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Old 31st December 2007, 09:54 PM   #21
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Porkchop61,

Ag ja (darn, must you have such an appetising nom-de-plume), main problem with much of the reasoning in Electron Stream is that field strength progression from cathode - anode in tubes is not as simple as assumed there. Grid contribution is also dependent on density of wire mesh vs a solid plate, etc. I have accurately measured electrode distances for several power tubers and they do not correspond to what was given there somewhere, etc. etc.

Leave the diode out. The G2 always draws current, so it gives no more than a continuous but unreliable little voltage drop. Assuming that it passes dc and not ac is nonsense.

Tubelab,

Oh yes? (regarding wire-wound load resistors). Thanks, I will then have a look at that myself (vs non-inductive R). Perhaps another new year's resolution is to build a 100W equivalent loudspeaker load.

AND, AH YES! REGARDS AND LOADS OF FORTUNE TO ALL FOR THE YEAR 1111101100! (Jeepers, am I that old?)
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Old 31st December 2007, 11:42 PM   #22
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Why test an amp with a non inductive resistor when a speaker is inductive itself?
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Old 1st January 2008, 02:01 PM   #23
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Quote:
Why test an amp with a non inductive resistor when a speaker is inductive itself?
In order to make a meaningful and repeatable measurement of an amplifiers performance a purely resistive load should be used. Yes the amplifier will be operated into a speaker system which is far from a resistive load. Since every speaker system behaves differently, which one do we use as a "standard" for measurement? A speaker system with a complex crossover network may appear inductive, resistive, or capacitive at a given frequency, and this is not even considering back EMF effects.

If you are performing any measurements on an amplifier a pure resistive load should be used to obtain true readings. Why? Consider a frequency response test. an inductive load would have an impedance that rises with increasing frequency. It may be 8 ohms at 1 KHz and 10 ohms at 10 KHz. A tube amplifier will generally produce a higher output voltage across a higher load resistance, so the amplifier would show an increase in output as the frequency is increased. This would lead to a frequency response plot that has an upward tilt throughout the audio range.

I did some frequency response, power output and efficiency testing on my 833A amp a few years ago. http://www.tubelab.com/833SE.htm

During that testing I claimed a 51% efficiency at 212watts output. You can see the large 500 watt wirewound load resistor on the right. I was informed that those measurements might be in error due to the load resistor. Since I do not have any other load capable of withstanding the output of this amp (and I dissassembled this WMD before it could cause harm) I decided to do some testing on a lower powered amp. I found two sources of error. The inductive load caused the output voltage reading to be high by about 1 or 2 % at 1KHz even though it measured 7.95 ohms. The cheap DVM that I was using to measure the output voltage is off by 3 or 4 % compared to 60Hz. The error is not linear across voltage levels either. So these two error mechanisms caused my 212 watt reading to be optimistic by 5% or so. The resistor alone probably caused the slight upward rise in the mid band frequency response plot shown.

Are these errors a big deal. For the average DIY amp builder probably not. So your readings are off by 2 or 3%. They generally make your amp look better than it really is.

I have since constructed a load bank using premium (TO220 case) resistors mounted on a large heat sink, and I got my HP8903A audio analyzer fixed. I use this "good stuff" to make final measurements on a known good amp. I still use my indestructable 500 watt loads and a cheap DVM during the experiment phase and whenever something could go wrong.
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Old 15th December 2011, 05:30 PM   #24
adason is offline adason  United States
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Hi George,

I know this is old thread...but I have a questions about simple SE and the operation in triode/pentode/ultralinear mode when its comes to the sonic qualities. Can you comment on the sound, not just power? Are there any audible differences besides the total output power? What is the prefered mode for hifi?

ed

Last edited by adason; 15th December 2011 at 05:56 PM.
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Old 24th February 2012, 05:15 PM   #25
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ummm...seriously, here, DO NOT FLIP THE DAMNED SWITCH WITH THE AMP ON.
you're playing with the b+ supply to the power tubes here, straight off the output transformer, and the voltages are LETHAL. just cuz someone "got away with it with some junk tubes" does not excuse obviously unenlightened experimentation like this. THIS COULD KILL YOU or your amp, as well...just DON'T DO IT.

you COULD however, do it with the amp on standby, as at that point, the B+ is disconnected and the only thing that SHOULD be recieving voltage is the heaters of the tubes. i would reccomend, particularly if you're not intimate with the circuit in question (and never knowing what someone before you might have done inside it) to do this with the amp off.

not to be insulting, but someone could die doing the afforementioned behaviour. don't let that dumb person winning a darwin award be YOU.
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Old 24th February 2012, 07:50 PM   #26
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Quote:
Originally Posted by pinkjimiphoton View Post
not to be insulting, but someone could die doing the afforementioned behaviour.
How?
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Old 24th February 2012, 08:00 PM   #27
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If it is a metal shell switch in which the shell is in contact with the chassis (earth) there is no safety issue that I see.

That said, I don't know of any switches rated for this type of operation short of vacuum switches.

Operating a 250VAC switch with 350V DC on it is a sure recipe for destroying the switch due to contact arcing.

At some point in the switching operation you will lose the connection to the screen, which is a good way to destroy the tube and possibly burn up your OPT.
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Old 24th February 2012, 08:32 PM   #28
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Quote:
Originally Posted by leadbelly View Post
How?
never assume, unless you've been inside an amp, that it's wired right. i've seem amps come in for repairs where people had the chassis hot after "converting" them to grounded mains leads or "installing" new 2 conductor wires, etc.

people make mistakes..even people that have done this for decades.

as the gimp said as well, it could easily arc the switch as well...if that switch shorts, say goodbye to your output transformer as well.

imho, better to be safe...i've been bit enough times that i prefer to be pretty anal about safety. 500+ volts across the chest can make your heart beat kinda funny for a while.

just not a good practice. again, your mileage may vary. but to me, that's not a particularly smart thing to do...but by all means, feel free if that's what you choose to do.

peace
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Old 24th February 2012, 11:47 PM   #29
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It's a bit dramatic, and no doubt confusing for some, when a post shouts THIS COULD KILL YOU. Perhaps limiting the cautions to just the basic facts would be more beneficial rather than heading off in to paranoia land.
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Old 25th February 2012, 12:26 AM   #30
adason is offline adason  United States
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Hi George,

I know this is old thread...but I have a questions about simple SE and the operation in triode/pentode/ultralinear mode when its comes to the sonic qualities. Can you comment on the sound, not just power? Are there any audible differences besides the total output power? What is the prefered mode for hifi?

ed
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