No, shorted turns might not affect the DC resistance very much although it depends on the exact winding scheme. It will be lower, so the DC drop across the primary will be smaller but the change could easily be smaller than that caused by mains voltage variations.
The AC current will be bigger because the AC anode load will be much smaller, but who measures AC current in his output stage?
The AC current will be bigger because the AC anode load will be much smaller, but who measures AC current in his output stage?
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.
Vacuum tube amps really, really, really don't like operating into open outputs. That will cause primary OPT flyback that can generate thousands of volts. Given that the 300B's were glowing brightly suggests that this has happened. The primary windings probably won't stand up to that, and some of the turns flashed over. Shorted turns in the primary won't be so obvious with an OPT, since the finals limit the current, but the current flowing through the shorted turns will reduce the inductance. That's likely the cause of the new distortion.
Since most VT amps aren't supposed to operate into an open, the designers don't necessarily check for stability into open secondaries. If that thing took off and oscillated, it would have fed the OPTs enough signal to cause an over volt flyback.
That's why I always make sure to put shorts across the input and output of the channel I'm not working on when doing anything with stereo VT amps (and SS amps as well -- can't be too careful).
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The magnetic field does not have to collapse, just change quickly enough. It is not the disappearing of a field which induces a voltage, just the rate of change. In this case I suspect that a high frequency parasitic oscillation was the cause, as it would have fast changes - the output stage would become a high power Class C oscillator (a radio transmitter with no antenna). All that RF power has to go somewhere, and it has probably fried something in the output stage - the most likely being the OPT.
Any change in current or voltage on a winding will produce a back EMF. This is because the magnetic field arround the coil is changing. It is this change in field that induces the back EMF. This change can be AC or DC which changes in amplitude.
Blown OPTs
I agree with the previous posts. If the tubes went bright blue it seems to me to indicate that the plate voltages went out of sight. A high frequency oscillation and an open secondary are consistent with both the tube glowing bright blue as well as shorted windings in the OPTs.
Also, I would add that not having anything plugged into the input could have caused oscillations in the driver which propagated to the output stage. I've had amps that oscillated without an input connected. If that's the case no damage would be caused unless BOTH the inputs AND outputs were left disconnected. I wouldn't turn a tube amp on without ANYTHING connected to both the input and output. At the very least ground the input if you have no load on the OT. Better yet, NEVER turn on a tube amp without a load on the OT.
I hope that's not the case but it sure looks like it. Bummer. Unless the plates got really hot or there was arcing in the tubes, the tubes are probably still OK.
My 300B tubes (Electro-Harmonix) have only a very slight blue tinge, and only when being pushed hard. The blue light comes from ionized residual gas in the tubes.
I agree with the previous posts. If the tubes went bright blue it seems to me to indicate that the plate voltages went out of sight. A high frequency oscillation and an open secondary are consistent with both the tube glowing bright blue as well as shorted windings in the OPTs.
Also, I would add that not having anything plugged into the input could have caused oscillations in the driver which propagated to the output stage. I've had amps that oscillated without an input connected. If that's the case no damage would be caused unless BOTH the inputs AND outputs were left disconnected. I wouldn't turn a tube amp on without ANYTHING connected to both the input and output. At the very least ground the input if you have no load on the OT. Better yet, NEVER turn on a tube amp without a load on the OT.
I hope that's not the case but it sure looks like it.
Bummer. Unless the plates got really hot or there was arcing in the tubes, the tubes are probably still OK.My 300B tubes (Electro-Harmonix) have only a very slight blue tinge, and only when being pushed hard. The blue light comes from ionized residual gas in the tubes.
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I see. Not nice at all then.
Perhaps I'd better add some reversed diodes across the primary of my own OT.
I use MOVs on the secondaries of my KT88 PP amp. I checked the amp with a scope while it was driving speakers and measured the voltage where it finally clips (maximum output voltage), then used an MOV rated about 40% higher in voltage so it will never conduct unless I'm careless enough to turn the amp on without speakers, in which case it could save my OTs.
The protection can be on either side but it's very easy to get exactly the MOV voltage you need for most amps if you put it on the secondary side. The voltage should be measured and the MOV voltage determined using the speakers connected to the lowest impedance tap on the secondary of the OT (if it has taps). This will load the transformer the least and give the highest output voltage on the OT. The voltage can be measured on any tap as long as the protection MOV is placed on the same tap. I put the MOVs on the 16 ohm taps (highest voltage) so it protects the entire winding, but putting in on any of the taps as long as the voltage is adjusted should suffice.
I hear no sonic differences with the MOVs on or off. It's easy to hook it to the binding posts externally so you can take one lead and connect and disconnct the MOV to listen for any difference. If you can hear a difference either you used the wrong voltage MOV or your ears are better than mine.
Maintaining some sort of load is useful, especially at HF. Something like a Zobel network would help, with higher impedance than you would use for a solid-state amp. Grid stoppers also ought to stop this sort of thing - does the amp have stoppers on the output valves?
