Jet City PicoValve schematic?

[steeve_a]

I picked up one of these at GC during the summer (thinking it was a once in a lifetime deal that I couldn't afford to pass up ). I have the THD Univalve schematic which is quite different from the Picovalve.

I will post any mods I come up with here. The thread I started on mods for the Crate V18 is up to 54 pages- 801 posts:

http://www.thegearpage.net/board/sho...d.php?t=464873

Thanks in advance!

Steve Ahola

[shan564]

Any updates?

[sirbergersworth]

Anyone have any luck finding some mods?
I am looking for one to take the preamp gain control and make it about 60% of the stock gain.

[shan564]

Well, you could probably do that by swapping the 12AX7's for some lower-gain tubes... maybe a 12AU7 on one side and a 12AT7 on the other side.

[Rhythmtech]

I just picked up one of these also. Trying to reduce the gain a bit too and make it a bit less muddy and open it up a bit. I found this video, the guys says he did a Bogner 1959 XTC mod by swapping a few resistor and caps. Seems like it's a bit more like I am aiming to get it to sound. Anyone have any ideas on exactly what he might have done?

[sirbergersworth]

I did try the tube swap and tried 12ay7, 5751, 12au7 with no luck really. Just didn't sound as good. I just want the gain knob more usable for me.

[steeve_a]

I did locate the original schematic drawn by THD which had been modified by JCA to include the Bright switch, however in doing that the 1M grid leak resistor for V2A was inadvertently eliminated. Adding a 1M resistor from V2/Pin2 to ground right at the tube socket allows the amp to work as it was designed.

For a no-solder mod some people have replaced V2 with a 12AT7 which seems to work better than the rest of the 12AX7 family. I had first tried extensive "tube rolling" in hopes of getting a decent clean tone but that did not work. Even with the lower gain preamp tubes the signal never lost its muddiness.

Steve Ahola

P.S. I will post a link to the original schematic. (I ended up rewiring my amp as a single-ended Trainwreck rocket which did require extensive cutting of the copper traces so it is not for the faint of heart. I call it my PicoRocket.)

[gammalord]

I've got the Picovalve sounding real nice, cleans included - but it took a LOT of work. Here's what I did:

1) River City Amps Picovalve mod. I don't know what is done specifically, but I believe a couple caps are switched out for different values in the tonestack. The amp was more responsive to EQ, less muddy (but still muddy), and more open up top. That awful pop on standby was reduced by 50% with this mod.

2) I have a tung-sol 6L6 for power (the Tung-Sol 6550 caused problems, amp would not come out of standby, weird popping, etc). And a 12AT7 in V1 and a 12AX7 in V2. This was a big difference. The AX7 in V1 was overdriving the AT7 in V2 and causing this unbearable grit. Now, headroom is respectable, and the grit is gone. Muddiness reduced as well.

3) I used the stock speaker cabinet which was underwhelming. There was a "honk" in the mids that sounded like I had a wah pedal engaged constantly. I swapped out the speaker for an EV Force 12 (similar to the EVM12L) and lined the back, sides, top and bottom with about 1" of polyfil sheets. This was the most significant improvement in tone. The mud is gone, the highs opened up, and lows dropped down nearly an octave. Clean headroom noticeably increased.

I know I'm changing several variables at once so it's not the most scientific approach - but the combination of these mods has turned the picovalve into a little screamer. The 2w mode is now useable and has really sweet harmonics coming through when the amp is in overdrive.

I'm disappointed that all this needed to be done to get the thing sounding good. I'm confused as to how the company could release a product so profoundly muddy (which is the main problem with this amp).

If I didn't have that speaker laying around, the combination of these mods would have exceeded (plus the initial purchase) the cost of buying some of the more well-regarded amps in the same category. That is frustrating. If I could do it all over again I would have gone with Egnater's Tweaker. But I was determined to get a good sound out of this and I did. Just took a lot of work.

[steeve_a]

Originally Posted by gammalord

I've got the Picovalve sounding real nice, cleans included - but it took a LOT of work. Here's what I did:

1) River City Amps Picovalve mod. I don't know what is done specifically, but I believe a couple caps are switched out for different values in the tonestack. The amp was more responsive to EQ, less muddy (but still muddy), and more open up top. That awful pop on standby was reduced by 50% with this mod.

I recommend the River City mods for anyone who wants to get a better sound from their Picovalve.

I used the stock speaker cabinet which was underwhelming. There was a "honk" in the mids that sounded like I had a wah pedal engaged constantly. I swapped out the speaker for an EV Force 12 (similar to the EVM12L) and lined the back, sides, top and bottom with about 1" of polyfil sheets. This was the most significant improvement in tone. The mud is gone, the highs opened up, and lows dropped down nearly an octave. Clean headroom noticeably increased.

That cab is a joke. Speakers need room to breathe. With a good speaker the cab would be handy if you didn't feel like bringing out the big guns. (I've had good luck with the Eminence Red, White and Blues for about $80.) Hello Music kept offering the Picovalve stack for $260 but I had to pass on that because it came with the cab.

