Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: EKDENTON on April 01, 2016, 09:58:58 pm
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I'm working on a layout for an amp. The amp has an effects loop circuit. The picture is the guitar input jack. Two questions...is the jack in the drawing just a stereo jack? and is the drawing correct? There are two legs and the bottom leg when plugged in goes to to pin 2 on V1A and it looks to me like the top leg is drawn to ground so that when you plug in the guitar it will short the signal to ground? Or am I not understanding the schematic?
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It's that type of Jack that has the pin that makes contact with the tips leg when nothing is plugged in, and when you plug something in, the pins, or Legs, separate, and it lifts the ground. I think? Like the speaker output Jack on a Fender. It has a name, but I can't think of it? Or I could be wrong entirely?
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Two questions...is the jack in the drawing just a stereo jack?
No, it is a tip-ring stereo jack but it also has two "NC" normally closed switches. Unusual. If you do not need the second switch, what this jack does can be done with a much more common (and thus hopefully cheaper) part.
and is the drawing correct?
I can't really say. It seems odd that something from farther in the amp (can't tell what it is) is being fed to the guitar input. Normally neg feedback goes to a much later point and gain stage in the amp circuit and would be way too strong a signal for a V1 grid.
There are two legs and the bottom leg when plugged in goes to to pin 2 on V1A and it looks to me like the top leg is drawn to ground so that when you plug in the guitar it will short the signal to ground?
No it will UNSHORT the signal to ground. V1 is grounded with nothing plugged into the amp to cut noise, very common. When the 1/4" plug is full inserted, the tip is unshorted away from ground and now connects to your V1 grid.
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... The picture is the guitar input jack. ... is the drawing correct? ...
As Eleventeen says, no way to know unless you show us the whole schematic (or at least where everything connects).
I agree with Eleventeen that it's a TRS (tip-ring-sleeve) stereo jack with 2 normally-closed switches.
The "tip" is the part which connects to R1 and keeps amp noise down with nothing plugged in by shorting that grid to ground.
The ring part is between the tip and the heavy black line (that heavy line being the grounded sleeve). The switch runs off to some other part of the circuit, and is likewise grounded when you have nothing plugged in.
But your mono guitar cable has a grounded sleeve which is the length of a TRS sleeve & ring combined. So when you insert your mono guitar cord, the grounded sleeve engages that ring contact. However, all it is doing is disconnecting that switched circuit from ground so that it is active; it has nothing to do with the guitar's input signal. Just like a typical mono switched input jack, when you insert the plug, the plug contact lifts away from its associated switch contact.
So is it right? I'd need to see what circuit is being ungrounded by the ring switch.
These schematic symbols are for normally-closed switches on the jack. There are other schematic symbols to indicate normally-open switch contacts, or for other, more elaborate, switching mechanisms.
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Here is the photo of the other end, it just an effects loop. Maybe that is the design of the circuit but it looks to me like if I wire it as it is drawn in the previous photo, I will be grounding the signal even when plugged in? In the previous photo when plugged in should the top NC switch open and the line go to the effects return instead of to ground?
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here is the whole schematic
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When you plug in wont the signal follow this path? What am I seeing incorrect?
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It shorts signal from FX Return to power amp when you take the input plug out.
You can ignore the extra stuff and it will work fine. I suspect some tester had some annoyingly noisy FX in the FX loop and wanted the amp to go silent when he pulled the guitar plug. I'm not sure that is reasonable, or essential. You could build it the "usual" way and decide for yerself before you go on a hunt for a switched stereo jack (not a common part).
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Oh yea, now I see what you guys mean.
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> Oh yea, now I see
Arg. Well, I drew this, maybe it will help someone else.
> wont the signal follow this path?
Guitar plug TIP goes to the furthest-in finger. This is normal guitar. When plug is removed, finger falls on switch contact, shorting amp input, absorbing stray buzz.
FX Return comes to the nearer finger. With plug, it does nothing. No-plug, it hits switch contact, is shorted to ground, absorbs all crap coming from FX Return.
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Given it's an Orange amp, undoubtedly the input jack is a 6-lug Cliff-style jack, such as the CL1220A. Get one in hand and insert a cord plug to see how it works. You could also get a 4-lug Cliff-style jack, knowing there is one switched contact instead of two (the CL1220A has 3 switched contacts, but the Orange circuit uses 2 of them).