A diode across the primary is of course useless as it will conduct on half-cycles of audio. MOV is better. An alternative is a diode from anode to ground. This will normally do nothing. It won't even protect the valve from high voltage (well, not directly). What it will do is damp any HF oscillation as the anode will no longer be able to go negative of ground. I don't have one of these protecting my EL34 outputs, but I did put one on the OPT side of the HT fuse, together with a small ceramic capacitor. The idea is that if a valve shorts internally, thus drawing a large current through the OPT, when the fuse blows I don't want a huge voltage to develop which could cause an arc in the fuse. The capacitors slows everything down and limits the voltage, and the diode conducts when things go negative so the highest voltage across the fuse is just the HT.
A diode across the primary is of course useless as it will conduct on half-cycles of audio. MOV is better. An alternative is a diode from anode to ground. This will normally do nothing. It won't even protect the valve from high voltage (well, not directly). What it will do is damp any HF oscillation as the anode will no longer be able to go negative of ground. I don't have one of these protecting my EL34 outputs, but I did put one on the OPT side of the HT fuse, together with a small ceramic capacitor. The idea is that if a valve shorts internally, thus drawing a large current through the OPT, when the fuse blows I don't want a huge voltage to develop which could cause an arc in the fuse. The capacitors slows everything down and limits the voltage, and the diode conducts when things go negative so the highest voltage across the fuse is just the HT.
i have a fear that the muddiness hat i'm perceiving is actually the sound quality of the amp itself. after all, the model is only rated at 85db SN ratio. and the chinese reviews that i translated all state similar experience of sound stage collapse with complex/fast music. is tube rolling some that could remedy this if i'm replacing some truly terribly made chinese stock bulbs?
I am confused. How much did you listen to the amp before the incident? Did it sound really good before and now sounds like crap at the same volume levels? If so then something has been damaged. Now if you haven't had a lot of listening time it could be that it has always had a problem. I can't find much on this amp but a lot of the Chinese amps have a reputation for using crappy output transformers enclosed by fancy looking boxes to hide the fact. A crappy OPT would lead to flabby bass at high volume among other things.
Maybe you can find someone with a know set of known good tubes (outputs and rectifiers) that you could substitute (after checking for proper voltages with the old tubes) and see if it helps. Unfortunately those output tubes and OPTs are expensive to just swap out without any clue as to their health. The very first thing I would do is to check the voltages on the plates and cathodes of all of the tubes in the amplifier (not just outputs) and see if they are reasonable. If the outputs are fixed bias then you will need to check the bias current and grid voltage.
Sure with we had a schematic.
Maybe you can find someone with a know set of known good tubes (outputs and rectifiers) that you could substitute (after checking for proper voltages with the old tubes) and see if it helps. Unfortunately those output tubes and OPTs are expensive to just swap out without any clue as to their health. The very first thing I would do is to check the voltages on the plates and cathodes of all of the tubes in the amplifier (not just outputs) and see if they are reasonable. If the outputs are fixed bias then you will need to check the bias current and grid voltage.
Sure with we had a schematic.
Presapian,
When your transformers have survived try to measure dc resistances of the output transformers once you have your DVM.
The muddied sound might well be caused by (too) high dc resistances of the output transformer.
SE amps are not famous for damping factor, and a bad output transformer (primary dcr 150 ohms or more; secondary dcr over 0,3 ohms) worsen things.
A good quality transformer (primary dcr 50 to 60 ohms; secondary dcr not over 0,2 ohms) has a major (beneficial) influence on the sound quality.
Pieter
When your transformers have survived try to measure dc resistances of the output transformers once you have your DVM.
The muddied sound might well be caused by (too) high dc resistances of the output transformer.
SE amps are not famous for damping factor, and a bad output transformer (primary dcr 150 ohms or more; secondary dcr over 0,3 ohms) worsen things.
A good quality transformer (primary dcr 50 to 60 ohms; secondary dcr not over 0,2 ohms) has a major (beneficial) influence on the sound quality.
Pieter
On 18th October here the OP described the amp as "new".
There isn't much about it on the web, apart from advertising blurb. The transformers are described as "hand-wound", but that is not necessarily a good thing. Given competent construction, whether by machine or real people, the important things in an OPT are design and materials. The amp is said to be 20W out, at 2% THD (1kHz). I am not a SET fan, so I don't know whether that is plausible for a single 300B. It would be interesting to see what happens to distortion and power at frequency extremes.
Maybe the OP will get more help over there.
There isn't much about it on the web, apart from advertising blurb. The transformers are described as "hand-wound", but that is not necessarily a good thing. Given competent construction, whether by machine or real people, the important things in an OPT are design and materials. The amp is said to be 20W out, at 2% THD (1kHz). I am not a SET fan, so I don't know whether that is plausible for a single 300B. It would be interesting to see what happens to distortion and power at frequency extremes.
Maybe the OP will get more help over there.
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