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Old 9th January 2016, 05:25 PM   #541
Zen Mod is offline Zen Mod  Serbia
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if you're thinking of fiddling with (red marked) 10K resistor - there is no need for any kind ofpot ; just decrease it to first lower value if you're doubting that upper gate is demanding slightly more juice for non- oscillating work

also - if you look at :

Click the image to open in full size.


output impedance of your LDR attenuator thingie is nowhere near goo denough for feeding Papa's brick amp ; put regular 2SK170BL/2SJ74BL buffer in between
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Old 9th January 2016, 05:33 PM   #542
ci11 is offline ci11  United States
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Originally Posted by Zen Mod View Post
if you're thinking of fiddling with (red marked) 10K resistor - there is no need for any kind ofpot ; just decrease it to first lower value if you're doubting that upper gate is demanding slightly more juice for non- oscillating work

also - if you look at :

output impedance of your LDR attenuator thingie is nowhere near goo denough for feeding Papa's brick amp ; put regular 2SK170BL/2SJ74BL buffer in between
OK!

I do have some K240BL's left so I will find a buffer circuit. Any ideas?This has been what I thought all along.

On the supplies, I have the variac, iso transformer and caps all ready. I asked the question to Papa in the thread about which way to go - bench supply or cap bank and his reply was a "DC-coupled, low impedance" drive is needed for the circuit. He also suggested to start with the bench supplies first.

Well, my cap bank would have been 4x 68,000F/80V for each channel so not having to build it and risk certain death with one wrong move is a good thing. But I am ready to take it on if that's what's called for.
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Old 9th January 2016, 08:00 PM   #543
2 picoDumbs is offline 2 picoDumbs  Australia
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Originally Posted by ci11 View Post
ZM,

What - you don't like my AB 10K pot
The only reason to lower 10k resistor is if you have less than the desired current through the constant current source. This is the least of your problems at the moment.
Focus on proper power supply first. Once the amp is operating correctly you can consider lowering 10k resistor if you need more current.
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Old 9th January 2016, 08:06 PM   #544
2 picoDumbs is offline 2 picoDumbs  Australia
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OK!

I do have some K240BL's left so I will find a buffer circuit. Any ideas?This has been what I thought all along.
Use the buffer in Sony Vfet article Part 1
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Old 9th January 2016, 08:38 PM   #545
ci11 is offline ci11  United States
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Use the buffer in Sony Vfet article Part 1
I still don't know why the bench supplies are not working with the circuit the way it is set up. Until I do, how would I know what to avoid? In an earlier post, I asked Papa if the CSX1 PSU would work well with this, and to the best of my ability to decipher, his suggestion may be something else. I will look into it.

If it does looks like I have to build the cap bank after all, that should keep me busy for a bit. When that's done, I will start tackling the buffer.
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Old 9th January 2016, 09:29 PM   #546
ci11 is offline ci11  United States
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try with plain PSU - variac , xformer , cap bank

I had problems with lab supply feeding steady high Iq in past ...... and I don't care what was in question

using variac + big Donut + big cap bank is saving me of too much thinking
ZM,

The R&S lab supplies are pretty tough and are rated at 32V 5A continuous, 10A for an hour and 20A for 1mS. In my stress test using a 8 Ohm Frizlen braking resistor, they did not break a sweat and were rock steady - they drew exactly 7.1A at +60V.

What's puzzling is the current throttling that I am seeing on the supplies' meters, especially after the devices warmed up. Something in the circuit must be causing this to happen. And I would like to know why to avoid it.

By the way, I had already laid out an AC/DC PSU with the Variac before deciding to build this amp. It would be a 1kVA box with an iso transformer so I can use it to play music or safely fix amps. Attached is a print from the FPD file of the front panel. The 2 smaller circles are for power and AC/DC switches and the 4 larger circles are for Simpson panel V and A meters. Just haven't built it as the amp project got my attention after BAF.
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File Type: png Variac PSU.png (39.4 KB, 162 views)
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Old 9th January 2016, 09:39 PM   #547
generg is offline generg  Germany
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May I repeat two suggestions...

1. In Aleph P1.7 the Zener in the input caused oscillation with me and some other people.
Take it out.

2. Lynn analyzed in the F6 circuit a tendency of motorboating by simulation and he adviced as far as I remember to reduce the 100uF cap near the Jensen to a smaller value, 220uF.
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Old 9th January 2016, 09:43 PM   #548
Zen Mod is offline Zen Mod  Serbia
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well , in that case (bench supply proven) , you can exclude it's responsibility to oscillation

do you have a scope , by any chance ?

to exclude problems with bias generator , you can go with 9V battery and 2K2 trimmpot across it , as temporary test substitution
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Old 9th January 2016, 09:45 PM   #549
ci11 is offline ci11  United States
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Originally Posted by generg View Post
May I repeat two suggestions...

1. In Aleph P1.7 the Zener in the input caused oscillation with me and some other people.
Take it out.

2. Lynn analyzed in the F6 circuit a tendency of motorboating by simulation and he adviced as far as I remember to reduce the 100uF cap near the Jensen to a smaller value, 220uF.
Hi Gerd,

Thanks - I will look into it!
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Old 9th January 2016, 09:53 PM   #550
ci11 is offline ci11  United States
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Originally Posted by Zen Mod View Post
well , in that case (bench supply proven) , you can exclude it's responsibility to oscillation

do you have a scope , by any chance ?

to exclude problems with bias generator , you can go with 9V battery and 2K2 trimmpot across it , as temporary test substitution
ZM,

Well, I think the bench supplies are good, but it is possible that there can still be unintended interactions. This is why I am trying to understand the root cause before attempting a fix. And I will try the 2 suggestions from Gerd to see if that makes any difference.

The least destructive/disruptive test is your first suggestion, changing the 10K in the bias loop. Tomorrow, I will hook up my pot and see if I get anywhere. If the bench supplies still motor-boat or oscillate then it's time to build the Variac PSU. Not looking forward to it because the front panel has to hold the weight of the Variac, which is about 25 lbs. Total weight, 80+ lbs.

I do have a scope - but it's in storage. I am waiting for the QA401 to be released so I can get a competent, poor-man's AP to use both for time domain and THD+N measurements.

Last edited by ci11; 9th January 2016 at 09:57 PM.
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