Jet City Pico Valve (THD design)

[Steve A.]

I just picked up one of these on clearance at GC and would like a schematic for it. I haven't tried switching out tubes (this will take 6L6's, EL84's, KT66's, KT88's without having to rebias). I believe it is similar to Andy's UniValve only it is mass-produced in China. [NOT TRUE- the circuits are quite different. The PicoValve was a completely different circuit that Andy had designed with the only similarity being that it was a single ended design which could take several different output tubes.]

Thanks for reading this!

Steve Ahola

[J M Fahey]

Did you get the Univalve schematic?
It *should* be (almost) exactly the same, the same way as the JC20 something is the spitting image of the equivalent Soldano.

[Steve A.]

Did you get the Univalve schematic?
It *should* be (almost) exactly the same, the same way as the JC20 something is the spitting image of the equivalent Soldano.

No, I don't have the Univalve schematic. I finally got around to playing it for awhile the other day (I've been busy with a lot of other projects). I mostly use a clean sound for steel guitar these days (which the Picovalve lacks) but the distortion is very thick and creamy so I may want to keep it anyway.

Thanks!

Steve

[J M Fahey]

Ok Steve, here you have it.
It *should* be close enough for your repair/modding purposes.
It lacks some values, but, since you already have it on your bench ......
Good luck.

A *VERY* clever amplifier, I might add.
We were talking a few days ago about the scarcity of real *new* (and at the same time good sounding) design.
Well, this is a fine example.

[Steve A.]

A *VERY* clever amplifier, I might add.
We were talking a few days ago about the scarcity of real *new* (and at the same time good sounding) design.
Well, this is a fine example.

That is true- it certainly isn't the VoxaFendaMarshall design of the day, nor a Dumbleano...

The Picovalve doesn't have all of the bells and whistles of a Univalve; there is a single input jack, 3 speaker jacks, a 5W/2W switch and the gain/bass/treble/middle/master quintet of controls you normally expect to see.

Thanks!

Steve Ahola

[Blind Walker G]

Nice tube setup for Jet City Picovalve

Hi, this is a pretty old thread, but maybe someone has a look, anyway. I bought the Picovalve second-hand recently and was very unhappy with that one trick pony distortion sound, so I had a pretty radical change of tubes.
Actual setup is: Pre 1= 12AU7 Pre2= 12AT7 Master= 6V6GT, all NOS American tubes. With this setup the amp puts out a real nice crispy clean tone 'til gain at 4. Give more gain and a smooth crunch breaks in, give it the full gain and it still roars like an old elk looking for trouble, all settings played with master at full. I've tried a lot of different tube setups with the amp, but this was by far the best to make it usable for different kinds of music in recording sessions. You can use the 5w output, which sounds better, more open and less compressed than the 2W output, all the time cause it has lesser output than the original 12AX7/6L6 setup. Perfect for recording and bedroom sessions, if you want to use this little puppy on stage, try something else.
Regards
Blind Walker G

[Steve A.]

Actual setup is: Pre 1= 12AU7 Pre2= 12AT7 Master= 6V6GT, all NOS American tubes. With this setup the amp puts out a real nice crispy clean tone 'til gain at 4. Give more gain and a smooth crunch breaks in, give it the full gain and it still roars like an old elk looking for trouble, all settings played with master at full. I've tried a lot of different tube setups with the amp, but this was by far the best to make it usable for different kinds of music in recording sessions.

I guess I didn't post my findings here. In any case it looks like JCA screwed up in adding a Bright switch to Andy Marshall's design, as they removed the grid load resistor from V2 (Pre2). In my own amp I added a grid load resistor right on the tube socket but another amp tech discovered (as you did) that a 12AT7 will work in that circuit missing the grid load.
After figuring out what was wrong with the amp I tore it apart and rewired it as a Trainwreck.

The 5W/2W switch is a pentode/triode switch which was totally useless in that circuit- I think that they must work better in push-pull amps where I have been pleased with the result. So I rewired the 5W/2W as a boost in the preamp, and also reconfigured the Bright switch as something that is usable.

Steve Ahola

P.S. It's been a few months and I need to write up my mods before I forget what I did!

[Blind Walker G]

Picovalve Modding

I guess I didn't post my findings here. In any case it looks like JCA screwed up in adding a Bright switch to Andy Marshall's design, as they removed the grid load resistor from V2 (Pre2). In my own amp I added a grid load resistor right on the tube socket but another amp tech discovered (as you did) that a 12AT7 will work in that circuit missing the grid load.
After figuring out what was wrong with the amp I tore it apart and rewired it as a Trainwreck.

