Not sure if you answered before, but have you tried someone else's house AC outlets, just as a grounding sanity check? The buzzing in the video you posted is obtrusive and unless you use single-coils, or live under a radio tower (this happened to the Talking Head's drummer), it's hard to believe all 3 amps would have this issue. I get the "cheap amp" comments but still.... My Orange TinyTerror doesn't buzz like that.

Ah - OK, I think I had my threads crossed (someone who had just pulled an old amp out of a closet, on another one, sorry). Usually the bad cap issue takes time, but failures on new components can and do occur. For sure, this blatant problem would rarely get past QA. Or maybe it was a late day at the factory and someone wanted to go home. Could have been damaged/jiggled during shipping... This is a great site to bookmark - lots of knowledge here from some amazing techs with decades of experience (g1 and phatt, especially, have some serious chops!)

Ok - if you hum along with the hum, that's right around Bb - B, which is 60Hz or 120Hz, so bad filter cap(s) would be my first guess. If it's been a long time since you played it, this makes a bit of sense, dried out electrolytics. If you know what you're doing, open it up and measure AC volts across the power supply filter caps (across the cap leads, NOT from each lead to ground). You should get 0.00Vac since caps pass ac voltage. If you get a measurable AC voltage across a filter cap, there's the culprit. If caps look good and clean, then I'd try to hunt down a ground problem, which by nature can be frustrating, so lots of patience... On occasion I've run across bad filter caps on op-amp supply pins, and FET drain pins (when bypassed), but that's been more rare for me. Hope that helps. If you have any trepidation at all about opening the amp or measuring those caps, take it to a tech, not worth getting hurt over a silly cap. - Will

Is the pitch of the hum between a Bb and a B?

Well, looks like the power amp is blown, which sucks. I've seen something similar to this in the past, but the guy plugged one amp's speaker output into another's FX return - amazingly the power amp actually survived in that case, probably due to good protection circuitry.

But I agree with Phil here - if you've never changed a power transistor, especially the T0-3 (round) power transistors, take it to someone qualified. It will save you time, pain (getting shocked with DC supplies sucks), and maybe money too. The collector on the T0-3 BJT is tied to case itself, so you need mica or rubber insulators as well as a lot of heat sink grease (thermal compound) to properly mount and isolate/insulate the collectors from ground (the heat sink aluminum bar), and even the slightest mis-match or offset will light it right back up. In addition you need at least a light-bulb protection circuit to plug into; a variac is better, because you can ramp up the supply voltage while monitoring Vbe on the transistors and catch the collector current ramping, and stop it before they fry. If that all sounds like more strange jargon, take it to a tech.

Oh - and Phil, one of my personal favorite amps, with horrible jack placement, is the GK400RB -- all identical 1/4" jacks in a rectangular block, crammed next to each other on the back...

Will

1. is power supply OK? check all supply rails for proper voltages

2. if PS is ok, is output fried? The FX return should have a high enough impedance that hopefully the power amp isn't fried. Is there DC on the speaker outputs? Is Vbe >>> 0.7V on power transistors?

3. If amp OK, check the FX return DC coupling cap, which is probably rated at a lower voltage than what you may have fed it. If it is fried, it may have been the best outcome as it would be acting like a sort of fuse. Plug an oscillator into the FX return jack and check to see if that cap is passing the signal with a DVM.

4. If cap is good, check the first op-amp in the FX rx circuit by checking the op-amp output pins - if its fried they'll likely have either one of the supply rail voltages, or a non-zero DC voltage on them, if not the you'll find the oscillator signal there (an oscillator app for your phone will work here).

With luck, it might be as simple as replacing the DC coupling cap, but it is always worth your sanity to check the PS and power amp first.

Hope that helps, Will

Yep - I didn't get the name of the company who did all of them, but was told something similar about the power amps, and that each model was designed in partnership with each respective company, so there are differences in the preamps that could be interesting to study. Have not seen any schematics for any of them tho.

Dang! I like it! And, that guy makes great guitar-player faces.

Now, who's got the schematic?

The overdrive channel is where it's at.

Yes, it's a cheap POS. Yes, their copy is full of hackneyed BS.

