JCM 800 Mods

Paolo_M · 04-08-2008 10:53AM #1

The purpose of the mods was for warmer, thicker, meatier sounds with more gain at bedroom volumes. In this vid the MV is set to 4 with the Hot Plate @ -16dB and dialed down to half way.
The vid description gives details of work so far.
Any idea's on use for the third input hole drilled by the previous owner? He had fitted an additional input but had wired it for a fixed low volume and all the dials were by-passed which I felt was utterly useless to me so I removed all of his circuitry. I got this otherwise all stock ('81) amp cheap because of this.
Anyway is was thinking of a resonance control or maybe a PPIMV so I can hit the PI harder at lower volume settings, any thoughts/suggestions?

http://www.youtube.com/watch?v=7lsnwmeWvNg

Seziertisch · 04-08-2008 07:47PM

I don't know if this is possible with the circuit, but maybe a pot controlling a negative feedback loop from the speaker out back to the preamp could work well, like the presence control on a lot of amps.

I basically understand this concept but have no actual idea about how you would go about realising it in this case.

Eoin Madsen · 05-08-2008 09:23AM

The JCM800 already has a presence control that works off the negative feedback circuit.

You could add a resonance/depth control to the NF circuit or maybe a variable NF resistor? A PPIMV might be nice, but seems maybe a little bit redundant when you have an MV and an attenuator already.

A rotary switch for a variable slope resistor in the tone stack would be my first thought anyway.

Or, I'd put in a second input volume pot, rewire V1 into a parallel gain stage, and rewire the existing high/low inputs to brilliant and normal inputs. If high/low was something I used I'd add a mini-toggle high/low switch. And then I'd probably pull the MV and replace it with a PPIMV. And I'd probably have to tweak the cathodes on V2 to make up the lost input gain. And sure then it'd practically just be a 1959 with a PPIMV...

Paolo_M · 05-08-2008 11:17PM

Eoin Madsen wrote: »

The JCM800 already has a presence control that works off the negative feedback circuit.

You could add a resonance/depth control to the NF circuit or maybe a variable NF resistor? A PPIMV might be nice, but seems maybe a little bit redundant when you have an MV and an attenuator already.

A rotary switch for a variable slope resistor in the tone stack would be my first thought anyway.

Or, I'd put in a second input volume pot, rewire V1 into a parallel gain stage, and rewire the existing high/low inputs to brilliant and normal inputs. If high/low was something I used I'd add a mini-toggle high/low switch. And then I'd probably pull the MV and replace it with a PPIMV. And I'd probably have to tweak the cathodes on V2 to make up the lost input gain. And sure then it'd practically just be a 1959 with a PPIMV...

I was thinking of installing a foot-switchable 10k lift on the middle of the tone stack for a kinda solo boost a bit like the Mesa Boogies, but a selectable full frequency lift/reduction might also be interesting.
Nice to see a fellow JMP fan out there but I do also have a JMP50 1987 so going the 1959 route doesn't seem so enticing to me.
I was thinking PPIMV so I could get full gain out of the PI while still having the output tubes running at a moderate setting (4/5/6) and then use the Hot Plate when at home/smaller gigs.
Thanks for the suggestions, nice to see someone else into modding amps as my girlfriend doesn't really get it!! :pac:
Have you installed a resonance control before? How well does it work? I'd like a really tight bottom end for metal sounds. It's hard to tell how well it works from on-line vid examples as the sound is so compressed anyways.

Eoin Madsen · 06-08-2008 09:58AM

No, I haven't done a resonance control yet. It's definitely on my list when I have my own build up and running.

I replaced the insides of the ('76, formerly ailing) 1959 living in my studio recently - but it's not mine, so making holes in the chassis to facilitate modding as well was not on the cards. I'm going to tweak the circuit a little before I close it up for good though. I might see if I can install a PPIMV in the spare speaker jack socket.

Only thing that makes me hesitate with putting in a footswitch is that you need to put in a relay and, maybe more problematically, a power supply for the relay. Unless you make a custom footswitch and put a battery in it or something like that.

I ran some simulations with the Duncan design tone stack calculator (google it if you don't have it already!). Adjusting the slope res is definitely something worth looking at in terms of the overall gain and the TS curve. A lot of people seem to change the treble cap at the same time - there's a few common combinations knocking around. Again, I've yet to do it myself.

Paolo_M · 07-08-2008 11:29PM

Eoin Madsen wrote: »

No, I haven't done a resonance control yet. It's definitely on my list when I have my own build up and running. I replaced the insides of the ('76, formerly ailing) 1959 living in my studio recently - but it's not mine, so making holes in the chassis to facilitate modding as well was not on the cards. I'm going to tweak the circuit a little before I close it up for good though. I might see if I can install a PPIMV in the spare speaker jack socket.

Only thing that makes me hesitate with putting in a footswitch is that you need to put in a relay and, maybe more problematically, a power supply for the relay. Unless you make a custom footswitch and put a battery in it or something like that.

I ran some simulations with the Duncan design tone stack calculator (google it if you don't have it already!). Adjusting the slope res is definitely something worth looking at in terms of the overall gain and the TS curve. A lot of people seem to change the treble cap at the same time - there's a few common combinations knocking around. Again, I've yet to do it myself.

Yeah, I had downloaded this alright but I found something a bit odd. When altering the slope resistor in the sim, reducing the value seems to boost the overall frequency gain yet all the forums seem to suggest that increasing it will do it, 47k and 56k seem to be popular suggestions?
At 56k the calculator shows the 1k responce down about 2dB vs the 33k value, for example, with each control set to noon?!?
I'm a bit puzzled, do you know what the crack with this is?

I probably have it all arse-ways as usual but if you can help an idiot out that would be great!!:pac:

Edit: I suppose the obvious thing to do is wire a 100k lin pot in there a see what happens...

Eoin Madsen · 08-08-2008 12:28PM

Ah, a pot probably isn't a great idea because there's no coupling cap blocking DC between the cathode and the slope resistor (I was once chastised for similar reasons for having bypassed the treble cap to troubleshoot a problem :pac:). Em... as far as I know, raising the value should increase the level as less signal is divided around the treble pot into the bass and mid pots of the tone stack. It should flatten out the response... I think. I'll run the program myself later and see if I get the same results.

JCM 800 Mods

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