Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tyrannocaster on July 21, 2012, 08:06:02 am
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How can I do the Hi/Low Fender style inputs with just one input? Is the easiest way to create each signal path (2 68k making a voltage divider on one and 2 68k in parallel [ie, 33k] plus a 1 meg on the other) and then switch between them? I have a hole right under the volume control that could easily be enlarged for a mini toggle for this, I'm just not sure what the best way to do it is.
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Do I understand you correctly that you need the two different levels of sensitivity?
I ask because when I do a single input, I just have a 1M to ground and a grid stopper for the input tube (takes the place of a 68k/33k resistor on the input jacks).
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Yes, I would like to have both levels available for the single input. It's for high output vs. low output guitars and also for getting a cleaner sound sometimes vs. a dirtier one. On the amps that have the dual jack setup I use both of them; I use the high gain one the most but it's surprising how much I use the other one and right now with this Tweed FrankenPro I built that only has one input I'm stuck with the high gain level, which is fine for what it does but not so good for cleaner sounds.
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Okay. I'm not sure the best way to implement this, but here's an idea to try:
Take two 500k resistors (or 470k, 560k or whatever is a close standard value) and place them in series from input jack hot to ground.
Use a SPDT switch, with the switched pole connected to your input tube grid. Connect the other lugs to the input jack hot and the junction of your two resistors. That will allow the switch to select either full signal (input jack hot) or 1/2-signal (resistor junction) to be passed to the input tube.
I'm not sure if you'll hear popping when switching this (try and see). If so, maybe connecting a high value resistor (like 5-10M, 1/8w or so) across the lugs will help.
You could go a step farther:
If you amp is perfect with single-coils, not with humbucker, you could sub a 1M pot for this setup for a test. Find out how far you need to "turn down" to get the humbuckers happy, or responding similar to the single-coils. Then measure resistance from wiper to each end to determine your ideal values for the voltage-divider setting of your input switch.
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Thanks, HBP. I'm going to have to draw that out to visualize it; I'm awful at seeing circuits from an oral description. Sounds like a job for Shielded Cable Man! [Best cartoon voice there]
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This will simulate the hi/lo jacks about as closely as you can get...
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Oh, thank you for doing that! It's so much easier for me to visualize this way. That's a different approach than HotBluePlates'. Have you done this one? How effective is it?
HBP, I wonder if you could leave that pot in the amp for a constantly variable sensitivity. (I have an extra pot on the front of this thing.) This must be a bad idea or someone would have done it already. :laugh:
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Have you done this one? How effective is it?
Never done it. It behaves exactly like the Fender hi/lo jacks.
Look at page 6 of this pdf for a little more discussion on the hi/lo jacks...
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf)
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Oh, thank you for doing that! It's so much easier for me to visualize this way. That's a different approach than HotBluePlates'. Have you done this one? How effective is it?
HBP, I wonder if you could leave that pot in the amp for a constantly variable sensitivity. (I have an extra pot on the front of this thing.) This must be a bad idea or someone would have done it already. :laugh:
It's basically what Fender did on the "Low" input of the JCM800 amps. The gain control comes before the first gain stage.
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Is the first stage getting over driven by your guitar?
I'm just trying to understand why a voltage divider before the first stage is better then turning the volume pot on the amp down(voltage divider after the first stage) and why there is a need for hi/lo unless your guitar is so hot it's overdriving the first stage.
I guess what I'm trying to say is the lo input is just a voltage divider before the first stage so it does the same thing your volume on the guitar does. Do you want a lo so you can keep the guitar on 10 and not have to adjust the volume knob? Unless your overdriving the first stage, why not just turn down the amp's volume?
Plug in your hottest guitar with the volume on 10 and the amp on 1. Is the sound distorted?
Now put the guitar on 5 and turn the amp's volume to get the same volume. Which way sounds better?
This is basically what the lo input is doing.
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Is the first stage getting over driven by your guitar?
I'm just trying to understand why a voltage divider before the first stage is better then turning the volume pot on the amp down(voltage divider after the first stage) and why there is a need for hi/lo unless your guitar is so hot it's overdriving the first stage.
That's a valid question but I have a good answer for it. This amp has an "unused"channel (it's a Tweed Deluxe 5D3 preamp) that still affects the guitar. It's just like having a second input shorted to ground as far as the amp is concerned and I did it that way on purpose because I like the odd effects I can get out of my real '55 Deluxe via the interactive volume controls, which change the tone remarkably. So if I turn the volume down on the amp I also change the tone. This interactivity is a blessing/curse of this design and a lot of guys just get rid of it but I wanted to exploit it in this amp. So I have two volume controls, one for each channel, even though one of them is hard wired to ground like a jack with no guitar plugged in.
Hope that makes sense.
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I was just thinking divider before the input=more S/N ratio. didn't know about second channel I'll look at 5D3 to see what you mean.
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Here's the schematic:
(http://i425.photobucket.com/albums/pp332/Tyrannocaster/RATweedProLuxe4.jpg)
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Sluckey's schematic exactly captures Fender Hi/Lo jack switching (and he's got better Visio graphics on tap; might have to ask to borrow his schematic stencils).
What I proposed is similar, but different. It's not better, but does save one resistor. It might also allow more range of adjustment in the relative senstivities.
I'd suggest making sure the total of the resistors adds to 1M, or whatever you want your input impedance to be.
Jeff is also right in that adding voltage dividers should make the S/N worse when they are before any amplification. You'll just have to try it to see if it adds too much noise in your situation.
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That one is kind of elegant. :laugh: I am going to implement one of these...after I get the damn tremolo working.
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Sluckey's schematic exactly captures Fender Hi/Lo jack switching (and he's got better Visio graphics on tap; might have to ask to borrow his schematic stencils).
What I proposed is similar, but different. It's not better, but does save one resistor. It might also allow more range of adjustment in the relative senstivities.
I'd suggest making sure the total of the resistors adds to 1M, or whatever you want your input impedance to be.
Jeff is also right in that adding voltage dividers should make the S/N worse when they are before any amplification. You'll just have to try it to see if it adds too much noise in your situation.
While elegant, there is no grid stopper when the Hi input is selected. You need sluckey's extra resistor.
Respectfully,
Chip
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One could argue you have no real grid stopper in the Fender either, since the 68k resistors are either mounted on the eyelet board or the input jacks.
But sure, you can add a grid stopper right at the input tube grid.
I guess I said it right the first time: basically same as Sluckey's, just different (but not saving a resistor now!). :icon_biggrin:
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Well, I already have a 33k grid stopper there, so it's money in the bank.
Thanks for both of those solutions.