5F6A low volume

[Helmholtz]

There may still be a mistake in the phase inverter tail wiring where the presence circuit is. Extra elevation there would cause higher voltage.

On the other hand, the Marshall 2204 schematic in the link somewhere above shows 45V at the PI cathodes, albeit at a higher supply voltage.
Of course it seems necessary to check for correct wiring/resistor values, good solder joints and tube socket contacts.

[vinceg]

I recorded a video showing the pops around V3. As you can see when I hit the baseboard near V3 with a wooden stick there is a strong pop. I assure you that live is much stronger. Also if I touch V3 cathode wire or resistor and grid wire or resistors.

About hiss it seems caused by V1 12AY7. If I put a 12AX7 amp is more silent

https://www.youtube.com/watch?v=lbl5gcZomBQ

[Helmholtz]

As you can see when I hit the baseboard near V3 with a wooden stick there is a strong pop.

It seems that the amp completely drops out for a second or so, when you hit a certain spot. Is it possible to monitor a change in voltages when this happens? Unfortunately a DMM might be too slow. An analog needle voltmeter would come in handy.

[vinceg]

It seems that the amp completely drops out for a second or so, when you hit a certain spot. Is it possible to monitor a change in voltages when this happens? Unfortunately a DMM might be too slow. An analog needle voltmeter would come in handy.

I do not have an analogue tester. However, it is difficult for me make this measurement.
I should get somebody to help me.
What voltage should I control?
I noticed that everything I touch with a chopstick the amp have pop. Also if I touch ground bus.
Anyway the amp sound great.

[Helmholtz]

As already mentioned you should look for bad solder joints/contacts or some wire shorting something. Don't forget to check the caps board under the metal tray on the other side of the chassis.
Freeze spray may help to localize an intermittent contact.

What voltage should I control?

Hard to say. I would start monitoring the voltages at V3.

[vinceg]

As already mentioned you should look for bad solder joints/contacts or some wire shorting something. Don't forget to check the caps board under the metal tray on the other side of the chassis.
Freeze spray may help to localize an intermittent contact.



Hard to say. I would start monitoring the voltages at V3.

Tomorrow I'll buy a freeze spray. Can you tell me how to use?

[Pedro Vecino]

Probably coming from some bad contact (welds, bad contact on a preamp socket pin or something of the style) but It´s not a dry sound. It´s accompanied by something like a reverb tail that sounds like a microphonic power tube.

[Chuck H]

The noise from the tapping is definitely microphinic tube noise. That's not uncommon and it's nothing to be concerned about. The volume going away and then returning is probably an intermittent fault (unconnected circuit that should be connected) or short (two circuits connecting that shouldn't).

[Helmholtz]

Tomorrow I'll buy a freeze spray. Can you tell me how to use?

Spray-freeze suspicious joints/contacts/components and watch for noises, drop-outs and voltage changes. Don't overdo. Too much spraying collects water. Don't spray directly at hot tubes.
Sometimes a hairdryer used in the same fashion also works.
The principle is thermal contraction/expansion of (contact) materials.

And get some insulated contact clips/hooks/clamps for your meter to free your hands while testing.

Make sure the your tube socket contacts are clean and tight.

Slightly wiggle tubes in their sockets and listen for noises through speaker.

Exchange preamp tubes and watch for changes (e.g. voltages).

Failure analysis requires preparation, care and a lot of patience.

[vinceg]

Probably coming from some bad contact (welds, bad contact on a preamp socket pin or something of the style) but It´s not a dry sound. It´s accompanied by something like a reverb tail that sounds like a microphonic power tube.

The noise from the tapping is definitely microphinic tube noise. That's not uncommon and it's nothing to be concerned about. The volume going away and then returning is probably an intermittent fault (unconnected circuit that should be connected) or short (two circuits connecting that shouldn't).

Can freeze spray help me find one of these possible causes? I never used freeze spray, how do I recognize a problem?

[Reader]

Also tell us if the pop noise is affected by any of the front panel controls. Knowing that could help isolate the problem to specific sections of the circuitry. After that is known, we can also pull tubes to further isolate the problem area.

[Chuck H]

Can freeze spray help me find one of these possible causes? I never used freeze spray, how do I recognize a problem?

Yes it can. I don't use it, but I trust Helmholtz. He is saying the same thing we are. We were just supporting his idea of what the problem could be.

Right now I think the problem is when the amp goes silent. While you can hear that the amp is on, if you spray a connection or component and the amp goes quiet there is reason to suspect that connection or component.

I usually just push or poke on leads and components with a wooden stick (like the one you were tapping the board with). If I poke something and the problem happens then I have a place to check.

My concern right now is that the amp going quiet may be due to a short that is intermittent. Having the wrong voltage in the wrong place can silence the amp until things normalize again. I think this is why Helmholtz wants you to monitor voltages while testing.

The problem is more likely to be a fault. A connection that isn't staying connected all the time.

EDIT: I'm sorry if any of this comes out awkward or strange. I feel like a silly American typing slowly and loudly to be understood

[Helmholtz]

I was wondering about the long drop-out time-constant of a second or so. I think this points either to e-cap charging or - even more likely - an intermittent heater contact. Please check and resolder heater wires and especially the 10µ/450V cap in the middle of the board. Also the heavy wire jumper from turret of the minus pole (10µ/450V) to the ground bus wire doesn't look well soldered in/at the turret, judging from the picture.

[vinceg]

I noticed that if I move V3 tube there is same sintoms with pop. Do you think the socket is bad?

[vinceg]

I was wondering about the long drop-out time-constant of a second or so. I think this points either to e-cap charging or - even more likely - an intermittent heater contact. Please check and resolder heater wires and especially the 10µ/450V cap in the middle of the board. Also the heavy wire jumper from turret of the minus pole (10µ/450V) to the ground bus wire doesn't look well soldered in/at the turret, judging from the picture.

Perhaps I could identify the voltage drop. While I moved the V3 tube, there was a pop and and the amp was silent. Then I measured pin's tube and I detected 0V at pin2 and pin7 grid, 0V at pin8 cathode, while the voltages at pin1 and pin6 plate are increased from normal 255V to 420V. What could be the cause?

Heater wires seems right, also jumper wire from cap to ground.