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Old 18th January 2011, 01:48 PM   #21
TheGimp is offline TheGimp  United States
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Quote:
Originally Posted by godfrey View Post
....or if the amp likes to oscillate with no load.

...snip....

edit: oops, I missed eeyore's post. Seems like accidental oscillation is a very real possibility after all.

I can't help wondering why the designer didn't use a Zobel (or something) to make stability less load-dependent. Seriously. It can be done. In solid-state-land, you don't see stickers that say: "Caution, this amp may explode if the speaker is disconnected".
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.
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Old 18th January 2011, 02:20 PM   #22
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Originally Posted by kavermei View Post
As long as your amp was not actually outputting any music, no harm done. Don't lose any sleep over it!
..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

Last edited by richwalters; 18th January 2011 at 02:24 PM. Reason: more info
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Old 18th January 2011, 03:06 PM   #23
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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.

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I can start a new thread if Horsebox feels this is a hijack.
Not at all, go ahead. I wonder if you might get a better response with a more apt thread title though.
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Old 18th January 2011, 03:19 PM   #24
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by TheGimp View Post
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.

Have a look at post #29 on-of this thread. Some interesting ideas!

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

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M. Gregg
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Old 18th January 2011, 03:52 PM   #25
TheGimp is offline TheGimp  United States
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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).
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Old 18th January 2011, 04:07 PM   #26
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by TheGimp View Post
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).
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).


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Old 18th January 2011, 04:28 PM   #27
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Quote:
Originally Posted by trobbins View Post
all tubes are microphonic
except 6C17K-B Click the image to open in full size.

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.
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Old 18th January 2011, 04:42 PM   #28
TheGimp is offline TheGimp  United States
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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.
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Old 18th January 2011, 05:07 PM   #29
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0.2A? That limits you to 160mW into 8 ohms, if 200mA is the peak current. Could you clarify? 35V RMS is only 153W; not enough for some people!
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Old 18th January 2011, 05:08 PM   #30
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by TheGimp View Post
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.
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.

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