Humming pultec / HT voltage

[ioaudio]

samuel, look at jacob´s layout i posted above.

 

[dagoose]

[quote author="Samuel Groner"]

You mean run the 6.3 V voltage into an analog input (could also be a mixer then) to check if that gives a flat clean signal?

Yes--if you have it digital you can check the waveform as well, similar to a scope but with much limited bandwidth (which should probably not be much of a problem for this case).

I'm not familiar with the design but I think there is some confusion about the fuse. Is there one or more fuse? And before or after the transformer? On the first page you said that when you remove power the hum's gone (which is not much of a surprise). Later one you said that when you remove the fuse for the heater the hum is gone (which is something entirely different as it seems to narrow the problem to the heater).

Samuel[/quote]

removing the fuse just removes the 6.3v (in my case the way i connected right now) and when i power down all power is shut down bu the caps slowly empty so sound is coming through for a few seconds.

I will try with the 6.3v to see waht the PSU looks like, running it into my computer might be best indeed, i can check what the waveform looks like.

 

[keefaz]

removing the fuse just removes the 6.3v (in my case the way i connected right now) and when i power down all power is shut down bu the caps slowly empty so sound is coming through for a few seconds.

I don't understand, how the tube could give sound with no heater voltage ?
Are you sure there is no other 6.3VDC path in your circuit that heats the tube ?
Also I think it is bad for the tube to run HT voltage through it without heater voltage provided

 

[dagoose]

[quote author="keefaz"]

removing the fuse just removes the 6.3v (in my case the way i connected right now) and when i power down all power is shut down bu the caps slowly empty so sound is coming through for a few seconds.

I don't understand, how the tube could give sound with no heater voltage ?
Are you sure there is no other 6.3VDC path in your circuit that heats the tube ?
Also I think it is bad for the tube to run HT voltage through it without heater voltage provided[/quote]

It's just for a few seconds when the 4700uf elco in the 6,3v PSU is discharging so that's why it's working for a few seconds
Do you think it's bad for the tubes to have it run for some time without the heater on? (so no sound, just 250v)

 

[keefaz]

I would think it will short the tube life quickly
You have a voltage limit to not exceed between heater and cathode
You can measure cathode voltage at the output capacitor (4u7 250V) leg
(the leg that is connected to the cathode, not the leg connected to the
output transformer)

What is your tube model/brand ? On tesla/JJ E88CC tube, the voltage
limit between heater and cathode is 120V as seen on its datasheet
That mean you must not have voltage greater than 120V
when you compute Cathode Voltage - Heater Voltage

Regarding measuring HT voltage, I assume you do it safelly,
like put one hand on the back while using the voltmeter probe with the other

 

[Samuel Groner]

I checked the linked layout and it looks to me as if wiring the transformers as to not have a fuse for the HV circuit is pretty dangerous unlike you have a primary fuse (which I'd recommend anyway).

Samuel

 

[dagoose]

I picked up a scope today (philips pm3232 for 45 euro's) and found out that there is no ripple in the 6,3v section.
I see some ripple before the LM317 but after the lm317 all i measure is DC, 6.3v so it looks like the problem is not coming from the heater section.
My probe is just x1 so i can't measure the 250v to check that for ripple but it has 2 big caps whih i think should be enough.
Right now i'm also using the machine with a loud input and it's quite acceptable, about 60db SNR but i really want to get it down way more.

Any ideas what i can do right now?

 

[Samuel Groner]

My probe is just x1 so i can't measure the 250 V.

Oh sure--use AC coupling.

Need more time to think about the other problem...

Samuel

 

[radiance]

Would it not be better to build one power supply and feed both boards from that? Now you have two (actually 4, but two for each voltage) bridge rectefiers connected to one toroid...that's supposed to be a no no... (don't know why though)

You did try to power only one board right? (not only by taking one tube out...)

 

[dagoose]

[quote author="radiance"]Would it not be better to build one power supply and feed both boards from that? Now you have two (actually 4, but two for each voltage) bridge rectefiers connected to one toroid...that's supposed to be a no no... (don't know why though)

You did try to power only one board right? (not only by taking one tube out...)[/quote]

That does make sense indeed..
I was thinking of that too, i will try tonight or tomorrow, i just remove the rectifiers on one board and add the DC of the board together after the other rectifier.
Maybe the answer comes from about 2 kilometeres away while this forum is worldwide

Keep you posted!

 

[CJ]

I have heard of three cases where somebody left the soldering iron on.
Guess who one of the freaks was?>

 

[radiance]

[quote author="dagoose"]
i just remove the rectifiers on one board and add the DC of the board together after the other rectifier.
[/quote]

Well, first try to power only one board, disconnecting the power to the other board entirely. This should give you one hum free board if the rectefiers were to blame.

 

[dagoose]

[quote author="radiance"][quote author="dagoose"]
i just remove the rectifiers on one board and add the DC of the board together after the other rectifier.
[/quote]

Well, first try to power only one board, disconnecting the power to the other board entirely. This should give you one hum free board if the rectefiers were to blame.[/quote]

Just picked up bigger rectifiers anyway, i think the ones i have right now are critical in voltage and current, i bought 6a 600v versions now at Stuut
To me it sounds like this could be the answer to my humming machine, i haven't REALLY tried removing the power of one board, al least not completely, still had some wires going there which made the rectifiers connected.
Now i want the two big rectifiers seperated from the board and parallel the DC voltage to the two baords.

 

[dagoose]

Hail the big Bridgde rectifiers!
It works! The noise level is back from -60db to -75db and more and you don't notice it anymore now!
What is also did is i put the rectifiers offboard on top of the trafo's and so in the shielded section which means only DC is running through the box and AC stay's within the AC section.
Both DC voltages are paralleled on the boards now and it works perfectly!
I might try to do so grounding rewiring to see if i can get it down even more but i'm very happy with the result.

Thanks everybody!
Pic of the machine coming up soon, first i'm going to do some playing with my new knobs :twisted:

 

[ioaudio]

now you´ve eleminated the electrical problems :thumb:

measure (listen) noise with 60 hz full boost for improving the magnetic pick-up interferences, probably beeing responsible for the low level noise.
i suspect the wires coming from the high-cut section might pick up hum from touching the shielding.
also try additional shielding between the high-cut switch and the power switch / wires.
you could dismantle the high-cut switch and move it to the middle to see if it´s the culprit.

culprit is a strange word for a switch. or for anything that induces hum. damn, cj.

 

[robinhuntmusic]

This pultec is now owned by my and happy to report it still works like a charm. For tracking its just bonus