I'm disappointed that all this needed to be done to get the thing sounding good. I'm confused as to how the company could release a product so profoundly muddy (which is the main problem with this amp). If I didn't have that speaker laying around, the combination of these mods would have exceeded (plus the initial purchase) the cost of buying some of the more well-regarded amps in the same category.

On the plus side the amp is built like a tank, with huge transformers. It is unfortunate that the original design by Andy Marshall at THD was screwed up so badly when JCA decided that they wanted to add the Bright switch. Especially since it could reflect badly on Andy and THD. The River Amp mods helps to correct those mistakes.

Steve Ahola

[Zingeroo]

The first thing I would look at is the preamp stages and their plate resistors.
I'd bet a pack of guitar strings they went with 220k plate resistors in an attempt to get more gain... but that also makes the amp lose a quick attack. If you've played a Mesa Rectifier you can feel that loosey-goosey feeling that I hate about those amps and when I played the picovalve that's what I found also, a loose, spongey, unresponsive feel.

[steeve_a]

Originally Posted by Zingeroo

The first thing I would look at is the preamp stages and their plate resistors.
I'd bet a pack of guitar strings they went with 220k plate resistors in an attempt to get more gain... but that also makes the amp lose a quick attack. If you've played a Mesa Rectifier you can feel that loosey-goosey feeling that I hate about those amps and when I played the picovalve that's what I found also, a loose, spongey, unresponsive feel.

Send me a PM so that I can give you the address to ship the pack of guitar strings- make that an Ernie Ball set of strings for a E9 pedal steel...

Here is a link to the schematic as drawn up at THD:

https://www.box.com/shared/static/02...977380ef43.jpg

The schematic shows a 1M grid leak resistor for V2:1. However that resistor was used instead as a series resistor from the coupling cap of V1:2 to the grid of V2:1. When the Bright switch was activated it would add a 470pF cap in parallel to that 1M resistor to allow more high frequencies to pass. After doing that there was no longer a grid leak resistor for V2:1 which caused it to go bonkers (to use the technical term.) You can add a 1M resistor right on the tube socket from Pin 2 to ground to correct that mistake. Using a 12AT7 for V2 will work to some extent for one reason or another.

I drew up a corrected schematic last fall but it looks like chicken scratches even to me. (I need to get TurboCAD up and running again so I can crank out some drawings that are legible.)

After correcting JCA's screwup I proceeded to tear my PicoValve apart and wire it up as a single-ended Trainwreck Rocket... I call it my PicoRocket!

Steve Ahola

[alberob]

The River City Mod definitely improved the sound of my mine .Don't expect too much in the cleans department,but then I don't ever use a 5 watt amp when I want cleans.I use it when I want over the top high gain at a less than deafening volume.

[gammalord]

Agreed that the amp is well built and transformers appear robust. The cabinet is built fine, the materials are OK - but the dimensions are better suited for a ported box instead of sealed.

Im curious about the RWB and other Eminence offerings - the Swamp Thing seems like it might tame the mids more and pull out the lows and highs. We'll see.

You can add a 1M resistor right on the tube socket from Pin 2 to ground to correct that mistake.

Steve - what does adding this resistor do to the amp's performance/sound?

I've read about your PicoRocket and if only it wasn't so dang complicated - I'd do it in a heartbeat. Love to hear some samples just for fun.

[Zingeroo]

Originally Posted by gammalord

Steve - what does adding this resistor do to the amp's performance/sound?

I'm not the most knowledgeable guy in the world, definitely not as much as Steve, but the grid leak resistor has to do with the tube's bias. It sets some sort of reference point. And without it, as Steve said, the tube goes "bonkers." Which I think he means it goes unlinear in lots of screwy, unpredictable ways.

Steve, I probably owe you more than a pack of strings for all the info you've made available over the years

[PALYGAP]

Originally Posted by steeve_a

Send me a PM so that I can give you the address to ship the pack of guitar strings- make that an Ernie Ball set of strings for a E9 pedal steel...

Here is a link to the schematic as drawn up at THD:

https://www.box.com/shared/static/02...977380ef43.jpg

The schematic shows a 1M grid leak resistor for V2:1. However that resistor was used instead as a series resistor from the coupling cap of V1:2 to the grid of V2:1. When the Bright switch was activated it would add a 470pF cap in parallel to that 1M resistor to allow more high frequencies to pass. After doing that there was no longer a grid leak resistor for V2:1 which caused it to go bonkers (to use the technical term.) You can add a 1M resistor right on the tube socket from Pin 2 to ground to correct that mistake. Using a 12AT7 for V2 will work to some extent for one reason or another.

I drew up a corrected schematic last fall but it looks like chicken scratches even to me. (I need to get TurboCAD up and running again so I can crank out some drawings that are legible.)

After correcting JCA's screwup I proceeded to tear my PicoValve apart and wire it up as a single-ended Trainwreck Rocket... I call it my PicoRocket!

Steve Ahola

I did the corrections of the 1M resistor and that really made a big change. The amp is now usable especially in crunch/dist area whereas initially it produced a lot of mushy stuff.

steeve_a, where could I find a description of the Trainwreck Rocket mod that you did? I might give it a try.