Some HMMM on my SRPP

[mod_evil]

Dear DIYAudio Friends,

Tomorrow I was finished my Pré amplifier, with the simplest topology of SRPP. The pre-amplifiers tubes
is russian 12au7.

The power supply is two 12+12 2A back-to-back transformers. The 220vAC is Full Bridged and Filtered with a 330uf capacitor.

The power supply of the Filaments is a 12+12vAC retified and Filtered with 3300uf, and regulated with a 7812.

I haven't noise on the output, the pre-amplifier is opering okay, but I have some HMMMMMMMMMMM.

Thanks,
Felipe Navarro

 

[Geek]

Watchout for h-k breakdown on that top tube.

Use of two tubes for stereo, one top tube, the other the bottom. Float the top tube and bypass it to ground with a cap for noise (2.2 to 4.7uF OK). If you lift the bottom tube's heater 30V or so, AC or DC makes little difference unless the heater insulation is leaky. Since the top tube is a CF, I found that lifting its heater above cathode voltage doesn't make a difference.

 

[mod_evil]

Hey Geek,

I stay using one tube per channel.

Why I need to float the tube's heater to 30v? How I do this? A friend asked me to make a B+ refference and put this refference in the GROUND of the Tube Filament.

Is this correct?

Thanks in advance.

 

[EC8010]

You really do need to change your arrangement so that one valve is used for the lower triodes and the other valve for the upper triodes. It's because you have approximately 150V on the cathode of your upper triode, but at the moment your heaters are connected to 0V. That means that the upper triode has 150V across its heater cathode insulation and that is exceeding its rating. If you use one valve for the upper triodes, you can use a separate heater winding to heat it and connect that winding to +150V. Even better, connect it to +180V to switch off the unwanted diode between the cathode and the heater. Keep the lower triodes fed from their existing heater connected to 0V.

 

[mod_evil]

Hi Friends,

I will modifier mine amplifier to this schematic how I put in this post.

This will work fine? Without HMMMs?

Thanks in advance,
Felipe Navarro

 

[mod_evil]

I have puted a referece of the B+ on the Filament GND. How the schematic I has posted on my last post.

The HMMM persists...



Oh My God.

What I Do?

 

[EC8010]

Try a 47uF 250V capacitor across R6.

Of course, the hum could be something else entirely. Post a photograph of the underneath...

 

[mod_evil]

A photograph.

I will put a 47uf 200v on the R6.

And call the results in here.

Thanks

 

[EC8010]

We need a slighter looser shot that shows more of where all those wires go...

 

[mod_evil]

Friend,

In this photo you can see best than other a best vision of all circuit.

I have putted a 47uf 200v on the R6 and the HMMM persists.

This is a old photo without the Referece of the B+ on the Filament Supply.

Thanks,
Felipe navarro

PS: Congratulations for the U.K. soccer team. The team stay playing well, and GOD SAVE THE QUEEN

 

[EC8010]

OK. There are various reasons for your hum.

(1) You haven't built it on an earthed meta chassis. As a consequence, you have nothing to screen it from hum.
(2) You have long input and output wires that are unscreened. You might get away with that in a metal chassis, but not here.
(3) You are probably getting hum from your mains transformers. Try moving them away, or putting an earthed metal screen between them and the audio.
(4) I hope that isn't a mains switch taped on at the front, with wires trailing under the audio...

You need to read som old (50s or 60s) construction articles to show you how to build audio that doesn't hum.

I'm afraid I'm not interested in football.

 

[mod_evil]

EC8010,

I have mounted this plate to make some tests with valves

You really think the problems is this the transformers or the cables?

Is a good idea solder the GND on the metal of the transformer? ( To shield it )

The switch taped in front is a B+ switch, how I put on when the Filaments will be hot!

 

[EC8010]

Your transformers should certainly have their cases connected to mains earth - for safety, let alone hum! I think your long unscreened leads are probably the problem. If you're just experimenting and aren't worried about looks, why not rebuild your test circuit on a baking tin? It's cheap and will solve a number of your problems.

 

[Geek]

Hey Geek,

I stay using one tube per channel.

Why I need to float the tube's heater to 30v? How I do this? A friend asked me to make a B+ refference and put this refference in the GROUND of the Tube Filament.

Is this correct?

Thanks in advance.

Here is an example of an SRPP filament arrangement that I've used for years, humfree. It shows the floating filament for the top as well as the 32V raised for the bottom. You do not ground them.

** edit **

I see I cropped the values... 470K top and 47K bottom for a 300-350V B+

 

[mod_evil]

Mr EC8010,

I can't rebuild this SRPP.

I recabled the inputs and reorganized the voltages cables.

And the HMMM persists

Thanks and Best Regards,
Felipe Navarro

 

[mod_evil]

Here is an example of an SRPP filament arrangement that I've used for years, humfree. It shows the floating filament for the top as well as the 32V raised for the bottom. You do not ground them.

** edit **

I see I cropped the values... 470K top and 47K bottom for a 300-350V B+

Mr. Geek,

I stay using 169k and 69k in the floating. I have 70v reference.

See the scheamtic how I have uploaded in this post.

Thanks for your reply and Best Regards,
Felipe Navarro

 

[EC8010]

Don't lose hope! Let's see a photograph of it as it is now. There are lots of reasons for hum...

 

[Geek]

There are lots of reasons for hum...

And it vexes the best of us.

Good luck! We'll try and help best we can

 

[mod_evil]

Friends,

I have changed my tube. I putted a 12BH7 made by RCA NOS in the place of 12au7.

The HMM get low, when I puted this new valve. But no much ( I loud some classical music, and with HMMM isn't good )

Thanks,
Felipe Navarro

 

[Zen Mod]

Friends,

I have changed my tube. I putted a 12BH7 made by RCA NOS in the place of 12au7.

The HMM get low, when I puted this new valve. But no much ( I loud some classical music, and with HMMM isn't good )

Thanks,
Felipe Navarro

try to make AC short near or at tube bases-to be sure that hum is in or is not in input cables;

if not-then ......is hum gone in exact moment when you kill mains AC,still with (few seconds,at least) music singing?


few questions for start........