Jet City Amps Appreciation Thread!

[zainprox]

Ok! thanks!

 

[CyanideJunkie]

@cyanidejunkie: How diff will that sound? I think one of my tubes blew:/ I can turn my amp on, power tubes are still glowing, the three preamp tubes that I can see are still glowing, speaker cable connection's good. I'll try to use another power chord, I read somewhere that that could contribute to this problem, but not so sure yet:/

Not sure about sound, but it will make the bias in the power tube section more "intelligent", self-adjusting the bias to fit the current, helps to prolong tube life. Most decent tube amps are already cathode-biased in the preamp section, but usually only the higher-end ones comes with cathode-biased power tube section.

Did anything peculiar happen prior to the amp dying? ie. Any popping sound? Any smoke or strong burning smell? At best, you could have a loose tube. At worst, you could have blown the transformer, but highly unlikely lah.

 

[J.custom]

I see. Hmm, nothing happened to the amp leh. I was playing the night before, woke up, flicked the switch on but no sound.

I'll open it up tmw morning to make sure all the tubes are glowing. So far I can only see the 2 Power tubes and 3 preamp tubes, the other two have to open up the back to be able to see them.

Do you know how to bias it?

One day I wanna try KT66s in this amp. FastRedPonyCar on YT did it with his JCA100H and it sounds great! No bias required. He then rebiased it and then put in KT88s in there, insane!

 

[J.custom]

Hey I noticed that 2 of the 12AX7s had two rather distinct glows to them, but the other one only had one side glowing, do you think that could be the faulty tube?

 

[dodgethis]

Yes, the tube is dead. Replace immediately.

 

[J.custom]

Thanks! Oh man, that was fast. Looks like I'll be making a trip down to Well Audio Labs tmw

 

[CyanideJunkie]

Do you know how to bias it?

There's a trim pot on the PCB, should be in the same area where the power tubes are on the flipside. That's your bias adjustment. You can't bias the individual tubes though, so THEORETICALLY it's best if you purchase a matched pair if you want your tubes to last longer. You can use a digital multimeter to measure the plate current(it's not the norm, but YouTube has vids on how to do it) and adjust accordingly, or heck it and let your ears be the judge. In any case, it's best to let a certified tech handle it as it involves running the amp hot with LETHAL(no joke) amounts of current.

On the tube issue, dodgethis is most likely right, but bear in mind that you will not see tubes with large plate structures like the Sovtek 12ax7-lps and Mullard 12ax7 glow, as their filaments are encased within the larger plate.

 

[J.custom]

I see, yeah I don't think I'll touch it if it works fine.

Don't think I'll be able to afford a Mullard! Will probably be getting a JJ to replace that dead tube.

 

[CyanideJunkie]

Which position is the faulty tube in?

 

[J.custom]

V1 if I'm not wrong. The two preamp tubes with the metal casing around them is the phase inverter and the rectifier tube? Not sure

 

[CyanideJunkie]

The JCA50H has a solid-state rectifier section. And the tubes with the retainer clips are power tubes! Tall ones right? Don't mix them up lol!

Not sure if the JCA50H has the same configuration as the JCA2112RC even though they share the same preamp/power tube ratio, but it should be something like this: one tube for the Normal channel, another for the Overdrive channel(most likely v1&v2), one tube for the FX loop, another for the phase inverter, and one last one for the gain return. You should email Jet City if you want confirmation on the specific positions.

 

[J.custom]

Yup power tubes are the tall ones but there are 2 other preamp tube-sized tubes behind the tranny which have some sort of metal casing around them, so can't really see whether they were glowing or not.

Yeah I think what you said sounds somewhat correct.

OH yeah, something for you to check out

http://acapella.harmony-central.com/showthread.php?2639458-Jca20H-Mod-thread-and-owners-club.

This really got me going until I figured I could get the JCA50H by selling the amp and saving the amount of money I would have spent on the mods.

 

[CyanideJunkie]

Hmm, you better email Jet City for clarification then.

Cool thread. The only mods there I would consider would be the choke, and maybe the depth knob one. The channel switch mod would be cool, but there's something magical about a single-channel amp la! One less switch to step on too lol.

 

[Ratboy]

Not sure about sound, but it will make the bias in the power tube section more "intelligent", self-adjusting the bias to fit the current, helps to prolong tube life. Most decent tube amps are already cathode-biased in the preamp section, but usually only the higher-end ones comes with cathode-biased power tube section.

Did anything peculiar happen prior to the amp dying? ie. Any popping sound? Any smoke or strong burning smell? At best, you could have a loose tube. At worst, you could have blown the transformer, but highly unlikely lah.

That's not exactly right, also not exactly wrong
Cathode bias uses the cathode current to derive a bias current, so it appears to self bias
and allows user to swap tubes without re-biasing.
This method although convenient, does not squeeze every bit of potential out of the tube.

Fix bias has a fix bias current supplied to the tube
usually optimized to extract the max performance of the given tube model.
So since the bias current is fixed, you cannot anyhow swap tubes
as different tubes bias differently.

Read more about tube biasing here
http://www.aikenamps.com/WhatIsBiasing.htm

 

[J.custom]

Thanks for the interesting thread! Enlightened me I still haven't bought my tube yet though:/

 

[dodgethis]

=Ratboy

Actually, fixed bias amps can have their bias adjusted, assuming a variable resistor is used in the bias circuit. Some companies, like Mesa Boogie, use a fixed resistor and forces the consumer to buy their own power tubes. Smart owners immediately swap out this fixed resistor for a variable one to make their tube buying a lot more flexible.

The naming 'fixed bias' IMHO can be rather confusing at times. It means fixed because once set it at that level, it will stay at that level unless you adjust the variable resistor again. However, it should be noted that the bias can drift a little bit, depending on how much the power tubes are burnt it.

Also, cathode biasing has one very big flaw, in that you cannot dictate the bias value that is optimal for the power tubes that you have in your amp. To add on to ratboy, cathode biasing can also squeeze out too much potential out of the tubes and leave them with a very short lifespan, which was the case with the previous generation Vox Custom Classic amps.

 

[CyanideJunkie]

@RatBoy & dodgethis:

Thanks for the heads up! First time I've heard of the 'tube potential' issue in regards to cathode-biasing.

Thanks for the interesting thread! Enlightened me I still haven't bought my tube yet though:/

Aiyo, don't dilly dally! Your amp collecting dust liao lol!

 

[J.custom]

Haha wanted to drop by ytd, but ended up hanging out at sound alchemy till 7.30 lol. Do you stack the dist of your amp with any of your ODs?

 

[dodgethis]

Because I can't edit my post. The proper terminology should be 'biased too hot or too cold'.

 

[CyanideJunkie]

Haha wanted to drop by ytd, but ended up hanging out at sound alchemy till 7.30 lol. Do you stack the dist of your amp with any of your ODs?

Yeah, I usually stack it with my AC Booster for rhythm and kick in my Andora's Box for lead tone. Fat and searing is probably the best way to describe the result haha.