8 October 2013

JCA22H Scoop (aka SRV) Mod

Back for ANOTHER instalment. So the third switch along in the last post is a mid scoop mod. HOWEVER that is not all that is going on here. We need to keep in mind that what was the crunch channel is now able to get pretty clean. I'm not sure how much I liked switching this mod on with more gain... anyway!

The scoop mod is again stolen from other Soldano products. Saying that... it's a pretty old concept and probably has a chapter of its own in the old "RDH4" as it's known (A kind of valve nerd bible). 

The mid scoop is activated by grounding a capacitor between two resistors which are bypassed by a second smaller value cap. This is called a Bridged T Filter. The top cap is usually small allowing high frequencies to the skip the filter. The lower cap is large enough to bleed off relevant frequencies but not large enough to bleed low end. Low end is attenuated as normal through the resistors. By disconnecting the cap from ground, we end up with a normal RC (Resistor Capacitor) Network. Highs skip the resistors and everything else is attenuated. The values of the components determine the effect.  

I wanted to fit this into the amp is a nice way. Luckily the PCB has lots of spots with space between tracks. After removing the board I drilled holes for two eyelets and removed the original parts I planned on replacing. 

Here are the eyelets put into place. It is awkward doing this with a populated PCB so a lot of care is needed! Don't do it if you shake around like Scooby Doo in an old fairground. 

Here is the T filter in place. The yellow cap is the Bridging cap, and the resistors and the little blue cap for the T. The purple wire goes off to the DPDT to switch the mod on or off. Now in the background of this photo you can also see a green wire which is connected to ground that goes off to the switch and another purple wire.

These wires form the next part of the mod. The green wire provides a ground point for filter when the switch is in the on position but it also acts as ground for a 22uf capacitor which when active, sits across the cathode resistor of the second gain stage in the Crunch channel. The reason I added the ground here is to keep this cap grounded at the base of that stages cathode resistor when in use.

 The 22uf Capacitor creates more gain at that stage for almost all audible frequencies. The graph below is just show as an example of a 22uf Cathode Bypass Cap on a stage with a 1M grid resistor. The effect of this cap coupled with the scoop mod means that not only are the mids scoop out but the lows and highs are also boosted which gives are really nice warm clean tone. I like this mod!

6 October 2013

JCA22H Crunch/Clean and Bright Switches

Next up! Adding the SLO100 style Crunch/Clean and Bright/Normal switches to the Crunch channel of the JCA22H.

These work by dropping in place of R5 which is just to the side of the Crunch channel gain control of the amp. The output of C2 feeds into the On-On DPDT switch via the purple wire on the left. When that switch is down the signal goes through that brown 475K resistor mounted vertically on the switch and then goes across to the next switch. This switch is an On-On SPDT with another 475K resistor. The signal passes through this second resistor and back to the DPDT were the white cable feeds the signal back to the PCB and to the Crunch gain pot. At this point there is an added 39K resistor connected between the signal and ground also by the DPDT switch. The two 475K resistors in series and the 39K resistor to ground form a "potential divider". This potential divider decreases the level of the signal going into the Crunch gain control. Reducing the gain of the channel at that point. 

When the switch is flipped up into Crunch mode the first 475K resistor is bypassed and the 39K resistor is no longer connected to the signal. This means there is now only a single 475K resistor in series with the Crunch gain control and no potential divider. Increasing the gain of the channel. 

The bright switch is very simple. It switches in and out a 470pf capacitor across the second 475K resistor. That resistor is always in circuit unlike the other resistors. The capacitor creates a way for high frequencies to bypass the resistor so the tone is brighter. Simple! 

This idea was directly stolen from the SLO100. If you are going to take ideas from any amp... you may as well do it from the SLO. Enough other people have!

The last switch on the right is the bright switch for the Overdrive channel. All this does is switch the bright cap that is usually across the pot (C6). I swapped both gain pots for 500K's so the bright cap in my amp is 1000pf. When the switch is down the amp is in its 'normal' mode so the capacitor is allowing high frequencies to partially bypass the Overdrive gain control. When the switch is up the cap is disconnected. In the SLO100 this is called the switchable Haynes mod.

This Bright switch is different to the other. The other bright switch is across a fixed resistor which means in effect it always works. The Haynes mod however doesn't. As you turn the gain pot up and decrease the resistance the signal is passing through, there will be a point where the frequencies not skipping through the cap will really be just as loud as the ones that are. At this point the cap is no longer adding any benefit to the highs, so switching the cap out will not dull the amp. For this reason this kind of switch works better at lower gain settings where the effect of the cap is much more noticeable.

The other DPDT switch we will leave for another day!

5 October 2013

Jet City JCA22H Choke Mod

Another common mod of the JCA22 or JCA20 is the 'Choke' mod. A choke is an inductor that sits in the high voltage power supply and aids noise filtering. Noise in the power supply can be things like rectification transients and other horrible stuff. In some situations a poorly filtered power supply can ping all kinds of horrid noises into your tone. Nobody likes horrid noises in their tone! The power supply is the heart of the amp can affect the feel and tone as much as any resistors or controls your guitar signal might go through. The high voltage supply in an amplifier is usually referred to at the B+ or HT (High Tension). The terminology comes from other areas of technology when valves were more widely used.

