Humming pultec / HT voltage

[dagoose]

Hi all!

I'm in the middle of testing and finalizing my stereo extended pultec but i do have a really annoying 50/ 100 hz mains hum.
I read some stuff about it that the 25v voltage to the tubes might be to high, mine reads about 270v after the resistor (so on the tube) and 300v before the resistor which is the standard 3k3, i think that is a bit high isn't it?
Do you think it will help to change the resistor to a different value to lower the voltage? and what resistor should i use then?
When i power down the machine the hum is instantly gone and i have a perfectly working EQ for a couple of seconds so i don't think it's a groundloop hum.
I tried moving the toroids without result, no differnce at all...
I also tried to reverse the second transformer polarity with no result as well..
When i remove one of the tubes (when i have just one channel connected) it gets a bit quieter.

I'm waiting for the input transformers to come in but i don't think that will solve the problem completly.

Anyone?

 

[ioaudio]

voltage is ok, as long as your parts (caps) are rated high enough.

if the hum is below the signal level and changes a lot while boosting 60hz
try some shielding between the power switch/wires and the high cut section.
try removing the high cut switch so the wires dont touch the shielding.


if the hum is very strong/loud, so you basically cant work with the eq, it might be electrical gounding problems or ripple from the psu.
-max

 

[Samuel Groner]

Posting a WAV sample (30 s) of the noise might help.

Samuel

 

[keefaz]

Do you have hum with EQ bypassed as well ?

 

[dagoose]

[quote author="ioaudio"]voltage is ok, as long as your parts (caps) are rated high enough.

if the hum is below the signal level and changes a lot while boosting 60hz
try some shielding between the power switch/wires and the high cut section.
try removing the high cut switch so the wires dont touch the shielding.


if the hum is very strong/loud, so you basically cant work with the eq, it might be electrical gounding problems or ripple from the psu.
-max[/quote]

I think, like i mentioned, that it is indeed the PSU section.
I'm using 2 big shielded toroids. http://www.nuvotem.com/en/products/std_encap230v.shtml en then the one with 2 x 12 v scondary.
What is do right is paralled the 2 x 12 sec so that the coltage stay's 12v but it uses all available power, according to the specs this should not be a problem.
The second trafo for the 250v section is used in the same way, so 2 x 12v parallel and then use the prim for the 250v section.
I hear sum humming coming from the trafo's itself but it looks like all voltage's are ok.
Maybe i'll try to use one sec section for one channel and th other sec for the other channel and then one of the sec to 1 sec of the second trafo to get to 250v.

I made a really great drawing of the stuation, i'm a real photoshop designer hero as you can see :razz:
But just wondering if that might solve the problem.

 

[dagoose]

Just tried the situation as is drawed and still get the hum... so that didn't work... too bad..
I made a soundexample with the hum, first seconds with hum and after a couple of seconds without beacuse i turn the powerswitch off (mains out) and you can hear with boosting at 60hz and 5khz the hum stay's the same so i'm sure it's the tubestages.
Anyone? http://www.dagoosemusic.nl/temp/humtest.wav

 

[Samuel Groner]

Sorry, I meant a sample without music, input shorted. That way we can look at the waveform and spectrum of the noise which tells one or the other thing.

Samuel

 

[dagoose]

OK, going to record that tomorrow, it sounds like a 50hz hum coming from the PSU of the tubestage, like i said.. when i power down the hum is gone instantly while the sound goes on for a couple of seconds (elco's).
I'm waiting for the input trafo's, i think i will get 'm tomorrow so i think i'll wait for that, balancing the signal on the input might be helping too.

 

[Val_r]

[quote author="dagoose"]Hi all!

I'm in the middle of testing and finalizing my stereo extended pultec but i do have a really annoying 50/ 100 hz mains hum.
I read some stuff about it that the 25v voltage to the tubes might be to high, mine reads about 270v after the resistor (so on the tube) and 300v before the resistor which is the standard 3k3, i think that is a bit high isn't it?
Do you think it will help to change the resistor to a different value to lower the voltage? and what resistor should i use then?
When i power down the machine the hum is instantly gone and i have a perfectly working EQ for a couple of seconds so i don't think it's a groundloop hum.
I tried moving the toroids without result, no differnce at all...
I also tried to reverse the second transformer polarity with no result as well..
When i remove one of the tubes (when i have just one channel connected) it gets a bit quieter.

I'm waiting for the input transformers to come in but i don't think that will solve the problem completly.

Anyone?

[/quote]

Hi,

Your hum does not come from hi voltage, but it is much like the heaters to disturb you....

if you have a large current demand from the heaters of both channels, remember that IC's like LM317, 350, 78xx etc. go crazy if you do not supply some volts more than needed,
100Hz is coming from the diode bridge,
too much current request for the 317 regulator, and it gets the ground dirty.

You really need to supply with a larger voltage before the regulator, best done through a Pi filter, or better a common collector electronic filter with a power BJT, such as BD231 or whatever similar NPN you like.

Try that and let me know.

Remember to star ground all of the ground from heaters (i.e. only 1 point from heaters ground to your main ground).

Let us know......

Keep up the good work.

Respect,

Val.

:wink:

 

[dagoose]

[quote author="Val_r"]


Hi,


You really need to supply with a larger voltage before the regulator, best done through a Pi filter, or better a common collector electronic filter with a power BJT, such as BD231 or whatever similar NPN you like.

Try that and let me know.

