I suspect there is the concern that anything added to the design will comparomize the audio quality.
I've been thinking about how to protect an amp from playing it without speakers attached.
I can start a new thread if Horsebox feels this is a hijack.
..obviously with no signal, well designed Tube amps should be bomb proof stable with load disconnected on the o/p or even short circuited.
However, I did come across a famous kit name which went into self oscillation with the speaker removed which damaged the o/p tube screen grids and stewing the screen resistors. It could be this creating the strange noises on warm up, ultimately leading to increased distortion.
I've got loads of SV KT88's with damaged screens, fit for triode operation.
Unless one has THD equipment to measure the effect of scorched screens, the effect on quiescent currents can be very small but geometrically slightly misaligned to create higher THD. Replacing tubes is the only option and beware of the problem next time round !
richy
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Well I took the bottom off this morning, had a good poke around and found nothing untoward so I've gone as far as I can really. It seems there's a possibility of cooking some of the OPT primary windings but I have no means to test.
Thanks everyone for the helpful advice.
Not at all, go ahead. I wonder if you might get a better response with a more apt thread title though.
Thanks everyone for the helpful advice.
Not at all, go ahead. I wonder if you might get a better response with a more apt thread title though.
Have a look at post #29 on-of this thread. Some interesting ideas!
http://www.diyaudio.com/forums/tube...rned-off-without-load-now-sound-veiled-3.html
Regards
M. Gregg
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I'm trying to figure out how to pass UL60065, sections 4.3.4 and 4.3.5 which basically state you have to run an amp at full output into the most unfavorable load (including open and short circuit). Furtheremore, they call out shorting or opening any surge suppression varistors as part of the test.
The tests have to run for 4 hrs or until power is no longer drawn by the unit (fuse blows).
The tests have to run for 4 hrs or until power is no longer drawn by the unit (fuse blows).
On first "Quick" look this is a requirement to prevent electric shock should components fail, if open or short circuit conditions should occur. Also in the most difficult situations. Am I correctly interpreting the standard?
If so is the double insulation standard applicable in the O/P Tx? (Isolation and limitation of max voltage on secondary windings).
Regards
M. Gregg
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except 6C17K-B
To prevent damage of output transformer in parallel with output tubes in push - pull amps should be soldered high voltage high frequency SS diodes, in reverse polarity. Diodes from TV horizontal flyback are fine.
In addition to preventing shock, it is concerned with the possibility of fire, hence the 4 hr time limit requirment for the test.
The secondary voltage limit is set at 35V Peak (RMS or DC) on the secondary of the OPT. They are also conecerned with the current and specify a max of 0.2A for 2 minutes which some amplifiers may exceed under short condition of the secondary.
The secondary voltage limit is set at 35V Peak (RMS or DC) on the secondary of the OPT. They are also conecerned with the current and specify a max of 0.2A for 2 minutes which some amplifiers may exceed under short condition of the secondary.
Components that comply with 14.1 and 14.2 are exempt. So control networks and components "within this category" that limit output voltage should comply. Also Tx's 41.3 insulation. An output Tx should have a high flash over voltage.
If this a requirement of domestic appliances then do O/P Tx's not already comply with this requirement. If so then only the voltages and current are of concern, linked to current capacity and clearance times of conductors (Fire) and fusing. So if you can limit O/P voltage with components the short circuit current in the O/P and primary winding conductors could be covered by ADS (Fuses / CB).Voltage is low at short so current limitation should be the issue!
I guess you have already covered this.
Regards
M. Gregg
except 6C17K-B
To prevent damage of output transformer in parallel with output tubes in push - pull amps should be soldered high voltage high frequency SS diodes, in reverse polarity. Diodes from TV horizontal flyback are fine.
Au contraire, even transmitter valves have mechanical resonances of the least supported part (grid) - however probably not in the audio range! I collated some interesting info at Microphonics
The freewheeling diodes only clamp one half winding below 0V, and rely on good coupling and low leakage inductance of other half winding (a reasonable assumption for OT's at lower frequencies, but maybe not for transients due to say a fuse blowing).
Ciao, Tim
Since my Hobart has cSAus, I suspect they could also carry cULusa .
My battery charger is older and is UL listed.
6S17K-V is planar triode that worked in rockets on acceleration up to 500G, and up to 10g vibration. I doubt you can register any microphonic in audio band. I could not, I have breadboards of mic pre and MC phono preamp.
Did I wrote that it is for PP output stage? What fuse blowing if the question was about disconnected speaker? And how fuse blowing can affect PP transformer?
For SE amp one more diode + powerful Zener is needed.
Yes well the phrase all valves are microphonic was meant to be a reminder, given kevinr's comment, that although you may not 'hear' a valve being microphonic in your amp, it will be contributing a microphonic signal (howefver small) due to vibrations given the generic construction of valves. Even the frame grid structure will vibrate. The point being not to gloss too liberally over the issue, even though it was not the main post topic.
I suggest an SE configuration is inherently lower risk wrt OT insulation breakdown from speaker side transients.
Ciao, Tim
I think you did target PP configuration. Yes I should have used disconnected speaker as the more relevant example of a transient. Whether a step change in load, or a fuse blowing, if there was current flowing in the OT primary at the time of the transient then all that energy has to go somewhere. I'd anticipate one winding end is nominally clamped (a freewheeling diode, or zener, or conducting tube, or B+ if both PP's are in cutoff), the other end of the winding flys in voltage (mainly limited I suggest by whatever parasitic capacitances are around). It's a good topic to discuss and worth appreciating how it may impact the commonly used circuit configurations.
I suggest an SE configuration is inherently lower risk wrt OT insulation breakdown from speaker side transients.
Ciao, Tim
The Russians stuck to valves in their planes for a very long time as they are immune to EMP (electro magnetic pulse) from nuclear explosions.
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