Crack construction queries RESOLVED

[Banzaiburrito]

Hi I’ve just finished building my Crack and would be grateful for experienced thoughts

1) My resistances checked out ok give or take 0.1 ohm except for the centre poles of the RCAs.  Both were within the correct range but there was a 4.5 ohm variance between the two (black 98.2 and red 102.7) is this significant or acceptable?

2) Voltages were again all within the stated ranges but there was again some variance in the readings where you would expect similarity.   All of which were stable and repeatable.  Terminal 1 was 78.5 and 5 was 87.5. 7 was 111v and 9 was 109v.  Does this indicate an issue?

3). On listening there is audible warm up in the left channel (similar to that of a pipe clanging as it heats) none in the right.  Also there is very slight back ground noise in the left channel. If this is becomes more audible, a slight tap on the chassis brings this back to being acceptable albeit noticeable where the right is quiet.  A gentle chassis tap also produces micrphonics through the left channel only. Could the voltage and noise issues be one of the tubes is poorly??

Many thanks

James

[Paul Birkeland]

1) My resistances checked out ok give or take 0.1 ohm except for the centre poles of the RCAs.  Both were within the correct range but there was a 4.5 ohm variance between the two (black 98.2 and red 102.7) is this significant or acceptable?

This is very normal.  The carbon tracks of the volume pot have a tolerance around +/-10%. 

2) Voltages were again all within the stated ranges but there was again some variance in the readings where you would expect similarity.   All of which were stable and repeatable.  Terminal 1 was 78.5 and 5 was 87.5. 7 was 111v and 9 was 109v.  Does this indicate an issue?

There's no issue with that.  You may also find that those voltages change after the amp has been running for a few hours.  If you had 75V at terminal 1 and 100V at terminal 5, that would be cause for concern.

3). On listening there is audible warm up in the left channel (similar to that of a pipe clanging as it heats) none in the right.  Also there is very slight back ground noise in the left channel. If this is becomes more audible, a slight tap on the chassis brings this back to being acceptable albeit noticeable where the right is quiet.  A gentle chassis tap also produces micrphonics through the left channel only. Could the voltage and noise issues be one of the tubes is poorly??

Noises generated by tapping on the top plate is almost always indicative of a poor solder joint.  Microphonics from tapping on the top plate isn't particularly uncommon, and most of our users don't tap on the top plate when using their amplifiers, so this is not considered a problem.

[Banzaiburrito]

Thanks Paul for your reassurances on the voltages.  With regards the noises through the left channel, I’ve reflowed the joints and all still look ok. The hum remains.

The noise on warm up is only through the left channel and is a clanging Sound. The permanent noise is a mellow hum. I’ve tried using shorties so it’s an issue with the Amp. I’ve switched the plugs over on my Senns and the hum switched so it’s not the headphones or my ears. Also the microphonics are prevelant through the amps left channel.

Any advice on where to look next would be appreciated.

Thanks

James

[Paul Birkeland]

Can you listen to a 60Hz, then a 120Hz tone and let us know which is representative of what you're hearing?  Also, posting photos of your build can be helpful.

[Banzaiburrito]

It’s definitely 50/60 htz range it’s not pronounced but noticeable in slow passages.  Photos to follow..

[Banzaiburrito]

Testing photo upload - more to follow

[Banzaiburrito]

Part 1

[Banzaiburrito]

Part 2 Let me know if you need anything else.  Thanks for your support Paul

[Paul Birkeland]

Another useful test is to plug a pair of cheap headphones into the Crack and poke around with a wooden chopstick at the connections on the bottom to see if you can find a spot that changes the noise you hear.  This will direct you to where you might want to reheat solder joints or otherwise focus your efforts. 

[Tom-s]

Please remember to do this poking around with something non-conductive. A wooden chopstick or similar.

[Banzaiburrito]

Another useful test is to plug a pair of cheap headphones into the Crack and poke around with a wooden chopstick at the connections on the bottom to see if you can find a spot that changes the noise you hear.  This will direct you to where you might want to reheat solder joints or otherwise focus your efforts.

Thanks Paul I have tried that, there was no one connection that stood out. All of the connections between the pot, 9pin socket and jack appeared to create a noticeable microphonic reaction when gently proded. It seemed to me this was mainly in the left channel. Working towards the back this dissipated.

I wonder whether one of the valves has not enjoyed its journey from the West Coast to Great Britain and is no longer in tolerance or properly balanced?  Perhaps the microphonic nature is minimised towRds the rear due to the mass of the transformer? Resisting vibration?

I was planning my first valve roll anyway so I have ordered some GEs and will see if they make a difference!  In the meantime I will suspend investigations put my iron away and enjoy listening to it!!

I’ll let you know how that goes. Thanks again!

[Banzaiburrito]

Please remember to do this poking around with something non-conductive. A wooden chopstick or similar.

Ah that explains why my socks caught fire!!!    Thanks for the heads up, I took extra precaution and stood on my wife’s yoga mat also!!

[2wo]

That will  teach you to wear your nomax socks...John

[Banzaiburrito]

RESOLVED I received two new valves today a Telefunken GE 6080 and a long plate GE 12AU7.  As BB King once sung, ‘ the hum has gone ‘. 👍🏻 The only problem now is that it’s clear I need some Senn 800S’ and a Chord Dac 😂. 

The amp is ace, many thanks to all at HQ for the enjoyment the kit has and will give me.  Now where did I put the speedball..🤔

Cheers