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Problem with the high hum in a tube amplifier for a guitar

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Thomasdj

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grounding point in tube amp

Hi Frank,

I'm thinking of building an amp switcher into the footswitch chassis (I use two amps, - one for high gain distortion and digital effects and one for warm clean sound with analog effects). Currently I use a passive A/B-switcher but it is not "dead" sead silent. I made this circuit with inspiration from the switching matrix. Do you think it would work as a completely noice free switcher?

Best regards

Thomas
 

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Re: Tube amp hum problem

Hello Thomas,

the circuit can be expected tot to generate switching noise. I don't see a purpose of the octocoupler B.T.W.

The problem is, that the amplifiers have to high input impedance. The LDR doesn't reach a resistance high enough to completely turn-off the signal. Additionally, I fear a considerable noise from the high-ohmic resistors. A ground connected (around 50-100k) load resistor after LDR may give better behaviour.

Also, a MOSFET has rather high low-frequency noise, don't know if it's disturbing.

Regards,
Frank
 

    Thomasdj

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Re: Tube amp hum problem

Hi Frank,

I see! ..the impedance could be too high. I plan to use these LDRs with very low light-resistance (a few ohm) and very high dark-resistance (<50 Mohm):

http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf

(on page 43)

I tried to make the preamp and it actually didn't sound as good as I thought. Tried many BS170 but didn't make much difference (Think this could have something to do with the high low noise you mentioned, - which I didn't knew at all!). I guess I only need a buffer, to compensate for loss signal in the LDR or only to gain a little before the amplifiers. I've also tried this preamp:

http://www.till.com/articles/GuitarPreamp/

, which actually sounds good! I've tried BF245, J210 and 2N5952 and found that the 2N5459 is the one I like the most (it's very discrete). I also installed a volume potentiometer instead of the 51k resistor, however it's a bit noisy!! I guess I will have to remove the pot again and replace it with two series resistors to lower the output.

You're right, the optocoupler shouldn't be there..

Thanks Frank

Thomas
 

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Re: Tube amp hum problem

Hello Thomas,

A BF245 or another JFET would actually be my first choice for the preamp. You have to assure a suitable bias point for maximum undistorted AC output. Actually I don't know what's the maximum input level is, I had thought of 100-200 mV.

I know, that LDR achieve high resistance, but with some delay. Also they have a parallel capacitance that causes feedthrough with high load resistances. An resistor noise is probably a limitation. I think a lower impedance analog switch with a preceeding amplifier could be a better solution.

As a general consideration: When you shut down the input signal, the unused amplifier still generates some noise. If you shut down the unused amplifier itself, you won't need to switch the input signal, just need a distribution preamp.

Regards,
Frank
 

    Thomasdj

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Re: Tube amp hum problem

Hi Frank,

I installed the J174 today and this actually improved the switching issue. The noise is almost gone, - I only get a little tik-sound when switching. The real improvement is that the tik-sound don't get louder when turning up the amp! It stays at the same low level. When using 2N546x the pops/clicks raises with the volume.

About the preamp, I see what you mean. I think I'll try removing the preamp from the circuit and think it all over again. Besides LDRs I could use the mute circuit from mesa. Currently I use a 3PDT switch to switch between the two amps (one pole for each amp and one pole for LED indicators A/B). It's an ok passive switcher which shunts the signal to ground. It does not pop/click when we are rehearsing but when we playing live the problems occurs which are really loud.

Thanks

Thomas
 

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Re: Tube amp hum problem

Hello Thomas,

Sounds good.

Regarding switchover in general: If you want to operate the switch with signal present, a smooth operation (short fade out/fade in) would be preferable. LDR can achieve it, also FET (which also acts as a variable resistor like a LDR). I've been using FETs (BF245) recently in a limiter that kept PA amplifiers from clipping.

The transition of FET control voltage can be slowed down by a RC circuit as needed, the same with LDR controlling LED current.

Another important point is, that the switch circuit must be free of any spurious DC voltage, otherwise clicks and pops are hardly to void.

Regards
Frank
 

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Re: Tube amp hum problem

Hi Frank,

The amp is now stable and works as it should!

However, I cannot figure out how to make the switcher circuit. The attached pdf is the A/B-switcher I use now, - can you see anything that should indicate why the circuit pops when switching? is there something I can do to improve the circuit?

Best regards

Thomas
 

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Re: Tube amp hum problem

Hi Thomas,

I'm still lurking here and the suspense is killing me... :wink:

Presuming the switch is a break-before-make type the output will float for a short instant before connecting
to the other channel.

I haven't paid full attention so I'm not sure where "input" comes from but you could try to connect
a 100k resistor from "input" to ground - just so that the electrons have somewhere to go during the switching transient.

A simple experiment at least...

/Ram
 

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Re: Tube amp hum problem

Hello Thomas,

To my opinion, pops are most likely caused by spurious DC voltages respectively currents present at the implied instruments. E. g. the MESA Bogie input jack is DC coupled to preamp tube grid. If some grid current is present here, it causes a DC shift of the input, depending on the connected resistance. Voltage drops along the ground nets in amplifier may be another cause of DC voltage at the input. So an additional AC coupling at both switch outputs would help. Connect e. g. 1 Mohm to ground at switch output and a 0.1 - 1 uF series capacitor to output jack tip contact.

