* WARNING * WARNING * WARNING *

Amps contain lethal voltages, even when turned off and unplugged. This amp has bleeder resistors that SHOULD drain the filter caps within about a minute of the amp being turned off. However, it is possible that lethal voltages remain due to broken circuit traces, or other mods and or mistakes made while modding. Therefore, CHECK THE AMP FOR STRAY VOLTAGES before sticking your hands in it. If you cannot read a schematic well enough to figure out where and how to check, DO NOT WORK ON YOUR AMP.

Proceed at your own risk!
These mods have been tested by me. I'm a hobbyist, not a professional. Let me repeat that: I'm a hobbyist, not a professional. Some of these mods I made from seeing other amps and trying similar circuits, some of them I just designed and tried. Perform any mods AT YOUR OWN RISK. The mods below roughly describe how to do this; you NEED TO READ THE SCHEMATIC. I'm assuming you know not to run B+ through a pot and which side of the coupling caps have B+ on them.

YOUR SAFETY IS YOUR RESPONSIBILITY!!!

Here is a video demo if the amp now:

Here's a sumary of v2 of the mods:

I backed out the previous mods I had made. The amp sounded like it had a wet blanket over it and wasn't touch responsive. I never played it. I figured I might as well make some serious mods to it because it was collecting dust.

I like the amp so much now that I took it to a gig. It's incredibly touch responsive and LOUD.

Here is a summary of the mods. I've attached a schematic with the current mods. I'm not going to provide step-by-step instructions; if you know what you're doing, you can easily figure out how to make these mods from the summary and schematic. If you don't know what you're doing, you'll destroy your amp, or worse, electrocute yourself or cause a fire.

TL/DR: Don't mess around with amplifiers if you don't know what you're doing.

v2B - The most significant thing done here is move v2b to the other side of the tone stack, as a recovery stage. This involved breaking the trace under the board, which is located under the tube socket. Rather than remove the tube socket, I shined a light under the socket and marked it on the top of the board. Using a drill bit attached to a dremel, I drilled until the trace between pin 1 and 7 was broken. It took two holes to get it right. You might seek out a great image on the 'net that someone put together showing the traces under the board superimposed on the top of the board. Unfortunately, but understandably, this image doesn't have the trace under the tube being shown.

Other traces needed to be broken in order for the v2b triode to be configured as an ordinary gain stage. I don't recall which spots I needed to do this exactly, but there was one underneath the board. To get to it, the board needed to be removed. I used a cutting wheel on the dremel to cut the traces. I found spots where I need to connect to the B+ as well as the ground. Some holes ended up with two components in them. You'll figure this out if you do these mods.

The input of the tone stack has been rerouted to the grid of v2a and the output of the tone stack (vol) goes into the grid of the newly freed v2b.

See the schematic for the conections.

v2a has all the stock silliness removed and has a 1M grid leak resistor. Going into the grid (out of v1b) is a 480pf/520k in parallel into a 470k. All of these are configured by standing parts straight up into the board in existing holes and solder the tops together.

The cathode of v1b has a 10k resistor. There is a 2.2k/1uf that is switched in and out using the old 2w/5w switch (makes sense, since it gets MUCH louder with the added gain). I'm not crazy about the overdriven sound, but it's usable.

Since I reconfigured the input to v2a, the original bright switch was no longer used. I broke the trace (on top of the board) and used it as a bright switch (120pf) across VR1. It's currently too bright, and I may lower the value at some point so it's more usable.

As far as tubes, I found the JJ 12AX7 works best in both sockets. For an output tube, I went with a 6550. A 6L6 was my next favorite. An EL34 and 6V6 were tried, but I didn't like either. What I'm finding is that a single-ended output tube driven to overdrive sounds splatty and not musicial. The 6550 doesn't quite get into overdrive on this amp, even when it's dimed.

I changed the cathode resistor on the output tube. The stock one is a 250 ohm and 2w. Although I measured just under 2w of dissapation with the stock 250, it was starting to get brownish. I went with a 250 ohm 10w, which makes the output tube run a little colder and the 10w resistor can certainly handle the power.

Here is the preamp schematic as it exists now in my amp:

* WARNING * WARNING * WARNING *

Amps contain lethal voltages, even when turned off and unplugged. This amp has bleeder resistors that SHOULD drain the filter caps within about a minute of the amp being turned off. However, it is possible that lethal voltages remain due to broken circuit traces, or other mods and or mistakes made while modding. Therefore, CHECK THE AMP FOR STRAY VOLTAGES before sticking your hands in it. If you cannot read a schematic well enough to figure out where and how to check, DO NOT WORK ON YOUR AMP.

Proceed at your own risk!
These mods have been tested by me. I'm a hobbyist, not a professional. Let me repeat that: I'm a hobbyist, not a professional. Some of these mods I made from seeing other amps and trying similar circuits, some of them I just designed and tried. Perform any mods AT YOUR OWN RISK. The mods below roughly describe how to do this; you NEED TO READ THE SCHEMATIC. I'm assuming you know not to run B+ through a pot and which side of the coupling caps have B+ on them.

YOUR SAFETY IS YOUR RESPONSIBILITY!!!

Standby and 2w/5w Switch

Most of the mods are in the preamp, with the exception of adding a 22uf 500v filter cap for the screen between R7 and R24.

