Jet City Picovalve thread!

Modern Saint said:
After rereading your note, I will explore to see if it will be necessary.
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Actually, it's only 1M. I put a half watter in there. Super easy and really cleaned up a lot of the slop in the amp. It was how the amp was designed, but between design and manufacture something went awry. LOL

If I weren't an incessant tinkerer that might be all I would do, outside of tube rolling. However... I really want to do that switchable cathode bypass cap setup. LOL
 
Woohoo, mines here. Nice box for $100! Recorded it stock, then changed the tubes and recorded it again. Those stock tubes, are, um, crap. Still, sounds/feels a little muffled and, I dunno, stiff?

It's opened up, soldering iron is warming up....
 
What tubes did you swap in?

Are you limiting yourself to just the grid leak resistor at first?

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GilmourD said:
What tubes did you swap in?

Are you limiting yourself to just the grid leak resistor at first?
Click to expand...

2 JJ12AX7's and a Sovtek 6L6WXT+ - the sovtek was short lived though, I ended up w/an EH6L6 and the two JJ's. Have a few more preamp tubes to try, but it's bundled up for the night.

I did do just the grid leak resistor, opting for a 560k from V2:1 to ground. This put the grid leak value at 427k w/the 2M in parallel. Big difference, but still a little muddled.

Added a 10uf across the cathode of v1:1 (R5) - better.

Then I put a 1uf across V1:2's cathode resistor (R9) - WAY better.

I took off C1, which I believe is local feedback and removes some of the highs coming in. This adding a little more usable high end.

I also took off the 22pf cap across V:1 pins 6 and 7, but had to put it back as I believe it was producing blocking distortion without it and was killing the signal.

I also tried just jumering the 1M in the bright switch (extra grid leak), but it left the amp sounding kind of thin. I kind of like the tone of the bright switch. If I take the board out, I may change that circuit a little.

Put in a JJ6V6, a little quieter, but just didn't have that zing the 6L6 has.

Amp sounds great right now. Cranks up pretty good, and the clean tone is usable.

I'm thinking about designing a switch to swap the pre-amp around to a fender silverface champ circuit, and the current circuit. I don't think it will be that hard actually. It would take right off the plate of v1:1 (probably after the coupling cap) and into the tone stack, where it's getting its signal from the cathode follower. I dunno, have to think about it and draw it out.

Also fwiw, I found a 220uf cathode bypass cap across R11 that isn't on the schematic.
 
wagdog said:
2 JJ12AX7's and a Sovtek 6L6WXT+ - the sovtek was short lived though, I ended up w/an EH6L6 and the two JJ's. Have a few more preamp tubes to try, but it's bundled up for the night.
Click to expand...
I have to say, I liked the EH6L6 in mine.
wagdog said:
I did do just the grid leak resistor, opting for a 560k from V2:1 to ground. This put the grid leak value at 427k w/the 2M in parallel. Big difference, but still a little muddled.
Click to expand...
Where is this 2M?
wagdog said:
Added a 10uf across the cathode of v1:1 (R5) - better.

Then I put a 1uf across V1:2's cathode resistor (R9) - WAY better.
Click to expand...
What sort of range did the change occur in?
wagdog said:
I took off C1, which I believe is local feedback and removes some of the highs coming in. This adding a little more usable high end.
Click to expand...
Hmmm... I'll have to take a look at that.
wagdog said:
I also took off the 22pf cap across V:1 pins 6 and 7, but had to put it back as I believe it was producing blocking distortion without it and was killing the signal.

I also tried just jumering the 1M in the bright switch (extra grid leak), but it left the amp sounding kind of thin. I kind of like the tone of the bright switch. If I take the board out, I may change that circuit a little.
Click to expand...
Well, isn't it that the 1M is in series between the coupling cap and V1:2 with no connection to ground that's the problem?
wagdog said:
Put in a JJ6V6, a little quieter, but just didn't have that zing the 6L6 has.

