JTM45 Clone Bright channel

[ilfungo]

Hi
I've built a JTM45 Clone
The amp sound good just a little to bright for me.
After few ivestigation with oscilloscope I have disover that the bright chanel have a strange sine waveform.
The normal channel not.
I've changed tube, caps and resistors on the bright channel but nothing help.
If I remove the bright cap and bypass cap on 270k mix resistor the problem go away...
I don't know if it's a problem, but someone with more experinces can help me?

 

[dmp]

Probably layout and wiring. Can you post a pic of your build?

 

[My3gger]

Probably layout and wiring. Can you post a pic of your build?

Would you explain why would problematic layout give scope picture like for bright channel? I'm asking because i was mostly lucky with layouts and never seen such picture because of it.

 

[dmp]

I haven't seen your layout to say

 

[ilfungo]

Tomorrow I will post some pics...
It's strange only on bright channel.
Same tube and few other component before the summing resistors and I have replaced all with same result.
Thanks

 

[dmp]

More likely to get high frequency feedback with the 'bright' cap

 

[AnalogPackrat]

Ceramic or film cap?

 

[ilfungo]

Ceramic, but I've tried also Mica, same result...

 

[ilfungo]

inside JTM45

 

[AnalogPackrat]

Looks like a very neat wiring job. Does anything change on the waveform when you poke around or tap various components with a wooden chopstick?

 

[ilfungo]

I've tried but nothing change,
also moving the cables do not change.

 

[Matador]

What does your square wave response look like?

 

[ilfungo]

Hi Matador
this is with 100mv square wave input
I 'm not an expert with oscilloscope, I know only see the signal but not decifrate it...
But the problem seems on both channel but more on Bright?

 

[musipol]

What's the trace look like with no input and bright vol at full? Is it oscillating?

 

[ilfungo]

This is with no signal input , at max level

 

[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.

 

[musipol]

Looks ok, like noise, not osc. What is that switch you have on the back panel? It looks to be connected to the 1st tube thru some caps......I can't quite tell

 

[abbey road d enfer]

After few ivestigation with oscilloscope I have disover that the bright chanel have a strange sine waveform.
The normal channel not.

The only differnce between the Normal and Bright channel is the 100pF and 500pF on the Bright channel. What happens if you disconnect them?

 

[ilfungo]

Looks ok, like noise, not osc. What is that switch you have on the back panel? It looks to be connected to the 1st tube thru some caps......I can't quite tell

It's simple 3 position cathode bypass cap, I've disconnetted it tu be sure but is not th problem

The only differnce between the Normal and Bright channel is the 100pF and 500pF on the Bright channel. What happens if you disconnect them?

I've tried and the channles are near the same but analyzed only with sine wave...
I need to try with square wave, the problem is more evident.

 

[AnalogPackrat]

Can you provide a link to the schematic of the version you've built? Also, what frequency were you running in the scope shots you attached (sine and square)?