I'm just interested in seeing what they are using for their non-linear stuff, mainly in the distortion channels, to see if they are offering something interesting, even if it subtle (the Quilter's bias shifting circuit would be a good example), or to see if they are just re-hashing the same tube-screamer-style crap.

Right now, the Katana-style "5 or 6 basic digital models, simple interface" and "digital models on your iphone" amps are dominating the SS guitar amp market. I think that this amp is a risky proposition, if they are just throwing yet another amplified distortion box at the marketplace, even if the price point is low. But, if they've got something unique to offer, then I'm curious to see what they've got going on.

Thanks,

WP

Ok - curiosity is getting the best of me; I know these new amps are built by Acoustic (now owned by Guitar Center), and they look super-cheapo, but I'm curious about the "100% Analog" circuitry, especially for the distortion channels. No luck finding schematics (yet?) and a quick US patent search turned up nothing. Anyone here got any technical info on these?

TIA,

Will

https://www.musiciansfriend.com/amplifiers-effects/gamma-g50-1x12-guitar-combo-amplifier/l86074000000000

For the MC1436, pin 8 is a no-connect (N.C.) so you can ignore a connection there.

However, be careful. Often, the supply pins are omitted from the main schematic page(s) because layout techs usually put all the op-amp power supply connections and all of their bypass caps in one area, usually separated from the other pages. This makes the schematic cleaner and easier to read, but can be easily mis-interpreted.

- Will

The input amp is configured as a kind of multiple feedback 2nd order HPF. R6, R7 and R8 set the filter gain.

The C1, C2, R4 R5 network is the HPF. Changing the 510k resistor will alter the LF edge of the HPF (changing any of those 4 components is going to adjust that frequency).

As is, the HPF fc is 60Hz, well below the ~88Hz low E-string on a standard 6-string guitar. This stage has no HF compensation in the feedback path, so the HF breakpoint is super high, well into the MHz range (basically, the op-amp internal compensation is setting the 1st order HF edge frequency, so the op-amp itself is going to set that part of the FR).

This is interesting as I would expect to see the standard 22pF - 100pF cap in the feedback loop to stabilize. If you want to soften the HF response, add a cap in parallel with the FB resistor.

Have you determined if the brittle-ness is an EQ thing or a lack of Nonlinearity in the input stage?

Will

Place a graphic EQ (or multi-band parametric) in the FX a loop and set for Clean channel.

Does adjusting this EQ help the brittleness problem? Is there any setting that makes things tolerable/good?

If not, then EQ or filtering mods probably won't do what you want. I'd look at adding a FET based "warm up" stage to the input preamp. My favorite FET is the J113 and I've made many of the Danyuk-based FET triode sim circuits with them - with the Danyuk equations, the J113's only get about +12dB of gain, and you and resistor-divide that back down to unity if needed. And, only requires a few parts, and can be built easily on a small piece of a proto-board.

If EQing fixes the issue we'll enough, then I'd put the preamp and post-limiter circuits in a SPICE simulator, and see what I could tweak/add to mimic the EQ.

My gut feeling is that the lack of warmth is a non-linear issue, and not EQ. But this way you can test that idea. You could also place very low distortion tube-screamer style boxes in the FX loop, or at the input to the amp (normal stomp box style) to further suss out the issue of EQ vs. nonlinearity.

Hope that helps,

Will

You could replace VR10/11 with log pots if you want to - I'd probably leave them alone.

To get back to the original issue, The input preamp has a max gain of (47/2.2) + 1 = 22.36 = 27dB

If you want to lower the gain of the preamp which is the first op amp after the input, you have to either lower the 47k feedback resistor R4 or raise the 2.2k shunt resistor R5. To experiment, you could replace the 2.2k resistor with say a 10k tweaker-pot and raise the resistance above 2.2k while playing through it till you get what you want.

If still too much gain, change to a 50k tweaker-pot. When you get the gain where you want it, either leave the tweaker pot in place, or measure it and replace with a fixed value.

With 6.8k, you get ~18dB max gain. Note that raising this resistor value also lowers the minimum gain, when the 22k gain pot is at max resistance. So changing that one resistor will alter the overall gain range as well as the max gain.

Will

Agree with Phil - simple attenuator in the FX loop is the least intrusive option.