hash noise on start-up?

[luthierwnc]

Hi All,  I'm investigating an unfamiliar noise on a homebrewed Dumble-style amp I made 10 or so years ago.  It uses a solid-state rectifier with fairly heavy filters driving a pair of 6L6's at around 455 VDC.  I've attached the schematic below.  When I kick it on the power is very low for a couple minutes and then the amp makes a loud rasp like being between stations on an old-fashioned radio.  That lasts about ten seconds and then the amp functions fine.  It is not heater dependent because I've left it on stand-by for better than 10 minutes and it still goes through the low-sound then hash before full power.
The tubes (Chinese or Russian) all have less than 50 hours on them.  Sockets are clean.  I looked closely at all the solder joints and they look OK too. All of the components were new at build.  My next step will be to haul a speak cab down to the basement and check voltages during the boot-up.  I'll put it on a scope at the same time to spot differences but have to work fast and then let it cool-down until the next check. 

Next would probably be to re-flow the joints in case the heat of playing improves a cold joint. 

I've dealt with issues like this before but they didn't get better by themselves.  If anyone has seen this before, I hope you'll write what you found.  Thanks, Skip

[pdf64]

Starting with  the valve in V1, how far along do you have to go for the noise to be eliminated?

[tubeswell]

Filter caps

[luthierwnc]

Thanks gents.  I'll take a speaker cab downstairs to try and catch it in real time.  It is a short window of time so it might be a while before I have much to report.  sh

[PRR]

There's a LOT of switching and maybe seven "volume pots". What state is it in when it acts-up? Start with 'Recover Output' full down.... does it "rasp"? It will take a while, but work your way back to the 'Clean MV' and 'Trim' pots.

[luthierwnc]

Hi PRR,  At start-up none of the relays are active so no OD, no PAB.  I've got the Volume at about 9 and the clean master around 2.  Both the Send and Return are around 11:00.  For the first couple minutes after turning off the stand-by the volume is very weak.  Then you get the harsh hash (a mix of popping and schhhhhhhhh) for 5-10 seconds and a few seconds after that the amp works fine.  If I turn it off a minute or two and back on it is fine.  That happens whether there are any FX in the loop or not.  This is a really loud, clean amp and that comes through.  It might be harder to tell with something gainy. 

I'll have some bench time tomorrow and report back with what I find.  Cheers, sh

[luthierwnc]

More news may have to wait.  I took it downstairs last night and plugged a guitar in one end and a speaker cab in the other.  It worked flawlessly.  This morning (after significant others were up) it did the same thing.
This has happened before - not with this amp but generally speaking.  I'll keep poking and prodding.  Cheers, sh

[luthierwnc]

Bad solder joint on V1 grid.  Sorry to bother ya'll.  sh

[tubenit]

Zero bother .................              This post was a good thing.

In fact, this type of post is incredibly useful and instructive to the rest of us!  It totally fits within the theme of tweaks and repairs.

I am super appreciative that you posted the resolution to the problem!  Sure wish more people would do that on the forum.

Thanks,  Tubenit

[luthierwnc]

Thanks.  And yes, something like that is usually the gremlin.  It is also an object lesson to other builders.  I've never made the same amp twice.  And when you start replacing components on a prototype build, it is easier to lose track of virgin solder joints  Cheers, sh