Lesson Learnt (then forgotten within 3 months)

Richard Higgins · Post by **Richard Higgins** » Thu Dec 06, 2007 12:53 pm

When we make mistakes at work, no one is to blame as long as we all sit down and discuss the lessons learnt, so these are my latest mistakes.

Lesson 1 A 630 volt Cap can blow at 370 Volts.My 2A3 monoblocks have a choke loaded power supply, the first cap is 1.66mf made up of 1mf plus 3 x .22mf caps. Three of the caps were 1000v and one was an Audyn .22mf 630 volt.
Over the last couple of years 2 of the Audyns had failed in use, becoming shorts and blowing the main fuse. I should have got the message and changed to a different make, but I had several.
Last month one blew at switch on and took out the rect. valve as well.
I replaced the Audyns with Soniq 630 volt caps and now all appears well. Also I had put one speaker sounding quieter down to me/room/music, they were within 8 volts (273v265) across the caps.
On replacing the caps, the different voltages remain, but the loudness now seems the same, and sound better.

Lesson 2 The level of improvement is not necessarily related to cost.
I came by some 8mm felt and reading Peters articles decided to try it in the Impulse speakers behind the bass drivers. I eventually lined most of the first part of the transmission line. This was successful, improving bass and treble and reducing mid-range muddle.

Lesson 3 It pays to recheck old work.
I decided to check over my PSUII/PreII/PhonoII to see if there was anything I could improve it or “bad practice.”
I had not looked in the PSU for ages, I found I’d fitted Black gates throughout and replaced the valve heater rectifier with diodes followed by a Phillips cap.
I had also bypassed the 100mf caps with 1mf Solens.
I removed the Phillips and Solens caps.
Result, a definite improvement.
Just to confirm what has been said many times, Black Gates are worse when bypassed.

Next stage? New m/c step-ups, the original WADs seem a bit exposed. If I can get the guy at Cinemag to reply to e-mails I might get a Christmas present.
I believe that the main difference between PhonoII and PhonoIII are the 2 caps added for stability, does anyone know the value?

Regards Richard (Higgins)

Post by **Nick** » Thu Dec 06, 2007 1:16 pm

Lesson 1 A 630 volt Cap can blow at 370 Volts

Always worth considering what ripple current will be going through them, also what peak voltage they might have seen.

Did you have ballancing resistors on them?

Cressy Snr · Post by **Cressy Snr** » Thu Dec 06, 2007 1:17 pm

Richard Higgins wrote:When we make mistakes at work, no one is to blame as long as we all sit down and discuss the lessons learnt, so these are my latest mistakes.

Lesson 1 A 630 volt Cap can blow at 370 Volts.

Tell me about it!

Two generic 630V caps on HT decoupling duties became shorts and destroyed half of the output stage of my ill-fated EL509 OTL project.

Two more of the same caps doing the same HT decoupling developed shorts and took out the driver stage power supply in the 6AS7 amp. I was at the corner shop at the time. Luckily my daughter had had the presence of mind to pull the plug from the wall or it could have been serious.

I don't decouple HT any more.

Probably should have had balancing resistors between them but I'm too scared to try it again.

Steve

Paul Barker · Post by **Paul Barker** » Thu Dec 06, 2007 1:49 pm

Yes as said before I've found that these problems become more frequent more dangerous and more offensive to the ear the further up the voltages you attempt to climb.

It is best to stay below 500v.

I think it was Neal who video'd me saying we shouldn't really be using all this old stuff as I connected up 1k5v through a bunch of 50 year old parts, all of which I could give examples of destructive failure.

When people see a working amp of those voltages unless I tell them I very much doubt they have any idea the amounts of dangerous and unexpected experiences lie behind them. Not to mention the destructive effect on sound of such a component misbehaving but not yet revealed.

andrew Ivimey · Post by **andrew Ivimey** » Thu Dec 06, 2007 3:04 pm

And not putting the HT+ clip lead back on, switching on and wondering why the diodes blow - great fuses!

And being gently reminded by enthusiastic friend that 'we all make mistakes sometimes' having admitted I had soldered to pin 6 what shold have been on pin 5.

