• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

HELP!: my SET amp was turned off without a load and now it sound veiled!

:( i had no idea hifi amps were subjected to same precaution as guitar amps, when i turned on my SET amp without speakers connected to it. the amp is Spark 743D, and has slow start indicator- red then green. the amp went to green after usual warm up period. a minute later the 300b tubes suddenly went bright blue then the amp went back to red- protection mode. i turned off the amp immediately when i saw that. there was no smell of any kind and the transformers were cool to the touch. but now the amp sounds veiled and muddy- everything sounding like an AM radio broadcast. did i fry the 300b tubes? oh boy.. please let me know.
 
no there was no audio to the amp. input cables weren't even connected. is it really possible that the transformers are fried? i smelled nothing burning and they were cool to the touch. i have no means of checking the bias. though this problem makes me regret not getting a multimeter sooner. the amp was on green for less than a minute when it kicked back into protection mode. more inputs appreciated. much thanks, gents.

p.s. the 300b tubes are unequal blue glow between the pair. one has a barely noticeable blue glow and in different location- the bottom plate. the other one glows mainly around the top plate... just a clueless info since i have no knowledge about these things...
 
Last edited:

Brit01

Member
2008-07-04 10:35 am
Yes tubes can glow blue due to the electrons.

Did you see the tubes glow red hot for a while?

Does it have fixed or auto bias? I've had a faulty fixed bias occur in prototypes and you want to pull that power supply before you can say ....oh dear me. Fuses at the correct rating are essential in those cases.

As mentioned a schematic will help.
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
Is this parasitic oscillation? I guess this could happen without a load, but it would not persist once the load was back in place unless the high voltage has caused insulation breakdown in the OPT so now it has shorted turns.

If it was then I would have expected a problem maybe on one channel, however PreSapian has not said if it is on one channel or both. Whats the chance of both output Tx's going?

Regards
M. Gregg
 
Given the same condition on both channels it is possible to get the same behaviour on both channels. Any oscillation could continue until an OPT short killed it off. The bright blue glow mentioned by the OP could be a symptom of oscillation. The blue glow sometimes seen normally in output valves is usually much fainter.
 
Is this parasitic oscillation? I guess this could happen without a load, but it would not persist once the load was back in place unless the high voltage has caused insulation breakdown in the OPT so now it has shorted turns.

so do you mean that the tube glow being back normal with the load is a sign that the OPTs aren't damaged. mine are so.

Yes tubes can glow blue due to the electrons.

Did you see the tubes glow red hot for a while?

Does it have fixed or auto bias? I've had a faulty fixed bias occur in prototypes and you want to pull that power supply before you can say ....oh dear me. Fuses at the correct rating are essential in those cases.

As mentioned a schematic will help.


i know most 300Bs are suppose to have a blue glow. on mine, when the protection mode came on, the blue glow became intense enough to tint the whole glass with the color. it was not a red glow but blue. the filaments didn't seem to get any brighter but the room lights on so it's hard to tell.

Well if the tubes won't bias or the fault persists then a schematic is needed.
Lets hope its something easy!

Regards
M. Gregg


so if the tubes are able to bias to spec, would that mean that everything is a OK?

and is it possible that the rectifier tube is the one that took the damage? i've already ordered a sovtek 5u4g to replace the generic chinese one which im hoping to improve the muddy sound. this tube doesn't have a bias pot for it so it should be the cheapest experiment without investing in a multimeter.


once again, thanks for your expertise, gents.
 
Last edited:

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
PreSapian,

You don't need the ultimate accuracy multimeter here. A basic 3.5 digit meter will work just fine. Such a meter can be had for less than a dollar and less than $5 by the time shipping is included. You can probably find one locally for less than $20 if you look at an auto parts store or electronics shop. It'll need to be able to measure up to at least 500 V.

If you spend a bit more you get something that actually looks quite useful. Like the VC97. Heck, I might pick up one of those just for the capacitance function and thermometer... And at $27 shipped, it's probably about what you paid for the replacement rectifier tubes, and certainly much, much less than a pair of 300B's.

~Tom
 
Last edited:
^^thats great news, Tom. i'm currently in china though so it wouldn't be in dollars but much cheaper, still. :) i should be able to order one next week.

muddiness is equally present on both channels. the tubes i have in the Spark 743D are all chinese save the pair of 6922 which are sovteks- another bad choice as i'm finding out from my research. i remember most tube gears with chinese tubes fitted in them would always sound terribly harsh until i rolled them. perhaps it's the opposite with this one for change? terribly dull and muddy until i roll out the chinese tubes?

also, my speakers are 87db homebrew- tube amp friendly impedance curve and only being driven to low volume levels. the amp sounded impressive with a same 87db sensitivity dynaudio speakers before. i regard my diy pairs higher than the dynaudio so i should suspect the problem is with the amp...
 
Last edited:

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
Shorted turns would not affect bias. Everything would seem normal, except the volume would be rather low and distortion greater as the valves tried to force some signal through the OPT. The test would involve measuring the primary inductance with the secondary open then shorted. Some DMMs have an inductance range.

Agreed,

However the current will rise through the tube if the turns are shorted so if you have the original value of current, and it has changed dramatically it may reflect shorted turns.