#002 Steel String Singer - almost there

[Lothy]

Hello all,

it was a lot of work, but also fun to build.

IMG_20240308_190744.jpg

IMG_20240308_210511.jpg

More pictures here:

https://photos.app.goo.gl/pPRyMSgD9XeZF1EQ9

If you can see any anything wrong, I would appreciate your comment.

Cheers
Gerhard

[rccolgan]

Looking good!!!

[Raja_Kentut]

beautiful, thanks for sharing !
Aus welcher Ecke in Deutschland kommste ?

[Lothy]

Done, except the FET and Reverb footswitch connector.

IMG_20240309_185627.jpg

IMG_20240309_185633.jpg

I power it up tomorrow. The last 2 solder joints had to be redone twice. I no more in the right condition for high voltage hassle.

Cheers
Gerhard

[martin manning]

...

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[Lothy]

...
popcorn.jpeg

Well, I think I need something stronger.
I got a terrible hum:

https://photos.app.goo.gl/GURBzRK223r8BSjd8

It's there, even if I pulled all the preamp tubes. When I pull the first PI tube, it's gone, no wonder.

Ok, what's different to Ryans layout drawing:

- jumpered all grounds of the preamp board and hooked it to the ground on the input jack

- the reverb jacks are grounded to the chassis + connected to the input ground

- The transformer has a seperate winding for the bias, one side is groundet to the main ground at the power transformer -> that's my bet...

I have scraped of the coating of all the jacks on the back and used lock washers for grounding.

I changed the OT wires on the plates, no difference.

The amp works and it sound really good, from what I can hear through all the hum...

Any ideas, what I can do next?
Thanks a lot!

Cheers
Gerhard

[martin manning]

I got a terrible hum:
It's there, even if I pulled all the preamp tubes. When I pull the first PI tube, it's gone, no wonder.

So that eliminates the CF driver and its power supply.

- the reverb jacks are grounded to the chassis + connected to the input ground

I would ground them at the jacks only.

- The transformer has a seperate winding for the bias, one side is groundet to the main ground at the power transformer -> that's my bet...

If you have the +170V at the driver plates and -170 at the junction of the 100k Rk's it's all good.

Any ideas, what I can do next?

What happens if you ground the PI input (wiper of the MV pot)? If it goes silent that eliminates the PI.

[bepone]

Well, I think I need something stronger.
I got a terrible hum:
https://photos.app.goo.gl/GURBzRK223r8BSjd8

not good filtering for the CF tube?

[Lothy]

What happens if you ground the PI input (wiper of the MV pot)? If it goes silent that eliminates the PI.

No change.
When I lift R64 NFB resistor, the hum is almost gone.
Did I miss a ground some where?

[Richard1001]

Dit you put in the ground wires to the capacitor board?
Try to measure if you have 0 ohm between all the ground connections and chassis.

[martin manning]

When I lift R64 NFB resistor, the hum is almost gone.
Did I miss a ground some where?

OT secondary/speaker ground?

[Lothy]

When I lift R64 NFB resistor, the hum is almost gone.
Did I miss a ground some where?

OT secondary/speaker ground?

Thats it... those damn speaker jacks got no connection to the chassis, even with lock washers.
Thanks god, the OT ground was wired to the jack I'm still using. Otherwise bye bye OT.

Thanks you so much...

Gerhard

[martin manning]

Great!

As long as you have a load connected the OT will be fine. Good practice to connect the OT common directly to the jack as you did, in case the jack comes loose.

[Raja_Kentut]

a cheap way to protect the OT against secondary open condition is a MOV (metal oxyd varistor) just solder it directly over one of the output windings. The rated voltage approx. 2x the highest expected output voltage. They look like a disk with about 15…20mm diameter.
The good thing with this MOVs:
when they fail because of overload, they fail almost always to short. (0 Ohm -> safe for OT)
and they give you two ways of alarm:
They release a fog with a stink of old fish and glow orange

[Lothy]

Here is a quick demo:



Cheers
Gerhard