Re: Turns out the Hum on my 2204...


"Xtrchessreal" <XtrChessreal@xxxxxxxxx> wrote in message
news:38131d6d-864b-4ea3-a843-e4bfdbe56428@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jan 18, 8:17 am, "Phil S." <psymo...@xxxxxxxxxxx> wrote:
"Xtrchessreal" <XtrChessr...@xxxxxxxxx> wrote in message

news:1b430725-dd52-4d9a-97d3-2f5b8a927e10@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



So my parts came in. I placed four 10uF two 20 uF and one 40uF all
rated at 600V totalling 120uF at the input filter, this replaced the
single can 450V 50uF + 50uF (a nice looking can).

Before the new config change initially I measured an AC 120 Hz signal
on V1b plate at about 30 mV not distorted with a positive half cycle
and then flat. It doesn't really give you an alarm unless you realize
that the same leg is Cap coupled to the V1a input as well. So with no
input signal on V1b you can hear the 120Hz hum very easily because it
is making its way through the back door.

This was not the case after adding the new Cap config. the voltage
completely disappeared and I have no hum on my 2204. It actually
sounds just like the Marshall built 2204 now. I am amazed that this
cap config has provided this level of quality to the circuit, it is
that different. I cannot measure a voltage any longer, with my Oscope
to its most sensitive 2 mV per division. I am sold on this cap
filtering method.

I bought a bunch of small value caps at 50V for the bias I will
replace the current config with that tomorrow. Now my issue is
becoming HISS But I can handle that for now. My guess is that the
caps give you hiss but I haven't read that, its just you never get
something for nothing.

Later ppl

X

Congratulations on solving the problem. I bet this feels good.

On the hiss, in my limited amateur experience, hiss comes from a few very
basic things.

On a high gain amp, any little bit of tube hiss is amplified to extreme
levels and is difficult to eliminate. Some people put a small value (maybe
10pf) across the plates of a the dual triode. I don't think the 2204
qualifies for this sort of thing. You might also benefit from putting
shields over your preamp tubes, thought this tends to be a solution for
hum
rather than hiss.

Old carbon comp plate load resistors can be very noisy in any amp. Replace
those with metal film resistors. There is no tone on the plate load
resistors, so changing the material from cc to mf should not influence the
mojo of the amp one bit. Do all of them.

You've probably done this a few time already, but you might give it the
once
over looking for bad solders, loose jacks/pots, etc.

Coupling caps have an inner foil and outer foil. I am a bit of a skeptic
on
this one, but Randall Aiken (who I freely acknowledege is light years
ahead
of me) says it matters which way the cap is installed. In particular, I
don't know if you can see the hiss on your scope, but, if you can, you
might
find that reversing one cap makes a difference. Aiken discusses noise, not
hiss, but I guess hiss is one kind of noise. Read it for
yourself.http://www.aikenamps.com/OutsideFoil.htm

Wow, that Aiken knows his stuff! I hadn't read his info in a while.
The information is dead on correct too. I just didn't think about
that before. His tip of how to check the shield side is a great tip.
One of those things you never see in a book.

I have more work to do now. I have a lot of hiss and I suspect I
could benefit from this Cap orientation. I did keep all the resistors
aligned the same as I was building it but if they are all the wrong
way then that would benefit me greatly. So I guess I should first
double check all of the alignments, looking for one that is not
aligned the same and use that one to test the orientation, keeping a
note of how it was in the circuit and then correcting it, if it needs
it. If it doesn't need correcting then all the rest do but I will
still check just in case to save unnecessary work.

In general I know the 2204 has hiss so I will also check this amp
against my Marshall Built 2204 to verify the hiss level. If they both
bother me then I won't do anything. If the one I built is
significantly louder with hiss then I will continue with plan A above.

As far as solder joints go I have been wondering about them. Is there
a reference you are aware of that can give me close up pics of good
soldering techniques so I can compare that as well. I feel I do a
good job but then I have never had someone with a fair amount of
experience look at my work. Generally I don't over solder or leave a
mess but if there is a technique or a look I can shoot for then I can
make corrections. I suppose it can be a bit like welding where the
beads needs to be uniform etc.

Thanks for the tip and the info

X

=================================
Lots of stuff on youtube. You need to form a nice shiny cone. Try this one
(many examples of nicely done joints, or search:
http://www.youtube.com/watch?v=I_NU2ruzyc4


.

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