Need help - black out while the amp. was on

[Megaudio]

We had black out while my tube amp. was on.After that the bass was not so tight any more and the highs were stronger.Now comes strange part:all voltages are ok,all caps are ok,all resistors seems to be ok...can be the tubes?...service men told me I should replace all tubes...does anyone knows what happens when short but big overload (spark) hit the amp?..the curious part is that I have phono stage and it shows no changes in the sound..so it must bi power stage in amp. and more stronger curent then in phono stage..or?...what do you think?

[DF96]

Very unlikely to be the valves, as valves don't set the frequency response of an amp. If all voltages are OK then all valves are OK - maybe you should find a competent service man!

It is just possible, although unlikely, that a mains voltage spike could have damaged an output transformer. This would not show up in DC voltage tests. You can check by measuring inductance.

[quikie22]

@DF96: I think if the spike was great enough to kill/damage the OPT, the speakers would be in all likelihood damaged/dead too...
My two cents: It could be some blown caps due to the "overvoltage" right before the blackout. Maybe not enough to totally destroy them but make them leaky enough to have a reduced capacitance. A reduced capacitance could lead to a weak bass.
Tubes are generally very robust creatures and do not succumb.
Are you sure the difference is very noticeable. Do play your familiar CDs/songs and make an objective comparison. Sometimes the anxiety that something may have happened clouds our judgment enough to make us hear differences that may or may not be present.

[Megaudio]

Well..it's not the first time I had the black out..I have here time to time small black outs but always with the same result..I already "lost" 1 amp..this is my second amp...I thought the tube amp is not so sensible on sparks but I am wrong....and yes..I feel the diference and both channels show the same problem..it's just that dinamic is not so good like before and even my friend ask me where is this deep strong buss...I change then 2 big caps - 3300 mf each and small one paralel to it...0,47mf...but no result..then I change 4 smaller caps - 22 mf each - again nothing....but phono stage amp. shows no demage so I think this is because of the smaller voltage and only few mA strong power..the amp has 450 mA...SOMETHING is damaged ....no doubt in that...how to find out is for me nightmare...the service men is very good,is well known as a tube "guru"...we checked all voltages..all in perfect condition..realy a mistery....to buy an UPS double conversion with perfect sinus vawe is maybe the solution but for now I want my old sound back...only what to do..where to start????...it is curious that both amp. shows the same degradation in sound after black out...just the same... the sound picture lost the
tightness and deepness.The instruments have not this strong structure as
they normaly have...realy do not know what to do next

[mastodon]

Get your repairman to hook up a sine wave generator to the input of your power amp, a dummy load to the output and a scope to the in- and output. Then measure the frequency response of the amp. That way, you won't have to rely on subjective observations.

Then again, if your repairman is really a tube guru, then there's no doubt he has already done the above and establishes the nature of the problem...so what did he find? If he didn't do this, then find another 'guru'. One who can actually do some decent measurements.

[quikie22]

sounds like some blown Output trannies....as per DF96 suggestions..

or else the speakers may have blown/burnt.... check with another pair of speakers or connect another known working amp to the speakers... loss/poor function of one speaker could cause loss of stereo imaging and bass response...

[Megaudio]

Originally Posted by mastodon

Get your repairman to hook up a sine wave generator to the input of your power amp, a dummy load to the output and a scope to the in- and output. Then measure the frequency response of the amp. That way, you won't have to rely on subjective observations.

Then again, if your repairman is really a tube guru, then there's no doubt he has already done the above and establishes the nature of the problem...so what did he find? If he didn't do this, then find another 'guru'. One who can actually do some decent measurements.

Yes we did all measurements....only few resistors are not checked in power stage..he said they should first go out as in circuit is not possible to measure them...so..the instruments show ok..but my ears are better instrument....I am musiker and I know when the sound is different...the loudspeakers are ok because we conected with amp by my friend...perfect...even he said to me..what happened with your terrific bas??...if I do not locate the damage,I will sell it and by UPS with double conversion - on line machine and start with new amp...I do not see other solution...anyway thank you guys for your help...i realy appreciate that..thanks

[indianajo]

My st70 has taken some lightening strikes over the years, tempered by the refrigerator or furnace blower motor of course. Have lost a couple of power switches arced and the 1000V turn off pop capacitor I installed in the early seventies. Don't know why your rig should be more sensitive to surge, unless some wiring fool connected you to 220 instead of 110 for a while, or 3 phase analog. (sqrt 3 times voltage delta over wye). UPS are a complete waste of time, my previous employer tried them to protect the label printers from lightening strikes. They were less reliable than the naked label printers. For serious overvoltage protection install an obsolete SOLA constant voltage output transformer.

[Conrad Hoffman]

Tube amps are in fact extremely robust. Either you have a problem that a competent tech should be able to locate quite quickly, or more likely you've been burned once and have sufficient paranoia that the amp can't sound right. The only way to fix that is do a blind comparison with some amp you regard highly. We're all subject to FUD (fear, uncertainty & doubt) and the results on our perception are very real. BTW, where are you?

[Megaudio]

Originally Posted by indianajo

My st70 has taken some lightening strikes over the years, tempered by the refrigerator or furnace blower motor of course. Have lost a couple of power switches arced and the 1000V turn off pop capacitor I installed in the early seventies. Don't know why your rig should be more sensitive to surge, unless some wiring fool connected you to 220 instead of 110 for a while, or 3 phase analog. (sqrt 3 times voltage delta over wye). UPS are a complete waste of time, my previous employer tried them to protect the label printers from lightening strikes. They were less reliable than the naked label printers. For serious overvoltage protection install an obsolete SOLA constant voltage output transformer.

Hm...my only problem are very short black outs ,all other issues I have under control.Now if You read manuels it's allways stated that switch must be in off position whan conecting to the mains.If You look the switch inside the amp You can see small capacitor is always there protecting amp from tha sparks..well this spark has to do somehow with all this story...the problem is that the spark do not destroy part but only change something in it...now..we compare my amp with the same amp at my friend and he first noticed the sound difference so I know egzactly what I am speaking about.Besides he has the same situation few years ago and could not find out where is the problem and sold his amp as I will probably do the same...I came on many forums to see if anyone has the same experience but so far no...anyway I do appriciate your help and time You invested,thanks