I disabled the standby switch and 2w/5w switch, repurposing them for other features. Remove the standby wiring and add a jumper on the board. The standby is not necessary on this amp, and it pops anyway.

Same with the 2w/5w switch, just move one leg of R22 so it is permanently connected to the screen supply node (and add a 22uf 500v filter cap to it - probably optional).

Schematic

Schematic source is from JetCity Picovalve - markings over the top are mine. Google has several links to original. Click for full size.

V1

Starting with the first preamp tube, v1:

Remove both local feedback 22pf caps on the tube itself, c1 and c3.

Why: These caps feedback high frequency signals to the grids of their respective triode, canceling them out. This reduces some of the brightness of the amp. Note that if you just remove these and do not reduce the gain of the second stage, you will get undesirable high frequency oscillations.

Using the 2w/5w switch, toggle between a .47uf and 22uf bypass cap across R5, the cathode resistor for v1a. This will pop when engaged. If the popping bothers you, wire a 50k resistor between the bottom of each cap and ground.

Why: Using a cathode bypass cap allows certain frequencies to pass through the tube. The .47uf allows more highs, while the 22uf is a full bypass, resulting in a fuller sound. In heavily overdriven situations, the 22uf can be too much and result in a "woofy" tone.

Change C2 to a .022uf.

Why: C2 is a coupling cap. The .0022 that is there prevents some bass frequencies from passing on to the next stage. A .022uf will allow more signal through. It is worth experimenting with this value cap, or NOT changing it.

Add a filter after C2 consisting of a 470k resistor bypassed by a 470pf cap.

How: Just after C2 (lift up one leg - the one closest to the input jack), insert a 470k resistor with a 470pf cap across it. Run the junction of those to where you lifted the leg of C2.

Why: The 470k resistor will filter some highs off and also reduce some gain. The 470pf cap will allow frequencies higher than the cutoff frequency the 470pf cap to pass by.

Change C7 to a .001uf

Why: C7 is a treble bypass cap, allowing high frequencies to pass by the gain control when the gain is turned down. Unfortunately, the value that is there (220pf) results in a very thin/wimpy tone when the gain is turned down. A .001uf allows more frequencies to pass, allowing the amp to keep its fullness as the gain is turned down.

Change R9 to a 10k.

Why: There is a lot of gain already in the preamp. Changing R9 to a 10k reduces the gain of that stage significantly.

Using the standby switch, wire a 3.3k resistor from the top of R9 to the switch, and the other end of the switch to ground.

Why: This is a really nice boost switch as it restores most of the gain lost when R9 was changed to a 10k. This will pop when engaged. Turn the boost off when you want a clean sound, and turn it on when you want more gain.

Drill a hole and mount a 500k audio taper pot. A push/pull is nice as it lets you add another switch for a cathode bypass cap later.

How: Lift the leg of C5 (on the other side of the plate of v1b) and wire that to the top of the 500k pot. The bottom goes to ground. The wiper goes to the leg where C5 was lifted.

Why:There is a pretty hot signal going to the next preamp stage and overdriving it. This lets you control the level of it and clean up the amp if desired. Also, having the push/pull switch is nice.

V2

Moving on to V2. There is a lot of talk on the 'net about Jet City "leaving off the grid leak on V2a. This isn't the case. They did configure it with a 2M grid leak, which was probably a mistake, but it's there. Let's reconfigure this and the "bright switch":

Remove R12 (old 1M)

Change R14 to a 470k

Change C13 to a 470pf (bright switch bypass cap)

Put a 470k from ground to pin 2. This is now the grid leak resistor of V2a. You can run this to the junction of R14/C13, or right to pin 2.

Remove the 220uf cap that is across R11. This cap (labeled C12 on the board) is not on the schematic. It is located just to the left of R11. This cap adds A LOT of low frequencies back into this stage and results in a lot of the woofiness heard with the stock amp.

On my amp, I made this change to support a mod that I took out. You should probably NOT make this change, but I'm leaving it here for the sake of completeness. Lift the bottom of R11 (ground) and insert a 47ohm resistor to ground and to R11 (teepeed: /\) - again, this isn't necessary and probably is not audible.

Using the push/pull from the 500k pot you added, add a 2.2uf on the switch across R11 (above the 47ohm above, or ground if you didn't add the 47ohm resistor).

Why: This is a switchable cathode bypass cap. It sounds nice when engaged when the amp is cleaned up. It also adds a bit of depth when the gain is cranked.

Change the mid cap, C10 on the schematic to a .01uf.

Why: This changes the way the EQ works and responds. I was hoping to add some adjustability to the bass control. I can't confirm that this change is audible - try it, or not.

Tube Selection

In V1, I'm running a JJ ECC83S (12ax7). Several people have reported good results using a 12AT7 to knock down some of the gain. I've found with the mods I made above that using a 12ax7 worked best.

In V2, I'm running a ECC823. This is a hybrid tube, which has a single triode of a 12ax7 on the "a" side and a single triode of a 12au7 on the "b". The rational is that the "b" side is configured as a current driver (cathode follower) to drive the tone stack and a 12au7 is better suited to the job. I'm not certain there is much audible difference, but theoretically it should be better. Worth experimenting with.

I'm running a GT6550 as the power tube. The 6550 sounds MUCH better than any 6L6 I tried in the amp - really made it sound bigger and cleaner (when the preamp is cleaned up).

Clips of the amp

Clean

Dirty

This was taken just before adding the 2.2uf across R11.