JTM45 Clone Bright channel

[ilfungo]

First pics 1000 hz sine vale
Second 1000hz square wave
Last no signal input but full level.
I've used the original schematic but it's difficult to read and
I've found this attached.

 

[fazer]

Looks like your using tantalum bypass caps for the 820 ohm. Cathode resistor. The values would be less than the 300 uf shown on the schematic. Are you sure the are correct polarity? Have you tried a 300 uf aluminum only and observed scope? More importantly listened to tone. JTM’s are a darker sound. Not a super bright. Amp

 

[ilfungo]

Yes I've used tantalum cap, the polarity is correct.
Could be' a material of cap the problem?

 

[AnalogPackrat]

Have you traced the signal path backwards through the amp to determine at what stage the anomaly on your bright channel sine signal appears?

 

[ilfungo]

All this misuration are on the output jack.
If I come back with oscilloscope the signal is perfect until the summing resistors point
after the problem come back.

with square wave and load on output I can ear hi frequency ringhing from output transformer.

 

[AnalogPackrat]

Sorry, missed that. What type is C4? Have you tried removing it or replacing with a different type?

 

[ilfungo]

Yes I've tried Ceramic and Mica Cap for C4.
I will try Poly type.

 

[AnalogPackrat]

While C4 is out, try it with nothing there.

 

[ilfungo]

This is the situation without C4.
Square wave.I don't know if it's normal but with load on output I can ear very

 

[AnalogPackrat]

How does the sine look?

 

[AnalogPackrat]

With a sine input have you traced the signal all the way backward to the input jack? And one other important question...what is the voltage you've been applying to the input? Guitar maxes out in the 1-2Vpp range.

 

[ilfungo]

Tested with 100mv and 200mv on input

 

[AnalogPackrat]

I wouldn't worry about the output transformer "singing." My unpotted hand-wound API312 mic pre output transformers do the same thing at high levels. The little glitch on the sine doesn't seem normal to me.

When you say the amp sounds too bright, what are you comparing it to? Do you have another amp you can plug into the same speaker & cabinet using the same guitar? Do the tone controls operate as expected?

 

[ilfungo]

It's difficult to explain, bright it's not really correct,

When the level go up the amp start to produce heavy hi-mid .
And seems those frequencies are resonants.

 

[AnalogPackrat]

Resonant like fixed tones or harmonics related to the input?

 

[AnalogPackrat]

Grid stoppers might help if it's oscillating when driven hard. Reflow ground connections. Was this a kit build?

 

[AnalogPackrat]

https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

 

[Matador]

There is significant ringing on the square wave: I would open the feedback loop (disconnect the feedback tap from the output transformer, leading to the 27k feedback resistor), and then ground that lead, then retake the square wave response.

Feedback really should be tuned with the layout and actual output transformer, but if it's got too much bandwidth open loop then you'll be chasing your tail all day.

 

[ilfungo]

Hi
This is the situation with 1000 hz square wave with feedback resistor to ground and after to 16 ohm tap...

 

[ilfungo]

News
if I put a 1000pf cap across 100k plate resistor of V1B the ringing go completly away...
Could be this a possible solution or this could compromise something?
Thanks!