Does This Switch Wiring Work For What I Want?

Discussion in 'Tech Talk - Electronic Repair Modifications DIY' started by JakeAC5253, Nov 26, 2016.

  1. JakeAC5253

    JakeAC5253 Active Member

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    I want to break a signal path which has no discrete components in it as of now. I want a 3 position toggle switch to have the options of Stock (signal pass through), Resistor 1 (signal path broken and routed into Resistor 1), and Resistor 2 (signal path broken and routed into Resistor 2). I've been pondering this, but I think I have figured out how to do it. Can anyone look this over and see if this looks correct? The switch is an ON-ON-ON DP3T switch. Forgive the sloppy MSPaint work, I'm a mechanic, not an artist :D
     

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  2. Boswell

    Boswell Moderator Distinguished Member

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    Hi Jake - you have asked us to comment on a solution to a problem without telling us what the problem is!

    The simplest way of wiring your solution would be to use a single-pole centre-off switch and two resistors, one of which is the lower of the resistance values and the other is the difference in required values. You wire the resistors in series and use the switch to short out the difference resistor, both resistors or none (centre position).

    Give us a bit more background and we could help further.
     
  3. JakeAC5253

    JakeAC5253 Active Member

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    Hey Bos, long time no see. (I was Guitarfreak here many years ago)

    What I want to do is install a switch into my 1981 JCM800 2203 between the tone stack and the master volume. The Stock setting will be a pass-through with no resistor like how it is now. The Resistor 1 setting will put a resistor in the signal path to lower the output volume. Resistor 2 will do the same, but it'll be about 4x larger than the value of resistor 1 to cut volume by a lot. Right now I'm thinking values of 120k and 470k.

    2203u.gif
     

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  4. paulears

    paulears Well-Known Member

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    Don't forget that a simple resistor on it's own often doesn't change the volume, because it's going to limit the current, that's all, so do do a volume control arrangement needs to be a bridge between the output and ground, with the point between two resistors setting the voltage. In a high impedance circuit, then two 47K resistors produces half volume at the mid point - like a volume pot. The output resistor just limits any current flowing, and with valve designs often for safety. So you could experiment with resistor value that will reduce the output, but I'd expect a small tonal difference too if you load the circuit too much. I suspect I might put two variable resistors on the back panel to set the two different gains. Quite a bit of trial and error here to make sure you retain the tone.
     
  5. JakeAC5253

    JakeAC5253 Active Member

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    Well the position of the resistor just prior to the MV pot would act along with the 1M value of the pot, causing a downward shift to the point at which the output comes from the voltage divider network. But you have actually given me an alternate idea. I could wire up a switch to modify the value of the 1M resistance of the MV pot using parallel resistors. A 1M resistor would make the pot into a 500k pot, and a 115k resistor would turn it into a 100k pot. Also an interesting idea, though I do feel like that may have a pulldown effect on the tone stack or may screw up the voltage seen by the phase inverter. What do you think?
     
  6. paulears

    paulears Well-Known Member

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    I see your thinking - I wonder how much of a shift will happen - I suppose you could wire the switched resistors between the wiper and the ground end and the wiper and signal end - giving you the options, and fairly simple switching. Worth having a go, as all the mods could be tried direct onto the pot solder pads? I'd wonder if their action was going to be that silent though - might get some pops if the capacitors discharge rate changes suddenly when the load gets higher? So at worst, maybe a couple of capacitors to watch over the switch??
     
  7. Boswell

    Boswell Moderator Distinguished Member

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    Hi GF - good to see you back again!

    This is what I meant about needing the complete picture. You shouldn't change the value of the 1M MV pot by putting single resistors in series or parallel with it as that will affect the tone control circuit.

    What you could do is have alternative L-pads that you switch into circuit with a multi-pole rotary switch. The parameters that you need in order to calculate the pad values are an input resistance of 1M (when loaded with the 1M MV pot) and your required attenuations. An L-pad is a series resistor at the top and a shunt resistor to ground.

    Be aware that a consequent effect of padding at the MV pot is that you may cause signal overload in the stages prior to that point.
     
  8. JakeAC5253

    JakeAC5253 Active Member

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    Jesus, this is getting too stressful lol. I'll just buy an attenuator haha. Thanks for the help though, I knew coming here I would get good advice. This is like the 4th version of this same mod I've devised, and every time I look deep enough, there are strong reasons not to mess with it. Attenuator it is. Anybody here use attenuators for tube amps? I was looking at the Weber Mass 200 because it seems the most transparent. I think it simulates a reactive load instead of being purely resistive.
     
  9. Boswell

    Boswell Moderator Distinguished Member

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    Maybe we should have started here: what do you want to achieve?
     
  10. JakeAC5253

    JakeAC5253 Active Member

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    I have had the JCM for a while but it was in disrepair from the previous owner until recently when I repaired it and rebiased it and etc. It sounds great now, but it really is an absurdly loud amp, 1 on the MV is already "other-end-of-the-house" loud. Given my current living situation, I can't have the MV any higher than say 9 o clock, and that's just when the tone starts to bloom. I was looking for ways to take some edge off of the volume so that I could run the master a bit higher and explore more of the amps true character. I looked into using a Variac to drop the line voltage down a bit, also heard that this would cause the amp to sag a bit, which I like, but I was told that would mess with the bias on the stages and was not the best thing to do to a classic amp (this one is from first year of production of the 2203 series).

