Tone loss

pocus

Hi guys. I appear to have lost the rich lunch sound I used to get from my boogie mark v.
I use it with Voodoolab GCX and ground control. Good quality pedals and connected with George l cables. Used to sound out of this world and then it just lost its quality for some reason. Also noticed that my cable makes a noise when vol up on guitar when on overdrive channels. That never happened before. Not cable as I've tried with many. At a loss

ICBM

Assuming you've eliminated any other cables and FX as a possible cause, try replacing the first preamp valve (V1), closest to the input jack. If you don't have a spare, swap it with any of the other ones.

A DC leak from the first valve stage is a common cause of crackling or handling noise from cables or the guitar pot, and it often means the valve is on the way out and may not be sounding as good as well. Sometimes it will work perfectly well in another part of the amp circuit, but if it does prove to be the cause it's probably best to replace it.

ecc83

ICBM said:

Assuming you've eliminated any other cables and FX as a possible cause, try replacing the first preamp valve (V1), closest to the input jack. If you don't have a spare, swap it with any of the other ones.

A DC leak from the first valve stage is a common cause of crackling or handling noise from cables or the guitar pot, and it often means the valve is on the way out and may not be sounding as good as well. Sometimes it will work perfectly well in another part of the amp circuit, but if it does prove to be the cause it's probably best to replace it.

I have never understood why amp input valves are invariably DC coupled? Saves a capacitor I suppose but is there some deep, tonal reason?

Dave.

ICBM

ecc83 said:

I have never understood why amp input valves are invariably DC coupled? Saves a capacitor I suppose but is there some deep, tonal reason?

Not sure. I suppose I should test it. It's always been done like that traditionally, but plenty of modern amps do use a cap… on the other hand generally ones I don't much like the sound of! I can't be bothered to work out the roll-off frequency but I would have thought it would have to be a pretty small value to make any difference.

I agree it's not really good practice not to have one though, since not only does it cause this problem (assuming that's what it is this time) it also exposes the valve to possible bias problems and/or failure if you have a pedal with a DC leak on the output.

57Deluxe

...Tinitus?? Comes on in an instant...

Cirrus

When tone is lost, where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

AAAAAAAHHHHHHHAHAHAHAHAAAAAAAAAA

When tone is gone, where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Where does it go?

Does it go to ground?

Or is it living, in capacitors

Or hiding beneath the scratchplace

Waiting in the gulf between the jack socket contacts?

The empty chasm way in there

Could be filled with tone

Does it spark into our fingers and hearts?

Does it wait for us in the dark?

Does it condense into dust around the voice coils of our speakers?

Or flitter into nothing?

jpfamps

ICBM said:

ecc83 said:

I have never understood why amp input valves are invariably DC coupled? Saves a capacitor I suppose but is there some deep, tonal reason?

Not sure. I suppose I should test it. It's always been done like that traditionally, but plenty of modern amps do use a cap… on the other hand generally ones I don't much like the sound of! I can't be bothered to work out the roll-off frequency but I would have thought it would have to be a pretty small value to make any difference.

I agree it's not really good practice not to have one though, since not only does it cause this problem (assuming that's what it is this time) it also exposes the valve to possible bias problems and/or failure if you have a pedal with a DC leak on the output.

For the standard 1M input impedance, a 100 nF cap would give a -3db frequency of around 1.6 Hz. This rises to around 24 Hz for the 68k input impedance of the second input on a Fender. 

These calculations don't take into account the source impedance which will reduce the turn over frequency. Source impedance could be as high as say 125k for a guitar with a 500k volume pot turned down to half it's value.

As the source impedance will affect the turnover frequency, I suggest that this is not a good point in the circuit to try to affect the frequency response of the amp.

ICBM

I would assume that the idea is to use a large enough cap that it doesn't affect the frequency response for any practical value of source impedance.

AlanP

It is really just an issue of DC blocking.

It seems to me there are a lot of slightly questionable practices in Amp design.  A minute or so ago I was looking at the schematic for the Fender Excelsior (I just got one...)  DC bias for the 2nd stage (2nd half of V1) depends entirely upon the wiper to track contact of the volume pot....

Cirrus

Those rogues!

ICBM

AlanP said:

It is really just an issue of DC blocking.

It seems to me there are a lot of slightly questionable practices in Amp design.  A minute or so ago I was looking at the schematic for the Fender Excelsior (I just got one...)  DC bias for the 2nd stage (2nd half of V1) depends entirely upon the wiper to track contact of the volume pot....

That's fairly normal with interstage gain controls. Worse is when in amps with a post-phase-inverter MV, the bias voltage for the power valves goes entirely via the pot wipers, eg Marshall DSL201/401.

The Excelsior is a bit of a nightmare in other ways too… it's very valve-sensitive and also a complete pain to work on.

ecc83

ICBM said:

I would assume that the idea is to use a large enough cap that it doesn't affect the frequency response for any practical value of source impedance.

Indeed. I don't see how a high source resistance will affect the LF response with the addition of a series 100nF cap? Agreed it will cause some HF loss due to the input Miller capacitance.

Then, just about every pedal on the planet has a solid sate front end and a DC blocking cap and 100nF is a fave' value (have an S1 pre amp board in hand as I type!) seems to be ok?

Dave.

jpfamps

With a 1 Meg input impedance and 100 nF cap there would no effect in the audio band.

Increasing the source impedance will reduce the turnover frequency (not by much with a 1M input impedance, but significantly with a 68k input impedance); my point here being that this was not a good place to try to influence the frequency response of the system.

However, with a typical 10k input impedance used in line level studio gear a 100nF cap will give a -3db point of 159 Hz, which will be audible.

ecc83

Nope missing something here J!

The wideband attenuation of a 250k source into 1meg is about 2dB. I cannot see that the extra 32K (XC at 50Hz say) is going to make a lot of difference?

With 250k + cap into 68k (why?) the whole thing becomes a current source?

Dave.

jpfamps

ecc83 said:

Nope missing something here J!

The wideband attenuation of a 250k source into 1meg is about 2dB. I cannot see that the extra 32K (XC at 50Hz say) is going to make a lot of difference?

With 250k + cap into 68k (why?) the whole thing becomes a current source?

Dave.

The -6db input on many amps is a 68k input impedance.

When looking at RC filters you always have to consider the source impedance as it a circuit.

ICBM

jpfamps said:

The -6db input on many amps is a 68k input impedance.

It's actually 136K .

The two 68Ks are in series.

jpfamps

ICBM said:

jpfamps said:

The -6db input on many amps is a 68k input impedance.

It's actually 136K .

The two 68Ks are in series.

Indeed.