Attitube G20

[big-ez]

HI guys, Wow, I haven't posted here in a LONG time. I build a few clones years ago (don't remember what user name I used), and you guys really helped me out.

I've been wanting a new beater amp to play with and found this weird one on Reverb the other week (for $108!!). It's called the Attitube G20, out of Korea. 2 12AX7, 2 EL84. Kind of a cool looking enclosure, and very neat dressing. The guy that shipped it did a really crappy job, and the switches on front panel were busted in transit. I'll try to fix those today and test this out, but I did take some pics and draw up a quick schematic.

It looks like it has a Tweed Princeton-style tone stack (after both preamp gain stages). The power section looks a lot like a Ghia. There are no cathode bypass caps at all.

One interesting thing: it has an on-off-on switch that does something I haven't seen before (if I drew this correctly). It changes the circuit between the gain stages:

In OFF: the signal from the output of V1a is: 0.0047 -> 330 pF -> V1b Input
ON1: 0.0047 -> 330 pF -> (here is adds in the nonamplified guitar input signal to this amplified path) ->V1b Input
ON2: 0.0047 -> 330 pF/bypass (330 pF is still in circuit, but there's also a bypass around it) -> V1b Input

Anyone ever see this (particularly the ON1 setting)? I'm attaching some pics.

I hope this is interesting to you guys....

Schematic: https://photos.app.goo.gl/RCRX67LFq6kLBoUK9

Guts: https://photos.app.goo.gl/khNCM2VcfCjc9Uxn7

Ed

[kagliostro]

Very nice construction   


--


is it this version ?





Please, if you can, a bit more detailed photo of the inside, thanks

Franco


p.s.: BTW, I'm not sure, but seems that the small e-cap that is connected to the pin of the second socket (starting from left) is a bit bugly on the top, may be it is to be replaced before of problems

[big-ez]

I think that cap is OK, that it's just the camera angle. Looks OK in person. Here are some links to higher-res photos:

https://photos.app.goo.gl/mNF4x3xafk4LocBD6

https://photos.app.goo.gl/16p6pyLfv5XRpU3D7

https://photos.app.goo.gl/DMkQYB5tHTbwaDte8

https://photos.app.goo.gl/wTRaVWGVnNKsNpvb7

I got a replacement switch today and fired it up. It's really pretty noisy. Haven't had a chance to diagnose much yet. Solder joints look decent all around, and wiring is tidy. Any suggestions, I'm all ears!

I measured DC power supply voltages at x, y & z (in my schematic). They are:

x: 294V
y: 309V
z: 368V

Ed

[kagliostro]

Many Thanks for the photo Big-ez

It's really pretty noisy

which kind of noise ?

Franco

[big-ez]

First, I forgot to answer your first question: yes, that is the amp in the pic you attached.

Noise: hum mainly, and loud. I just put in all new tubes. Hum increases with volume, even w/out guitar plugged in. I'll poke around when I get a chance and report back. The noise changes of course as I switch the settings between naughty/breezy/moody. From what I can tell:

OFF (this i breezy, very very bright and moderately loud): the signal from the output of V1a is: 0.0047 -> 330 pF -> V1b Input
ON1 (this is moody I think, less volume and dark sounding): 0.0047 -> 330 pF -> (here is adds in the nonamplified guitar input signal to this amplified path) ->V1b Input
ON2 (naughty, very loud, like a big volume boost, the most noise/hum [which makes me think the issue is somewhere early in the amp]): 0.0047 -> 330 pF/bypass (330 pF is still in circuit, but there's also a bypass around it) -> V1b Input

Ed

[kagliostro]

Are you shure the connections to the input jack are good ?

Good ground connection on the input ?

When there is no guitar jack on the imput the amp is silent ?

Franco

p.s.: Can you draw a bit better the schematic at the imput with the switch ? (I don't see a ground switch on the input jack, only a gnd connection, I mean on your schematic)

[big-ez]

New schematic posted below. Yes, there i a ground connection to input (I was just lazy in previous post). I added voltages to this diagram.

It hums when guitar is plugged in or not, and it doesn't matter where the volume knob is. The most alarming noise (I think) comes after the amp is warmed up for about 30 secs. I think it's coming from the last output tube (though it -could- be the transformer). There's a kinda high pitched buzzy/ticking noise, very fast, that kicks in. It is definitely mechanical, and is coming from the amp (not the speaker). When I remove the last EL84, it stops. I tried different tubes, all with the same noise/effect, so it's not the tube. I also put my finger on that tube to steady it when the noise was happening, but it wasn't vibrating at all and that made no difference. Strange.

[PRR]

.

[shooter]

HA you beat me!

what about the V1A without a grid ref to ground 

EDIT: updated schematic, added .sch version 2nd update.  Semi-final update 

Made corrections to the PA cathode bias section from original

[big-ez]

OK, some good catches there. I had screwed up the tone controls a little, missed the resistor off of pin 6 in V2, and screwed up the connections from pins 1 & 6 on V2. I fixed all of it in new version below. However:

- there is definitely no 1M resistor to ground on the input
- that 68K resistor to ground off of pin 7 of V1 is there in the amp
- 80K resistor off of the el84s is also there in the circuit, going to the power rails/supply
- not sure what the 'ground?' comment is in red writing that PRR posted. the circuit as I depicted it looks correct in the amp

shooter, thanks for the .sch version. Note that there are some values in there that do not match the values in my (weak and pathetic) hand-drawn one.

[shooter]

adds in the nonamplified guitar input signal to this amplified path

As an engineer type this circuit bothers me 

Has anyone got an example beating outta phase signals to "create" a musical sound from the classic examples of old