Rumbling/Rustling Noise at Start-Up

[David Root]

Recently my 50W '70s ODS makes an odd noise for a few seconds immediately the HT is switched in. This is with all volume and gain pots turned fully down.

It is sort of an electrical cross between rustling and rumbling and lasts only a few seconds.

My AC/standby switches are set up so that the plate cap is charged before the standby switch goes to play mode. Could that have anything to do with it?

Also, when I turn up the pots and play, the sound is strangulated, distorted and low power for a while. If I turn up one of the gain pots higher, at some point there is a craaak! and full clean power is on. Sounds like an intermittent or dry joint maybe?

I'm not sure these two things are connected. Any advice appreciated.

[Ken Moon]

Assuming it's not a tube problem, or a defective filter cap, a bad solder joint is a definite possibility.

Lots of folks like to poke around with chopsticks, and you may get lucky this way, but it's hit and miss at best.

The way I do it, and I really should stress BE SAFE if you try this at home (put one hand in your pocket), is to use a short jumper with mini-grabber clips on both ends, and jump around each solder joint, starting with the power supply.

You can buy short jumpers like this from Pomona that work nicely.

If, for instance, you have a filter cap, to test if the ground connection has a cold or intermittent solder joint, you would put one end of the clip on the cap's negative lead itself, close to the body of the cap.

The other end goes to the turret that the same lead is connected to, or in the case of PCBs or eyelets, to the lead of another component that shares the same node.

So you are jumpering around the solder connection itself, and if there's a bad joint, the problem will go away.

For ultimate safety, you should power down, put the clip on, and power back up. Mark the components on the schematic that you have checked, so you don't lose track.

If you choose to do this on a live amp, please be super careful, and don't even try it if you have shaky hands. Again, put one hand in your pocket so you can't have a current path through your heart muscle.

...and did I say BE CAREFUL ?

[David Root]

If I can't find it any other way, I'll buy some of those jumpers, don't have any right now.

This is an odd one. It was running normally, I had my DMM plugged into the bias test jacks. When I removed the +ve prod from the jack, immediately strangulated, distorted low output. Another time the same thing happened when I put the +ve prod BACK in its jack. Yet another time flipping the Presence switch did it, also had it happen with Rock/Jazz, OD Manual/Pedal, and Brite switches.

I have checked connections on all of these switches, they seem to be good. Ground connections in these areas seem good too.

I checked the PS cap connections, they seem good. The PS caps are all good.

[David Root]

I finally put the scope to it. It was not acting up, and everything is kosher, nice clean sine wave on clean mode, rounded off square wave on OD. So I shut it off, maybe it will act up later, when it has cooled off....

I'm beginning to suspect a bad filter cap. I have five older Illinois caps on the preamp, loop and PI, and a ARS double can cap on the plates and screens.

I wonder if one of the IC caps is causing this. I checked them yesterday and they all read 2 ohms ESR except one which is 2.5 ohms. All read in the "marginal" area (yellow), between good (green) and bad (red) on my EDS88A electrolytic cap meter. All show "DCR normal" light under test.

Could one still be bad? Should I replace them all?

[Structo]

I have had preamp tubes sound like that and also be intermittent with the problem.
Like maybe a connection inside the tube was bad.

In reality there are only three things it could be.
A bad tube
A bad component
or a bad solder joint.

Finding it is the problem!

I was taught to troubleshoot from the power amp towards the preamp because of the amplification factors that compound.

[Jana]

I would also suspect a leaky coupling cap. It sounds like some DC is leaking someplace where it shouldn't. That also kind of fits in with your description of the strangulated and distorted sound.

[David Root]

Thanx Tom. It hasn't happened today or yesterday after I changed some preamp tubes around. I will have to see which one(s) are no longer in it.


Update: there were two '80s Mullard 12AT7s that I changed out yesterday to a Mazda and a R/T. Both Mullards test NOS, which they are, but if one is intermittent how would one tell?

[David Root]

Thanx Jana. I'll check that out.

Update--no leaky coupling caps.

Still getting the rumbling/rustling noise for a few seconds coming off standby, but so far today no low volume strangulated sound problem.

I am thinking about those Illinois preamp PS caps again. Anyone know which years were bad? I have the date codes. One is TTAA. these are all 500V 22uF but one is slightly smaller than the other 4.

[David Root]

I think I fixed it. Went thru it with a scope and a 400Hz sinewave signal. Intermittent connection between +ve and shunt on the Send jack. Hardly any tension when I put a plug in it, angles were wrong, nothing holding them together, they were barely touching. Replaced it, no issues so far.

Not an old one either. Switchcraft too.

In future that will be the first thing I check on BEFORE I put a jack in the chassis.

[Cliff Schecht]

My new KT88 build makes some pinging and other goofy sounds on boot-up. Nothing that I'm worried about, especially with a brand new build. No NOS caps (at least in the B+ filtering) but this seems normal with new tubes. The metals haven't had time to expand and contract with heating and cooling and are still finding their shape.

[David Root]

My goofy sounds are still there, but much less so. Power tubes are NOS RCA blackplates, so maybe they are responsible for this rustling & rumbling. It happens with all gain controls and master turned to off, so it has to be something aftyer the master vol.

[tubeswell]

Hi David, did you try pulling the pre-amp tubes one at a time to get closer to seeing within which stage the issue may be? Another thing you could try to detect the culprit area, is grounding the plate of each stage with something like a .1uF 600V film cap with an insulated 'gator clip on each end until the problem stops.

One thought that occurs to me is leaky voltage somewhere, either on your board, or in one of the components (like in one of the pots? - Not being familiar with that amp - has the amp got an LTP with a presence pot in the GFB loop?)

[David Root]

Leaky components is still a possibility I suppose. Most of the pots and jacks are recovered from the same '67 Bassman I took the iron from. Interestingly the bad jack was not out of the Bassman, it was a new one.

The rustle/rumble has to be in the power tubes or the power tube PS or bias supplies as those operate after the gain/vol pots.

I scoped it and that showed me the bad jack. When it did connect properly all stages showed a normal sine wave thru to the OT secondary.

Yes it does have LTP with presence within the GNFB but the presence is switched, not a pot, with a 0.1 uF/50V cap. See Dumble ODS #40 schematic.

[tubeswell]

So how are you sure it is coming from the PA? Did you pull the pre-amp tubes?

[passfan]

I scoped it and that showed me the bad jack. When it did connect properly all stages showed a normal sine wave thru to the OT secondary.

I've had that happen as well and I use cliffs on the inputs now.