Dummy Needs Help?

[The Ballzz]

Hi Folks,

I'm stumped, not because of difficulty, but because of lack of knowledge/understanding/brain power. I'm trying to work out a two jack version of the standard Fender/Marshall two channel, four jack setup as well detailed by Rob Robinette in the link below. Unfortunately, what I'm trying is not an option listed by Rob, though I'm fairly sure it can be accomplished.

I need to know how to wire two input jacks (preferably Cliff style) so that plugging into one of them gives the same function as guitar plugged into Normal Hi and jumpered from Normal Lo to Bright Hi. And then plugging into the other jack gives the same as guitar plugged into Bright Hi and jumpered from Bright Lo to Normal Hi. When I refer the "same function" I mean both V1A & V1B grids as well as the actual guitar input jack seeing the same impedances that Rob details. In other words, all the same grid stop and grid leak values as doing it with four jacks and a jumper!

https://robrobinette.com/How_Fender_Inp ... s_Work.htm

My brain explodes when I try to lay it/work it out, much like my brain would react to multi dimensional chess!

Thanks To Anyone Willing To Help,
Gene

[thetragichero]

so i would use a shorting jack like this (i don't buy from stewmac, this was the first Google result): https://www.stewmac.com/Pickups_and_Ele ... gK-bPD_BwE

use that for the "non-jumpered input" the third terminal connects to the second "jumpered input jack"
(actually i would use the same jack on the jumpered input with the third terminal going to ground to prevent noise)

lemme know if a picture would be easier to understand and I'll make something up in paint

[The Ballzz]

so i would use a shorting jack like this (i don't buy from stewmac, this was the first Google result): https://www.stewmac.com/Pickups_and_Ele ... gK-bPD_BwE

use that for the "non-jumpered input" the third terminal connects to the second "jumpered input jack"
(actually i would use the same jack on the jumpered input with the third terminal going to ground to prevent noise)

lemme know if a picture would be easier to understand and I'll make something up in paint

That would work, but it's not what I'm looking for. It's one jack jumpered one way and the other jack jumpered the other way. There IS a difference as to which channel is more dominant. I'm not for a "non-jumpered" at all.

Thanks For The Reply,
Gene

[thetragichero]

oh sure, have whatever the pad resistor is in between lugs two and three on that jack (tip and shorting lug)

[Stevem]

With any type of fully shorting jack like a Cliff type I don't know how you get your needed result without atleast having a dummy jack plugged into the input your jumping over to!

[sluckey]

Which amp do you hope to implement this mod?

[The Ballzz]

Which amp do you hope to implement this mod?

Not a mod "per se" but a new build. The current victim is a 5F8 preamp, although it is identical (prior to V1A & V1B) to a 5E3, Marshall 4-hole, etc, with the typical 1MEG/68K/68K arrangement, . Mr. Robinette's treatise is quite enlightening and shows how plugging into one channel and jumpering works out to be different/opposite than plugging into the other channel and jumpering. The plugged into channel becomes slightly more dominant. Now, I realize the changes/differences are small, but in this madness known as tone chasing, subtle changes can sometimes yield surprisingly big results.

This 4-jack arrangement is not as simple as it looks at face value. There are many factors working together and I believe that one of Rob's conclusions may involve some incorrect math, but the general idea is well conveyed. On the other hand, I've never seen anyone factor in the part about losing 3db(-ish) of signal strength when feeding two different inputs?

TIA,
Gene

[martin manning]

Four inputs and two jumper options for each makes 8 possible "jumped" configurations. The channel you jump from loses the -6dB divider effect of the low input since the parallel 68k are split into a series 68k from the guitar to the grid for that channel, and the other 68k is placed in series with the jumper to the other channel. If the guitar is plugged into the low input, there is a small divider effect into the jumper, formed by one 68k and the 1Meg to ground, but that is only -0.6dB. The jumper can then go to either the low or high input of the other channel, whose input network functions normally with either 34k in series and 1Meg to ground for the high, or 68k in series and 68k to ground for the low (resulting in -6dB for the jumped signal). Note that the 1M to ground (and both 68k grid resistors if the low input is jumped over) from the channel with the guitar plugged into it are now placed across the input of the jumped channel, lowering it's input impedance slightly.

By far the largest effect is the increase in the series resistance going into the grids, and the effect that has on the high frequency response due to Miller capacitance. The channel with the guitar plugged into it will have 68k in series (using either input), and the jumped channel will get an additional 68k. That means the source channel will have some HF roll-off, and the jumped channel will have even more.

The most common configurations in use are plugging the guitar into the high input of one channel and jumping the low input to the high input of the other channel. I can't readily think of a way to replicate those using two jacks, but the circuit shown below using one jack and a DPDT switch will do nicely, and it will use the same front panel real estate. All of the resistors could be mounted to the jack and switch lugs.

[The Ballzz]

Four inputs and two jumper options for each makes 8 possible "jumped" configurations. The channel you jump from loses the -6dB divider effect of the low input since the parallel 68k are split into a series 68k from the guitar to the grid for that channel, and the other 68k is placed in series with the jumper to the other channel. If the guitar is plugged into the low input, there is a small divider effect into the jumper, formed by one 68k and the 1Meg to ground, but that is only -0.6dB. The jumper can then go to either the low or high input of the other channel, whose input network functions normally with either 34k in series and 1Meg to ground for the high, or 68k in series and 68k to ground for the low (resulting in -6dB for the jumped signal). Note that the 1M to ground (and both 68k grid resistors if the low input is jumped over) from the channel with the guitar plugged into it are now placed across the input of the jumped channel, lowering it's input impedance slightly.

By far the largest effect is the increase in the series resistance going into the grids, and the effect that has on the high frequency response due to Miller capacitance. The channel with the guitar plugged into it will have 68k in series (using either input), and the jumped channel will get an additional 68k. That means the source channel will have some HF roll-off, and the jumped channel will have even more.

The most common configurations in use are plugging the guitar into the high input of one channel and jumping the low input to the high input of the other channel. I can't readily think of a way to replicate those using two jacks, but the circuit shown below using one jack and a DPDT switch will do nicely, and it will use the same front panel real estate. All of the resistors could be mounted to the jack and switch lugs.

Thanks for that Martin! I was fairly confident that someone here would "smell what I'm steppin' in" and was also fairly certain it would be you! That diagram makes it so much easier to visualize and understand what's actually going on. For that matter, I think if I stare at it long enough, I'll likely be able to figure out a way to utilize two switching Cliff jacks to do the duty of that DPDT switch, given that each of the jacks is actually a DPST.

Thanks Again!
Gene

[sluckey]

The right side of that switch appears to do nothing. All three lugs are jumpered together.

[ER]

When ever I've done an amp like that, I go with a single input and an A/B/Y switching configuration onboard the amp kinda like Martin describes. No use for the second jack really.

[The Ballzz]

The right side of that switch appears to do nothing. All three lugs are jumpered together.

Now that I spend a bit more time looking at it, you are correct! The drawing does not specify which traces are connected to each other and which ones bypass each other!

Thanks For That,
Gene

[martin manning]

The right side of that switch appears to do nothing. All three lugs are jumpered together.

My drawing convention is crossing lines do not connect unless there is a dot (and there are no connected crossings in this case).

[The Ballzz]

I see it now! The red dots I added helped me visualize it more clearly.
Thank You Sir,
Gene

Channel_Jump_Sw-1.png