Inverted back plates build

[grapplesaw]

Very unlikely on the bridge snubber's Glen...thanks for running the test.

Joe there is a typo- the 1000watt unit is 74.5uh

I used the Sencor lc77 tester with has an inductance test function.

I will reset everything and start again with all rows installed and a bit larger fuses. Not ready for those 8 amp we once talked about. I am running 5 amp in the other WO build.

 

[grapplesaw]

Joe there is a typo- the 1000watt unit is 74.5uh

I used the Sencor lc77 tester with has an inductance test function.

I will reset everything and start again with all rows installed and a bit larger fuses. Not ready for those 8 amp we once talked about. I am running 5 amp in the other WO build.

Just calculated load and looks like I was drawing 2.28571 amps
So now not to worried about fuse blowing

 

[laatsch55]

Shouldn't be, I blow 5's all the time...

 

[grapplesaw]

Shouldn't be, I blow 5's all the time...

Goes to show I don't push it hard enough
Thanks Lee
I guess we will go with 8 amperes if this thing ever fly's

 

[laatsch55]

8's are for sine wave testing, 5's for music....

 

[Northwinds]

This is a absolutely fascinating thread. I would have already thrown the amp across the room after all that shit!!! I admire your perseverance Glen

 

[grapplesaw]

This is a absolutely fascinating thread. I would have already thrown the amp across the room after all that shit!!! I admire your perseverance Glen

Thanks for the support. I would have possibly chucked it but it probably would has landed on my foot, these things are heavy.

The saga continues. The reason for the fuse blowing is one driver has blown on b- left channel which I was running at the time. I fortunly have been using the varic to power up each time and hear it load up right away. I switch to the DImm Bulb and confirmed the high draw. After finding the problem I fired it up with only left fuses in all was ok. This had to be what was causing all the issues, right?
I then put in the right fuses and find there is a short in that side. So will have to pull those to find driver gone in that side. Lot of damage done at not to high current with one channel only driven.

I will stop now till I have the 009 sil pads.

So did the driver blow on its own, I had an other order of on transistors come with a defective one ,
Or did the 007 sil pads do all this damage and leak the transient signal,
Or is there still a gremlin hidding in the bowls of it.
I am now hoping it is the pads, because that is the only difference in my material list that I can come up with.
The low wires , although did stick through to far , probably did not touch anything after all

 

[grapplesaw]

Glen you want a load bank that is wound as non-inductive. Wire wounds resistors are usually quite inductive (especially these big ones) which is why I do not use them for the emitter resistors (although PL did). Wire wounds can be had in non inductive but they are specially wound to cancel the inductance normally created when you coil up (spiral up) a lot of wire around a tube.
You probably do not have an LCR bridge to test the load resistors. As a quick test, see if a 0.1uF mylar or ceramic type cap tacked right across the resistor itself makes the situation better or worse to give us a clue.

How long are your test wires that go out to the load resistor (these are also inductive if not paired closely together)?

Update.
After many interruptions I have it running again

After a quick test, have to work today so not any real time for this today, I reversed the little .01uf cap we tried across the load resistor, as it had increased the distortion, and now it has eliminated the distortion on low voltage test with only on set of fuses in. And you though these were bipolar eh!

So I have made some progress I think, thanks Joe for your input. I will put in Sixth row and see what happens after I test the other channel. Still not clear on why I lost the one driver.

 

[laatsch55]

I would hate to think we are getting bad runs of 21196's/95's....

 

[grapplesaw]

I would hate to think we are getting bad runs of 21196's/95's....

for now this one is all on ME!

 

[grapplesaw]

All back together

Sine is a little fat on the crests but the snubber cap has settled it down

I will wait till I have the .009 SIL pads before pushing it

So Thank you to all and especially Joe for getting me through this

Mark it as "built"

 

[grapplesaw]

I am in Sunny California this weekend. So having a bit of down time I have been researching bias crossover hatch and proper bias settings. I had no idea how this was more than twisting a pot to .350 mv. I think Ed has only touched what to so I found may sites tha were very interesting. This site did clear a lot of the basic differences be tween amp types and quasi vs full complimentary amps.
http://www.qsl.net/va3iul/RF Power Amplifiers/RF_Power_Amplifiers.pdf
http://sound.westhost.com/articles/cmpd-vs-darl.htm

I found a way to much on bias adjustments. It is clear that THD and bias have a lot of dependency. As well the thermal issues with over compensating cross hatch distortion. Some good info is here

http://www.ieee.li/pdf/essay/rf_power_amplifier_fundamentals.pdf


This is very interesting focussing different distortion types and cures

http://www.douglas-self.com/ampins/dipa/dipa.htm#7

 

[grapplesaw]

I put this back on the bench today to rob the power transistors for my dual 500. Last time I worked on it Joe had pointed out the SIL pads were too thin. I was chasing a distorted sine wave curve discussed earlier in this thread.

So instead of taking the parts I replaced all the SIL pads and installed them back into the unit then retested it. Well there you go. Problem solved. At 2k I got 58.6 volts before clip and .029% THD over five octaves. It also had .031 THD+N at the same settings.

 

[laatsch55]

Well how about that.....

 

[Gepetto]

I put this back on the bench today to rob the power transistors for my dual 500. Last time I worked on it Joe had pointed out the SIL pads were too thin. I was chasing a distorted sine wave curve discussed earlier in this thread.

So instead of taking the parts I replaced all the SIL pads and installed them back into the unit then retested it. Well there you go. Problem solved. At 2k I got 58.6 volts before clip and .029% THD over five octaves. It also had .031 THD+N at the same settings.

Glad it worked out for you Glen. Parasitic capacitance is a tricky thing to get your head around and there is a relatively large plate capacitance to ground between that large collector area and the chassis ground/DC ground. Separation is your friend unless you are trying to make a capacitor.

 

[laatsch55]

So it's not a short of sorts, isn't current somehow being induced from one surface to another??

 

[Gepetto]

So it's not a short of sorts, isn't current somehow being induced from one surface to another??

Yes a proportional short for AC (but 90 degrees out of phase with the AC voltage) but not for DC Lee. The higher the frequency, the greater the short in the case of parasitic capacitance.

 

[laatsch55]

So parasitic capacitance only occurs in AC?

 

[Gepetto]

So parasitic capacitance only occurs in AC?

Hmmmm...interesting question Lee. Parasitic capacitance exists at DC as well but the size of the capacitor is (generally) so small that it generally goes unnoticed to the circuit involved. But that small capacitor at high frequencies can turn in to a real stability gremlin, especially in feedback related circuits.

 

[laatsch55]

So you can have instability in the amplified signal and rail voltage?