    Also, I love switchable mods. My 5150 has a gain cut switch that I designed and installed, and the JCM is going to have a 3 way mod switch installed when the parts arrive for stock gain, more gain, and way more gain, by altering the cathode bias on one of the gain stages. There's something about increasing the versatility of a piece of audio equipment, while also retaining its completely stock character at one of the switch positions that makes me happy.
     
  11. paulears

    paulears Well-Known Member

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    Ah - personally, I suspect that all these mods will be tone negative - until the amp starts to kick the tone will be horrible. Cutting down drive to the finals won't be a positive mod. Why don't you consider these kinds of products - much, much simpler.

    https://www.thomann.de/gb/attenuators.html
     
  12. JakeAC5253

    JakeAC5253 Active Member

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    Yeah, that's probably the route I'm going to go. I may still try that Variac tho :p
     
  13. dvdhawk

    dvdhawk Well-Known Member

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    Good to see you again, Jake! I thought you were the king of the isobox / re-amping setup, or is that not a part of your current living situation?
     
  14. JakeAC5253

    JakeAC5253 Active Member

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    Good to see you too man! Yeah, those were the days! That's correct, living situation changed a few years back and I had to give up reamping... That may change in the near future hopefully.
     
  15. Boswell

    Boswell Moderator Distinguished Member

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    I would strongly advise against using a Variac on the mains input. Going that way could drastically shorten the life of the output valves (tubes).

    If you really want to maintain the amps's tone when driven hard yet have manageable decibel levels, then the only thing that could do this is padding at the loudspeaker terminals. This would involve some careful calculations and high-wattage resistors.
     
  16. JakeAC5253

    JakeAC5253 Active Member

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    You mean like a U-Pad or an H-Pad? So like the parallel resistor is somewhere in the vicinity of 10x the cabinet load, and the series resistors are some value calculated against the speaker load which would cause a significant power drop? So if my speaker load is 8Ω and I'm going to make a U-Pad, then the parallel resistor could be 82Ω, and the two series resistors could be 12Ω each and very high wattage? I'm going to make the assumption that is not safely switchable. Ooh! I can make it into an external box though!

    I also know that the 8Ω rating is more of a colloquialism than an actual hard rating, and that the actual moment to moment "resistance," while reactive, is more likely in the 18-25Ω range. So that may change the values if those are the numbers we are concerned with.

    I'm just making guesses at this point, haven't done any math yet.
     
  17. paulears

    paulears Well-Known Member

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    No issues with the switchable aspect, as mains switching voltages are unlikely to be exceeded here. In most cases this design of item presents a higher than normal impedance to the amplifier output - s0 switching extra pads in and out doesn't cause any issues.

    Now I see you've been at this idea for quite a while, can I comment? You've had loads of suggestions on doing what you seem to want both internally, requiring mods and possibly extra holes in the amp chassis, plus external boxes - and you don't seem to have actually tried ANY of the - even the simplest.

    You are doing a Don Quioxte quest - getting nowhere fast, but constantly increasing your options, yet failing to try any of them. Borrow an external one, or buy one from a dealer who is happy to take returns and try it out. You are going backwards. Plug one in and play! All your choices simply increase time between desire and conclusion.
     
  18. Boswell

    Boswell Moderator Distinguished Member

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    Here's a page describing loudspeaker padding with a resistor calculator. I haven't checked the formulae, but at first glance, the second set of equations is what you would need. However, he doesn't give power dissipation ratios, so you would have to calculate the resistor wattages separately.

    As an example, if you took his table figures for a 12dB (16th of the power) attenuation at 8 Ohms, the series resistor would be 6 Ohms (e.g. 2x 12 Ohm or 3x 18 Ohm in parallel) and the parallel resistor 2.7 Ohms. The parallel resistance comes down to roughly 2 Ohms when the speaker is connected, so the 100W power output of your amp would be split as 75W into the 6 Ohm resistor and 25W into the 2 Ohm composite, of which about 20W goes into the 2.7 Ohm resistor when the speaker is connected.

    Sourcing resistors to carry this load is a slightly tricky task, and I would start by looking at the 25W THS range from TE Connectivity (Tyco). There is a value of 18 Ohm 25W, so 3 of those in parallel gives 6 Ohm at 75W. The shunt resistor is easier, so you could use the 2.7 Ohm at 25W. If there is a danger that the speaker could become disconnected from the pads, then use a higher wattage resistor for the 2.7 Ohm. Use thick wire for connecting the resistors, as it is very easy to end up unintentionally with higher resistance values if you use standard connecting wire.
     
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  19. JakeAC5253

    JakeAC5253 Active Member

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    Its been like two days dude, lol. I am still researching.

    I think you forgot to post the link. The site probably has an explanation for this, but why is the shunt resistor so small? Wouldn't that pull down the impedance seen by the power tubes in much the same way that running a head set for 16 ohms into a cabinet rated at 4 ohms would do?
     
  20. dvdhawk

    dvdhawk Well-Known Member

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    Hey Jake, hover over the word "Here" at the beginning of Boswell's posted. It is the link. I missed it at first too.
     
    JakeAC5253 likes this.

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