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Old 2nd September 2011, 01:13 AM   #1
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Join Date: Mar 2011
Default rotary switch for different cathode resistors


Does anyone know of any issues with using a rotary switch for different value cathode resistors? I tried this and really like the changes it had-crunch to clean. The only thing that is bad is the loud pop it makes with each turn, unless I turn down the volume.


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Old 2nd September 2011, 05:17 AM   #2
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Location: New Zealand
Hi Daniel

You need to use a make before break switch, one that makes the next connection before breaking the prior connection. If so you will avoid the switching noise. Another way is to use a LDR Light Dependant Resistor - if your final resistance is more than 35 ohms- 48 ohms a NSL32SR2 will provide all the settings you need. You can use the switch you have feed it with a fixed 16ma from a current source like an LM317 with 75 ohm resistor between its adjustment and output leg. to the Anode and place your switch in the Cathode lead with its final destination ground. A NSL32SR2 is two circuits in the one package a variable resistor in/ out, and a Cathode and Anode like a LED on the other. Varying the voltage then alters the resistance. LDR's work best with a fixed current
Cheers / Chris

Last edited by Chris Daly; 2nd September 2011 at 05:22 AM.
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Old 2nd September 2011, 05:58 PM   #3
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That's all?? That was easy, I have one-not correct, but I see how it works. They're pretty reasonable on ebay too. Thanks Chris,

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Old 2nd September 2011, 07:22 PM   #4
TheGimp is offline TheGimp  United States
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The other option is to place the max resistance from cathode to ground, then use the switch to select a parallel resistor to reduce the value.

Say 470R to gnd
with 2K = 380
with 1K6= 363.3
With 1K3 = 345
With 1K = 319.7
With 820 = 298.7
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Old 2nd September 2011, 08:01 PM   #5
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Wow! That's easy too. Just so I understand correctly: my set up is this...

with a 12AX7 wired in parallel-(values are half of what a single triode would be because of this)

200 ohms

Then, ground the 1.5K resistor and parallel the rest of the values with 1.5K.... yes?

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Old 2nd September 2011, 09:59 PM   #6
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A simple spdt switch with an open center position works well. For a single tube in the center position I would use 2.7k. Then switched to one side using a 3.3k resistor you get 1.5k, switched to the other side with a 1.2k you would get 830 ohms. These are standard values you would see in a guitar amp. For two triodes try 1.8k, 1.2k, 560R.
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Old 2nd September 2011, 10:53 PM   #7
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I like that too. there's not much difference between each value with the six I have.
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Old 3rd September 2011, 07:52 AM   #8
tubekit is offline tubekit  United States
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You could also switch the cathode bypass capacitor(s) for different tonal configurations. Use bigger capacitors to boost the lows or smaller ones to reduce them while retaining the overall stage gain of the standard bypassed configuration.

Changing the low frequency rolloff of the stage also affects the apparent loudness of the mids, since the greater the amount of available energy being used for deep bass, the less is available for midrange--conversely, raising the cutoff frequency tends to boost the mids by making more current available to amplify them. A two-pole six position would give you two different cap configurations for each of three bias resistors, or three different cap configs for each of two bias resistors.

Call the cap aspect of the setup a "fat/thin" switch.

A position with no bypass cap is useful if the stage has so much gain with the cap in place that it distorts with all values of cathode resistor. Pulling the cap out of the circuit reduces the gain, and could then be used for a clean/dirty option or to extend the range of possibilities between mellow clean and aggressive maximum distortion as also set by the choice of cathode resistor.

A resistor in series with the bypass cap is another possibility with its own set of possible uses.
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Old 16th October 2011, 06:09 PM   #9
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I am still having pop noise problems. I now have a footswitch that adds a bypass cap and a resistor (in parallel) to change the cathode gain characteristics. I am using a the Fender/Marshall values of 1.5K and then 480 when the switch is on. I have a 1M resistor across the switch and it helps with the cap (when no resistor change is made), but pops when the resistor clicks in.

I have tried to use a make before break switch and that did not work either. It seems the pop is the result of the change and not breaking off before making the next connection.

Any advice?? This is really a nice clean/dirty switch, but I'm concerned about the noise of the pop and someone using it live.

Does anyone know if a pop is 'standard' for some switching? I'm not sure if it could be heard while playing with a band.

Thank you...
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Old 16th October 2011, 09:50 PM   #10
benb is offline benb  United States
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You cannot change the DC resistance of the cathode resistor without affecting the DC bias and resulting in a pop. But you CAN change the "ac resistance" without changing DC bias. Use the largest value resistance you'll use for the cathode resistor. Put a large (maybe 100uF - somehting where the rolloff with the series resistor is below 40Hz) capacitor between each resistor you're switching in, and a moderately high (10k to 47k) resistor across the switch so the cap is always charged up to where it will be when the switch contacts are connected.

Part of the sound change may have something to do with the bias change as well as the AC gain, so doing this might not sound exactly the same. In that case it'll be more complicated to switch cathode resistors without making a pop.
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