Second gain stage

[xcortes]

I received my WM-D6c from ESL Labs and is a killer of a recorder/player for a battery based headphone system. Unfortunately it only outputs 0.25V which is not enough for my Quickie to drive headphones. So I'll install a second Quickie in the same chassis (sharing the batteries). When using this configuration (extremely short interconnects and driving a 100k grid or even 249k grid resistor) can I reduce the size of the output (in this case coupling) cap? I have some nice 0.1 uF ones.

And will the answer be the same if I use grid chokes instead of grid resistors on the second stage?

Thanks!

 

[Paul Joppa]

0.1uF into 100K gives -3dB at 16Hz, -1dB at 32Hz. You can place the gain control between stages that way - which will help with the microphonic problems that go with increased gain.

Remember to cut the bias resistor in half, since it will be carrying twice the current!

 

[xcortes]

I was thinking of using independent bias resistors in each stage thus maintaining the originals. Is that ok?

Thanks

 

[Paul Joppa]

Sure, but you'll need two more D-cells - the bias resistor floats the cathodes (and their power supplies).

If you're going to add D cells, you could probably get away with just one, to power a pair of 1G4GT which take 50mA each and the negative power supply can be grounded - no resistor, no bypass cap. They are octals and may have existed in the -G envelope before they came out in the -GT straight-sided version.

 

[xcortes]

Stupid me, I was thinking of the plate resistors and not the cathode ones. Now what you suggested makes sense to me: connect the cathodes of both tubes to one single bias resistor (2k instead of 1k), right?

And what would think of using grid chokes instead of resistors in the second stage? This seems like the perfect amp to add those having no AC whose hum can be picked up by the chokes.

Thanks

 

[Paul Joppa]

Umm ... V=IR. Twice the current, half the resistance. 500 ohms (499 is close enough ...  :^)

Good application for a choke, but then where do you put the gain control?

Incidentally, I've never experimented to find the right capacitance for grid chokes - and given the nature of ungapped magnetics, I don't think you can do this with theory.

While you are experimenting (or anyone else?), I wonder if a paralleled resistor, say 100K or 250K, would help or hurt? If the high impedance in the audio band is important, then it would hurt. But if it's the low DC resistance that is important, then a paralleled resistor won't make any difference except that it would give a reliable resistance for the coupling cap to work with.

 

[xcortes]

I want to keep the volume control before the first stage to keep the input transformers and Penny and Giles pot. I know that coul mean more microphony but sine this will be a headphone dedicated amp I hope this won't pose a problem.

On the resistor parallel to the grid chokes I can experiment. I'll post my impressions.
Thanks

 

[xcortes]

Task completed. It took me more than expected because I had to troubleshoot it. The good thing is I took advantage of the troubleshooting and corrected the heater wiring that was done the original way, before corrections were posted.

I couldn't find my caps stash so I installed instead a pair of .33uF as coupling caps. I'm using the grid chokes and there's plenty of bass. Maybe even too much so I will eventually try the smaller caps and the resistor in parallel. Plenty of gain, enough for my Walkman, no problem with microphonics and, a brief test shows excellent sound.

I'll post pics in another thread where I have earlier pics.

Thanks