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too much gain
too much gain
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Old 6th June 2010, 01:39 PM   #1
chopchip is offline chopchip  Canada
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Default too much gain

im on my way to finishing a preamp but the gain is too high could this be because the filament voltage is above 7 when it should be about 6?

thanks
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Old 6th June 2010, 01:43 PM   #2
Arnulf is offline Arnulf  Europe
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How much is too much ? Emission will be higher with increased heater voltage but not *that* much. higher.

You can use series resistor, a diode (if you're using DC heater supply) or two diodes (if you're using AC heater supply) to drop the voltage to 6.3V.
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Old 6th June 2010, 01:53 PM   #3
SGregory is offline SGregory  United States
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Does this preamp use feedback? What is the schematic of the preamp?

Filament voltage does need to be lowered using a method as Arnulf suggested.
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Old 6th June 2010, 02:33 PM   #4
ashok is offline ashok
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too much gain
You might be killing the life of the heater by using a higher voltage !
For modern day applications I think most tubes without feedback could give you much higher gain than required. Negative feedback could reduce the gain depending on your circuit.
If I were you I'd try to pick a tube ( for example) that can give lower gain like say a 12B4A than a 6DJ8 and avoid any feedback scheme.

If you post your circuit you might get more accurate replies !
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Old 6th June 2010, 02:44 PM   #5
Globulator is offline Globulator  United Kingdom
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If I were you I'd use a resistor to drop filament voltage, and use degeneration on the gain stage to reduce the gain.

I.e. remove cathode bypass caps and possibly increase cathode res value (with a bridge for the grid bias as required.)
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Old 6th June 2010, 02:54 PM   #6
chopchip is offline chopchip  Canada
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the gain is way too high, i can only turn the volume up about 30% and then it starts to distort. it doesnt use feed back it is a dual paralleled single ended triode with a cathode follower.

here is the schematic of one channel, the pot is 100k
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Old 6th June 2010, 03:03 PM   #7
SY is offline SY  United States
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too much gain
With a tiny fraction of a milliamp running through your cathode follower, it's not likely to perform well...
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Old 6th June 2010, 03:23 PM   #8
Arnulf is offline Arnulf  Europe
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This surely is a waste of tubes ... split it up and make it two separate channels or change the topology. If you intend to go the second route, have a look at Broskie's "Aikido" stage - 4 tubes put to a much better effect.

This has nothing to do with the filament voltage, it has everything to do with wrong solution to a problem. If you can desribe what you're actually trying to accomplish (besides building a tube preamplifier, of course ), I'm sure we'll be able to come up with a more suitable solution.
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Old 6th June 2010, 03:51 PM   #9
chopchip is offline chopchip  Canada
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Quote:
Originally Posted by Arnulf View Post
This surely is a waste of tubes ... split it up and make it two separate channels or change the topology. If you intend to go the second route, have a look at Broskie's "Aikido" stage - 4 tubes put to a much better effect.

This has nothing to do with the filament voltage, it has everything to do with wrong solution to a problem. If you can desribe what you're actually trying to accomplish (besides building a tube preamplifier, of course ), I'm sure we'll be able to come up with a more suitable solution.
there are only two dual triodes in this schematic. do you mean that they could be used more effectively?


i didnt design this circuit. this is a kit and im green to electronics. im just trying to reduce the gain. so i assume that making the plate less positive or making the grid more negative would result in reduced gain.

the grid controls the flow of electrons so the more negative it is the less current there is and the plate attracts the electrons so the less positive it is the less current there will be.

mu is the plate ability to regulate current flow over that of the grid.

is this correct?

so if i increase the value of the resistor to the plate then gain will decrease or if i change the cathode bias resistor to make it less positive relative to the grid would reduce gain. i guess decreasing its value would have the desired affect. or i could use a higher value potentiometer on the input i guess too.

am i right? is one method better than the other? i assume you dont want to change the operational parameters of the tube too much given its characterstics.
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Old 6th June 2010, 04:00 PM   #10
leadbelly is offline leadbelly  Canada
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too much gain
Quote:
Originally Posted by chopchip View Post
i didnt design this circuit. this is a kit and im green to electronics.
You said in one of the many threads you are starting that you are building a Decware kit. Since I doubt that hand drawn schematic you posted is designed by Decware and certainly not drawn by them, maybe you should back up and tell us what is going on?
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