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Old 6th August 2013, 08:15 AM   #31
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by ITPhoenix View Post
...the hiss is still there. The hiss sounds like white noise.
forgot to ask ...

is this with guitar plugged in ?

and does it change if you touch the strings ?

(when not playing my preamp control is always turned fully down)
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Old 6th August 2013, 10:40 AM   #32
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Quote:
Originally Posted by tinitus View Post
I suppose it will only reduce the lower frequencies that does not produce hiss

so, basicly it only means increasing the hiss, in relative terms
You don't really want frequency selective feedback in that way, the cathode decoupler should be chosen to give full gain at less than the lowest frequency you're going to be using.

Using too small a capacitor gives bass cut, which is effectively treble boost - making the hiss worse.
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Old 6th August 2013, 10:44 AM   #33
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Quote:
Originally Posted by ITPhoenix View Post
Unlugging V1 makes it virtually all disappear. I have to put my ear up to the speaker to hear it.
Which is what you would expect, hiss comes from the front end, and is amplified as it passes through the rest of the amp.

Replacing the front end with a single EF86 would probably help, noise is why you don't use triodes in the front end of mike or tape preamps. Also use metal film resistors in the front end, as they are quieter.

But for the gains and overdrive you're obviously looking for, hiss is probably fairly normal, and to be expected (and is part of the 'valve sound').
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Old 6th August 2013, 12:08 PM   #34
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Quote:
Originally Posted by Nigel Goodwin View Post
Using too small a capacitor gives bass cut, which is effectively treble boost - making the hiss worse.
yes

Quote:
Originally Posted by Nigel Goodwin View Post
You don't really want frequency selective feedback in that way, the cathode decoupler should be chosen to give full gain at less than the lowest frequency you're going to be using.
for music or PA .... no
but for instrument amp it can be used that way
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Old 6th August 2013, 01:50 PM   #35
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Originally Posted by tinitus View Post
for music or PA .... no
but for instrument amp it can be used that way
Even for instrument use it would be a bad idea - if you truly want to restrict low frequencies?, there's far better and more effective ways (the roll off rate is pretty poor).
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Old 6th August 2013, 01:58 PM   #36
tinitus is offline tinitus  Europe
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its well described on valvewizard.uk, in every detail, calculation and all ...

but why is that so interesting ?


I would rather hear the answer to my previous post

but maybe it doesn't matter
its clear now the preamp is too complicated to be just about cathode bypass caps

Quote:
Originally Posted by tinitus View Post
forgot to ask ...

is this with guitar plugged in ?

and does it change if you touch the strings ?

(when not playing my preamp control is always turned fully down)
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Old 6th August 2013, 02:27 PM   #37
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by ITPhoenix View Post
Unlugging V1 makes it virtually all disappear.
means removing both V1A and V1B ... right ?

have you tried to disconnect only one of them ?

btw ... regarding cathode bypass caps

isn't it normal with no bypass caps at all, in a guitar preamp ?
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Old 6th August 2013, 02:51 PM   #38
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Originally Posted by tinitus View Post
means removing both V1A and V1B ... right ?

have you tried to disconnect only one of them ?
It's a LOT more work to disconnect just one, than to simply unplug the valve.

The other half is only on the over drive channel anyway.

Quote:

btw ... regarding cathode bypass caps

isn't it normal with no bypass caps at all, in a guitar preamp ?
Some do, some don't - it really depends how much gain you want.

I suspect the main problem for the OP is that he's adding TOO much gain - just checked some Marshall circuits, and there's usually either one bypassed and two not, or two bypassed and one not (the cathode decoupler on the first stage was also 330uF, but the resistor was lower).
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Old 6th August 2013, 03:11 PM   #39
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Did the amplifier anytime worked of properly, or it is a new one?
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Old 6th August 2013, 08:17 PM   #40
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Check this out folks!

A bare wire was attached to V1 first stage input grid and held in open air. The hiss went away but picked up 60 cycle hum as it should.

I suppose that answers the question the source of most of the hiss: RFI.

My theory is that the literal antenna somehow cancelled most of the RFI in the circuit.

So it's there even with no cable connected to the input. This leads me to the power cord, or the power grid at my location, which is an apartment building. I will try the recommended 3-wire EMI/RFI filter and a shieled power cord.

Also, decreasing the value of grid leak resistor (R13) on V2.B noticeably reduced the hiss, and also reduced the gain slightly. At least none of the tone was lost. The reduced gain is really not a problem, since Normal Crunch is considerably lower in output compared with OD. So it makes for a simpler SEND level control.

Well, it probably wasn't any of the valves, so soon I will have a 5751 and a couple of Mullard 12AX7 reissues to play with!
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