Does jumping inputs give more gain?

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Does jumping inputs give more gain?

Postby keano » Fri Aug 01, 2008 3:07 pm

I read by doing this you obviously control the tones but can get more gain is this true?
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Re: Does jumping inputs give more gain?

Postby PCollen » Sun Aug 03, 2008 1:52 pm

keano wrote:I read by doing this you obviously control the tones but can get more gain is this true?


Look at the schematic to follow this response. When plugged/playing into Channel A and it's Vol is on 10 (just for discussion purposes :twisted: ),
and Channel B Vol is on 0 (wiper grounded) , the Ch A signal is sent across the 2 mixer resistors which act as a voltage divider, with their junction connected to V2a control grid. Each mixer (ideally the same exact value of 270K or 470K, depending upon amp) drops half the applied signal so V2a grid gets 50% of what's coming from the Ch A volume pot.

If Ch B volume is now increased , it adds resistance between Ch B mixer resistor and ground. Ch B mixer becomes larger than Ch A mixer, and drops more voltage now than Ch A mixer and so the signal voltage at the control grid of V2a becomes greater.

This interactive effect is more pronounced with 270K mixers than with 470K's . Mixer resistors are also in parallel with, and effect, the wiper-to-ground Vol pot resistances, but that is another story better left to Mr. Thevenin.
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Re: Does jumping inputs give more gain?

Postby Billy Batz » Sun Aug 03, 2008 2:03 pm

keano wrote:I read by doing this you obviously control the tones but can get more gain is this true?

Slightly more. Nothing to write home about but you may like it. It mostly adds in the sonic character of the other channl. If you jumped them internally youll get a lot more gain then jumping them with a cord.

When you use a y cord or jump from low to high inputs you couple the input resistors together from both sets of jacks which lowers the strength of the signal on the grids on V1. Also for the same reason each channel will sound slightly different then it normally does. Try plugging into the bright channel and set it to 10 and the normal on 0 then plug a jumper from the bright channl lo input- the tone changes slightly just at this point- then plug it into the the normal high input- even witht he normal on 0 it changes the brights tonality. The same if you reverse it. I actually prefer plugging into the normals high then jumping the normals lo to the brights high. It just seems woodier to me because theres more series resistance to the bright channels grid.

But even if you do use cords you will get a slight boost with both channels turned up youd just have a good deal more if it was jumpered internally. I used to do this a lot but I kinda quit doing it for a long time but I started doing it with my JTM45/100. I think it sounds sweet like that.
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Postby clarkydaz. » Mon Aug 04, 2008 9:28 am

is it fairly simple to jumper them internally? i wouldnt mind trying it if not too consuming.
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Postby Roe » Mon Aug 04, 2008 10:26 am

just jump em at v1's pins
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Postby clarkydaz. » Mon Aug 04, 2008 10:49 am

jumper pins 2 and 7. thats all it is?
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Re: Does jumping inputs give more gain?

Postby Guitarjb » Mon Aug 04, 2008 1:28 pm

Billy Batz wrote:
keano wrote:If you jumped them internally youll get a lot more gain then jumping them with a cord.




Is the above statement the result of having completely removed the two resistors from whichever channel gets disconnected?

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Postby SDM » Mon Aug 04, 2008 2:34 pm

If you just jumper pins 2 and 7, the unused inputs and their 68ks load the signal down to both channels. That's because the 68ks of the unused inputs are still attached and in parallel to ground (through the action of the switching jacks), so you end up with 34K to ground at pins 2 and 7.

Preceding that you have (when using a high input), a 1 meg to ground first, then through a 34K(total) grid stopper to the grid. Normally that'd be it, but now you have the 34K to ground there too due to the jumper, thus a voltage divider of 34K/34K halving your guitar signal in to both channels.

