NAD + Need some opinions on tube swapping

[GuitarIv]

Cheers guys,

so, some of you may already have read about my decision to get a Jet City JCA50H, today it finally arrived and so I give you a New Amp Day thread!  :D

I don't know where to start, I guess first of all I really have to give big props to Thomann for processing my request in such a fast manner, if my bank wasn't so lame and the money would have been transferred faster I'm sure I would have gotten the amp last week.

The packaging was solid and the head was secured inside in a way that only very rude behaviour could have damaged it, a Footswitch, a users manual and a Jet City sticker were included.

First impression of the head: solid and sturdy built, very tidy and nothing coming loose or indicating that it is "made in China".

So I plugged everything in, fired it up and started playing from Clean to Distorted, just kinda to go from good to better.

Cleans: Well, if you expect pristine Fender Cleans, this is the wrong amp for you. The sound isn't bad, it's a thousand times better than anything a 6505 head could ever give you, but it isn't exactly something you would be expecting from a Fender. Still way better than the cleans on the JVM 205 and somewhere near the EVH 5150. Very usable, throw in a Delay Pedal into the Effects Loop and there you are.

Crunch: Woah. I can't think of another word right now, the crispness this thing delivers is amazing, from Blues to Rock to all the AC/DC Hard Rock goodness this head slays. Very dynamic and responsive to pick attack and your playing, reacts great to the guitars volume and tone pot and note separation is superb.

Overdive: That's what I'm talking about. This thing spits mids, sounds warm and chunky, clear as a tear and sits somewhere in the old school JCM 800 Thrash Metal territory. A tubescreamer is a must as the Overdrive Channel isn't tight enough on its own, but as soon as you put the little green box in front everything works like a charm. If you're looking for a 6505 modern distortion tone then this won't suit you. I don't know how to explain it, the saturation the Jet City delivers is just something entirely different. As said it responds to your playing and thus it is unforgiving. I love it.

So to sum everything up: I can't believe I've gotten such an amazing amp for 300 quid. This is a bang for the buck and a huge improvement over my Peavey Valveking.

Cons: as with every piece of gear, there are some downsides:

This thing is bright. You want to keep the Presence and Treble Knobs on 5 or lower, otherwise it can get piercing. This is most likely the reason why a lot of people complain about the missing Depth Control Soldanos usually come with.

Fizziness: This is the reason I'm asking for some insight on changing tubes on the amp. I have read a lot of reviews about the JCA50H where people suggested to swap out the stock factory Chinese tubes for better ones. The fizz is not present in a way where dimming the highs would remove it, it's just constantly there. In the background. And it's a bit disturbing.



So my question would be: what kind of tubes should I get? Something that tames the brightness of the amp and is better quality than the stock Chinese tubes would be nice. The Amp has 5 12AX7s in the preamp section and two 6l6s in the power amp section. I read some threads about changing V1, V2 and V3 and how every of those positions affect your tone in a different way, I'm not an expert so some insight would be appreciated.


Cheers!

[Kiichi]

Congrats on the NAD! Cool little insight on the first impressions, read it with great interest.

Concerning the tube swap I am not an expert either, though I have done a bit of swapping myself. What I can tell you is that the V1 will have the biggest effect on the tone. That is where the signal hits and everything after it uses what it produces. So if you happen to have a 12Ax7 laying round you might just wanne pop that in the V1 and see how much it changes. If that does not cure the issue go on the the other stages as well.

[Toe-Knee]

v1 - tungsol RI
v2-4 JJ ecc83s

In the power amp id go JJ 6l6gc for a grindy thick tone tungsol for a brighter tone (not what you want right now but with different preamp valves it could work)

also for true cleans you need a small mod.

[GuitarIv]

Thanks guys!

As far as my understanding goes the preamp tubes have a bigger effect on the overall sound than the poweramp tubes?

I've read the users manual and it says I can safely swap tubes without having to adjust the bias as long as it's the same manufacturer and type of tubes. Does this mean as soon as I get JJs I will have to adjust the bias or is this obsolete?

Toe-Knee: thanks for your input, I always appreciate your thoughts very much. For the cleans I think they will do so far, I like a little bit of dirt on them. Anyway, are there any EU suppliers for tubes? Right now my only reference is Thomann and they have JJs, Mesa Boogies, Sovteks, TADs, AMTs, Groove Tubes and Engl Tubes for the 12AX7s, but no Tungsols...

[Toe-Knee]

Thanks guys!

