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Old 24th July 2009, 03:45 AM   #301
chrish is online now chrish  Australia
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Hmmm, those ebay 600uF 500V capacitors, the seller does not ship outside USA
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Old 24th July 2009, 07:20 AM   #302
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Quote:
Originally posted by chrish
Hmmm, those ebay 600uF 500V capacitors, the seller does not ship outside USA

Know anyone in the US who could ship them on? I'll split shipping with you if you do. Send me an email if you are keen, this is rather off-topic.
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Old 24th July 2009, 08:27 AM   #303
chrish is online now chrish  Australia
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It is probably a good thing... I have too much junk at the moment anyway and I am about to move house again

BTW, thevoice, you have your email switched off.

Regards,

Chris
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Old 25th July 2009, 03:10 AM   #304
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Quote:
Originally posted by tubelab.com


An FFT analyzer is excellent for capturing distortions that occur in the amplitude domain. THD or Total Harmonic Distortion does show up as unwanted harmonics. THD is simply a "total" of all of the unwanted harmonics. The FFT analyzer will let you see each harmonic. This is good since some harmonics sound much worse than others. It can be used to look at classic IMD or intermodulation distortion. This occurs when one signal mixes with another signal to create two or more new unwanted signals that are not harmonically related to either of the originals. The classic case is when a vocal (or other) signal (at say 1000Hz) gets blasted by a bass guitar signal (at say 60 Hz) creating two new signals at 940 Hz and 1060 Hz. This ALWAYS sounds bad. It isn't always your amplifier either I have found obvious IMD recorded into some CD's. Metallica's S&M comes to mind.

PIM is something different. It is intermodulation in the frequency domain. All electronic circuits have a delay associated with a signal passing through them. The signal comes out of your amplifier a few microseconds later than when it went in. As long as thes delay remains constant everything is OK.

Much of the delay is caused by RC circuits. Consider a mosfet with a gate stopper resistor. The mosfet has considerable input capacitance which forms a low pass filter against the gate stopper which will have a frequency response and a delay associated with it. Now what do you think will happen if the fets input capacitance varies with signal level. A large signal will cause the delay through the circuit to change with the music. In mild cases a heavy bass note could blur the edge of a transient or other signal with fast rise and fall times. In a bad case you are actually applying frequency modulation to a signal.
great explanation ! yet more questions arise... like why would the FET's input capacitance change? I can see how that would change the time constant (and phase) and wreak havoc ...

and then you only have to ask - doesn't whatever this effect is happen with tubes too?

Sorry for the off topic diversion...
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Old 25th July 2009, 12:50 PM   #305
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It's fairly easy to see how the capacitance can vary across a diode junction in reverse bias. There is a depletion region surrounding the junction that will vary in width with the applied bias. This varying width non conducting region sandwiched between two semiconductiong regions forms a lossy capacitor. I don't claim to understand exactly how this effect occurs in a mosfet, but it does.

The spec sheets of many mosfets include a capacitance VS voltage graph like the one included here. In a follower application the gate and the source remain at a nearly constant voltage with respect to each other, so the Ciss and the Coss specs aren't too important. The Crss spec is the gate to drain capacitance. The drain is usually bypassed to ground through the power supply, so Crss appears directly across the output of the preceeding stage.

Yes, a tube can exhibit some voltage modulated capacitance effects. There is an "electron cloud" or "space charge" surrounding the cathode. It is more conductive than the sorrounding vacuum. With a strong negative bias the cloud will be compressed around the cathode. As the bias is reduced the cloud expands. It also becomes less dense losing conductivity. This effect can cause the grid to cathode capacitance to be modulated with signal, but the effect is far weaker than it is in a semiconductor.
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Old 26th July 2009, 03:45 AM   #306
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I have IRF820s set aside for this amp. They are the MOSFETs recommended in "MOSFET Follies" Trust they are OK, as that is all I have
I put a pair of IRF 820's in the amp. One blew instantly, and the other lasted until I turned up the drive.

Most mosfets have internal zener diodes to keep the gate voltage from going high enough to blow the oxide. These fets do not have the diode, so the gate got blown in both of them. It may be possible to add an external diode.