To eliminate that voltage divider and get your full signal in to both channels, just remove the wire from pin 2 OR 7 - corresponding to the channel/inputs you won't use anymore. Just unsolder the wire from the tube pin, heatshrink or tape off the end so it cannot short to anything, tuck it out of the way. This way If you ever decide to remove the internal jumper, only need to remove the jumper on pins 2 and 7, resolder that wire back to pin 2 or 7, quick and easy.
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Postby Billy Batz » Mon Aug 04, 2008 3:07 pm

Yeah I forgot to mention that but its what I meant. Youd have to disconnect one set of inputs. If your used to plugging into Vol1 then disconnect the wire going to pin 2
Sound Clips
68 12xxx Series Super Lead/Bass Clone
67 10xxx Series Super Lead Clone
66 JTM-45/100 Super Clone
StoneAge 4x12B - Scumback H55's
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Re: Does jumping inputs give more gain?

Postby PCollen » Mon Aug 04, 2008 6:19 pm

Billy Batz wrote: I actually prefer plugging into the normals high then jumping the normals lo to the brights high. It just seems woodier to me because theres more series resistance to the bright channels grid.

.


Whether you plug into BRITE Hi and jumper BRITE Lo to NORM Hi,
or b) plug into NORM Hi and jumper NORM Lo to BRITE Hi, the signal path series resistance is exactly the same. It has to be, as the jack wiring and switching is identical (assuming stock jack wiring).
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Re: Does jumping inputs give more gain?

Postby SDM » Mon Aug 04, 2008 7:45 pm

PCollen wrote:Whether you plug into BRITE Hi and jumper BRITE Lo to NORM Hi,
or b) plug into NORM Hi and jumper NORM Lo to BRITE Hi, the signal path series resistance is exactly the same. It has to be, as the jack wiring and switching is identical (assuming stock jack wiring).


Gotta look at the switching jacks and signal flow. Using different channel inputs for the guitar and jumpering results in different circuits before the grids, not identical. As can be seen below, the input you plug into, sees a 1M to ground, then a voltage divider of 68K/1.068 Meg before it's grid. The other channel sees a 1 Meg to ground, then a 136K/1Meg voltage divider, then out through a 34K (parallel 68Ks) before it's grid.

So both a bit different there and the channel plug you into/jumper will effect things a bit.
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1959 inputs jumpered equivalent circuit.GIF
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Postby Billy Batz » Mon Aug 04, 2008 9:04 pm

PCollen wrote:Whether you plug into BRITE Hi and jumper BRITE Lo to NORM Hi,
or b) plug into NORM Hi and jumper NORM Lo to BRITE Hi, the signal path series resistance is exactly the same. It has to be, as the jack wiring and switching is identical (assuming stock jack wiring).

Its not symetrical. Meaning both input schemes are the same sure, but when you jump you alter the channels in one way thats not odentical and when you switch which channel is the original and which is jumped to it alters them in the same way but backwards as steve illustrates. When you start with the normal channel the brite channel is clearly a bit woodier, or in other words a little edge is shaved off because by the time the signal is on the brite grid its seeing 160k+ of series resistance vs the 34k you would see if you just plugged into the bright channel hi by itself. Thats a significant amount of high end reduction.

This old graphic I made helps to visualize things even though I didnt add int he extra jumping scheme. I made on of these that does show how it works but that was a few years ago now nds I cant find it.
http://forum.metroamp.com/download.php?id=1707
Sound Clips
68 12xxx Series Super Lead/Bass Clone
67 10xxx Series Super Lead Clone
66 JTM-45/100 Super Clone
StoneAge 4x12B - Scumback H55's
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Re: Does jumping inputs give more gain?

Postby PCollen » Tue Aug 05, 2008 6:47 pm

SDM wrote:
PCollen wrote:Whether you plug into BRITE Hi and jumper BRITE Lo to NORM Hi,
or b) plug into NORM Hi and jumper NORM Lo to BRITE Hi, the signal path series resistance is exactly the same. It has to be, as the jack wiring and switching is identical (assuming stock jack wiring).


Gotta look at the switching jacks and signal flow. Using different channel inputs for the guitar and jumpering results in different circuits before the grids, not identical. As can be seen below, the input you plug into, sees a 1M to ground, then a voltage divider of 68K/1.068 Meg before it's grid. The other channel sees a 1 Meg to ground, then a 136K/1Meg voltage divider, then out through a 34K (parallel 68Ks) before it's grid.

So both a bit different there and the channel plug you into/jumper will effect things a bit.


You are correct in this respect, and my original post was not contradictory to that.

Re-read my original response, and perhaps you'll understand what I said.
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