Remember to star ground all of the ground from heaters (i.e. only 1 point from heaters ground to your main ground).

Let us know......

Keep up the good work.

Respect,

Val.

:wink:[/quote]

Ok.. hmm.. how do i do that?
right now i have 2 toroids with 2 x 12v sec so i could put 'm in serie to get 24vAC but i think that might be a bit much to drive the LM317 for the heater?
I think 12v to the LM317 is enough, is measure 6.3v on the tube heater but it sounds indeed logical that the heaters are cauing the problem or am i missing it here? havn't really checked the DC voltage before the LM317, might be a lot lower then 12v?

Or can i use a different regulator then the lm317 or maybe some extra elco's to get rid of ripple?
I think a lot of people build the stage this way so i think the elco's this way should be enough.
What do you mean with a PI filter?

Thanks!

 

[ioaudio]

12 vdc unregulated should be plenty for 6,3vdc from the lm317.

 

[keefaz]

What are your power transformers specs? 2 * 12V, 24VAC ?

Also did you try to disconnect one board (the left board on your photo)
to see if the hum remains ?

 

[dagoose]

I tried some things tonight without 'enough' result (it is a lot better now but is still have a big hum. To me it still feels like it's the PSU, i think of the heater but it could also be the 250v power.

I disconnected one channel so the trafo's where connected to only one board, made no difference
Tried some rewiring of the ground, all grounds are star grounded to the frame now and tried without the mains ground.
I also added the input trafo's today, this helped quite a lot, the hum level is down a lot now but still very noticeable.

The trafo's are 2 x 12v secundary toroids, should do the job.. http://www.nuvotem.com/en/products/std_encap230v.shtml

I made an example of the hum again, this time with sound and without and bypassing (hard bypass with relais so input /output XLR connected) left and right seperately so you can hear the difference, the channl most away from the toroids is humming the most (the left board on the picture)

http://www.dagoosemusic.nl/temp/humtest2.wav

Anyone?



EDIT: i was thinking of something that might be worth trying tomorrow (going to bed now): i have a 270k bleeder resistors on the first 400v elco soldered directly to the board to get rid of the high voltage after switching of (takes about 10 minutes now to get to 0v).
Do you think this might help if i remove it? if the machine is working fine you don't need 'm anyhow..

 

[keefaz]

If you remove your bleeder, voltage may take more than 3 hours to drop from 300VDC to 30VDC (I experienced that).
Maybe you could improve your wiring layout ?
You could twist the AC power wires, use shielded cable for I/O connections..

 

[Val_r]

[quote author="dagoose"]
Ok.. hmm.. how do i do that?
right now i have 2 toroids with 2 x 12v sec so i could put 'm in serie to get 24vAC but i think that might be a bit much to drive the LM317 for the heater?
I think 12v to the LM317 is enough, is measure 6.3v on the tube heater but it sounds indeed logical that the heaters are cauing the problem or am i missing it here? havn't really checked the DC voltage before the LM317, might be a lot lower then 12v?

Or can i use a different regulator then the lm317 or maybe some extra elco's to get rid of ripple?
I think a lot of people build the stage this way so i think the elco's this way should be enough.
What do you mean with a PI filter?

Thanks![/quote]

Did you measure the current that the heaters draw?
Try also to check if ground is clean (with a scope)
Hope this helps.

Val

 

[CJ]

so this just went by everybody and you went on a wild goose chase?

<I'm waiting for the input transformers to come in but i don't think that will solve the problem completly. >

This is the first "Doh" award I have given out, admidtaly predjudice because of my affiniyty for xfmrs.

wake up u guys and gals

i wondewr whos gonna get the next "Doh" award?
:razz:

 

[dagoose]

[quote author="CJ"]so this just went by everybody and you went on a wild goose chase?

<I'm waiting for the input transformers to come in but i don't think that will solve the problem completly. >

This is the first "Doh" award I have given out, admidtaly predjudice because of my affiniyty for xfmrs.

wake up u guys and gals

i wondewr whos gonna get the next "Doh" award?
:razz:[/quote]

don't get it... i think you mean that with placement of the input trafo's (balanced) the problem was gone... i knew that it wasn't gone then, the output is humming so the input won't matter.. i don't think i deserve a DOH award
But hmm... why do i always have to read your post like 4 times and still don't get it? :shock:

But anyway.. just did some research again, rewired some stuff, the IO cables are shielded (thick original prefer cable). Addes some extra caps in parallel with the 10uf elco after the lm317, twisted the AC cables t the boards, tried switching polarity in all directions but hardly any result..
I don't have a scope (still on my wishlist) so i can't measure anything.

Right now i 'm thinking about the tubes, instead of the ECC88 tubes i'm using 6922EH electro harmonix tubes, they should be the same as the original tubes but i couldn't find any info about heater voltage and stuff. anyone?

 

[dagoose]

To give some idea of how the inside looks like right now, a high res picture.

 

[ioaudio]

just to be sure: you disconnected the filter network and shorted the input from the amp boards and still get the hum?

 

[CJ]

oh, now you add the input irron to make me look like a punk, ok, be that way.
:razz:
:grin:

:wink:

:?
:roll:
:shock:

:sad:

:evil:
:twisted:
:green:

Wow, I just did a scientology tone chart with the moticons,
tom cruz, eat your heart out you gay mutha f....

hey now....chill it.

OK, I will bring you back up...

:grin: :razz: :green: :razz: :grin:

That angry girl loked like she was gettin a nut!

:green: :green: :green: :green: