Old Philips-NEFA 2891 A Tubemicpre - Troubleshooting

[ruffrecords]

My HT350 board uses the same principle but with just three stages. It uses 470uF caps as these are made in large numbers for SMPSUs and so are relatively cheap. The reactance of 470uF at 100Hz is close to 10 ohms so with 1K resistors you get 40dB per stage for a total of 120dB.

Cheers

Ian

 

[Conviction]

Thanks, PRR. That was thorough!

Sorry for being unclear, but I think what the "Homebrew B+ supply" does is add a barely audible high pitched sound (which it should not).

And for the successive RC-stages: I think this has more to do with the fact that the preamp pentodes are 1) direct heated and 2) rather microphonic and hissy/noisy over all.
Naturally, I'll have to do some more research on this, but otherwise tinkering with the B+ filtering seems like a patch for a wound that doesn't need it.

I'll keep you updated.

 

[emrr]

Those same pentodes may not seem noisy in a 40-60 dB circuit, but in something like this it's unavoidable. 

 

[Conviction]

So. Tried another B+ supply today (switching), but soon heard a very audible hum. The output is adjustable, though, so I'd like to keep it.
It's the most convenient so far and, also, fits nicely in the case.

I'm thinking of building a filtering bank and put in the B+ rail, successive filtering. Any suggestion in values? 10 or perhaps 22uF caps and, lets say, a couple of hundred ohms in between?

 

[ruffrecords]

I'm thinking of building a filtering bank and put in the B+ rail, successive filtering. Any suggestion in values? 10 or perhaps 22uF caps and, lets say, a couple of hundred ohms in between?

Three stages will give you a really decent hum attenuation. For resistor values you can determine these based on the current draw and the voltage drop across them that you can afford. For the capacitors the bigger the better so use the biggest you can fit in.

Cheers

Ian

 

[emrr]

Some more thoughts on PSU's for direct heated tubes.

http://www.hificollective.co.uk/pdf/diyhs_filament.pdf
http://www.bartola.co.uk/valves/dht-filament-regulator/
http://www.diyaudio.com/forums/tubes-valves/38248-new-dht-heater.html?perpage=30&pagenumber=2
http://www.tentlabs.com/Products/Tubeamp/Tubefilament/index.html
http://www.diyhifisupply.com/opcart/index.php?route=product/product&path=179_136_139&product_id=559

 

[Baltazar]

Hello!!
Totally forgot about this thread, glad i found it again and cool to see another unit in use!!

My unit has been broken for a while now, haven't had time to look into it till now. And i think I need some help...
I'm really bad at tubes and how they work. So if someone have any good links with explanations about the basics i gladly read it!

Mine is now completely dead. All tubes dead (at least not light up), i believe its the PSU (original from 1950-something) thats gone bananas in the filament/heater voltage. Schematic says it should be 6,3v but mine is 16v.
I don't know where to start really, don't have the schematic for the PSU. Any suggestions on what could cause the voltage to change like that?

Attaching the schematic, got it from a electronic guru friend, but he's not really working with electronics anymore.

Best


EDIT: and also, if the heater voltage now is on that high level, should all the tube be blown now or could they still work if i get the voltage down?

 

[Conviction]

Baltazar,

I can most likely fix the PSU for you, if you care to send it to Gothenburg. Would love to retrace it and see how it's done.

I'm glad someone finally uploaded a good copy of the schematics. I took a photo of the inners (mine is glued to the chassis) and posted earlier in the thread.

I'm guessing your "guru friend" works at the tech department of SR 

Edit: the heaters should be in the 4,5V-range. 6V is for the VU bulb.

 

[Baltazar]

PM to you!

The guru friend came across a huge stock of old Swedish Radio schematics and SCANNED all of them..!

New-to-tubes-question: the heater is the one i marked with a red circle in the schematic attached, right?

 

[emrr]

Yes. It is both heater and cathode here, since they are direct heated tubes.

 

[Baltazar]

Thanks!
-
-
-
-
-
-
google > tube basics
-
-
-
8)

 

[Baltazar]

Some progress.
Replaced the two DL94 tubes, they are in series so now the DAF91 works. The jan 6418's are still dead, might have to replace them as well.

The voltage now looks much better but still not there, 8,5v instead of 4,5v.
There is a built in voltage meter and when as you can se on the attached images it says 4,5V in red.
Any ideas what to do next? i guess i shouldn't run the unit too long with the wrong voltage, cause the tubes will blow faster, right?


Edit: Replaced two JAN6418's now the voltage is 7,3v. Just have to wait for the last 6418, that would bring down the voltage to 6,7v - but that is still too much.
My guess is that it's been a few volts to much since i bought it, that was why one of the 6418's was out when i bought it. And i didn't use it that long before the complete breakdown.
Best way to fix it? A resistor?

Also noticed that there was no power to the lamp in the VU. Do i dare to hook it up on the same tap as the incoming (should be) 6v?
Soooo many questions...

 

[Conviction]

I linked the VU with the heater supply (on the top connector, "inre batteri") but unfortunately it caused a little hum. I'll do some research.

By the way: the output should really be padded about 20dB. The unit is so much more useful that way. The input is fine and should not need any padding - at least not for dynamics.

 

[Baltazar]

Ok! I've replaced the bad tubes, but the voltage was as expected still too high.. It's passing audio but there was a pretty "intense" background noise. Just for fun i linked the VU-lamps with the heater voltage.
Voltage is now down on 1,9v (think it was?) the tubes don't glow anymore but all the background noise is gone, passing audio and it sounds much better... Will try it in action really soon.

The tube in the PSU is called 90C1.

This is of course not ideal since if a lamp or two blow up the voltage will bump up again...

 

[Conviction]

Bumping up this thread a little.

The reason is that I'm looking for a shielded enclosure that will fit the top compartment of the box, measuring roughly 17x5x3 inches. This is for the power supply. Don't want the PCB's+transformer to be rattling around in there. Any ideas?

By the way: did you guys notice the ratio of the J/S input transformer? 50/200/500 ohms primary to 250K secondary. That's some ratio there! I didn't notice until today.