Impressive thump

[johnsonad]

I've got a pretty impressive start up thump through my speakers. The intensity of the thump varies with which pair of tubes I have in it. Not sure what I messed up but can someone point me to a solution to this?

[Grainger49]

Read FAQ #1.  This sounds just like the problem.

[johnsonad]

I wait a minute it so. I'm using Russian PIO caps and I wonder if they are partially to blame? Coupler is 0.1uF and PF is 2.0uF. I'm used to a very small thump but with this pair of tubes it enough to make me jump....

And it is the tubes in some way but I'm not sure how. I rolled in a different pair and there is a very light thump in both speakers. With the tubes in questions it's violent.

[Grainger49]

Aaron,

I have used KK PIO and they don't seem to charge slower than others.  So there is something else going on here. 

So your system is on for a minute or two then goes thump?  Does anything else happen at this time?

[johnsonad]

No, I warm up the pre as always the fire up the SEX. About 10 seconds later I get a thump in each speaker. This is nothing new but with this pair of tubes it's a much larger thump and it sounds different that the normal thump.

[Paul Joppa]

Do these tubes have a series heater? Some use a single heater wire for both sections, showing a short glowing segment at the top.

I'm speculating that with parallel heaters, the more massive power section cathode heats up more slowly, so that the driver has already stabilized - but with series, they might well heat up at the same time, so when the driver plate drops from B+ to its quiescent vale, that jump goes right into the power stage grid.

Do you also have C4S loads? That would make the above transition happen more rapidly.

[Grainger49]

And I have an answer for it.  The same switch I use to take the leakage DC out of my OPT.  It is a short to ground after the Parafeed cap, before the transformer primary winding.  You open the short after 30s of warm up.

[johnsonad]

Hi Paul,

No, parallel heaters.  I have a pair of series heated and they have nearly zero thump. Of the two pair of parallel heated tubes, one pair (Tung Sol) is much worse than the other (GE).  And yes, I have the C4S installed.

[johnsonad]

Thanks Grainger but I already have one switch on the PF caps (between the two sets of OPT's) and it isn't designed to go to ground. I could switch it to the none used pair of OPT to start and switch to the speaker OPT after warm up but that's a bad idea....

[Grainger49]

Aaron,

That might get rid of the leakage DC at startup.  Why not try it?

[johnsonad]

I'm tempted but is it a good idea to switch at full voltage?

[Grainger49]

If the volume is down there is no voltage on the output after startup.  I swap speaker leads all the time.

[johnsonad]

Well, it worked! I turned the volume all the way down, put the switches on the Stax OPT's, fired up the SEX, waited a minute or so the flipped the switches to the speaker OPT's. Zero thump, noise, spark or shock to the operator

I remember reading somewhere on the old forums that this was a bad idea. My switches have a good rating to them (and are more than needed for this).  Help me to understand why I shouldn't switch these as I just did?

[Grainger49]

Opening the output of a tube amp with feedback, i.e. no load, is a bad idea.  Could this be what you are thinking of?

[johnsonad]

I remember PJ had an amp with multiple OPT's on switches to a/b and if I remember correctly it had problems. Something with switching with the power on was a bad idea. I could be wrong...