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
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:
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
330
510
820
1K
1.5K
Then, ground the 1.5K resistor and parallel the rest of the values with 1.5K.... yes?
thanks
with a 12AX7 wired in parallel-(values are half of what a single triode would be because of this)
200 ohms
330
510
820
1K
1.5K
Then, ground the 1.5K resistor and parallel the rest of the values with 1.5K.... yes?
thanks
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.
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.
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.
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...
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...
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.
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.
Your answer contains a hint of a clue. The resistor across the switch helps smooth the transition when changing from cap to no cap (or cap to cap) because it allows the capacitor to charge gradually instead of suddenly.
By the same token, you can put a CAP across the switch to slow the VOLTAGE change and soak up the transient when changing resistors. A very small value cap in relation to the resistance value at audio frequencies (something analogous to a 1 Meg resistor to your cap value) should smooth out the transition from one voltage to the other without being an AC path around the resistor being switched, just like the 1 Meg resistor is not a path around your bypass cap.
Experiment to find a value that works to eliminate the pop without affecting the tone after the switch is thrown. I might start with something in the 100 pF range, or above or below depending on how that works. 100 pF has a reactance of about 1.59 Megohms at 1kHz.
The bigger the capacitance, the less the resistance at a given frequency, so 159 pF would have a resistance of 1 Megohm at 1kHz.
As long as that little cap has a reactance that is at least two orders of magnitude greater than the resistors at 10kHz you're unlikely to hear it. Another way to look at it is that as long as it is less than 1/100th of the value of your bypass cap, you're unlikely to hear it. So if you have a 10uF bypass cap, and use a .1uF pop filter, you probably won't notice its effect except when throwing the switch.
This is all theoretical, mind you, so experiment. But 100 pF to .1 uF is a pretty broad range to play around in, so there are lots of possibilities.
By the same token, you can put a CAP across the switch to slow the VOLTAGE change and soak up the transient when changing resistors. A very small value cap in relation to the resistance value at audio frequencies (something analogous to a 1 Meg resistor to your cap value) should smooth out the transition from one voltage to the other without being an AC path around the resistor being switched, just like the 1 Meg resistor is not a path around your bypass cap.
Experiment to find a value that works to eliminate the pop without affecting the tone after the switch is thrown. I might start with something in the 100 pF range, or above or below depending on how that works. 100 pF has a reactance of about 1.59 Megohms at 1kHz.
The bigger the capacitance, the less the resistance at a given frequency, so 159 pF would have a resistance of 1 Megohm at 1kHz.
As long as that little cap has a reactance that is at least two orders of magnitude greater than the resistors at 10kHz you're unlikely to hear it. Another way to look at it is that as long as it is less than 1/100th of the value of your bypass cap, you're unlikely to hear it. So if you have a 10uF bypass cap, and use a .1uF pop filter, you probably won't notice its effect except when throwing the switch.
This is all theoretical, mind you, so experiment. But 100 pF to .1 uF is a pretty broad range to play around in, so there are lots of possibilities.
Well I think everyone has covered the mechanical switching possibilities...
How about using FET's as switches to add the parallel resistors and switch the gate of the FET from the mechanical switch via an R/C network to add a time constant such that the FET comes into conduction gracefully. I think that might help with the noises when switching. The power dissipation in the FET would be essentially zero so they wouldn't get hot.
How about using FET's as switches to add the parallel resistors and switch the gate of the FET from the mechanical switch via an R/C network to add a time constant such that the FET comes into conduction gracefully. I think that might help with the noises when switching. The power dissipation in the FET would be essentially zero so they wouldn't get hot.
Thanks Mooly...
Since I had to look up what FET stands for and means, it's a bit over my head at this point. Later on down the road I can learn more about those. It sounds like a type of relay. Another circuit I'm not too familiar with, but I think I'd rather use a relay, only because it is older technology. It may mean nothing, but I'd want to keep the vintage tube circuit with a vintage switching circuit.
I started learning about tube amps one year ago and have built two, both of which are awesome. This is the last piece of it that I'm learning...
Since I had to look up what FET stands for and means, it's a bit over my head at this point. Later on down the road I can learn more about those. It sounds like a type of relay. Another circuit I'm not too familiar with, but I think I'd rather use a relay, only because it is older technology. It may mean nothing, but I'd want to keep the vintage tube circuit with a vintage switching circuit.
I started learning about tube amps one year ago and have built two, both of which are awesome. This is the last piece of it that I'm learning...
Oh, I did not know the relay would still pop. In that case I will be learning about FET's eventually because I do like this modification on the footswitch. In that schematic I found I didn't need the three tubes in the preamp. I made another similar with only two and it has the same gain characteristics with 2-12AX7's. My best bet would be to make a channel change using the third tube and a relay. At least for know. But for an easy mod to a footswitch already in place for boost, one could benefit from the FET switching a crunch resistor in the circuit. Thanks, again...
The main reason for my doing this is most every guitar player I've talked to never likes their second-lead channel. They use a good clean channel and pedals. The main difference between the Marshall crunch and the Fender clean is in the preamp cathode. It seems so easy to make both in the same amp w/o using so many effects pedals.
Thanks for your help...
Thanks for your help...
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.