V76 noise problem (pics inside)

[sodderboy]

Just a follow-up from my EMT fixperience here:
Measured at the rectifier output.
Original selenium rectifier- 321 rippled VDC, and bouncing around.
Replacement silicon rectifier- 333 rippled VDC, no bouncing.
The difference is an increase of 3.7 percent. Not enough to warrant messing with series resistors. Anything over 7% would make me thing differently.
Mike

 

[CJ]

I trhink it depends on how new the selenium rectifier is.
Don't they develop a greater volyage drop as they age?

This sound difference thing has me all wound up on doing A/B tests, Selenium vs Silicon, but what if I like the selenium?
Where the heck would I get a good one?
The Ollienator, perhaps?

 

[sodderboy]

I cannot imagine what the difference could be, except for a voltage difference hitting the filtering and regulation. That kind of comparison is like "those Duracells sound much better than the KMart batteries".
And if you REALLY need some, there are still SR manufacturers:
http://www.lphoying.com/selenium.html

Mike

 

[CJ]

I thought about how lame it would be to put a toxic time bomb back in the V72 just so the sound does not tweaked, then I woke up.

I am either going to rewindthe pwr trans for lower sec volts, or try this little UTC choke that will drop right in there.

Could also ggo with a tube rectifier if I rewind the xfmr for a 5 volt tap.

 

[martthie_08]

Some time has passed and I haven't really gotten around to fix up the V76, in the meantime I have moved to Basel, assisting in a recording studio. There is a really nice tech guy here (who is also a groupdiy member) and I am sure I can get this unit up and running 100% in the next weeks. I ordered the four main 32uF filtering caps from Reichelt and hope they will fit. If the problem persists I will go on for a step by step recap of the whole preamp. The schematic of the unit is on my site now, please have a look here: http://www.homepage.mac.com/marten.thielges/racking/V76_Schaltplan.pdf
to be continued...

 

[martthie_08]

ok, replaced the power supply electrolytics and most of the signal path electrolytics, while I was at it and the noise seems to be gone!!! At least I had the preamp running for about an hour and couldn't notice anything. Great!

Now another issue is on my mind: I have two V72 sitting in the same rack and they go up a good bit more in the highs as compared to the V76. Could this be because of the PI filter after the low cut switch in the V76? Can I totally leave this out of the circuit (remove the two caps, jumper the inductor)? I have found differing opinions with a google search...

thanks for all your help! cheers, Marten

 

[tommypiper]

Martie, my understanding is that the V76 has plenty of top end and clear highs with the stock filter. It is totally usable in either state, with or without filter. I have heard it both ways and one was not any duller sounding than the other. It depends on what one means by extended highs, but for practical purposes there should not be an obvious difference with your V72s with or without the filter in my experience. (I have not done controlled comparisons, just my impressions, so someone may shoot me down.) :grin:

Maybe your V72s are sounding too thin? Because a properly set up stock V76 does not lack top end!

 

[Michael Tibes]

[quote author="martthie_08"]Could this be because of the PI filter after the low cut switch in the V76? Can I totally leave this out of the circuit (remove the two caps, jumper the inductor)? I have found differing opinions with a google search...[/quote]

I was surprised to read somewhere that the pi filter absolutely needed to be there, I hadn't found any problems without - and the V76m anyway doesn't have it. I originally removed the filter on a few units because the coil was broken.
I also haven't found a serious difference in treble reproduction between V76s and V72s. My V76s are all entirely recapped (the el. capacitors).
Sometimes I get a feeling, that some people on the net make things a bit more mysterious - so that they appear more like magicians...
I do feel like V72s might sound a bit more 'open' - maybe due to the simpler circuit?

Michael

 

[martthie_08]

The V76 is fine, it was me not paying attention, the hi cut filter was in I have never have it in, so I don't pay attention to it, somebody must have touched it...
The V76 has been in a record session all day with a U47 and it's fine! These guys fit well together, will report back soon later with some more impressions.
Anyways, big thanks to all of you :thumb:

 

[CJ]

The pi filter, when it is working, will flaten the response a bit on the high end.
Without it, there is a 3 db rise, so I am told.

Those inductors use a ferrite pot core that is glued together. I have only seen one, the pot core halves came apart, the glue failed, and so with the gap, there was very little inductance. If this is the case with yours, then the inductor becomes , more or less a resistor, so your pi filter does not roll off where it used to.

I do not have it on my 76 and it sounds fine.

 

[mikka]

Bit OT but:

Could also go with a tube rectifier if I rewind the xfmr for a 5 volt tap.

I spoke to an old tech some time ago who told me they used to just stick 5volt rectifiers in 6.3 circuits and didn't give a raz. It shortened the life a bit but not enough to matter. Another option was just to stick a block resister in there to drop the filament voltage.

Edit: Think I dragged this in some time ago. Alzheimers is kicking in...