Preamp Design and Bypass Capacitors

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Jana
Posts: 1314
Joined: Sun Sep 07, 2008 10:40 pm
Location: Minnesota

Re: Preamp Design and Bypass Capacitors

Post by Jana »

"The bypass cap controls degenerative feedback through the cathode resistor.
The incoming signal causes the plate current to fluctuate. The bias voltage,
the voltage drop across the cathode resistor, will also fluctuate.
This condition is not desirable because it reduces the amplification available."

Unless you are tailoring the gain of the stage based on frequency, then this is desirable. The bypass cap gives a nice way to roll off low end rumble or mud simply by reducing the gain of the lower frequencies. One cap to roll off the lows and it is done without introducing any parts in the signal path. Pretty cool in my opinion. :)
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dobbhill
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Joined: Sat Jan 22, 2005 12:04 am
Location: Louisiana

Re: Preamp Design and Bypass Capacitors

Post by dobbhill »

Tom:
Fixed bias in a preamp tube is the same as a power tube: ground the cathode and apply a negative voltage to the grid. HiFi guys sometimes use batteries for that. And yes, you still have to use coupling caps between stages.
D
There are no stupid questions, just stupid people.......
Rick
Posts: 151
Joined: Sat Mar 24, 2007 11:46 pm

Re: Preamp Design and Bypass Capacitors

Post by Rick »

jjman wrote:HiFi guys are using LED biasing on preamp tubes. A red one provides about 1.6volts and no cap is needed. They run a few in series when more bias voltage is needed, or a different color. Those guys are adverse to caps in general so they know how to avoid them. If you don't want the drawbacks of a cathode cap, I'd check into it.
I use small signal diodes quite a bit for preamp cathode bias, mostly 1N4148s. they work really well, 2 in series gives a steady approx. 1.2V and it can give you some very noticeable change in tone and tighten things up it seems. They generally need no bypass either, hence the tonal difference. This probably works better in higher gain applications, but tastes vary and you should try it sometime so you know what you have in your tone arsenal. another 2 cents...
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Structo
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Location: Oregon

Re: Preamp Design and Bypass Capacitors

Post by Structo »

I have read about using diodes or LED's to bias a preamp tube but, does that mean you don't bypass the diode with a cap?

When I have experimented with gain stages I couldn't believe the difference in gain between a stage without bypass and one with a cap.

I'm just trying to get a better grasp of all this and make my amp sound better at the same time. :D
Tom

Don't let that smoke out!
j-po
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Joined: Mon Oct 13, 2008 11:58 am

Re: Preamp Design and Bypass Capacitors

Post by j-po »

A led is 1.6V or whatever fixed value with no regard to current flowing through the tube. Sorta like bypassing a resistor with a infinitely large cap.
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