Anode Follower Article

[bacaruda]

Hi,

I have used anode "follower" circuits from time to time as virtual earth amps/buffers etc and think they are a great tool.

I found this article about them: http://www.aikenamps.com/anode.pdf, and for the life of me I cannot understand how the circuit in Fig. 9 on the last page could function. With the cathode of the 1st triode grounded, and the grid referenced to a positive voltage, wouldn't it just burn up?

I think I've got a grasp on how anode followers work, what I'm confused about is how those two amplifiers could be DC coupled like that.

I'm hoping someone could educate me on what I'm missing here, as this specific circuit would do exactly what I need for an upcoming project (assuming it works).

Thanks

[kevinkr]

If you are referring to figure 8 there is a connection clearly marked "B-" in that schematic, so there are provisions for negative grid bias in the circuit design. I think it would be relatively difficult to get it to work, but I suspect the use of a fairly large plate load resistor would prevent the triode from drawing enough current to self destruct if it was driven into saturation. Or perhaps it is just drawn wrong..

[ruffrecords]

Yes, I think you are right. Either there needs to be a blocking capacitor in the feedback path or a bypassed cathode resistor for the first triode.

Cheers

Ian

[kevinkr]

Originally Posted by ruffrecords

Yes, I think you are right. Either there needs to be a blocking capacitor in the feedback path or a bypassed cathode resistor for the first triode.

Cheers

Ian

It's supposed to be a DC design, but there looks like something wrong with the schematic.. Looks odd to me too..

[kevinkr]

Why not just use a CF running off of a bipolar supply driven by a diff stage? (The circuit shown needs a negative supply anyway?)

[smoking-amp]

Figure 9 looks like it could use a VR tube in series with the cathode of the 2nd tube to lower the output voltage and give the 1st tube some operating headroom.

[kevinkr]

Originally Posted by smoking-amp

Figure 9 looks like it could use a VR tube in series with the cathode of the 2nd tube to lower the output voltage and give the 1st tube some operating headroom.

Looking at some of these circuits I'm starting to think that there are problems with a number of them.. I used a very similar circuit to figure 9, but there was an output cap on the follower and the feedback was taken after the cap.. Also cathode bias on the first stage.. Conceptually similar. FWIW with the right tube choice made a great no gain follower for a diff amp in a line stage for example.. (cable driver)

[DF96]

Best to regard fig 9 as a skeleton circuit, for the purposes of analysis. A real circuit would need bias, DC blockers etc. Possibly he has taken a BJT circuit and simply substituted valves without thinking?

[Chris Hornbeck]

Thanks for posting this - really takes me back. His phono equalizer with anode follower RIAA feedback around the second stage is still, to my mind, the best that can be done in the single-12AX7-per-channel-phono-EQ sweepstakes. Wasn't it Davey O'Brien who re-designed it with the first stage open loop?

Thanks,
Chris