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


"Xtrchessreal" <XtrChessreal@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


.

Relevant Pages

  • Re: Turns out the Hum on my 2204...
    ... rated at 600V totalling 120uF at the input filter, ... This was not the case after adding the new Cap config. ... caps give you hiss but I haven't read that, ...
    (alt.guitar.amps)
  • Re: What makes noise/hiss; redux?
    ... There is hiss. ... says, I think I agree that it is the amp, not the tube. ... cap, is it microphonic? ... The bright switch is SPDP. ...
    (alt.guitar.amps)
  • Re: What makes noise/hiss; redux?
    ... There is hiss. ... Clearly, some tubes hiss more, but it ... you that the major component of the hiss is the valve. ... cap, is it microphonic? ...
    (alt.guitar.amps)
  • Re: Turns out the Hum on my 2204...
    ... that the same leg is Cap coupled to the V1a input as well. ... becoming HISS But I can handle that for now. ... Old carbon comp plate load resistors can be very noisy in any amp. ... If it doesn't need correcting then all the rest do but I will ...
    (alt.guitar.amps)
  • Re: Turns out the Hum on my 2204...
    ... that the same leg is Cap coupled to the V1a input as well. ... caps give you hiss but I haven't read that, ... Old carbon comp plate load resistors can be very noisy in any amp. ... If it doesn't need correcting then all the rest do but I will ...
    (alt.guitar.amps)
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