Re: What makes noise/hiss; redux?


"Stephen Cowell" <scowell@xxxxxxxxxxxxx> wrote in message
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"Phil S." <psymonds_no_spam@xxxxxxxxxxx> wrote in message
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"Stephen Cowell" <scowell@xxxxxxxxxxxxx> wrote in message
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"Phil S." <psymonds_no_spam@xxxxxxxxxxx> wrote in message
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Sorry to beat a dead horse, but there is something and I'm not "getting
it". Thanks for bearing with me.

I'm on that Trainwreck Express variant again. There is hiss.

The V1 tube does matter, but I've got one 12AX7 (Siemens) that seems
about the same as a 5751. Clearly, some tubes hiss more, but it
suggests to me that I've got a pretty decent tube in there now, the
Siemens.

You should own a Sovtek 12AX7WB, just for
a reference. They're cheap (not that wonderful a
tone) but just about the quietest available... get
several, *all* tubes vary in hissiness to some
degree.

Thanks...this is a reasonable consideration, but reflecting on what mykey
says, I think I agree that it is the amp, not the tube.

Ah, yes... mykey. Whatever... I'm here to assure
you that the major component of the hiss is the valve.
If you're having a hiss complaint in the gain hole it's
normally the valve or the plate resistor going bad.


I don't see anyone contenting this point, maybe a bit of dithering around
it. Let's stipulate that it's a given. Changing the plate load resistor
has not effect. Changing the tube does, but even a 5751 is noisy, just not
as noisy as a 12AX7. I have three 5751's (GE, RCA), consider them used, but
they sound OK and don't exhibit significant microphonics.

It is possible for cathode resistors to contribute
to hiss... play around with your treble hi side
cap, is it microphonic? Are you using mica,
ceramic, poly?


OK, this is where the rubber might meet the road. I had not focused on
this. This was my first parts order from Mouser. I've you've ever tried
to manage an order from the website, you know what I mean when I say it's
one step removed from rocket science. Not everything I ordered was what
I had in mind.

These houses handle all customers... places
specifically for amps don't carry the short-leg
stuff.


The zealots who worship this amp say it needs Sprague Polypropylene OD caps.
I dunno from Jacques Chit, but tried to get those. It is the reason I used
Mouser, but then they were out of stock on some of what I needed. Heck, it
was an experience I don't recommend. Prices are decent, though.

The parts will work, just be a funny fit... but
better-sounding alternatives are available.


Yes, they are in the circuit 8^}

I prefer disc ceramic for the small caps...
tried silver-mica, it killed the sparkle. They
can be microphonic, but this is part of the
organic nature of the amp... valves themselves
are microphonic as well.


The schematic calls for a 500pf treble cap. For this, I unwittingly
ended up with a 470pf WIMA polypropylene film cap, mouser #
505-FKP1470/1600/5.I've got one 500pf silver mica. Do you think this is
a swap worth doing?

Sure... this is where you find out what works.
Try every different cap you can find here...
it's the only way to get the best tone.


OK, worth doing, if I can come up with some choices that don't cost me an
arm and a leg with postage. You know how that works, 10 cent part costs
$10.10.

[I've got the same WIMA cap on the bright switch , which sits across the
vol pot wiper and the same outer leg fed by the Treble pot. Sometimes,
you get lemons and you make lemoade. This WIMA cap is for a PC mount. I
gingerly bent the short stubby legs into loops and added some wire. The
other side of the bright switch is an axial (small orange) 100pf 630v
Vishay/Sprague polypropylene film cap, Mouser# 594-2222-464-31001.]

But you keep that bright switch turned off, right?
I don't have a schem... is that circuit fully switched
off? Or is some of it still working with bright off?


Sorry, apparently these are only published to Ampgarage members <your
favorite snorting noise here>. If this keeps up, I'll see if I can post it
somewhere in the open. AFAIK, there is no copyright.

The bright switch is SPDP. Down is the 100pf and up is the 500pf. I'm
running it with the 100pf in circuit. Funny, I didn't read the schematic
carefully before this. The proper switch is on-off-on. Maybe I'll get one
of those. Meanwhile, maybe I should lift these caps from the pot?


You can form a 'hiss probe' by putting a
500v .001uF cap on an alligator jumper. Shunt
(short to ground, via the cap) the signal
path at different places and look for the
drop.

Yikes! You think this might be worth taping to a chopstick? Or is this
something that requires power off between each test?

Oh, I don't know... big fat cap, insultated clip,
short lead on one end... it's been done. Best
is probably a ball-point-pen case with the
assembly inside.

Clever! And I saw your other post about using a 1mf cap.

__
Steve
.




.

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