Amp sounds great right now. Cranks up pretty good, and the clean tone is usable.

I'm thinking about designing a switch to swap the pre-amp around to a fender silverface champ circuit, and the current circuit. I don't think it will be that hard actually. It would take right off the plate of v1:1 (probably after the coupling cap) and into the tone stack, where it's getting its signal from the cathode follower. I dunno, have to think about it and draw it out.

Also fwiw, I found a 220uf cathode bypass cap across R11 that isn't on the schematic.
Click to expand...
Where is your 220uf cap? I'm 99% sure I didn't see anything wired across R11, unless there's a trace on the board that's putting it in parallel. I didn't look too closely at the traces and definitely didn't pull the board out.
 
GilmourD said:
Where is this 2M?
Click to expand...

On the grid of V1:2, there is a 1M resistor (w/the 220pf cap on a switch - the bright switc) and another 1M resistor in series to ground (the original grid leak resistor). That makes for 2M of grid leak (even if the bright cap is switched on). Installing a 1M from the grid to ground makes it roughly 667K rather than 2M. I used a 560k and measured 427k to ground.

GilmourD said:
What sort of range did the change occur in?
Click to expand...

The amp is just way more alive and responsive with the cathode bypass caps. I think you could go with a .47 rather than a 1UF and maybe a 1UF on v1:1's cathode. It just sounded so good I decided I wouldn't need a switch, I was staying w/the values I put in (10uf on v1:1 and 1uf on v1:2)

GilmourD said:
Hmmm... I'll have to take a look at that.

Well, isn't it that the 1M is in series between the coupling cap and V1:2 with no connection to ground that's the problem?
Click to expand...

If you're referring to r12, it's connected to ground, however it's making for 2M of grid leak. Check this post: http://www.mylespaul.com/forums/5470176-post204.html - it has a file called PicoOverlay_g.jpg that's really useful. The corrected schematic on the first page of this thread shows it as well.

GilmourD said:
Where is your 220uf cap? I'm 99% sure I didn't see anything wired across R11, unless there's a trace on the board that's putting it in parallel. I didn't look too closely at the traces and definitely didn't pull the board out.
Click to expand...

It's in this picture. R11 is an 820R and is the cathode resistor for V2:1 - look just to the left, you can see C12 which is bypassing it. It's a 220uf.

 
Had a thought hit me when I woke up this morning. The 1M grid leak was connected in the wrong place. The value, 1M is fine, but it should have been directly on pin 2 of V2A.

This is the original schematic (with the incorrect grid leak resistor setup):


Here is my mod:


Pretty simple to do: unsolder the top of R12 and pull it up (leave the bottom end in and grounded). Run a jumper wire from the top of R12 (NOT the board, the resistor that's now standing) to the top of R14 just before it runs to V2B:2 (grid). Can't really crank the amp right now, but I can tell it's going to be good. Bright switch works great too.

 
So, things to try this afternoon:

The 2w/5w switch is pretty much useless and can be re-purposed. Same w/the standby switch - no need. I'll "hardcode" those and then come up with some stuff to try w/the switches. I was thinking that maybe the 2w/5w switch would make for a good "blower" switch, lifting the middle pot from ground and disconnecting the tone stack - would certainly be a boost - might be too much.

The standby switch could be used as a cathode bypass cap switch though - definitely look into that.
 
Man, I wish I weren't at work browsing from my phone. LOL I'll go over those schematics when I get home later.

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Well, I ended up backing out this mornings change above. Didn't sound good cranked up. Did remove the standby and 5w/2w wiring but haven't used the switched.

This amp sounds good, but I think it's over saturated and could used some negative feedback... Trying to work that out.
 
Nfb experiments failed, will need more thought.

I did use the 5w/2w switch as a cathode bypass switch on v1a. Switching between 1uf and 22uf, the audible difference wasn't that great. It was noticeable, but not a huge difference. I think I'm going to try .47uf and 22uf.

