Modding an amp with an effects loop.

[markammon1979]

Hi, is there anyway of adding an effects loop to my Marshall JTM-45 Reissue? This amp has already had a few mods so I'm willing to take the risk by modding further. Also is there anyway of fitting a solo volume boost mod to an amp?

[Guitarist]

First try with inserting a simple insert after the MV pot. Just a plastic switching, stereo jack. Then build from there. Try some rack +4 type gear first. To boost try a Dano Transparent Overdrive. Report back

[pdf64]

What mods has it got, what do you hope to achieve with a loop (that the existing arrangement isn't allowing you to do), and are you hoping to run stomp boxes or rack fx?
More fundementally, why not change it for an amp that comes with loops and boosts?

[imaradiostar]

I'm curious how it's modded too- if it's been modded to be some kind of a high gain amp then a loop makes sense but unless you're using the amp wide open for power amp distortion it should be very pedal friendly just by putting all the effects in front of the amp.

jamie

[markammon1979]

Hi, thanks for everyone replying so far. The amp started life as a stock jtm45 reissue. I was never totally pleased with the sound when it was cranked, it always seemed to lack something. My Grandfather is an ex radar and electrical engineer and he told me he could completely gut out the inside and handwire it to the original Marshall Schematic which he has done. The amp sounds so amazing now but here is the problem. The other amps I have used in the past have all had Effects loops fitted. I have used the effects loop as a boost circuit in the past using a seymour duncan booster pedal in between the send and return jacks. This has given me a big control over the boost and doesn't seem to color the sound. When I place the boost pedal in the input of the amp I only get more distortion and not a clean volume boost. To overcome this I have been switching between two amps set to two different levels when playing live, but I'd rather be able just to take one head and cab with me. Is there anyway of getting the volume boost without having an effects loop?
Thanks.

[pdf64]

A JTM45 distorts primarily due to the power amp being overdriven. So a clean boost, ie increasing the level of the overdriven sound by XXdB, isn't possible. It just drives the whole amp harder, increasing overdrive, as you describe.
But I don't understand how you can have 2 JTM45 amps, driven to overdrive, putting out different sound levels. Unless you got speaker cabs with differing efficiency levels, eg green backs in one, vintage 30s in the other?

[markammon1979]

Hi, sorry I should have been more specific. I don't run two JTM 45's I run a Hand built Marshall 18watt alongside. I also run my JTM 45 with an attenuator. I also use an overdrive pedal on the JTM 45. I set the JTM 45 to about halfway so it's just breaking up and then add a touch of overdrive pedal. The 18 watt is running unattenuated for the lead sound while the JTM 45 is used for rhythm with the attenuator.

[pdf64]

OK, I can't see how an effect loop would necessarily be any benefit to you.
Try the following arrangement
guitar - overdrive - SD boost - JTM45 - attenuator - speaker
Try a link cable between the JTM45 channels to possibly get a richer tone.
With some careful level, gain, boost, volume and attenuation balancing, you should be able to achieve what you want.
Start off with the lead tone, turn the overdrive and boost on, set high, and set the attenuator for the overall lead sound pressure level. Then turn the boost off and set your rhythm sound. Then set the boost for your lead boost.
It might need a few iterations to get it right.
With the 4 inputs, you've got the option of using one channel for rhythm, and bring in the other for boost, with a link cable and volume pedal.
The other channel then effectively become your effect loop.

[markammon1979]

That's a good idea about using the other channel for a rhythm sound. I have an A/B swiching box which I assume I could use for switching between the two. The only trouble is the normal channel is a bit too muddy for my liking and only use the bright channel. Is there a way of making both channels identical in sound?

[pdf64]

Yes I know what you mean. With the JTM45, both channels have the same bass response, but channel II usually benefits from being brightened up a little.
That's quite easy, just fit a 'bright' capacitor across the channel II volume control (in the same way as the one across the channel I vol control). The value will be somewhere 100 and 565pF, silver mica type.
And remove the capacitor across the following Channel I mixer resistor, which will be 220pF or 556pF (connects on to channel I volume control wiper, center terminal).
The above will make both channels the same. Channel I will be slightly duller than before, because of the capacitor that's been removed. It has to go though, because in making channel I brighter, that capacitor makes channel II duller.
You can compensate for losing the mixer capacitor by making both the bright capacitors slighter higher, eg if 100pF, then try 120 or 150pF.
Or set the treble / presence at little higher.
The parts I've mentioned are noted on the schematic
http://www.raw-sewage.net/images/jtm45ri-schm1.jpg
under the 'Specification Differences On JTM45 Variants'.
eg add a second Cap A to channel II, and remove cap B.
If wanted, you could make the 'A' caps different for both channels, eg I use 120pF for rhythm channel, 220pF for lead channel.

Yes, an A/B box would be fine for switching between channels, or adding a second channel for lead boost (by inserting it in line with the channel link cable I mentioned).

[markammon1979]

Thanks for your help with answering my questions, it is much appreciated. I'll make the mods over the weekend and see how it sounds. Thanks again.