mod JCM 800 inputs into foot switchable channels

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ZeroPointEnergy
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mod JCM 800 inputs into foot switchable channels

Post by ZeroPointEnergy »

Looking for suggestions for turning my JCM 800, into channel foot switching.

I believe the main difference just one extra gain stage in the high input side. I do not mind if the two channels would have a volume mismatch. I just really like both sides of the amp, and just want them foot-switch selectable.

Thanks! BTW, this is the first time posting here in years and years. Been busy. I used to be 1way, waaay back when. hehe I hope SixString (with fence cable for guitar strings, hehe) is still going alright!

Glad so see this place is still here.
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Re: mod JCM 800 inputs into foot switchable channels

Post by JMPGuitars »

Isn't a JCM800 a single channel amp?
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Re: mod JCM 800 inputs into foot switchable channels

Post by ZeroPointEnergy »

JMPGuitars wrote:
Tue 04/28/20 4:33 pm
Isn't a JCM800 a single channel amp?
Yes, and no, if you consider the two inputs are a gain stage different sounding! That is no small change. But it is not foot switchable.

However some JCM 800's including diode foot switchable selection of the two "channels", so in that case, it absolutely is a two channel amp.

Mine is the all tube signal path like the former JMP's. On my all tube model, there is no (1)transistor distortion clipping added, and no (2)channel switching. My model is the 4010 combo, and I have another cab for the another 12" speaker. 8)

So it has been done before, and is desirable, so much so they even did this at the factory, so go figure, if it's a single channel amp or not. hehe

~~~

The JCM 800 is not just one model, it's a series of amp similar models. So it helps to keep that in mind.

Presently in stock form, to change the "channel", you need to manually change the guitar cable input, and that means:

(1)walking over to the amp, then (2)lowering the volume to do it safely, then (3)unplug from input, (4)plug into other input, (5)increase volume to taste, (6)walk back to the pedal board/performance.

So presently, you can have access to both channels but it's a bothersome SIX step process, that I would much rather have nicely settled in a foot switch.
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Re: mod JCM 800 inputs into foot switchable channels

Post by JMPGuitars »

That's easy. Use an AB (or AB/Y) box. No mods needed. You can use a Y cable and then just run a single cable.

You can make one of those yourself if you want. They're really easy. If you wanted to get fancier you could probably add a simple signal attenuator to the AB box so the channel levels can be adjusted.
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Re: mod JCM 800 inputs into foot switchable channels

Post by ZeroPointEnergy »

JMPGuitars wrote:
Tue 04/28/20 6:17 pm
That's easy. Use an AB (or AB/Y) box. No mods needed. You can use a Y cable and then just run a single cable.

You can make one of those yourself if you want. They're really easy. If you wanted to get fancier you could probably add a simple signal attenuator to the AB box so the channel levels can be adjusted.
I think that was true with the first generation of MV amps, the JMP's, however it is not true for my early 80's, JCM 800, model 4010.

The low input, is not it's own separate channel, it's mostly just one less gain stage of the high input channel, so the volume is not balanced, and is made operational by inserting the cable into the low input jack.

The plugged in low input, avoids the high gain input channel, as long as the low input cord is plugged in!

~~~

So you can not just run two cable inputs, from an a/b stomp switch. I am hoping to plug only into the high input side, and connect that to the low input signal path with a wire lead, that connects just AFTER the cord detection part of the low input circuit, and thus misses the first gain stage.

Then just put a foot switchable relay solenoid switch, to remotely connect or disconnect that optional signal path to the low input side of the amp.

It's only really changing how the inputs are routed, to avoid the cable detection of the low input. Just plug into the high input, and control that input, with a foot-switch that allows for the low input path to optionally function.
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Re: mod JCM 800 inputs into foot switchable channels

Post by JMPGuitars »

There are way too many versions of that amp, it gets confusing.

