F2a - C3m Monkey

[EvilMose]

Hello DIYers,

I am attempting to design an amplifier with a C3m wired as a pentode direct coupled to an F2a wired as a triode using the "Monkey" topology. Aware there are some non-standard resistor values in there. That aside, there are a few things I am struggling with:

I am having trouble figuring out how to nail down Vg1 for the C3m. Right now, I have a battery, but I am not sure it would work.

150v for Vg2 is something of a placeholder - plan to use an 0D3 in the power supply to get this. Any reason to wire the 0D3 into the amp instead? Perhaps use a voltage divider to also supply the 5V required for Vg1?

This is my first crack at this sort of thing so I would not exactly be surprised if there are errors or omissions, so anything you see that strikes you as odd, please let it be known.

Thanks!

[kevinkr]

Using a C3M in pentode connection with an inductive load may cause problems with LF response due to the rather high plate resistance of pentode connection. (You want low rp with choke loading) I've not seen monkey configuration used with anything but triodes I suspect for this reason.

You can use LEDs for biasing the C3M or conventional cathode bias with a bypass cap. (Again you don't want to do anything to unnecessarily raise the rp of the C3M.) You can also use negative bias at the grid, (input cap required) and if you make it adjustable you can then easily set the operating point of the output stage as well.

[mastodon]

G3 of the F2a is internally connected to the cathode; you can't connect it to the anode. Which is good, 'cause you'd fry the tube.

The datasheet here suggests 16mA through 1 250Ohm cathode resistor, which means a Vg of about -4V.

I'm not familiar with the 'Monkey topology'; does it have anything to do with drawing 16mA through the grid of the F2a? I have zero experience with this tube, so perhaps it's reasonable, but it strikes me as odd, especially since in your diagram, Vg1 of the F2a is effectively -21V (compared to the cathode of the same tube),and with those parameters, the tube shouldn't draw 71mA of anode current. So either I have no understanding of this circuit at all (which I don't, I admit), or there's something not completely in order with the values.

[kevinkr]

Originally Posted by mastodon

<snip>

I'm not familiar with the 'Monkey topology'; does it have anything to do with drawing 16mA through the grid of the F2a? I have zero experience with this tube, so perhaps it's reasonable, but it strikes me as odd, especially since in your diagram, Vg1 of the F2a is effectively -21V (compared to the cathode of the same tube),and with those parameters, the tube shouldn't draw 71mA of anode current. So either I have no understanding of this circuit at all (which I don't, I admit), or there's something not completely in order with the values.

The current isn't drawn from the grid, but from the cathode through the dcr of the choke and an additional series resistor. The plate current of the C3M through the series resistance of the choke and resistor is what provides the actual negative bias for the output tube. Can't talk to the value of effective bias on the F2a but the topology does work quite well with other tubes. The driver does IMHO have to be triode connected in order to work properly with the choke reactance if any sort of LF response is desired.

[mastodon]

Right, that makes sense, and I sort of get it. But in that case, the C3m should see a plate voltage of about 234V (241V of the F2a cathode minus the 16mA through the choke and series resistor, assuming the ). That means the grid of the F2a sees the same voltage, so the F2a is biased at -6.8V, and the tube should draw about 100mA. That in turn means that the cathode of the F2a won't sit at 241V, but at about 375V. This topples the whole bias again, exceeds the limiting values of the C3m... Can't really work this one out from the top of my head, guess I'm too stoopid for DC coupled circuits

Apart from the above, what is the advantage of this concept? The C3m effectively acts as a choke loaded stage, but what is the advantage of feeding it from the cathode of the F2a? Sort of a push pull effect? Could anyone link me to a topic/site where this is covered?

[eeyore]

Look up DRDin this forum. I went on a journey in understanding this topology, named Loftin White, Monkey, Free Lunch, DRD, Ultrapath, etc. I think there is a resistor missing between the cathode of the F2a and the series resistor and choke feeding the C3m. The vendor of this design is that it is a safer way to do direct coupling. The way I understand it is that there are two resistors at the cathode of the F2a, the top one sets the bias for the F2a and the bottom sets the plate voltage of the C3m. Running this at 670V+ means more caps (in series) in the PSU and everything is go to be rated at 1000V plus I reckon.

On another note, I am thinking of building a C3g-F2a amp, but using a 2:1 IT instead of this because of the higher voltage. I do have all the parts ready to build a 45DRD, so can comment later when I build it.

[eeyore]

I also reckon the load for the F2a is a wee low, I'd suggest looking at a 5K loadline.

[EvilMose]

I was (obviously) not aware inductive loading presented such a problem for pentodes - appreciate the heads up. Will look at subbing in a 6c45pi or a triode-strapped D3a in place of the C3m; new schematic will follow after I run the numbers.

eeyore - out of curiosity, are you planning to run the C3g as a triode or a pentode? Looking forward to seeing your design. I actually ran the loadlines based on F2a curves I entered into Steve Bench's spreadsheet (available here: http://greygum.net/sbench/sbench101/). I just picked a number based on what maximized power output, not what would give the best sonic performance (which is what I am ultimately interested in).

[revintage]

A C3m or any pentode without load will not work. So triodestrap that one too.

Oops, just noticed Kevin pointed this out.

[eeyore]

Yeah, I have only seen Triodes in this topology. Will be using the C3g in triode mode only. D3a might have too much gain at a mu of around 70, but that could mean you can remove a previous stage. The F2a doesn't need heaps of voltage to drive, around +/- 20~30V would be enough. You can even try some of the industrial dual triode, 5687 family, 6n6p, 6h30, 5842, etc. Lots of options.

When I get around to it, I have been sitting on this project for 2 years due to kids... Anyway, from what I understand, the higher the load, the lower the distortion as it approaches an infinite load. Similar reason, I suspect to why using CCS loads and inductive loads are better, the loadline is in the more linear region of the curves. But the lower the load, the easier and better it is to build a transformer. So I guess there is a balance between the two. Best way is to download the F2a Siemens datasheet and draw your own loadline. From memory, I had mine at 400V, -25V on the grid and 5k load. I think that gave around 8W power before positive grid current. Be careful also not to exceed the maximum plate and screen voltage for the F2a, I believe it is around 425V.

Have a good read around the net, as there are heaps of information on this topology and design. Also be careful of the higher B+ voltages. Good luck!