problem with this clone

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Bieworm
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Re: problem with this clone

Post by Bieworm »

Btw, I don't think you can jumper channels. It looks like you have one normal channel with 1/2 12ax7 and the lead channel will likely be the 2nd triode in parallel or series to fatten up the signal. This means the triode of the normal channel can only be used separately OR in series/parallel with the 2nd triode.
Maybe somebody else here with more knowledge says a different thing... but that's what I can derive from the photos.
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

if it can be that it works like that, with which the problem would be in the second channel.

Attached audio file the first two are from the first channel and the second part of the file corresponds to channel 2, first bridge pickup and then neck pickup where the problem with the Bass
AUDIO-2021-04-21-18-07-21.mp3
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Re: problem with this clone

Post by Bieworm »

Wow.. I think you need to check some voltages. This is with the amp fully dimed?..
The low end is out of balance...
Maybe Josh or someone else here knows where to start looking?
The cathode bypass cap seems rather small..try clip a large value in parallel with alligator clips. At least 1000uf 63V. It's the cap at V4. Watch out for the polarity when testing that... negative to ground

You could have more control over that by reducing the cathode bypass cap on V1, but with the pcb that's a pain in the a$$ when you don't have the know-how and skills
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

in the previous valves ? v1 or v2?

Thanks
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zaphod_phil
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Re: problem with this clone

Post by zaphod_phil »

Where's the schematic for whatever amp this is?
Bieworm wrote:
Wed 04/21/21 3:58 pm
Wow.. I think you need to check some voltages. This is with the amp fully dimed?..
The low end is out of balance...
Maybe Josh or someone else here knows where to start looking?
The cathode bypass cap seems rather small..try clip a large value in parallel with alligator clips. At least 1000uf 63V. It's the cap at V4. Watch out for the polarity when testing that... negative to ground

You could have more control over that by reducing the cathode bypass cap on V1, but with the pcb that's a pain in the a$$ when you don't have the know-how and skills
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Re: problem with this clone

Post by Bieworm »

zaphod_phil wrote:
Wed 04/21/21 4:26 pm
Where's the schematic for whatever amp this is?
Bieworm wrote:
Wed 04/21/21 3:58 pm
Wow.. I think you need to check some voltages. This is with the amp fully dimed?..
The low end is out of balance...
Maybe Josh or someone else here knows where to start looking?
The cathode bypass cap seems rather small..try clip a large value in parallel with alligator clips. At least 1000uf 63V. It's the cap at V4. Watch out for the polarity when testing that... negative to ground

You could have more control over that by reducing the cathode bypass cap on V1, but with the pcb that's a pain in the a$$ when you don't have the know-how and skills
Think that might not be an option...that will possibly remain a mystery. The OP should post this amp on various forums to trace the origin of this one.. since it's a pcb amp it should be built in certain volumes, so somebody must know something.
I looked up that OT.. it's 30W??? That's a skinny little iron, no? Maybe what we're hearing is that little one choking on its food? The PT ain't no heavyweight either for that matter. Sounds like this thing is running to its limits
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

Hello, the amplifier is 18W I do not have the electrical diagram

I thought it was faulty because on channel 2 with the bridge pickup it sounds very good and the problem appears with the neck pickup and only with the low notes

Thanks
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Re: problem with this clone

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cisatracurio wrote:
Thu 04/22/21 9:19 am
Hello, the amplifier is 18W I do not have the electrical diagram

I thought it was faulty because on channel 2 with the bridge pickup it sounds very good and the problem appears with the neck pickup and only with the low notes

Thanks
What's the value of the V1 cathode bypass cap? Is the problem still obvious when you turn the bass pot down? Low frequencies can turn your sound into blubber and mud on high gain settings
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geoff 1965
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Re: problem with this clone

Post by geoff 1965 »

the chassis is marked as "JMP" so if the preamp is plexi channel 1 will have a .68uf on the cathode of that triode "i can see that on the board"
channel 2 will have a 220uf on the cathode "this was intended for bass guitar" so you will get mushiness with the neck pickups.
notice the PT's secondary of 235-0 @ 75mA,that's too low and borderline for current rating.
cisatracurio are you confident at taking some voltage measurements?
be careful if you are unsure with this!
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

I will review what they have told me
yes yes yes I could take current measurements

Thank you so much
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

Don't you think it could be due to a fault in the bass filter?


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Re: problem with this clone

Post by Bieworm »

I'm afraid it's all guessing. And pretty complicated to troubleshoot without schematic . How does it behave with the bass pot turned down? It's a tough diagnosis when you don't answer questions...
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

it behaves the same and the filter if it complies if it works

as soon as I make the measurements I count them
and I'm going to try to get the electrical scheme

Thanks
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

Hello I am attaching the diagram that the constructor has sent me

Thank you

4BCE61DC-E48A-45B8-AA85-E739932F7BA1.jpeg
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

A8258E79-1F75-4C25-A753-B71931AFF901.png

Thanks
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Re: problem with this clone

Post by Bieworm »

this can not be the schematic of that amp. Your amp has 2 preamp tubes.. this schematic shows 3 preamp tubes
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cisatracurio
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Re: problem with this clone

Post by cisatracurio »

You are right, I had not noticed anyway in the brightness channel the 680nf capacitor gives a cutoff frequency of 86 Hz, perhaps modifying it?

I have asked for the correct scheme

Thank you
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Re: problem with this clone

Post by Bieworm »

cisatracurio wrote:
Mon 04/26/21 9:09 am
You are right, I had not noticed anyway in the brightness channel the 680nf capacitor gives a cutoff frequency of 86 Hz, perhaps modifying it?

I have asked for the correct scheme

Thank you
If you're referring to the 0.68uf cathode bypass cap... that's already a low value and likely not the problem. It's typical marshall value, intended to prevent the preamp from having too much low end boost. To be sure you need to untie the wire from pin 3 or 8 on V1 ( the one connected to the .68uf cap ) and put a 2.2k resistor in series with that wire, with the other end of that resistor to ground. That will tame the low end boost...but will change the character of the overdrive. But I doubt that is the problem.
What is the value of that electrolytic cap on the far left on the board? Looking at the amp from the front. If that is a 220uf cap like geoff implied that is going to be the possible bass pusher. You could temporarily disconnect that other cathode wire and put a lower value cap to ground, parallelled with a 1.5k resistor. Try a 50uf 50V cap first. Go even lower when needed.
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