A little help understanding the Express Pre-amp circuit

[jammybstard]

I've recently read up (Robert Megant) on how to design tube amplifier stages and as an exercise I'm going through some common circuits bit by bit calculating the operating point, gain, dissipation etc of the various stages. I started on the express (kelly 90) this morning and while V1a and V1b seem pretty straight forward apart from the lack of grid stoppers, I cant work out whats going on with V2a the third stage.
[IMG:153:289]http://i51.photobucket.com/albums/f368/JammyBStard/Capture_zpsd950a10b.png[/img]
The input resistance seems low at 150k, Rk seems very high at 10K. I can't place an operating point on the load line that lets me use 10k and there's no Ck which I would have thought would crush the gain and make the amp tinny. Obviously this doesn't happen so I am miss-understanding this stage.
I hope that makes sense
Could someone help me out?

[martin manning]

A 10k Rk will work, it just biases the stage closer to cut-off, which will cause it to clip sooner. Bypassing Rk is worth about 6dB, but there is no shortage of gain here. The 150k to ground at the grid will have more effect on the preceding stage by increasing the AC load. That will cause it to clip sooner on its cut-off side, but since the signal is inverted by each stage, that clipping will be on the opposite signal peak, and it will be frequency dependent (more on the low-mids and below).

[jjman]

Although mine is not a pure clone, the scope shows how the 3rd stage clips during cutoff on the bottom of the current wave. The screen in inverted for the 3rd stage so it is showing current which is upside-down wrt voltage.

The other line is the 2nd stage which is clean, showing that the 3rd stage clips 1st while turning up. I suspect the 2nd stage never clips unless the tone stack is modified or a booster is used from the guitar.

You can also see that the output from the 3rd stage is about 4x its input based on the scales.

[jammybstard]

I've had a look again and I think I get it now.
If:
The max plate voltage is 282v
The max plate current is 2.82ma
The plate is biased at about 250V and 0.3ma
The Quiescent power dissipation from the plate is about 75mW
The grid must be about -2.75v
Rk = V/I = 2.75/0.0003 = about 10K

Voltage gain is about 41 or 32dB less about 6dB as Rk is not bypassed is 26dB

[IMG1024]http://i51.photobucket.com/albums/f368/JammyBStard/ea986fea-d83b-4b04-b589-fadfd667c89e_zpsedac485f.jpg[/img]

Does that sound about right?

[martin manning]

That's about right. I would add the Rk to the Ra to calculate the max current as 282V/110k=2.56mA, and draw that load line. Since you know Rk, assume cathode voltage Vk is -2.5V, calculate current Ik=-2.5V/10k=0.25mA, and spot that point on the -2.5V grid line. Next assume Vk is -3V and calculate Ik=0.3mA, and spot that point on the -3V grid line. Connect those two dots and the intersection with the load line is the quiescent point.

[jammybstard]

That's about right. I would add the Rk to the Ra to calculate the max current as 282V/110k=2.56mA, and draw that load line. Since you know Rk, assume cathode voltage Vk is -2.5V, calculate current Ik=-2.5V/10k=0.25mA, and spot that point on the -2.5V grid line. Next assume Vk is -3V and calculate Ik=0.3mA, and spot that point on the -3V grid line. Connect those two dots and the intersection with the load line is the quiescent point.

Cheers Martin
I was trying to work out a way of deriving the point. Obvious now you've pointed it out.
That line did intersect with the same point I'd found by trail and error which is cool.

[EtherealWidow]

On a related note, here's a thread about the "cold clipper" stage. His results seem to differ from yours, though, jjman. He noted that the amp distorts from the back to the front starting with the power amp.

http://www.ax84.com/bbs/dm.php?thread=328710

http://home.polstra.com/amps/wreck1/scope/

[jjman]

On a related note, here's a thread about the "cold clipper" stage. His results seem to differ from yours, though, jjman. He noted that the amp distorts from the back to the front starting with the power amp.

http://www.ax84.com/bbs/dm.php?thread=328710

http://home.polstra.com/amps/wreck1/scope/

Looks like the 2nd vs 3rd stage scope results on the link are essentially the same as mine. Remember that my scope had the 3rd stage inverted in my earlier post.

Interesting that the link shows some phase shift from stage 2 to 3 which is strange since there is no cathode cap.