I spent all day tweaking on this amp. It positively rocks now. It is late now. so I will report further tomorrow morning.
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Old 26th July 2009, 03:49 AM   #307
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I see that you are using "the" surplus Plitron output transformer.
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Old 26th July 2009, 06:44 AM   #308
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Quote:
Originally posted by agent.5
I see that you are using "the" surplus Plitron output transformer.
I also see one of those huge ACS caps. I got 6, I'm still wondering how I'm going to safley mount them in any "regular" sized chassis.
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Old 26th July 2009, 07:09 AM   #309
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Originally posted by athos56


I also see one of those huge ACS caps. I got 6, I'm still wondering how I'm going to safley mount them in any "regular" sized chassis.

You just have to redefine "regular". You can mount them horizontally just like how they fitted inside the USPS box.
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Old 26th July 2009, 02:12 PM   #310
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Quote:
I see that you are using "the" surplus Plitron output transformer.
Yes, I usually perform all of my experiments that could end badly with a surplus "guitar amp OPT" that I have a bunch of. I also use a well worn pair of Chinese 6L6GC's This way I won't cry if I fry something. It also has been by standard test for years, so that I make comparisons.

I did fry one OPT too. The load resistor blew open during a maximum power test. The fire gods danced inside the OPT. That is why I now use the 500 watt load resistor standing on end behind and to the right of the ASC cap. Can't blow that one!

I spent the day tweaking and tube rolling, and last night I decided that it was time to hook up the good stuff. I made an attempt to clean things up (OK no clip leads in the signal path) and tried some better quality OPT's and tubes.

I had been saving the Plitrons for "something special". Could this be it? I had been thinking that I wanted something that could really use up some of the 400 watt at 20Hz rating that these transformers carry, but is that realistic? Would I actually use a 1KW tube amp? It would be cool, the ultimate bragging rights, but with the heat is south Florida, I couldn't turn it on for more than 5 minutes. That is about how long it would take to eat my speakers too!

I have used the Plitrons in several experiments before, but nothing really sounded good enough to build. I wired one of them into this amp and I was suitably impressed. I listened to this amp for several hours last night with everything from Dianna Krall to Metallica. These things sound good, really good. Last night convinced me to build an amp with this stuff. The reality of this is that a 75 WPC amp is more than enough and these transformers have been doing nothing useful for far too long.

For now my plan is to build an amp, maybe a quick and ugly amp, using two of these driver boards, two of the tag boards for the output tubes, the two Plitron OPT's and maybe an Antek toroid for power. I always design the power supply last.

I learned this about the Plitrons last night:

They are rated for 1250 ohms CT with 2, 4, and 8 ohm secondaries. In this configuration they claim 400 watt capability at 20 Hz.

They work very well at 2500 ohm CT with 4, 8, and 16 ohm secondaries and at 5000 ohm CT with 8, 16, and 32 ohm secondaries. The power rating with these configurations is far beyond anything that I could throw at them.

I used 6L6GC's with the transformers wired for a 5000 ohm load. Maximum power in triode mode was 40 watts. The usual 3 db frequency response plots at 1 watt are meaningless. The upper 3db point is 64 KHz. I tested frequency response at +0 -1db. The result 6.5 Hz to 44 KHz. Response over 20 Hz to 20KHz is within 0.1db!

I then tested the amp with the EH KT88's wired for a 2500 ohm load. The B+ was 450 volts and idle current was 65 mA per tube. The results are better! Response at +0 -1db, 5.1 Hz to 46.9 KHz. The upper 3db point is 66 KHz. The distortion at 60 watts and 20 Hz is 4.3 %. Output with input shorted, 0.8 mV.

Measured distortion :

2500 ohm load triode mode

THD at 1 watt 0.19%
5 watts 0.54%
10 watts 0.95%
20 watts 1.52%
40 watts 1.84%
60 watts 1.77%
70 watts 3.08%
75 watts 4.18%
78 watts 5%
90 watts 10%

The picture shows the scope at the edge of clipping, and the power meter reading over 75 watts.
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