Grid Bypass NOT Cathode Bypass!

LT spice, Curve Captor, PSUDII and whatever other sims you can think of.

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docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Grid Bypass NOT Cathode Bypass!

Post by docali »

Hello!

I was thinking already a long time about replacing the big cathode bypass capacitor in single ended stages. Ultra path might be a way. But one big disadvantage is that you need a really large capacitor to avoid the output impedance raise for low frequencies. I observed this for the GCA and UP circuits. The bypass capacitors needs to become very large to yield a flat frequeny response, 100uF or more.

Looking at some old WE schematics one can find a capacitor that bypasses the grid which normally needs an input transformer but which was used together with another capacitor coming from B+ or parafeed. But I think that we do not need the second capacitor.

The grid has normally a very high input impedance. It can be better bypassed ;-)

So, I simulated some things and ended up with the following. The input transformer can be grounded very soft with a 10k resistor if the grid bypass capacitor is large enough.

You can go to 47k and reduce the size of this capacitor. The big advantage for me is that both frequency response and output impedance seem to be much more flat comparing the size of the capacitor.

Any comments are welcome! If there are any errors in my sims please correct me.

I created the tube model for the EML 20a by myself. This tube has really incredible low distortion. Highly recommended!

Best regards,
Docali
Attachments
EML20A_XFMR_4to1_UP+_sweep.asc
EML 20A with grid bypass capacitor (Spice)
(3.74 KiB) Downloaded 230 times
EML 20A with grid bypass capacitor
EML 20A with grid bypass capacitor
EML20A_Grid_Bypass.jpg (126.97 KiB) Viewed 4954 times
EML20A_XFMR_4to1_UP+_zout.asc
Output impedance
(3.91 KiB) Downloaded 222 times
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