Circuit Analysis

[rlyach]

At the risk of boring everyone, I have some technical questions about the Toad design. Based upon the voltages shown in the manual I calculated the operating points of both tubes. The EL84 Looks fairly straight forward. I appears to be biased at Ia=60mA at Vg=-6V. This is a rough guess since the voltages measured are a little inconsistent. That also leads to my guess at the OT-4 looking like a 3K ohm load to the tube. This all makes sense but the EF86 is baffling. According to the manual, the plate voltage of the EF86 is ~30V, the screen is around 35V or 36V, and the cathode is around 30mV. The bias current is ~200uA. I don't understand this bias condition. First, with a ~0V cathode and a capacitively coupled control grid, the Vgk is 0V and part of the waveform will have positive grid voltage. Further, when I try and draw a load line for the EF86 it is operating at very low currents and voltages. I am very confused and would appreciate any help in unraveling this mystery.

[Paul Birkeland]

Hello Randy,

Measure the G1 voltage of the EF86 while operating, you will find that it is not 0V 

The EL84's have some variation, IIRC we wrote the manual and took measurements with an old school RCA EL84 before we sourced what we ship with the kit (the OT-4 will indeed look like a 4K load to the tube).

-PB

[Doc B.]

The point of the low plate voltage is to allow for direct coupling without having to go to astronomical power supply voltage, and running the current starved EF86 plate (about 200uA)  just about doubles the gain - important because we crank as much as 20dB of global negative feedback into the circuit and a typical single coil pickup puts out maybe only 250mV. And yes, in direct coupled circuits variations in one tube can pull the other tube's voltages around a bit.

Here's an article about the circuit that was the initial inspiration for the Tode design:

http://www.r-type.org/articles/art-003h.htm

[rlyach]

That make more sense. I forgot about the feedback and also that the input signal was so low. I have been spending too much time with stereo hifi amps. It is going to take me a while to adjust my thinking. Thanks for the link to the Mullard site.

[rlyach]

I spent some more time with the schematic and I was curious about the mid scoop control. I ran some simulations and here is the frequency response of the Tode's scoop circuit in each of the three configurations. My kit should arrive next week and I can't wait to get to building.

[Doc B.]

Yeah, that's pretty much how we describe the response of the circuit.