for choke load.

- One is that you proposed.
- Another one is that proposed by SomeJoe ... that consider the straight load line variable with the frequency (not always parallel as you tell) (see post at Tube DIY Asylum http://www.audioasylum.com/forums/tubediy/bbs.html

Where is the true ?

[Sch3mat1c]

Originally posted by Antonio Tucci
1) Parafeed.

No.

2) Capacitors (directely or indirectely) in the signal path.

*cough sputter*

Oh, and no.

3) Single power supply.

Yes very wasteful of power...

- Another one is that proposed by SomeJoe ... that consider the straight load line variable with the frequency (not always parallel as you tell) (see post at Tube DIY Asylum http://www.audioasylum.com/forums/tubediy/bbs.html

Where is the true ?

I don't know where your post is and frankly I don't care to dig through a *BBS* to find it, but if it is as you describe, it is false. If you must be completely exact about this, the line will be flat horizontal at resonance, eliptical off (due to varying amounts of inductance (at LF) and capacitance (at HF)) and how much depends on the reactance at that frequency.

To a basic approximation, you can ignore reactance if it is equal to the circuit's impedance (here defined by the plate resistance) at the lowest frequency of interest.

Tim