The 5W/2W switch is a pentode/triode switch which was totally useless in that circuit- I think that they must work better in push-pull amps where I have been pleased with the result. So I rewired the 5W/2W as a boost in the preamp, and also reconfigured the Bright switch as something that is usable.

Steve Ahola

P.S. It's been a few months and I need to write up my mods before I forget what I did!

Hi Steve, real interesting modding you did. Wish I could do something like this by myself, unfortunately my know-how is definitely to poor to try something like modding a tube amp, I'm just limited to play them.

[cpaz]

Actual setup is: Pre 1= 12AU7 Pre2= 12AT7 Master= 6V6GT...

I know nothing about messing with tube amp electronics so forgive this question, but the Jet City manual for this amp discusses user replacement of the power tube only. I am very interested in swapping out the pre amp tubes as you did - that said, can I literally just pull the old ones and drop new ones in or does anything else need to be done with regard to biasing, etc.?

[Blind Walker G]

Pre Amp Tubes

Hi there,
as far as I know, you can change the pre tubes without any problem, and this is where the sound changes. I tried 12AX7, 12AU7 and 12AT7 in the V1 stage (the pre on the left side), the AT7 softens it only a bit, while the real low AU7 for my taste works the best in this amp design. You will get some real headroom for clean sounds and still have the distorted possibilities. all these changes worked without any problem. So for the V1 stage you can choose one of these or a 12AY7 as compromise between AT and AU, for the V2 stage I would recommend an 12AT7, when you use AT, AY or AU in V1. This all works best with a 6V6 instead of the 6L6 in the power section. Would be interesting to try the old Vox layout EF86/EL84, will do this next. If you want any technical detail, better wait for Steve giving an answer. I'm not a tech anyway.

[Steve A.]

On most guitar amps you can replace 12A_7 preamp tubes with other members of the same family which may have more or less gain as per the following chart.

GAIN* FOR 12A_7 TUBES

12AX7. . . . 100
5751. . . . . . 70
12AT7 . . . . . 60
12AZ7 . . . . . 60
12AY7/6072 . 44
12AV7/5965 . 37
12AU7/6189. .20

These tubes are not completely identical except for the gain. The 12AT7 is biased differently from a 12AX7 which is why the Fender BF phase invertor is wired differently than an amp that uses a 12AX7 there. I also heard that the 12AU7 does draw more current than the 12AX7 which can create problems on some of the modern printed circuit boards that use plate or cathode resistors rated at 1/8 watt. But those amps are fairly rare.

I like to keep 5751's, 12AY7's and 12AU7's around to try plugging in to various tube positions on a guitar amp. While the lower gain varieties can clean up the preamp considerably, you can also use them to replace the 12AX7 in a phase inverter to reduce the overall wattage of the amp.

Steve Ahola

* The technical term is "MU" not "GAIN" but I wanted to keep things simple

[Blind Walker G]

Note about the Picovalve

Just one last thing. Okay, I'm not able to do any heavy modding on tube amps, but I tried a lot of different tube setups on the Picovalve. Compared to my Blackstar HT-5 Ministack the Picovalve with the matching Jet City 1x12 cab is just shit. I use the Blackstar over a year now on recordings, just changed the Sovtek pre tube to a TAD 12AX7 CX, and this baby is great for everything apart from metal. I did recordings from old Tellie Thinline to Duesenberg Starplayer TV, vintage Strat to old Gretsch Tennessean on this amp and everything sounds just great, clean, crunch, no matter. I've played mostly vintage Fender and MusicMan amps the past forty years, and in this tradition the Blackstar is just great for low watt recordings. Also had some hours on the Blackstar Artisan 15 combo, wow, what an incredible amp (no, I have nothing to do with the Blackstar company and they didn't pay me for writing this). I've got the Picovalve dirt cheap, so I use it for tube experiments, that's it. At least I've found some usable sounds, but...if the Picovalve is the only amp you use, sell it and buy something which is able to produce a variety of great sounds. I can send some links, if anyone like to hear the Blackstar. Didn't made any recordings with the Picovalve up to now, the Blackstar is just the better choice.

[bob p]

Can anyone re- post the schematic? It looks like it disappeared from the thread, probably when the attachments snafu went down a few months ago. thanks.

[Steve A.]

Can anyone re- post the schematic? It looks like it disappeared from the thread, probably when the attachments snafu went down a few months ago. thanks.

I just tried to upload it here and the attachment bug snatched it up.
I can email it to you if you like.

Steve

[bob p]

That would be good. You've got the email, right?

Also, what did you end up doing with your picovalve? I think I remember you saying that you 'wrecked it, which I think would be hard to do with a single ended amp. I never quite understood what you said you did. Did you ever draw it up?

In case anyone's interested, they're $99 at GC as I type this.