As far as my understanding goes the preamp tubes have a bigger effect on the overall sound than the poweramp tubes?

I've read the users manual and it says I can safely swap tubes without having to adjust the bias as long as it's the same manufacturer and type of tubes. Does this mean as soon as I get JJs I will have to adjust the bias or is this obsolete?

Toe-Knee: thanks for your input, I always appreciate your thoughts very much. For the cleans I think they will do so far, I like a little bit of dirt on them. Anyway, are there any EU suppliers for tubes? Right now my only reference is Thomann and they have JJs, Mesa Boogies, Sovteks, TADs, AMTs, Groove Tubes and Engl Tubes for the 12AX7s, but no Tungsols...

I use hotrox. And i've found in most cases the power valves make a drastic difference. And yes you should rebias really.Its a JCA50h right? Most JCA's that i've come across come biased cold so you can probably get away with it but its not the best thing to do. 

The bias could also be causing a good portion of the fizz.

Preamp valves you can swap without any adjustments

[GuitarIv]

Found the Hotrox site  :)

Ok, so my next question would be: how does one exactly bias an amp? Any special tools I need for this or any chance I might get fried in the process? Only thing I remember is a cold biased amp will extend the valves life and a hot biased one will sound better but consume the tubes faster.

[GuitarIv]

So, I've done some research and from the information I gathered all Jet City Amps are fixed bias, so switching poweramp tubes will require an adjustment. They have "an internal variable resistor that allows for fine adjustment when desired". Is this the so called Bias Pot?

So basically I need the right tools to measure the bias value between pin 3 and pin 8 on the socket, calculate the right value and then trim it to the value of choice whereas 50% will be "cold" and 70% will be "hot". This can be done by turning the bias pot with a Screwdriver.

Anyone mind to confirm this?


I won't attempt to swap out the Power Tubes on my own right now, at least not before I haven't done more research on the subject. I'll try out different Preamp tubes though as those don't need an adjustment from what I gathered.

Help is appreciated as always, thanks!  :)

[Kiichi]

Yeah, you are going about it in the right way. Powertubes should only be done when researched properly.

With the preamps you can do them without issue. The way I would go about it is focus on the V1, cause that, as I said before, is the baseline, the center of the sound.
It is very important that you get this one right. The character of the distortion can change really dramaticly, especially how it reacts to being pushed by a boost or OD.
So there pick something that displays the general character you want. JJs for example I find to be nicely full, round and roary, kinda like the Crawler among tubes.
After you have the right base character use the following stages to shape it further. The V1 is the meal, the others are the spices.
I for example like a JJ in the V1 of my Orange TT, but I need to follow it with a Tung Sol or Tube Town, which are mich brighter. In V1 I find them to be too trebly and hard for my taste, especially with the way I boost, but in V2 they add in air, sparkle and depth to the JJ.

Make a educated plan, get tubes and then try around IŽd say. Try every tube you can get your hands on (even if it is just borrowing a tube a mate got lying around) and try them in V1 to get a sense of what they do. Once you got the right V1, build on it.

[dave_mc]

Nice score :D

I haven't got round to swapping my tubes. I find using eminence v12 speakers tame a fair bit of the fizz etc., but most people seem to say that swapping tubes helps a lot, too, so it's probably worth considering. Toe-Knee will know way more than I do regarding swapping the tubes.

[GuitarIv]

@Kiichi: I will certainly get myself a bunch of preamp tubes and do some experimenting, after reading lots of great articles on the whole subject today this seems to be a safe and easy first hand solution. However first of all I'll get myself the 2*12 Palmer Cab loaded with Vintage 30s for the Jet City so I have a reference point as my Laney Cab loaded with Celestion 70 Eighties I'm using right now doesn't really do it for me, also I will have to open the cab up and recheck the wiring as there are some weird sounds coming out of it once in a while and being the third owner of the thing there might be some wear.

@Dave: Actually I'm still waiting for Toe-Knee to chime in as he knows his stuff  :D

Anyway, I have a feeling this head will finally give me the opportunity to explore the world of amplifiers in depth, I found out how to swap Power Amp Tubes and bias an amp today and downloaded the JCA50H wiring schematic, lots of great articles and forums on the Internet and in the end the whole subject gets a lot less scary once you start to do your research and understand how those things work (and what you have to do in order to not get fried  :lol: )

Cheers!

edit: I think I probably should ask someone to move this thread to the Tech section  :P

[Toe-Knee]

@Kiichi: I will certainly get myself a bunch of preamp tubes and do some experimenting, after reading lots of great articles on the whole subject today this seems to be a safe and easy first hand solution. However first of all I'll get myself the 2*12 Palmer Cab loaded with Vintage 30s for the Jet City so I have a reference point as my Laney Cab loaded with Celestion 70 Eighties I'm using right now doesn't really do it for me, also I will have to open the cab up and recheck the wiring as there are some weird sounds coming out of it once in a while and being the third owner of the thing there might be some wear.