If a DC voltage would be present at the input source, another series capacitor could be connected at the input side. No ectrolytic caps of course.

Regards,
Frank
 

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Re: Tube amp hum problem

Hi Ram:D

The input is the "guitar In" and outputs are for two amps. I don't know if it's a break-before-make switch but I do think so. I'll try to solder a resistor to ground, but if this does the trick, I really wonder why the "maker" of this circuit haven't thought about it. The circuit is from a well respected manufactor (I know it's very simple and straight forward) and I don't believe that I am the only one having this problem..

I've attached a sketch of my effect pedal setup/routing. I have four effects in front of the chain which is splitted via the A/B-box, selecting either Amp1 or Amp2.

Thanks

Thomas

btw Ram, the amp is rrrrroaring!!!!:D
 

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Re: Tube amp hum problem

Hi Frank!

Guess you wrote while I was answering Ram:D

I'll try that! It all makes sense Frank. However, this depends on how well maked my effect chain is!(see pic at bottom page 5 in this thread).. I think I will install yet another coupling cap directly at the V1 grids on both amps! guess I'll also have to verify that none of my existing pedals are causing pops.

Thanks

Thomas
 

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Re: Tube amp hum problem

Hello Thomas,

of course, there must be still a resistor to drain off the tube grid current. So a R-C-R circuit would be best. When selecting series caps to connect at the A/B-switch, the input impedance of the succeeding circuits must be considered. It may be less than 1 Mohm at the MESA Bogie, possibly rather 100 Kohm for transistorized effects. So the capacitor shouldn't be too small in value.

Regarding switch behaviour, they can be expected usually break-before-make by design, that shouldn't be a problem to my opinion.

Regards,
Frank
 

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Re: Tube amp hum problem

Ok Frank,

Tomorrow I will take all my pedals home and see how the input/output is wired. At same time I will try to improve another issue. the effect chain is humming because of ground loops via the supplies (wallwarts and daisy chain). I'm thinking of a new setup for the DC-supplies, using only the positive voltage from the wallwarts and the jack-cables as ground (negative voltage connection). Do you think this would work? - or should I use individual power supplies for each effect pedal? If so, would this eliminate the ground loops? I ask because I've read this somewhere on the net, but I've also read that it not always makes the hum go away..

Another problem is also that the pedals use different voltage and current. Most of them use 9v/0.1-0.2A but other use 9v/1.3A and 18v and so on..what would the best setup look like?

I know it's a lot of questions and different issues, but I am trying to get the best gear-setup once for all..

Thanks

Thomas
 

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Re: Tube amp hum problem

Hi boys,

I see you have a bunch of effect boxes in-line. Is the popping different if you plug the guitar straight
into the A/B switch compared to going through a switched-in effect box?

I wonder what style of music you play... Judging from the pedal setup I'd guess death metal. :wink:

/Ram
 

    Thomasdj

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Re: Tube amp hum problem

Hi Ram,

Haha..No not Death..but you're right..it's metal-style with a lot of heavy bounce rythms, special/weird bridges a.s.o...(can hardly wait to get the amp in the rehearsal!)

I actually can't remember..the effects are in the rehearsal room by now, and I'll bring them home tomorrow to check..I also seem to remember that some of them are poping when switched in and out. I think the whole chain needs a review..with the spurious DC in mind..

Thanks

Thomas

Added after 23 minutes:

btw...one thing I've notice, Frank/Ram.

Don't know exactly how to explain this, but if I step, tap or roll my foot on/over the cable (touching the 6 meter long cable) that goes from the guitar and to the first effect pedal, I can hear the scratching noise in the speakers. I don't know if this is just because of a poor shielded/quality cable....or if it also could hae something to do with spurious DC..

Frank, it's the same sound as when I turn the inserted volume pot at the little MOSFET-preamp..

Thanks

Thomas

Added after 6 minutes:

Just to make sure I've got it right Frank,

If I connect the guitar directly to the A/B-box and then directly to the amps..without connecting the 9 DC supply..then the pops should go away, right?
 

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Re: Tube amp hum problem

Hello Thomas,

I don't see, why individual supplies should cause ground loops, as long as they have no protective earth contact, and they usually don't. In contrast, using a common supply for mutiple effects with a split cable could cause ground loops. Either if using the signal connection as common ground or having regular two pole low voltage connectors, part of supply current will flow through signal cable ground and causes small voltage drops, can be an additional source of switching noise.
It can, but must not be necessarily. You should try.

Another point: Hum must not necessarily come from ground loops. It can be caused simply by insufficient filtered supplies. In this case, one high quality supply instead of many poor ones is preferable.

Regards,
Frank
 

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Re: Tube amp hum problem

Hi,

The 9V supply to the A/B box is only there for the LEDs so it should not affect the popping either way.

As for the power supplies to the boxes it sounds wise to have a single power/signal ground running
through the chain.

Way back I did some PA work and sometimes we had to insulate the protective earth pin in the
mains plugs to break up ground loops that picked up hum.

The human body is an excellent antenna for hum and the noise is capacitively coupled to the gear by touch.
Shielding and grounding is the only way to counteract that - unless you don't play dressed up in latex.
But then I guess it isn't metal anymore... :wink:

/Ram

PS I saw Frank's response now. I think it is best to just try different supply topologies
as a system with so many parameters (boxes) is hard to analyze.
 

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