Power Resistors used to make filters are fine. Lots of amps have them. The 5150 has one for example. It's a bit of a brutish way to squash noise. All frequencies in the HT will see the resistance. With a choke the resistance met by a particular frequency will depend on what the frequency is itself. It's a bit like going out to a bar. You head over to one place and they might be frisking everybody and holding everyone up and nobody is getting through but that means there are less people inside (Resistor). Or you go elsewhere where you can walk straight in, unless you're already a mess and then the door staff are going to keep you out! (Choke)

R52 is where the power resistor was. I removed the PCB and then removed the resistor. Another difference between the resistor and the choke is that when you crank the amp and play really hard the valves will start to demand more current from the power supply. If there isn't enough current then HT voltage will drop... this is voltage sag.  As the voltage drops the tone of the amp changes because the HT voltage usually has a nominal relationship with each gain stage to give you particular feel and tone. The power resistor has a large series resistance which increases the effect of sag. The choke however has very low series resistance and therefore lessens the effect of sag. The choke will therefore also increase the HT voltage after it in any circumstance. This is why in some amps when a choke is added as a modification the screen grid resistors on the power valves also need to be changed to account for the voltage increase at the power valve screen supply.

Here is the choke mounted just behind the power transformer. I opted for a 5Henry @ 75mA choke. Henry is a measure of inductance. The mA rating tell you what current can be drawn through the item. There will also be a maximum voltage that can pass through the choke too!

 Here it is soldered in place. Done!

The keen eyed will have spotted that in that last photo there are some extra blue bits around the rectifier. While I had a lot of 10nf 2kV ceramic caps knocking about, I thought I'd add them here. 10nf caps across each rectifier diode help to share the voltage being rectified (turned from AC to DC) between the diodes and to suppress switching transients. Not sure if I can hear a difference... I just did it and thought I'd explain why! If the power supply is the heart of the amp, this is like giving it a wheatgrass smoothie every morning.

3 October 2013

Jet City JCA22H Depth Mod

Decided to do a BUNCH of mods to the JCA22 all at once... probably a bad move if I want to know the result of each mod individually but I had a bunch of stuff kicking around from projects I never started and left-overs from other things I've built over the years... so while this thing is in bits I'm going to let loose on it and then see what happens.

Starting with the Depth Mod...

There is a lot of info out there on this mod on different forums so Google is your friend. The Depth or Resonance control is a great little gizmo for pumping up that low end up like a Sir Mix-a-lot music video. This is the way I have decided to do it.

I have added a 1uf capacitor in series with the speaker jacks and the pot. This will keep any DC gremlins from getting up in my pot and making scratchy noises that I don't want when I turn the dial. I mounted the extra cap on some tag strip screwed down under one of the PCB mounts. Check it!

I picked the 1uf because I had a bag of 1uf caps and I've done the same mod with that exact same value before in some other amps, so whatever. The wires to the jacks and to the pot will also be soldered to the tag strip.

Above is the pot. It's actually one of the original gain pots from the amp. I changed those pots for 500K's as part of other mods (which I will write about later) so I decided to recycle one here for the Depth. It also means that the PCB will JUST about squeeze back underneath this pot. A new bigger pot might not be the best fit here and would also be jammed right next to the Standby switch! I'd stick a big pot on the back.

The white wire will be going back to the PCB to feed what is known as the Negative Feedback Resistor. The NFB resistor limits the amount of negative feedback the amp is getting. In this amp it is 47K (If I remember correctly while typing this!). NFB is also variable at different impedance taps on the output transformer secondary, so using the 8ohm or 16ohm output should get you different results. I intend to try both and see which I like best but 8ohm is normally where the NFB comes from in this amp. Using the 16ohm output will increase the amount of NFB going back to the amp.

The Presence and Depth controls both function using Negative Feedback. In this situation the NFB is a signal from the output of your amp, which is fed back into the amp at the Phase Inverter (PI). This signal is actually 'out of phase' with what is going on at the PI (That is why it is called negative and not positive!) so when out of phase frequencies in the NFB meet their counterparts at the PI they tend to subdue each other.

When you turn your Presence control up you actually remove high frequencies from the NFB signal. The means there are now less out of phase high frequencies in the NFB to damp down those same frequencies in the PI. The effect is a perceived high frequency boost. The Depth Control works the same way, but with the low frequencies. Higher frequencies will pass through the 4700pf cap and skip the pot, but the pot will impede lower frequencies more as it is turned up. Thus reducing damping on the low end.

All this also depends on how loud your amp is. The louder the amp the more signal you get at the speaker jacks. Hence the stronger an NFB signal you will get. This is the reason why Depth and Presence (or NFB based controls in general) will not seem to work at low volumes until at higher points on the dial. Once you start to crank the amp up and the NFB is stronger, these controls will become more effective at earlier points on the dials. So when you start gigging and you take your amp out, be sure to reevaluate your use of those dials depending on the volume difference between gig level and practice level. Cranking the Presence at high volume could take peoples heads off with treble and you might sound muddy with a cranked Depth pot!