second input jack

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Tiny Daddy
I will work on all amps
Posts: 513
(2/27/04 10:55 am)
 second input jack
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 Some amps have 2 identical input jacks. Does anyone use both jacks at the same time? For what?
 
embotone
Junior tube assistant
Posts: 14
(2/27/04 12:35 pm)
 input jacks
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 The two jacks usually each have a 68k grid stopper resistor. If you look at how they are wired, you will see that when using one of the jacks, the two resistors are in parallel, giving 34k ohms resistance. This is usually used for lower output, single coil pickups.
I wonder why 68k is the "magic" value used in most amps. I am working on an amp now that only has one input jack per channel. I wired in a 50k pot as a variable resistor, in series with a 10k resistor, in place of a 68k resistor. The pot has a center detente (is that right? A spot at the center of the rotation that clicks) This gives me 35k ohms at the center, up to 60k or down to 10k resistance. I'll tell how it works when the thing is up and running, if you are interested.
 
GroundhogKen
Forum Moderator
Posts: 2301
(2/27/04 12:45 pm)
 Re: input jacks
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 One jack is hi sensitivity--usually for single coil pickups. The signal sees a 1 meg to ground resistor and then parallel 68K ( 34K ) to the grid.

The other is low sensitivy--usually for double coil pickups. The signal goes though a 68K resistor to the grid and there is a 68K resistor from grid to ground. The 1 meg is shunted.

It's a bit of a brain teaser to sort out what happens when you insert a plug into either jack. Keep an eye on those switch pins.

Ken
 
Tiny Daddy
I will work on all amps
Posts: 518
(2/27/04 1:24 pm)
 Re: input jacks
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 Oh yeah, one jack shorts the 1 Meg resistor if it's not used. I had to ask because I see people trying to connect a microphone or a bass guitar to the second jack with dismal results.
 
embotone
Junior tube assistant
Posts: 15
(2/27/04 1:55 pm)
 input jacks
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 Yikes, that puts a dent in my incredible variabe input resistance invention! Is there a ratio of values to be maintained between the input to grid versus input to ground resistance?
 
Tiny Daddy
I will work on all amps
Posts: 519
(2/27/04 3:26 pm)
 Re: input jacks
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 The 1 Meg resistor sets the bias for the tube and is not critical, meaning it could vary a lot, for instance 470K to 2 Meg and still work about the same. As for the 68K this provides frequency rolloff and keeps out interference from radio stations, but also if it is not used there will be a loud bang coming from your speaker if the guitar is unplugged with the amp on.
 
jbrew73
Junior tube assistant
Posts: 100
(2/27/04 3:29 pm)
 input jacks
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 i read somewhere once that the 68k was chosen because of a 10/1 ratio between the impedence of the source(guitar pickup roughly 6.8k or so) and the grid resistance 68k. the article gave other examples of this but i can't remember any of them or where the article came from. but that ratio is some "magic number" for any amplifing circuit.
it may have been in that AX84 theory article
 
HotBluePlates
I only work on Fender's
Posts: 692
(2/29/04 10:08 am)
 Re: input jacks
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 You're right JBrew.

It's an electrical "law". Basically, when you hook up the output of one device to the input of another device, you are trying to "bridge" the signal.

Maximum power transfer occurs when the output and input impedance are equal, like hooking a speaker to the proper impedance tap on a transformer.
Maximum voltage transfer occurs when the input impedance is 10 times the output impedance of the first device.

So 6.8k average output impedance -> 68k input impedance
 
embotone
Junior tube assistant
Posts: 16
(2/29/04 2:37 pm)
 input jacks
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 Here is an interesting article regarding grid resistors.
www.aikenamps.com/InputRes.htm
 
heresrobert
Junior tube assistant
Posts: 17
(3/3/04 1:43 pm)
 Use of second input jack
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 I always wondered about that, too. I use the jacks to bridge to a second amp so I can produce a clean tone from one amp while having low wattage amp cranked. I adjust the controls so one amp doesn't run over the other. It definitely makes an interesting sound.
 
 
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