Author Topic: Tweaking a Circuit's Response  (Read 210 times)

Offline steve

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Tweaking a Circuit's Response
« on: October 12, 2018, 12:53:03 PM »
For some just getting into tubes, or even with some experience, different tubes tend to sound differently. Here is a little tweak that does not cost an arm or a leg, yet can yield some very nice results, even making two different brand tubes, of varying cost, to sound nearly the same. I am sure there will be some one will not be able to match. By doing such, two different components can also be made to sound more similar, although not the same.

As my first example, suppose our system is slightly bass heavy. We have re-positioned our speakers, and still there is just slightly too much bass. Maybe the voices are just slightly muddy, too full sounding. Amps are usually easier to tweak than preamplifiers. YMMV though.

In our tube amplifier, we usually will have a coupling capacitor, C1, and following grid resistor, Rg1. Let's say C1 is 1ufd and Rg1 is 100k ohms. We can reduce the bass/lower midrange slightly by simply paralleling a resistor, Rg2, across Rg1 to reduce its value. This will thin out the bass/voices just a little, depending upon the value of Rg2, our added parallel resistor

The equation for paralleling resistors is R1 + R2 / R1 x R2 = Rtotal.

Let's say we parallel Rg1 with Rg2, a 1 meg ohm resistor. Our resulting resistance value will be 90.9k ohms. The bass and lower midrange response will be slightly less, thinning out the voices a hair.

One can use just about any value for Rg2, 500k ohms, 2.2 megohms, 220k ohms. The lower the value of Rg2, the less bass and thinner the voices will be.

The resistors, in general, are inexpensive. Just need a soldering iron, good solder, needle nose pliers. Minimal work involved.

Suppose one wants to increase the value of Rg1; there are two ways to go. Either remove the existing Rg1 and install a new value, such as 110k, 150k, 500k ohms etc. The other way is to remove one lead of Rg1, and add a small resistor in series with Rg1. The equation for series resistors is Rg1 + Rg2 = Rtotal.

Caveat: Usually there is a maximum value of Rg1 that may be used on a tube's grid, if one series resistors. Check your tube manual, such as Duncan's website. http://tdsl.duncanamps.com/tubesearch.php

Just something to experiment with.

Caution: Make sure all voltages in the component are zero volts. Lingering high voltage can kill.
I also recommend unplugging the AC cord from the wall.

cheers

steve

« Last Edit: October 12, 2018, 01:12:19 PM by steve »
Steve Sammet (retired, but manufacturing "V" ics again)

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