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Author Topic: What's the Difference Between Hi and Lo input jacks on some amps?  (Read 8291 times)

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Slimjim

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https://robrobinette.com/How_The_AB763_Deluxe_Reverb_Works.htm

Here is an exceptional article on the details and possible mods for the AB763. It covers the Hi/Low topic very well, as well as all of the other sections of an amp circuit. Much to learn, as always.
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Dante

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Awesome! Thanks SlimJim  :thumb-up:

I think this page sums it up pretty good, and explains the "bright" vs. "normal" on Fender amps

^^ That is a link ^^

Quote
The Hi and Lo inputs differ in signal intensity, input impedance and grid stopper resistance. The Lo inputs form a voltage divider using the two 68k grid stopper resistors. This voltage divider cuts the source signal in half (-6dB)--yes, half of the guitar signal is bled to ground. The Lo inputs are attenuated by slightly more than -6dB to offer less gain and more clean headroom. If you find you prefer the Lo inputs I recommend you try using the Hi inputs and turn down the guitar's volume control. You get pretty much the same outcome but you have control at the guitar but the Low jack's low input impedance can remove some high frequencies from the guitar pickup so the guitar tone is typically a little less bright when compared to the Hi jack.
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Kim

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Well now I think of it,with a JCM800 master volume (or the older JMP) 2203 and 2204 models the lo input bypasses one half tube.

I was checking out some other unrelated things, and came across this from the JCM800 2203 User Manual:   :lol:

9. High Sensitivity Input
This is the High Sensitivity input of the amp – in theory,
such an input is supposed to be used for low-output
pickups. This said, 99.9% of all 2203 users ignore this
and plug guitars loaded with high-output pickups into this
input, because it enables them to drive the pre-amp even
harder.
10. Low Sensitivity Input
This is the Low Sensitivity input of the amp, which was
designed for hot (high-output) pickups; we would
suggest you try both inputs and then decide which one is
best for you.
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MarshallJMP

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Great finding Kim  ;D
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Dante

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Sounds like each amp manufacturer had a different approach

Who woulda thunk?
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van Sinn

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Sounds like each amp manufacturer had a different approach

Who woulda thunk?

Mmnn..  you didn't expect DIN standards for tube amp inputs, did you now.. :lol:
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rnolan

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Seems so  :facepalm: .  IIRC hi and low inputs started when active PUs and built in preamp circuits took off. And the first I recall seeing were bass amps (Alembic were probably one of the first in the active space  :dunno: ).  I remember EMGs coming out way back but there were also a few others (Ovation ?, wow they had some bight  >:D , interesting guitars not sure if the PUs were active but they had a mean built in preamp circuit).  Played one through a BAEZ stack (great amps BTW made in my home town), you could mow the lawn with that setup  :metal: .

So seems some are just a PAD (cool the signal down by xdb) and some for impedance matching, or both.

Anyway, the advice in Kims post is good, try both, use the one you like that works for you  :thumb-up: , or use a Y lead and plug into both  >:D :whoohoo!:
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Harley Hexxe

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   I don't know if  a "Y" lead is such a good idea.

   Do you remember the old school amps with all the input jacks, supposedly to be able to plug the whole band into? When you did that, the instruments would fight for dominance in the input section.
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van Sinn

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   I don't know if  a "Y" lead is such a good idea.

   Do you remember the old school amps with all the input jacks, supposedly to be able to plug the whole band into? When you did that, the instruments would fight for dominance in the input section.

Well, that actually could be seen as an advantage, as it would force everybody to play correctly, and drop the ego ride :banana-guitar: :nono:  (yup, was a real  POS)
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rnolan

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Who knows what a Y lead would bring ? with just one input source it shouldn't be a problem, it will depend what's behind both inputs and then how they combine, so worth a try, may be good, may not be  :dunno: .  With my 73 Marshall, it has a lead and a bass input (well 2 jacks of each, old school small writing Marshall).  Input 1 is lead, 2 is bass.  So I bridge 1 & 2 (they each have a vol knob  :whoohoo!: ) and wind in some cleaner bottom end into the sound.  I know not the same as high/low inputs, but then depends how they work (many and varied it seems)
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Harley Hexxe

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Hey Richard,

    Give it a bash and let us know what you think
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rnolan

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Hey Harley, I don't have any amps/preamps with high/lo inputs to try.  I've always bridged the Marshall lead and bass inputs inputs and dialed in a fair bit of the bass channel (it has its own vol knob).  The lead channel in these heads is very toppy but including the bass channel rounds it out nicely (for me anyway). But no master vol on these babies, and crap does it mooove some air  >:D .
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Harley Hexxe

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Richard,

  What model is that Marshall head? I used to have a 73 Super Lead 100 a long time ago, but I don't remember it having a bass channel. Mine was modified with a rheostat in one of the inputs to act as a master volume. I'm assuming the input that was modified was one of the low inputs, but plugging my guitar into the input above the modified jack didn't give me more bass tone, just a weaker signal.
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rnolan

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Initially I thought it was a 1972 JMP 50w lead head.  When I changed the input tubes a while ago I noticed it was built in 1973.
It has 2 inputs (1 and 2) with 2 jacks each (paralleled) and a vol control for each input.  Also has presence, bass, mid and treb.
Output tubes are 2 x EL34 (6CA7).
I now have the bottom bass input jack wired direct to the power amp (as MJMP does with his 100 watters)
I think the super leads were a bit different  :dunno: (MJMPs area here  :thumb-up: ).  It was also around the time gear makers stopped having lots of different inputs, mic, bass, guit etc (ie run the whole band off one amp). But we used to have to do this, as a 14year old, I couldn't afford much of anything. I used to repair broken strings etc... and we made do with whatever we could get our hands on (and I soo wanted a Marshall or BAEZ (local amp maker) stack).
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van Sinn

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Erm.. are you sure about the two EL84's?

If so, I'd love to know which technology can make those produce 50 Watts.. :dunno:
Like, 1 Kv on the plates, -150 Volt cathode bias, 48 volts to the heaters, a power transformer featuring negative loss - and using liquid nitrogen cooling :lol:
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