Grainy/Trashy noise on tube amp

[Ampzone]

Fellows Ive got a noise problem on a tube amp that is stumping me. The amp notes have a grainy/fuzzy character when they trail off. When you hit a note, or chord with a strong impact the intial note is clean, but then you get a background grainy/fuzzy/trashy noise that comes in and out. It happens when the amp is set for lower or higher volume. Aside from this noise everyting else works fine, power, controls etc.

I thought this may be from the bias being to cold, but after biasing hotter the amp still makes this noise. So I replaced every tube, tried different speaker cabs, no change. No leaking DC on the coupling caps.


PS filter caps are good and new. The amp has a bias voltage doubler added.

Any ideas. It's probably something simple.

Thanks,

Ampzone

[Jazz P Bass]

What amp brand & model is it we are dealing with.
Do you have a scope?

[Ampzone]

Thanks for the response...it is a DR103. No scope I'm afraid. Ive been looking for possibly a loose tube socket contact but nothing yet. I'm about to do the old wood stick tapping test to see if I can find a broken/cold solder joint.

[Alex R]

One common cause of this is oscillation, internal electro-magnetic feedback. Which could be caused by a tube, or lead dress inside the chassis, or a component (or a ghost in the machine). A scope might help diagnose it (by displaying resonating frequencies feeding back that are beyond the range of human hearing), but in the end most of us are reduced to moving wires around with a chopstick, that's if tube swaps don't sort it, so you may as well try that first and see if you can affect the symptom.

[Ampzone]

Alex - the interesting thing is it did not originally do this until I added a bias voltage doubler. Which I foolishly forgot to mention. Anyways I tried two sets of output tubes but the noise remains. Could a voltage doubler cause this?

Chopstick method coming tonight. I'm hoping it's a loose solder joint or failing resistor.

[Wilder Amplification]

Crossover distortion can also cause that...which would make sense since you just now mentioned that you added the bias voltage doubler. Where is your bias currently set at and what is your plate voltage?

[Ampzone]

Wilder - thanks for chiming in. The Plate voltage is appx 497v. Bias is set appx 35ma. This is my second set of tubes to try to see if the problem was the tubes. To triple check I have some new KT-88's that I am going to try now to just too see if the noise persist. With the voltage doubler I may have enough voltage for the KT88's.

[Wilder Amplification]

35mA is WAY cold for KT88s at that voltage. If that's 35mA on each side of the OT (17.5mA per tube) you're ICE cold.

KT88s running at 500V can be safely biased up to 50mA (absolute limit).

The DR103 is a dual rail amp...have you worked on dual rail style amps before? They don't require nearly as much negative voltage at pin 5 since the screen voltage is much lower so more than likely you probably didn't even need the bias voltage doubler.

[Ampzone]

Wilder - I understand. It is an EL34 amp. I am trying to add do the KT88's now...I should have a report shortly.

[Ampzone]

Ok - Good news - it biased up with the voltage doublers extra juice - Bad news - I set the MA to appx 45ma but I am still getting the crossover buzz. I am going to try raising the bias. Wilder where would you max out at? 51-52ma? Ive never tried 88's in this type amp.

[Ampzone]

Whoops.. I just saw your reccomended bias for 88's. I will try a little more...

[Wilder Amplification]

BTW what is the screen voltage on this amp?

Also when you're biasing, you wanna get it up to 50mA, recheck the plate voltage, then redo the 70% calculation. As you bias hotter plate voltage will drop (plate voltage and plate current are out of phase).

[Ampzone]

Ok at appx average 48ma on each 88 it still makes the intermitent buzz, which happens when the intial notes starts to decay...that is either when you play loud or soft. It's like a mild-med fizzy buzz

The doubler was added bc I could not get the 34's to get in a safe range. With the stock setup the bias was too hot for the set of tubes I tried.

Stumped...well I still have more of the chopstick test to do. I am doing that now...nothing so far.

[Wilder Amplification]

Belay my last...I just looked at the PSU schematic and it is not a dual rail amp. I was confusing it with the DR405.

So that being said, is this something the amp never did then all of a sudden started doing? Or has this "fizz" been there the whole time and you're seeking to get rid of it?

[Ampzone]

Wilder - Ok the screens are appx 3-4v less than the plate current. The noise... well the amp originally did not do this but with the stock bias circuit the tubes were running very hot. I was maxed out on the stock bias. So I added a bias doubler you see below.

[IMG][/IMG]

With the doubler I was able to get a new set of tubes in a safe range...that's when the noise started, and continues with the 88's installed now. I am still doing the chopstick test...nothing has given it up yet...