Low frequency thump

[rockpassion]

Hello all,

I have had my Bee Pre  for about a year and a half now.  I have had no problems until just recently.  About 4-5 months ago I began hearing a low thump after turn on of my whole system.  At that time I had a pair of Quicksilver monoblocs attached and also the Eros phono preamp.  I changed out the Quicksilvers with Dynaco MKIII monoblocs thinking this may account for the thumps. 

The thump only comes from the right side so I began changing around things right to left starting with the 300B's but to no avail the thump was still present.  In the process of changing around things I also noticed a lot of crackeling.  The crackeling became so bad that I could touch the BeePre chassis and it would begin.  To get it to stop I would just keep taping the chassis until it stopped.  That indicated to me that I had a short somewhere.  So, I proceeded to check my connections on the right side and I found a resistor in one of the boards to be loose.  As a result I resoldered all the connections on all the boards on the right side, plus most of the connections throughout the BeePre.  I also made sure and resoldered all the connections on the volume, balance pot and selector switch. 

The result was the crackeling has completely disappeared.  It is also much quiter, meaning the hum has decreased considerably.  At my normal listening levels it is quite and I can only hear a hum if I put my ear right next to the speaker.  But, I let it warmup for about 1/2 hour and played some music.  Once I stopped playing the music I heard the thump again.  Very low frequency like before, kind of like someone tapped a bass drum. 

Any ideas on what might be causing this.

Thanks in advance.

Richard

[Paul Birkeland]

The crackeling became so bad that I could touch the BeePre chassis and it would begin.  To get it to stop I would just keep taping the chassis until it stopped. [...]  So, I proceeded to check my connections on the right side and I found a resistor in one of the boards to be loose.  As a result I resoldered all the connections on all the boards on the right side, plus most of the connections throughout the BeePre.

A+ work there!

Very low frequency like before, kind of like someone tapped a bass drum. 

Can you swap 300B's again?  I had a Chinese 300B do this to me, then one day it just quite working (problem solved I guess).

Also, about how often does this occur?  And does it occur with all sources?

[rockpassion]

Paul,

I will give it a try, swapping the 300B's.  The problem happens about every 15-20 minutes and yes it happens with all sources. 

One question I do have, if the 300B goes out could it do damage to my Dynaco's amps.  I just had them repaired and the gentlemen who repaired them said the Curcio mod's are very sensitive to voltage fluctuations especially at turn on.  Per my technicians suggestion I have installed a Variac so that I gradually increase the voltage. 

Thanks again Paul for your help.

Richard

[Paul Birkeland]

When the 300B goes out, the signal will just fade away into nothing.  The Curucio ST-70 board shouldn't be bothered by a turnon thump, but you can always just switch the amplifier on last (or at the same time as the 300B, the BeePre warms up instantly, the ST-70 does not).

The 15-20 minute interval is really reminding me of my 300B experience, but I wouldn't be surprised with either result of your tube swap.

[rockpassion]

Paul,

Just wanted to say thanks.  I believe your diagnosis was correct.  I swapped the 300B's left to right and the thump followed.  It is not as loud on the left side but it is still there.  The good news is it is not happening as often. 

I will watch for any failures but I want to thank you again for your help.

Regards

Richard