Jet City Violence Ep.3 - External / Variable Bias Mod.
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- เผยแพร่เมื่อ 17 ต.ค. 2024
- So, if I'm going to keep this amp, I have to be able to bias it, without having to open it up. Just plug in a meter, turn a knob, and giver. Power Tube Hot Swap. Jet City JCA50H
What a way to wake up this morning and see another episode.
Yeah, I filmed these all at once pretty much, a single project file so I decided to hammer them out.
hi,awesome video dude. Is the 56mA reading for both tubes ore only for one? I have the same amp with JJ 6l6 using a measuring socket adapter and found in the net that 28mA per tube would be a good bias for 6l6 on a SLO 100, can you please give me😂 short feedback on that? Cheers Markus
Did I get 56 ? On which tube, that sounds too high to me. This is an older vid now so I don't completely remember what went on. That would be for one tube.
The bias of a tube is proportional to the amps operating voltage. SO without knowing what voltage your particular amp is running at I can't be sure. It sounds about right. My Jet with the current PT runs at about 480v. Modern 6L6 is 30w, 30/480=62mA, multiply by desired Idle 50 to 70% to taste, I would be min 31mA to max 43mA on my amp. If running a 6550 or KT88 then it's always 50% bias.
Nice mod, and the commentary is hilarious ! 😆
Hah, thanks. This kind of stuff can be pretty dull, so I'm trying to make it entertaining.
Learning a lot from your content man! Keep it up. If you decide to let this amp go, I’ll make you an offer🙋🏽♂️
Thanks. Hah, this amp is kind of priceless at this point. Or more than a new one. A local store has a 100watter.
I have a whole series about this amp coming, It's had a lot of work done.
Ive got a jca100 with Mercury Magnetics transformers in it. I thought that KT88's were more of the el34 family. I didnt think you could go back and forth between el34 and 6l6 family. Explain please.
Nah, a KT88 is closer to a 6L6, like a 6L6 on steroids.
More octal power tubes are pin compatible. You just have to bias correctly.
Beware some amps use spare pins an jumper points, and 7027's have connections on all the pins.
IN the case of the JCA, it will take just about any octal type tube, including the EL34 and a fav of mine, the 6CA7 with little modding. You just have to rebias the amp for that tube set. A stock amp might not have enough bias range so might require modding to the bias circuit. In some cases, if you order something like the KT88, and can choose what bias range the set comes in, you can request a cooler one, like in the 30's if you order from Tubestore.com.
For KT88,, bias it at 50% of it's rated 42 watts, not 70%, and you should be good.
why 45mV? did you check the voltage on the PCB?
Yes and no, I use 45 as a generic starting point often to set the tubes real quick when I'm working on an amp, then I fine tune it after.
Mind you, for that amp, with the transformer it had at the time, 460v, if 6L6's, 30watts, 45 would be good for 70% bias, and KT88 at 50% bias. Bit hot for the 6CA7.
I miss that Jet City mods video serie.
You miss what ?
nice mod bro,planning to do it on my jca22h but it has no jp2 and jp3..any piece of advice? thanks in advance
22h? What power tubes does it use ?
@@DATT el84
@@lailaniemata5402 Ohh, usually they are biased differently, but according to a schematic I found for the 22h, yeah, they do use the same negative bias system. So this mod would be pretty identical. I don't know if the board layout is the same. Hopefully you can add some 1 ohm resistors.
For test point, You could look at test point jacks like the 105-0803-001 and 105-0802-001. Punch those part numbers in digikey or mouser.
@@DATT thank you bro..
@@lailaniemata5402 👍
how do i adjust the bias without soldering anything? With the onboard bias thing
Off the top of my head, the JCA did have a bias knob hidden inside. So you can make the adjustment, however, it did not have test points. I had to remove some jumpers and add in 1ohm resistors so I could measure the bias. Without that you'd need to buy a special tool.
A quick search on ebay for "tube amp bias test tool" gives you a visual of what you are looking for. It's a kind of interception socket, you piggy back onto the tube socket.
You hook that up to a meter and set it to test MA.
I would avoid the ones with the switches unless you know what you're doing, as flicking that switch with the meter in MA mode can result in a hell of a light show or worse.
@@DATT so if i put in the resistors i can test ma?
@@alexcharamis Yeah, 1 ohm resistors, I'd use at least 1w, 2% or greater metal film.
They you test the voltage drop across them with a meter in mV mode.
@@DATT sorry for all the questions 😂 but, are the capacitors discharged when i plug the amp off or will i get electrocuted?
@@alexcharamis The design of the amp will discharge them, just not right away. Give it a min or two should be good. If there is anything left it shouldn't hurt you. Basically you can use your meter to test for voltage to be sure.