Need help diagnosing speaker thump

[Kw6]

Hey guys,

I have an integrated 300b tube amplifier fairly new. Turn on amp all normal but when I am ready to turn off amp after hrs of playing I hear a soft thump through my left speaker only. Is it faulty capacitor a bad output tube. Just want to be knowledgeable of problem before I contact dealer? Thanks!

 

[RayDunzl]

I have an integrated 300b tube amplifier fairly new.

I'll expose my ignorance:

Which amp?

Single ended or push pull?

Transformer output or OTL?

Is there a DC voltage on the left channel when idle (no music playing)?

Is there a DC offset adjustment?

 

[Kw6]

I'll expose my ignorance:

Which amp?

Single ended or push pull?

Transformer output or OTL?

Is there a DC voltage on the left channel when idle (no music playing)?

Is there a DC offset adjustment?

Ok thanks!

Two mth old Cayin A 300b Mk II integrated.
Single ended class A has output transformer.

I don't clear instructions to measure DC offset as dont want to make a mistake and blow up amp.

 

[RayDunzl]

With a transformer output, DC not expected at the speakers.

So something before the transformer is emitting a little bump as you turn it off.

---

That's the end of my long-distance expertise. Not had an audio tube since 1973, and would have been clueless then.

Soft thump unlikely to hurt anything.

---

Looking at generic 300B schematics doesn't give me a clue.

Sorry.

Maybe someone with better credentials will reply.

 

[Kw6]

Ok thanks Ray your information has been useful. Curious what would cause left side only. Could it be rectifier tubes or output 300b tubes?

 

[RayDunzl]

Something would have to create a little change in voltage across the primary side of the output transformer translating to a little voltage change on the output side which you hear as a thump.

What would do that on one channel, don't know.

Are you sure it's not just a louder thump on the left?

 

[Kw6]

I will try again next time I listen. What happens if thump appears on both channels?

 

[RayDunzl]

What happens if thump appears on both channels?

Then it could simply be the normal result as the hundreds of volts present at the 300B and connected transformer "relax" when power is removed.

Cahnge in current flow through the primary of the transfomer shows up on the secondary connected to the speakers. That's what makes it work, and might give a thump as it shuts down.

Or something earlier in the circuit wiggles the voltage on the grid (varies the 300B current flow) of the 300b while it is still charged at shutdown, or, ???

I'm no expert, just guessing, till somebody smarter comes along.

Example of some possibly similar circuit:

The three big caps at the bottom will store some energy, and will drain down over a short period (seconds?) of time through the 8k, 82k, and 37k resistors (blue path). "Off" isn't instantly off.

For grins, try turning it off while something is playing. Bet it works for at least a moment. Might depend more on the storage for the input section, though.

I have an SS amp (Acurus A250) that will play on for 30 seconds at a moderate level after power is removed, until it finally (literally) fizzles out.

It has some rather big storage caps:

 

[Kw6]

What you say makes sense but I asked another fellow on a forumbwho has same amp as mine and his is 3mths old too and his speakers are similar efficiency and he hears no sound when he turns amp off. Also another Cayin owner but his amp is push pull KT88 and he says no sound pn turn off.

Really puzzling. Do you think one of my output tubes could be the cause or faulty capacitor?

 

[RayDunzl]

Really puzzling. Do you think one of my output tubes could be the cause or faulty capacitor?

Don't know.

Sorry.

 

[Kw6]

Thanks! I appreciate you trying to solve this mystery!

 

[Kw6]

Issue resolved I think. Someone with my amp said move the left 300b to other side. And lucky the sound followed the tube.

Thanks for your time Ray. You know more then you let on!

 

[RayDunzl]

Thanks for your time Ray. You know more then you let on!

Oh well...

 

[Kw6]

Hey Ray seems I was wrong as when i rebias amp this morning because I swapped the 300b tubes around the soft pop sound comes from both speakers. Have any ideas now? Dealer now says could be some resistor or capacitor in power supply. What do you think. As I hate to send ot back. Alll they wil do is change out the part for me.

 

[RayDunzl]

If the idle current (it's a Class A circuit) through the primary of the output transformer at power off falls at a rate sufficient to collapse the magnetic field at a rate that can induce a voltage spike in the secondary that the speaker can respond to, I would expect a speaker "pop" on power off.

That's my long distance free analysis.

Seems normal, and if not normal, then likely, and if not likely, then at least possible, to me.

And that's without anything being "wrong" or "broken".

 

[Kw6]

I will try another set of tubes just to be sure. But the pop won't damage the speakers in the long run? This pic is similar to my amp in the power supply. My dealer says may need to swap out some resistors or capacitors. Would it be where I drew the circle?

 

[RayDunzl]

I have an integrated 300b tube amplifier fairly new.

Send it back if its under warranty and you aren't happy.

But the pop won't damage the speakers in the long run?

You said "soft pop". Is it louder than the whacks in the loudest drum solo you ever played? Does it make somebody in another room run in and say "WHAT THE HELL WAS THAT?!?!"

My dealer says may need to swap out some resistors or capacitors. Would it be where I drew the circle?

I'd get a second opinion here, first.

@restorer-john @solderdude

 

[solderdude]

Soft pop is not problematic, annoying perhaps.

Ask other owners if they experience the same thing

 

[yodog]

I would get a faulty capacitor and or a capacitors soldering joint is loose, causing DC voltage leakage.

 

[raindance]

If it sounds good to you, don't worry about the pop. It's old, imperfect, hand built technology.