S.E.X. amp seconde hand

[mwgrient]

Thank you for the link to the knob!

One more thing: the S.E.X. amp is performing wonderful with speakers! But when I plug in my headphone, there is microphonic sound and a hum. When I use the same headphone on my Crack everything is OK.
When I use a headphone adapter for the speaker-outputs, and plug in my headphone to the adapter it is wonderful!

So my question is: what is happening? Why microphonic sounds (tapping n the chassis is really noisy) and a hum when I plugin the headphone?

[Tom-s]

Could someone explain what this sound could be? What could cause it? Is this how an oscillating tube could sound?

[Paul Birkeland]

When I use a headphone adapter for the speaker-outputs, and plug in my headphone to the adapter it is wonderful!

Is the adapter straight wire, or does it have a pad?

[mwgrient]

... not sure what you mean. I have a picture of the cabin. It's Chinese made and I think with resistors in it. Its working great :-)

[Thermioniclife]

Hi mwgrient,
Regarding the noise on one channel when you turn the volume down, you suggested that perhaps it could be the pot and I think it possibly can be. A long time ago I built an amp and the wiper on one channel did not make good contact to ground when turned to zero volume, it was used pot and I sprayed contact cleaner inside and worked it in. that worked but I replaced the pot with a new one as I did no trust the used pot.
The problem you showed on your video can be indicative of a tube grid with no ground reference.
Maybe try turning the volume all the way down and measure the resistance from the center connection (wiper) and the outer connection that  goes to ground, this should be 0 Ohms or close to 0

[Paul Birkeland]

A resistive divider will pad down any noise at the outputs.  You'll probably want to use it in general with IEMs or super sensitive headphones.

We have done some tweaks over the years to bring the noise floor down, and one that you could do would be to change the output transformer wiring from 8 to 4 ohms. 

[Thermioniclife]

Oh sorry, I see that you fixed the oscillation problem, disregard last transmission.

[mwgrient]

Hi all, thanks for thinking! I will take a look at the manual about the 4/8 ohms.
Regards, Marco.

[mwgrient]

I have done the 8 to 4 ohms thing on the pc-3's, according to the manual that came with the amp. To reduce the buss/humming.

Now my right channel has stopt working. There is a c4s board in there. Do not know if this is good to now. And not all the leds are burning.

Am I in deep trouble now?

[Paul Birkeland]

You likely have a loose connection or a broken wire.

[mwgrient]

I have done some precise inspection, with a nylon stick... gould not find anything. Reflowed some soldering points. But nothing. I switched to the 8 ohms again, and still no right channel.

I am wondering: when is it advisable to rebuild the amp; from scratch? I totally lost here because I have no insight information from building it. I bought it build.

Another wondering: can I eliminate de C4S Upgrade easily? Just the plane amp t keep it simple...

What would you do?

[Paul Birkeland]

I am wondering: when is it advisable to rebuild the amp; from scratch?

No.  How about a check of the DC voltages?

Another wondering: can I eliminate de C4S Upgrade easily? Just the plane amp t keep it simple...

You could, but at this point your amp already isn't working, so making more changes that aren't aimed at repairing the problem may just cause new problems.

What would you do?

Check the DC voltages on the channel that isn't working and report any that aren't what they are supposed to be.

[mwgrient]

I have some measurements

Terminal - Voltage
1 - 205
2 - 0
3 - 0
4 - 410
5 - 204
6,16 -  385 / 390
7,17 - 0 / 0
8,18 - 0 / 0
9,19 - 0 / 0
10,20 - 409 / 410
11,21 - 303 / 352
12,22 - 0 / 0
13,23 - 0 / 0
14/24 - 0 / 0
15,25 - 17 / 17
A1,B1 - 0 / 0
A2,B2 - 362 / 365
A3,B3 - 17 / 17
A4,B4 - 0 / 0
A5,B5 - 298 / 70
A6,B6 - 0 / 2
A7,B7 - 0 / 2
A8,B8 - 3 / 3
C1 - 2
C2 - 3
C3 - 0
C4 - 2
C5 - 3

A5 is certainly to normal.
11 too...

[Paul Birkeland]

A5,B5 - 298 / 70
A6,B6 - 0 / 2
A7,B7 - 0 / 2

I suspect you have a broken wire, likely one of the ones connecting the C4S to the A side of the circuit.

[mwgrient]

A5,B5 - 295 / 72
A6,B6 - 9 / 2
A7,B7 - 2/ 2

I checked and replaced the A side of the circuit with new wires. To be sure.
Still got that weird A5 high value?
But the values of A6 is also weird.