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Old 28th March 2005, 08:47 PM   #1
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Default Gain/Schematic Question

Attached is a schematic of a gain stage following my passive RIAA network. Currently I'm seeing a gain factor of about 10 but I believe it should be much higher?

Any input would be very appreciated. Thanks...Philip
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File Type: pdf 12au7 schematic.pdf (11.0 KB, 51 views)
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Old 28th March 2005, 09:02 PM   #2
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WHY is the plate resistor so much smaller than the cathode resistor? Unless there is some DC bias not shown, you are starving the poor tube.

If the load is hi-Z (over 100K), the plate resistor could be larger, and the cathode resistor more like 1K, not 100K.

If the load is low-Z, like 10K, then one 12AX7 can't be very happy (few small triodes will like 10K loading).

Do you have a copy of RCA's Resistance-Coupled Amplifier tables?
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Old 28th March 2005, 09:17 PM   #3
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Quote:
Originally posted by PRR
WHY is the plate resistor so much smaller than the cathode resistor? Unless there is some DC bias not shown, you are starving the poor tube.
Do you have a copy of RCA's Resistance-Coupled Amplifier tables?
Nothing like putting together a post to help one figure things out! You are correct Sir! A 1K cathode resistor wakes up the whole thing, it all came to me as I hit the send button. Thanks, I have a 12AX7 Guitar Center special to try in the hole just for grins too.

No, I don't have a copy of the RCA tables, I've been getting my head around Morgan Jones & Bruce Rozenblit but oftentimes I see something working in their schematics without my having the experience to realize their big picture and apply correctly to mine. Building a little breadboard rig has really helped with the experimentation though.

Thanks again for the help.

Philip
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