Doobie GK15 guitar amplifier circuit

[LevinGuitar]

I'm repairing a small old Japanese (25w?) amp. It had a non working 50K pots, but after a small cleaning they are somewhat working again. Overall I would like to make it sound better.

1. It have a bit of noise (not an issue) that does not increment with the volume pot, where does it comes from so? I have replaced almost all caps but it did not make change about noise.
2. The spring reverb is too low, almost make no difference.
3. It sound too powerful and not so clean, distorts easily on lows (the speaker I guess is dying).

Looking for the datasheet of the LA4420 5.5W chip (there are two of them), I found that the circuit is almost the same as the datasheet of the chip.

For now I tried to ground the negative speaker lead to use only one chip as amplifier. It was a small improvement, and still powerful enough. The reverb is gone, I have to look how is wired into the circuit.

Any more ideas? To lower 17.7v to 13v as the datasheet main voltage would be nice I guess. Also should I try a 0.15uf speaker filter cap to ground as the datasheet have?

I find it an interesting amp as it's small, light, have a 12v dc imput (the battery compartment is missing in my sample) and two imputs so it can be a useful tool for casual gigs.

 

[user 137017]

i guess its just a cheap little amp and no matter what you do it will always be lacking in tone, and volume clarity and punch. you have an amp with chip beeter suited to a cheap home hifi . im also guesding ts something like an 8 inch speaker.
have a look for a second hand vox modeling amp they can often be pickedup second hand for about £40 uk $50 us. you wil not be dissapointed otherwise go highend and buy an all tube amp 50w or more

 

[LevinGuitar]

I also thought of changing the chip, I have a pair of LM4562 at home, even with complete different size and pinout I could wire it if the implementation is easy and it would sound nicer. I'll not use it as a guitar amplifier, just need a clean amp.
The speaker is 8ohm and 6.5" I guess

 

[Philbo King]

Curious:
Is Doobie the brand name? Or maybe something you found in the amp cabinet?

 

[LevinGuitar]

https://reverb.com/es/item/49824419...ge-japan-practice-amp-15-watt-w-spring-reverb

Same as this one but worse looking. Anyway that one is waaay too expensive.

Made by Fuji Denshi Onkyo

There are a higher model, G30 that seems to have little in common and sound a lot better than mine

 

[LevinGuitar]

I've just realized that the main voltage is higher than the voltage rating of power filter caps

Can a transformer change the output voltage because of some aging reason? Or the rectifier diodes?

Edit: 15ohm resistor from the diodes to the main circuit gave me a close to the datasheet voltage, have a bit less noise and maybe a better sound, still have to change the speaker.

 

[abbey road d enfer]

To lower 17.7v to 13v as the datasheet main voltage would be nice I guess.

Would it? This chip is an automotive one, hence the 13.2V typical application, but the datasheet indicates it can safely be used with a higher voltage, at least 16V.
You would not gain anything in reducing the supply voltage.

I've just realized that the main voltage is higher than the voltage rating of power filter caps

This could result in a long-term problem. You can probably replace them with a higher voltage version. Progress in electrolytic capacitors result in smaller and smaller imprint.

Can a transformer change the output voltage because of some aging reason?

No, unless it utterly fails.

Or the rectifier diodes?

Diode failure modes are quite simple, open or short.
If one diode goes open, rectification goes from full-wave to half-wave, which usually results in more 50/60 Hz hum.
If one diode goes short, the supply voltage gets low and noisy, and it eventually takes the fuse or the transformer.

Edit: 15ohm resistor from the diodes to the main circuit gave me a close to the datasheet voltage, have a bit less noise

That is quite possible, because the supply voltage gets somewhat cleaner.

and maybe a better sound,

I doubt it.

still have to change the speaker.

No doubt a good speaker is probably way better than what's in there. The problem is a good speaker will set you back much more than the amp's market value. I've seen a few selling for €25.
In addition there is not much choice of good guitar speakers in this size (6" I guess).

 

[LevinGuitar]

Thank you! I was asking about the voltage rise causes because I hardly can imagine that 17.7v was the original voltage supply with 16v caps (all was original).

Edit! The reverb is connected through small caps to the positive lead and pin 10 of the second amp (it's not grounded vía that cap, it's a mistake) if I connect the negative speaker lead to the ground as for the 5.5w datasheet circuit, where should I rewire the negative lead of the reverb? Also to the ground through the cap?

 

[abbey road d enfer]

Thank you! I was asking about the voltage rise causes because I hardly can imagine that 17.7v was the original voltage supply with 16v caps (all was original).

Designers of cheap stuff sometimes take liberty with safety limits. Typically, a 16V cap is usable up to about 20V. It results in reduced life expectancy, but it's not a problem for the manufacturer since the equipment will be out of warranty when it happens!

Edit! The reverb is connected through small caps to the positive lead and pin 10 of the second amp

That would mean the drive is connected to the amp's output and the return is connected to te input of the other amp. That would probably mean that the direct signal is handled by one amp and the reverb signal by the other. It seems weird.
A proper schemo is needed.

 

[Gus]

IMO

Often a low cost solid state amp can sound much better with a speaker replacement.

Years ago I installed some very nice 12" speakers(I forget what brand but they were expense) in a 2 X 12 fender clone cab for a friend.
They did not bring an amp however, there was a old Peavy 8" practice amp available and we used that to test the speakers. We were amazed at how good it sounded with the amp connected to the 2 X 12.