Mojotone Plexi Kit: Why you're NOT getting the tone you want!
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- čas přidán 21. 08. 2024
- How to troubleshoot unbalanced output tubes in your MojoTone Plexi Kit to resolve distortion issues.
Multimeter I use:
-Neoteck True RMS 9999: amzn.to/4aoUcI2 (Amazon)
Where to get the Mojotone British Style 100W Lead / Bass Head Amp Kit:
www.mojotone.c...
Contact info:
diyaudiogarage@gmail.com
/ diyaudiogarage
Disclamer:
Please note that this video is for informational and entertainment purposes only. I am not a professional or certified expert in amplifier modifications or electronics.
The amplifier I'm working on in this video operates at high voltages, which can be extremely dangerous and pose a risk of serious injury or even death. This video is not intended to be a tutorial or guide. If you choose to modify your amplifier or engage in similar activities, you do so at your own risk.
I am not responsible for any damage to equipment, property, or any injury sustained as a result of attempting to replicate what is shown in this video. Always prioritize safety and, if in doubt, consult or hire a professional with the necessary experience and qualifications to carry out these types of modifications and repairs safely.
I believe you took out the standby switch which isn’t needed in most amps but that Marshall has a cathode follower tone stack and is solid state rectified which puts a lot of stress on V2 without standby. Eventually you’ll burn out that tube if you don’t allow V2 to warm up in standby.
Hi Jimmy! Thanks for the comment. I will definitely
look into that. I am planning on moving the master volume to one of the input jacks and permanently jumper the inputs. The I can install the standby switch. That would be a future mod.
@@DIY_AudioGarage if it’s a PPIMV it will be noisy next to the input jacks - I know from past experience. You could put that in the indicator hole and drill an extra hole for the indicator I believe. Good luck with the build! Such a cool amp!
I didn't connect it to the input jacks right away because I had a feeling it might cause some noise. And well, it turned out I was right! I was hoping to avoid drilling new holes, but I'll follow your advice. Thanks for the heads-up.
Sounds Good and i do have some advice since i just built a similar amplifier. There are some different ways to do this and remove noise. The easiest way is to use shielded wire. Jason from Headfirst recommends using shielded wire in any "high gain amp" and with the no.36 sir mod you turned the jcm800 into a high gain amplifier. I recently did the same and shielded the entire amp and it sucked. too much "tone suck" and honestly RG174 is too thin and cant handle the voltages and signal correctly. it basically burned up every coax like a filament in some spots, like a fuse. i recommend using mil spec 27500 aircraft wire. i used the 22 awg wire and its great because there is no impedance so you dont lose any treble or high frequencies. i also recommend using a hot shield. If you use a very good 8 inch Hot shield then you are Golden. I used one but it was too long. around 15 inches since i went around the board instead of under the board. some fixes will reduce hum, hizz or buzz but almost all will take a bit of treble away. You can use a capacitor of 100pF or more across the Anode Plate resistor of 100k. But do not use 470k if the anode feeds into the coupling cap followed by a 470k/470pF. That will cause some weird things to happen, in that case you have to use a smaller value. The first gain is the most important and the quietest way to quiet the amp is the miller effect. that means using a hot shield or a 10pF capacitor from the anode to the plate. The recommended value is 7.5pF from factory. But capacitors fail and may cause either a short or an open break. a short is very bad which is why i recommend a 10pF in the mixing stage. The Triode with a cathode of 820ohm and 680nF you can use the miller effect to remove the hum. If you look at the AFD100 schematic they used a 1nF capacitor across the 100k resistor at the triode with the 820ohm/680nf stage too. the DSL 100 uses a 1nF to ground from the anode of that same gain stage as well. Mesa boogie also uses 120pF capacitors across both the 82k and 100k in the Phase inverter. You have to be careful that if you use any shielded wire that you do not create any ground loops. you definitely have to do something about your ground loop hum. i recommend removing every chassis connection connected to ground. you should only have one place where all the grounds connect to the chassis. at least for the preamp section, not the power tubes but it does help if you do that with the power tubes too. my first attempt at building my amp was pretty good but i made so many problems with the ground. i read some tec hnical document on grounding which helped a lot. grounding an amp is almost an art form