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Old 22nd April 2013, 12:42 PM   #1
rtsang is offline rtsang  Hong Kong
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Default Reducing gain of a WE91 type amp

Just want to ask what is the best way to reduce gain of a 300B amp with a pentode driver followed by a cathode follower to drive the 300B. Way too much gain for my volume control in the preamp to manage. I am already using a passive transformer volume control.

I had thought of several options, want to see the pros and cons of each.

1. Add a potential divider in front of the input.
Not sure if there is any down side to this.

2. Remove the cathode bypass cap of the pentode.
This may increase the output impedance of the driver, though this should be buffered by the cathode follower.

3. Reduce the anode load of the pentode.
This may decrease the voltage swing of the driver.

4. Add feedback.
Probably not my cup of tea.

Thanks in advance.

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Old 22nd April 2013, 03:39 PM   #2
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Remove the preamp and mount the transformer volume control at the input of the amplifier. The preamp simply adds noise and distortion and is unnecessary.
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Old 23rd April 2013, 12:34 PM   #3
rtsang is offline rtsang  Hong Kong
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The preamp is a passive preamp with a selector and a transformer volume control. Unfortunately there is not enough space in the amp to add a few RCA sockets, a selector and put the transformer volume control in it to convert it to a integrated amp.

I had kept the interconnects short so not much noise should be added by this passive preamp.
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Old 23rd April 2013, 06:50 PM   #4
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Well, if you have a passive preamp, then there may be a problem with your transformer volume control. What is the input sensitivity of the amplifier and what is the slope of the volume control, e.g. is it an audio taper or linear taper? It sounds as if your volume control doesn't give you enough control at the lowest settings. A voltage divider at the input will serve to reduce the input voltage, but that seems like a bandaid repair rather than an engineering fix.
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Old 23rd April 2013, 09:09 PM   #5
chip647 is offline chip647  United States
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Default "Pentode Driver"

Well, there is your gain problem. Your pentode has too much gain. Try a lower gain pentode or, gasp, a triode. The buffer means that the tube is not driving any load of significance so a triode will not be harmed.

Your amp likely has full power with 0.1 volt input. You never want to throw away more signal than you have to. This is a system mismatch problem.
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