Brazilian tube amp - gato preto classics vira lata

[Woolybolly]

Thought I would share some photos of the inside of this odd little Brazilian guitar amp while I’ve got it opened up (trying to track down the source of some annoying crackle).  Looks like this company(Gato Preto Classics) was pretty short lived, not a lot of info out there that I could find.  I would guess this amp is about 10 years old, probably about 10 watts, the model is the Vira Lata.  Single ended (EL34) but with two ECC83s.  I was surprised by two preamp tubes in a single ended amp but I’m not an amp buff at all — is that a thing?  Some random googling got me to a photo of a Bell PM-10, which the Vira Lata resembles.

[bmccowan]

I've been hanging back in case somebody knows this amp. I do not. But the second preamp tube is likely there to drive something else, rather than gain. My guess is tremolo, just because it is so common. Several pics of that amp online, but all the ones I found show no markings on the pots/controls. I like the way it looks, but the PCBs are likely a problem, but impossible to say. Can you sketch out what you understand of the circuit? 

[PRR]

Overall it is hard to imagine it isn't a "Fender Champ" of some type.

The dual pot on the end *could* be a trem LFO, Wien or phase shift. That is at least one extra triode.

The amp seems to be widely available both on Brasilian sites and even on Reverb.com. Interesting that none of the images I peeped had labels on the controls. Surely they could use Br-Portugese or (as Japan does) English.

[dwinstonwood]

Someone wrote what the knobs are on this one.

[Woolybolly]

Oh, right, that’s another odd thing about this amp — no markings at all except someone has sharpied in 4 ohm and 8 ohm for the speaker jacks.  The controls are volume, bass, mid, treble and gain, the dual pot being the gain.
I’ll try to sketch out the circuit but I’m new to PCBs, so it might take me a minute.
I messaged the company on instagram but it doesn’t look like the account has been in use since 2017, so not expecting much.

[dwinstonwood]

There are also lots of YouTube videos for it, so it's not too rare. Just search for "gato preto classics vira lata"

[shooter]

trying to track down the source of some annoying crackle

those preamp sockets might be a good start, have had problems with them.

[Woolybolly]

Here's a detailed description of the crackle and what's going on with the amp:

- When first turned on, there is no crackle, but there are some peculiarities with how the amp sounds.  Turning the volume up past 7 causes the signal to drop out.  If I set the volume right at the edge of the sound dropping off, the signal sounds choked and starved of voltage.  If I back the volume off a hair, so the signal sounds normal, but turn any of the tone controls up, the signal will start to break up.

- Up until the signal starts sounding choked, it sounds normal and good, except the overall volume seems low.  Also, there is a faint rustling as the volume nob turns (I've cleaned the pot), but up until being turned to 7, there are no drop outs or inconsistencies in the volume.

- If I set the volume higher than 7 and turn the treble control up, the amp will suddenly start crackling very loudly, and the crackling will persist even if I back the volume off.  At this point, beneath the crackle, the volume pot will smoothly increase the signal from 0 to 10 without cutting out or sounding starved, but the overall volume of the signal seems lower than it should be.

- Turning the amp off and letting it sit for 20-30 mins will cause the crackle to go away.

[sluckey]

I'm guessing this is not a rush job?   

[Woolybolly]

Ha yes, there was actually dust on the circuit board it had been so long since I had taken a look at it...

I had the thought that the crackle might be due to DC leakage across the tone caps but then I realized the tone stack circuit was a cathode follower, and the plate of the preceding triode is being fed directly into the grid of the last preamp triode (no capacitor or resistor).  Is there a good place to read up on cathode follower tone stacks?  I'm having trouble conceptualizing how the signal flows through them.  Could a leaky cap in a CF tone stack still be the source of the crackle, and how would I check for leakage when there's DC voltage on both legs of the caps under normal circumstances?

(I am not an amp buff -- does having 4 preamp gain stages, a cathode follower tone stack, and a direct coupled cathode follower make this a "Marshall-style" preamp circuit?  Because there is no schematic for this amp, I have been trying to reference similar schematics to try to understand what's going on circuit wise)

Second line of questioning -- Because turning the treble control down rolls the treble off the crackle, I have assumed that the issue would be in the preamp circuit, not the power tube.    Is that a safe assumption?  And if not, any tips for what to check for first when trouble shooting a single ended EL34 circuit?

[Woolybolly]

Disregard the above mention of DC on both sides of cathode follower tone caps, it was my first time looking at a schematic for those and was briefly confused...

Just to button this up, it was indeed a faulty treble cap, the DC voltage was flowing across is almost unencumbered. Here's some info about this amp that I learned while working on it, so that it's out there in case anyone else is curious about it or needs to work on one.

- It is not a Champ clone.  There are two preamp tubes and 4 stages of gain.  After sketching out the circuit, I realized the preamp circuit resembles that of a Marshall Superlead.  I am not an amp guy, so it's not like I'm drawing from a huge database of knowledge, but I came across that schematic and realized they were at least similar.  Maybe it's based off a different amp or a different Marshall, etc., but if anyone else is wanting a rough idea of the circuit, that will get you in the ballpark.  But there's only one power tube -- high gain, single ended amp.

- Of note -- There are no cathode bypass caps, except for the power tube.  The volume pot is at the very end of the preamp circuit, before the power tube.

- The amp is probably about 5 watts.  Now that it's working, it sounds very good.  Clean tones are so quiet as to be almost non-existent, but the gain control sounds great, it's playable even maxed. Cool amp.  Would be curious what the other amps made by this company before they went under sound like.