I used the standby to toggle the bypass on v1b's cathode from none to .47uf - that had a bit of effect on gain when the amp was driven.

Ran out of time last night and have a gig tonight. Probably won't get to work on this til the weekend now.
 
Ah... Head colds... Keeping me away from being anywhere near coherent for days. :)

After looking at a bunch of things... There's definitely conflicting information about which component is which. Every schematic seems a little different and none of them seem to be what's really in the amp. I think we're going to need to see both sides of the board to see where everything goes.

Where did you insert the NFB loop and what value resistor did you use? And was there a bypass cap on that?

I'm going to spend some time cranking the snot out of my Pico this weekend. A review of it will be one of the three articles I'm writing for Seymour Duncan's blog this weekend (which I really should have had done by now, but this week decimated my brain). I should get some nice recordings of it done. I'll
 
I don't know if you saw this, but a dude at the mylespaul forum made a nice overlay of the bottom of the board superimposed on the top of the board:

http://www.mylespaul.com/forums/5470176-post204.html

http://www.mylespaul.com/forums/att...t-city-picovalve-mod-thread-picooverlay_g.jpg

(I think you have to be logged in there to see it).

The only thing missing are the traces that go under the tube socket.

Anyway, I'm pretty much done. This thing sounds killer. Here's my mods. I'm running a JJ6L6 and 2 old peavey 12AX7's. The amp now cops a real nice marshall vibe, and cleans up fairly well. These mods clean up the furry/fuzzy distortion and really open the amp up, imo.

Please note for random people reading this: Do not attempt these mods if you don't know what you're doing. My notes may be sketchy or incorrect. Tube amps contain electronic charges that can kill you. Although this amp has bleeder resistors, mods, mistakes and malfunctions may render them useless. Always confirm that the amp is safe before proceeding.

Here's my schematic and notes:



Remove wires from 2w/5w switch and standby switch and hardwire them. This opens up some space. The 2w setting is useless and there is no need for a standby switch on this amp.

Use the 2w/5w switch to switch the cathode cap across R5. I'm using a .47uf and 22uf. The 22uf is too much when the preamp is cranked. Good w/clean settings.

Remove c1.

c2 (v1a coupling cap): Remove original c2. Use a .022uf, install in same hole as r2 that connects to plate of v1a and leave other leg up. Attach a 470k with a 470pf across it to the open leg of new coupling cap and install other leg on run that goes to gain control (where other side of c2 used to go across to).

c7 (treble bypass on gain control): change to .001uf - this fixes the anemic tone when turning the gain down.

R9 - change to 10k

Use standby switch to toggle a 3.3k across r9 for a nice boost switch. This will "pop" when the switch is toggled.

Add a 680k from the run that v2a's grid connects to at r14 and run it to ground. This fixes the overly large grid leak issue. This is running from v2a's grid across the bright switch and across r12.

Change the mid cap (labeled c10 on schematic, but different on board) to a .01uf.

I tried quite a few different things and this set of changes does sound great. I've got the amp buttoned up and am going to play it for a week or so. There is quite a range of tones available by adjusting the master vol and gain. I find that the amp really picks up depth w/the master above 7.
 
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Made a few tweaks today. Wasn't happy w/the bright switch configuration. Here's the new schematic



Took about R12 completely. Put a 470k from the grid of V2a (actually sharing the hole w/R14). R14 gets changed to a 470k and C13 is changed to a 470pf. The difference with the switch is pretty subtle but it's a much better tone than the stock 1M/220pf.

Running all JJ tubes now (12ax7's and 6L6) and really pleased with the range of tones I can get out of the amp.

The only thing really bugging me is the lack of responsiveness out of the bass control; it's either no bass, or it snaps on about 2 on the dial and doesn't change much after that.
 
And in the never end mod's dept... added a 500k audio taper 2'nd gain pot right after c5 and a bright cap on the push/pull part of that part. Here's the current schematic:

It sounds killer now. With that second pot I can crank the master volume and tame the preamp down for some nice clean tones.
 
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