... which leads on to Paul's point. This lark can be very dangerous. I need to remember that. Over the years I have shocked myself maybe four times when it has been serious enough to rearrange my central nervous system - or that's what it has felt like. I have been able to walk away, burnt myself once, left well enough alone, calmed my racing nerves and gone back eventually. It isn't the shocks that stops me, though it might be one day.

Richard, your three examples are good ones; instructive too!

pre65 · Post by **pre65** » Thu Dec 06, 2007 3:11 pm

Hi-i now put HT bleed resistors on all my builds after getting a nasty shock from my Quads that had been powered down for quite a while.

Paul Barker · Post by **Paul Barker** » Thu Dec 06, 2007 3:22 pm

I get a shock at work every week but haven't had a dc shock to do with my hobby since the one back about 2002 400v dc my hair stood on end for two days and nights. Stood straight up it did.

Now think very hard while working on amps.

andrew Ivimey · Post by **andrew Ivimey** » Thu Dec 06, 2007 3:35 pm

But Paul, your hair still does stand on end and before you get back at me, it is much better to have hair than not to have hair! I know, I know.

colin.hepburn · Post by **colin.hepburn** » Thu Dec 06, 2007 3:59 pm

Hi All
I have not had a shock from my valve stuff up to now touch wood but did get a suppress

rummaging in my parts box from a 500v 330 cap it had been in there some 6 month had one from the speaker line from a 1000watt SS amp fixing the speaker plug back in my Disc Jocking day that lifted me of the deck that job was given to the roadie after that

Paul Barker · Post by **Paul Barker** » Thu Dec 06, 2007 5:54 pm

Just done some rewire work for a guy who used to work on 10kv stuff in a locked room full of capacitors storing that voltage up at Filingdales (where they were supposed to find out with dread they were all going to die within minutes and we lot would have been blissfully ignorant, but meant our lot could anialate their lot out of spight sort of place)

Anyhow there was an interlock on the door which cut the voltage and gradually lowerd an earthing prong onto the caps in the time it took the bleader resistors to discharge them safely before you went it the room.

The interlock failed he walked in, his hair reached for the sky, he everso gingerly like he had entered a minefield, back tracked!

Richard Higgins · Post by **Richard Higgins** » Fri Dec 07, 2007 7:29 am

Nick wrote:
Lesson 1 A 630 volt Cap can blow at 370 Volts
Always worth considering what ripple current will be going through them, also what peak voltage they might have seen.

Did you have ballancing resistors on them?

Hi Nich
After the valve there is the 1.66mf group of caps then a choke, then a 10k shunt resistor then 100mf cap, then positive to the op transformer.
But you have reminded me that orioginally there was some low frequency instability solved (?) by adding 100mf to the 32 mf cap after the transformer take off.
Time to have a fight with PSU Designer i guess. Also some snubber caps round the choke might be a good idea.

Regards Richard (Higgins)

Post by **Nick** » Fri Dec 07, 2007 8:06 am

If you have serial caps in a power supply, you always need to add equal rersistors across each cap, to form a potential divider, so the voltage is divided equally.

Dave the bass · Post by **Dave the bass** » Fri Dec 07, 2007 8:45 am

The worst shock I've ever had was working in a school in the 1980's when I copped 400HZ off a 100V line speaker PA system going into Fire alarm whilst I was working on it live (twit). That made me feel really rough. Not a big voltage (I've had higher) but I'm guessing it was that frequency that made me all jubby.

The PA system doubled as a Fire Alarm and class-change 'pip's, the pip's were 1KHz or so pulsed but the Fire alarm was a continuous 400Hz squarewave.

I fell off the ladder too I was working on.

That didn't help either.

I was doing some dull PAT testing yesterday and copped 2 back EMF's accross the mains plugs of Fridges, they sting a bit too.

DTB

Richard Higgins · Post by **Richard Higgins** » Fri Dec 07, 2007 8:57 am

Hi Nick
Yes I (just about) understand that balance resistors are required for caps in series, but these are in parallel.
I used the original WAD transformer and adjusted the voltage by a group of caps in parallel to give a choke loaded supply, following advice from Max and Paul.

Regards Richard (Higgins)

Post by **Nick** » Fri Dec 07, 2007 9:39 am

Sorry Richard, I should have read your post first

audio-talk

Lesson Learnt (then forgotten within 3 months)

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