@Dave: Actually I'm still waiting for Toe-Knee to chime in as he knows his stuff  :D

Anyway, I have a feeling this head will finally give me the opportunity to explore the world of amplifiers in depth, I found out how to swap Power Amp Tubes and bias an amp today and downloaded the JCA50H wiring schematic, lots of great articles and forums on the Internet and in the end the whole subject gets a lot less scary once you start to do your research and understand how those things work (and what you have to do in order to not get fried  :lol: )

Cheers!

edit: I think I probably should ask someone to move this thread to the Tech section  :P

You have the general idea of biasing down.

However you measure the plate voltage at pin3 but you measure the actual bias voltage across a 1ohm resistor from pin 8 to ground. It isnt the most accurate method but it's the simplest.

again though this isn't something that should be done recklessly. Pin 3 will have around 480v on it which isn't something you want to mess with!

Eurotubes do a nice bias meter thats $99 i believe that measures both the plate & bias safely.

Once you have the plate voltage its time for some simple calculations but i'll go into that later :)

Focus on the preamp now.

V1 is your input & crunch channel gainstage & V2 is the lead channel (this is a semi educated guess)

Those two should be your initial focus.

The others whilst not as critical do still have some effect.

[Dave Sloven]

Get the DVD Tube Guitar Amplifier Servicing and Overhaul - Gerald Weber.

If you can't bring yourself to pay whatever it costs you can find the torrent online somewhere.

It has a lot of stuff in there, including how to avoid being shocked!

[Alex]

So, I've done some research and from the information I gathered all Jet City Amps are fixed bias, so switching poweramp tubes will require an adjustment. They have "an internal variable resistor that allows for fine adjustment when desired". Is this the so called Bias Pot?

So basically I need the right tools to measure the bias value between pin 3 and pin 8 on the socket, calculate the right value and then trim it to the value of choice whereas 50% will be "cold" and 70% will be "hot". This can be done by turning the bias pot with a Screwdriver.

Anyone mind to confirm this?


I won't attempt to swap out the Power Tubes on my own right now, at least not before I haven't done more research on the subject. I'll try out different Preamp tubes though as those don't need an adjustment from what I gathered.

Help is appreciated as always, thanks!  :)

No, typically fixed bias implies that it cannot actually be adjusted. If it similar to what Peavey does, it is a quite "cold" bias that is safe for power amp tubes without biasing.

[Dmoney]

So, I've done some research and from the information I gathered all Jet City Amps are fixed bias, so switching poweramp tubes will require an adjustment. They have "an internal variable resistor that allows for fine adjustment when desired". Is this the so called Bias Pot?

So basically I need the right tools to measure the bias value between pin 3 and pin 8 on the socket, calculate the right value and then trim it to the value of choice whereas 50% will be "cold" and 70% will be "hot". This can be done by turning the bias pot with a Screwdriver.

Anyone mind to confirm this?


I won't attempt to swap out the Power Tubes on my own right now, at least not before I haven't done more research on the subject. I'll try out different Preamp tubes though as those don't need an adjustment from what I gathered.

Help is appreciated as always, thanks!  :)

No, typically fixed bias implies that it cannot actually be adjusted. If it similar to what Peavey does, it is a quite "cold" bias that is safe for power amp tubes without biasing.

Actually, fixed bias can be adjustable. The 'fixed' bit just refers a constant bias voltage being delivered to the grid of the output valve, which in a JCA50H, Soldano Hot Rod, Marshall JCM800 and a million other amps... it is. Those are 'Adjustable' fixed bias amps. The Peavey 5150 is fixed bias but not adjustable, the dual rec is like this too. Cathode Bias is the common alternative and is not set using the supply of a fixed/constant voltage to the output valve grids.

EDIT. Like Tony said though, Pin 3 has one of the highest voltages in the amp on it. I use Pin 3 to do bias but it means poking around at over 450volts and doing some light maths. I would get the bias tool for ease if you want to be as safe as possible.   

[Alex]

Wow, they really should rethink the term "fixed" then, it's kind of misleading for most people with a non-technical background.