I'm assuming this is the correct schematic since you said early 80s (the top is supposed to be the 4010):
jcm800_2204_4010.jpg

That's an interesting preamp design. At quick review, it looks like the low input uses half of the preamp tube, while the high input cascades the two halves of the preamp tube together.
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Re: mod JCM 800 inputs into foot switchable channels

Post by JMPGuitars »

After thinking about that preamp, here's an idea I came up with for switching channels without modifying the amp. It might be stupid (I did get only 3 hours of sleep lol), but it might work for you.
JCM800 Switchbox by JMPGuitars.jpg

I assume you can work off of a schematic. If this works, it can be done with a DPDT or 3PDT switch. I drew it with a 3PDT switch in case you wanted to add a 9V source, resistor, and LED to indicate which channel is active. As drawn, the switch would connect the LED negative to ground where it says "E."

The idea is simple. One guitar input goes to either the high or low input of the amp. The only input that needs to be grounded when not in use is the high input, as the low input's ground is lifted when the channels are cascaded (high input in use). Click to low input, high input is grounded. Click to high input, low input is not connected to anything in the switch box.

Assuming you're using good quality shielded instrument cables, I don't think there should be any issues. Maybe somebody smarter than me has another opinion. ;)

Thanks,
Josh
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Re: mod JCM 800 inputs into foot switchable channels

Post by ZeroPointEnergy »

Sounds good. So,,, I guess you are suggesting to do the switching, before the amp, and even before the inputs. That seems like one way of doing it.

I do not understand wiring schematics, as well as I do wiring diagrams. What jack does the guitarist plug into?

Because if the low input jack is used, that disables the high input jack, so you can not have both jacks plugged in, you would at best, only get the low input working, no more higher gain input.
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Re: mod JCM 800 inputs into foot switchable channels

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The amp does not work right when you plug both cables in. Everyone who has that amp seems to agree, you can NOT plug two cables into the amp, and expect it to work right, it certainly does not.

It is unclear what happens, as presently my amp is waiting to be serviced back into life, hopefully within the next week or so, but I believe either you only get the low input channel, or you get nothing, because after you also plug in the low input, doing so knocks out the high gain channel.

I think it would be better to do the mod after the inputs, only plug into the high input, and just create an optional, foot-switchable signal path to the end of the low input signal path.

I think you want to defeat the cable detection in the low input side, so the signal can travel to the second gain stage without problem of no cord plugged in. Without making some kind of change, you can not plug two guitar signal cables into the amp and it works, no, it does not work that way.
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Re: mod JCM 800 inputs into foot switchable channels

Post by JMPGuitars »

Oops, you're right. I just looked at the schematic again. I forgot the cascading was done through the shunt on the low jack. So when a cable is plugged in, the high input is defeated.

My switch won't work as drawn.
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Re: mod JCM 800 inputs into foot switchable channels

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Where did everyone go? I used to post here all the time about a decade ago. This place was all the rage. So much new great amp makers, and guitarists too, lots of interest in lower powered tube amps.

I never wanted a 100 watt tube amp, maybe a 50, but even just 10 watts tube power, when cranked, gets too loud for most home players. Thanks for the discussion. Is Zaphod Phil still here? I kinda remember him from before.
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Re: mod JCM 800 inputs into foot switchable channels

Post by JMPGuitars »

I dunno if you were around when the site was getting attacked and hacked, but that happened. The site was basically private and locked down until I rebuilt it in 2012. The community has been rebuilding since then.

Zaphod_Phil is still around. I dunno if he'll pop into this thread or not.
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Re: mod JCM 800 inputs into foot switchable channels

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Zaphod_Phil This is 1Way from way back when. I could not resurrect my account, Miss you brother, its been a long time! Major respect for your amazing amp knowledge. I was hoping to see SixString around here. But I did not know about the cyber attack, and all that.
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Re: mod JCM 800 inputs into foot switchable channels

Post by JMPGuitars »

ZeroPointEnergy wrote:
Fri 05/01/20 5:23 pm
Zaphod_Phil This is 1Way from way back when. I could not resurrect my account, Miss you brother, its been a long time! Major respect for your amazing amp knowledge. I was hoping to see SixString around here. But I did not know about the cyber attack, and all that.
You can PM me if you want help recovering that account.
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