Jet City JCA20HV Valve Change (how to questions?)

not_the_dj

I've been gigging every couple of weeks for a couple of years with my JCA20HV and I'm starting to think I should replace the valves.

Are Karltone and ampvalves the places to shop these days?

It's still running the stock valves, and I bought it second hand so I've yet to look at replacing them....is it a case of removing the chassis or just the back cover?

Any recommendations on the valves to buy, I'm not after any drastic change in tone, just a bit of preventative maintenance as it's my main gigging head.

simonk

I’d take the chassis out as there’s a 12ax7 (V1 from memory) tucked right in front of a transformer and it’s a bugger to reach still in the shell.
I put JJ’s in mine and they sounded fine.

tekbow

you talking pre-amp or power valves?

not_the_dj

Thanks @simonk ;

How easy is it to take the chassis out?

(I've searched but couldn't fine any photos or vids online, and I'm sat day dreaming about amps whilst at work ;-) )

simonk

A few screws IIRC and a bit of jiggling as it’s probably stuck to the tolex in places.

ICBM

If it hasn't started losing power or tone then don't change them, although it's worth having spares. Valves don't necessarily need replacing 'regularly', just when they wear out or fail - and in fact, the failure rate is worse with brand-new ones than with ones that have been used for a while, at least until they get really knackered.

This applies to preamp valves even more than power valves, so if you're feeling that it's not as loud or as good-sounding as it used to be, change the power valves first. This is an amp which will need the bias checking and probably adjusting for new power valves, so you'll need to know how to do that.

not_the_dj

ICBM said:

This is an amp which will need the bias checking and probably adjusting for new power valves, so you'll need to know how to do that.

I've been confused by that before with this amp as the manual states you don't need to have it biased.

In fact I started a thread soon after getting the amp and @digitalscream asked Doug at Jet City Directly about it....

http://www.thefretboard.co.uk/discussion/56428/


*Edit* P6 in the manual: http://www.synthmanuals.com/manuals/jet_city_amplification/jca20hv/owners_manual/19f8da_af5c6098cf5d721948c31c5d72006c8b.pdf

JMP220478

on my old 20v I was able to remove front and back panel easily to facilitate valve changes - and as the front panel wasnt particularly pleasing to my eye I replaced it with automotive grill mesh for a poor mans soldano look .. great amps for stupid cash ..  only sold to buy a custom 22 for fx loop n two channels ..

ICBM

not_the_dj said:

ICBM said:

This is an amp which will need the bias checking and probably adjusting for new power valves, so you'll need to know how to do that.

I've been confused by that before with this amp as the manual states you don't need to have it biased.

In fact I started a thread soon after getting the amp and @digitalscream asked Doug at Jet City Directly about it....

http://www.thefretboard.co.uk/discussion/56428/


*Edit* P6 in the manual: http://www.synthmanuals.com/manuals/jet_city_amplification/jca20hv/owners_manual/19f8da_af5c6098cf5d721948c31c5d72006c8b.pdf

In my opinion the way it's phrased in the manual is deliberately misleading.

Yes, it's a fixed-bias amp, which means - somewhat counterintuitively - that the bias may be adjustable. Whether or not it needs to be depends to some extent on how conservatively it's set at the factory, but with the variability of modern valves it is not wise to assume that it won't need to at least be checked.

It's *cathode* biased (AKA 'self-bias') amps which do not need to be adjusted. 'Fixed bias' is so-called because the bias voltage is fixed by an external supply circuit and does not vary with valve current, unlike cathode bias. The bias supply may or may not be adjustable, but the Jet City is - this is the adjuster:



Whether you want to use it is up to you, but it's there for a reason.

p90fool

I've had quite a few 20w Jet City amps and they are biased extremely cold from the factory. 

None of the EL84s I've ever bought would've pushed any of mine over the edge, but I'd still be uncomfortable about throwing valves in there without checking bias. 

It's an odd one tbh, Doug White insists there's no real need to bias, and we all know he's kind of wrong in principle but it seems to work in practice. 

Me? It's a gigging amp, just do it, it only adds ten minutes to your valve changing time. 

clarkefan

I don't think these amps can be biased without using the transformer shunt method, which I've been told a thousand times is proper dangerous and only for pros (which I am definitely not and have never done). 

They need to be modded to be able to take the relevant measurements the "safe" way (bottom of page, cutting connections, drilling the board and adding 1 ohm resistors off the EL84s):

http://markweinguitarlessons.com/forums/threads/jet-city-jca20h-choke-mod.25897/

Didn't we discuss this very thing some months back @ICBM  ? If I remember right you thought it would only increase volume or something? Damn, will do a search and come back...

ICBM

clarkefan said:

I don't think these amps can be biased without using the transformer shunt method, which I've been told a thousand times is proper dangerous and only for pros (which I am definitely not and have never done).

Don't ever use that method - it's very dangerous and also inaccurate.

You can do it by the transformer voltage drop method, which is safer and very accurate, although still dangerous by comparison with any cathode-current method.

Easiest and safest is to use a plug-in adaptor between the valve and the base, and measure the cathode current externally.

clarkefan said:

Didn't we discuss this very thing some months back @ICBM  ? If I remember right you thought it would only increase volume or something? Damn, will do a search and come back...

In general if the bias *is in the right range* then it only really affects the volume slightly, but that's not the problem - it's if it's outside the right range, either too hot (bad for the valves) or too cold.

It's certainly possible that you might not need to adjust it if the amp is biased cold - but it's also possible that it will be too cold, which will produce a nasty distortion that you don't want.

As p90fool says, it may not be absolutely necessary but it is still good practice.