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A Very Compact Hybrid Amp - Page 70

post #1036 of 2203
4/29/09 at 5:06pm
ok, all change.

pulling both opamps the results are almost identical. This effectively turns the input into a grounded cathode amp i think. it indicates both CRD's are doing the same thing.

from alex

"However, looking at your numbers, the opamp on the bad channel is trying to make the cathode more positive to turn off the tube. Yet it is detecting an anode voltage over 100V. What it should be doing is trying to decrease the cathode voltage to get more current to flow in the tube and to pull the anode voltage down. So the opamp is behaving exactly the opposite to what it should be doing. Have you tried replacing C2R?"

cr2 swapped and no change.

still debugging this one, it should be simple but something is miss behaving.

looks like a servo issue. next thing is to replace the crd with a resistor (11k) determine if the servo is indeed dodgy.
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post #1037 of 2203
4/29/09 at 6:32pm
Can you pull one end of the CRD and put an ammeter on it? I'm somewhat doubtful it's the servo. It could also be possible that there's an issue with the board, a trace perhaps?

You could also just swap the CRD from the good to the bad and see if the behavior changes.

I stated my hypothesis in an earlier post, and I think that is what may be happening.

One way to test is to change the starting parameters so that the servo pushes current, instead of sinking. That is to put the 3K resistor in parallel with the 1.5K cathode on the bad channel. That should bring the starting point down. You can even use a 2K resistor if you like. The opamp will have to source more current to bring the cathode voltage up, to bring the plate voltage up.

The plate resistor would definitely pin it.
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post #1038 of 2203
4/29/09 at 6:43pm
I'll try the resistor tomorrow and report back. thanks for the help.
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post #1039 of 2203
4/30/09 at 4:12am
I spent ten minutes ohming each side to check both are 100% the same.

Checked out fine. component values seem fine, this also allows me to rule out a bad trace or a short.

unfortunately this points the finger at a bad component, so tonight i will replace the crd with a resistor and go from there.
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post #1040 of 2203
4/30/09 at 4:57am
Until I get more time, this $4.50 rat shack plastic box will have to do

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post #1041 of 2203
4/30/09 at 5:17am
It actually looks pretty good in that case.
I could have swore I threw a nut and washer for the pot in the bag with the rest of the parts from that Mini3....
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post #1042 of 2203
4/30/09 at 5:26am
Yeah, it's there somewhere but I just happened to grab the first nut that fit. Gotta get a little volume knob as well. As long a it works..

Just hope the box doesn't melt.
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post #1043 of 2203
4/30/09 at 5:42am
dude!..you better hope that plastic is made of Bakelite or something..atleast make sure the regulator IC with the heatsink on it isn't touching the plastic..that IC gets to 70+ deg C on mine.
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post #1044 of 2203
4/30/09 at 5:52am
Yeah, I chose that box as it has plenty of space around the board. There are 12 big holes in the bottom, bigger than the top ones so it's getting more ventilation than the metal cases and the amp/box doesn't get nearly as toasty as your did while I had it. The board is on nice long standoffs and the air can circulate very nicely.

I don't forsee any problems with it at all. All working spiffily
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post #1045 of 2203
4/30/09 at 8:46am
Quote:
Originally Posted by MisterX View Post
It actually looks pretty good in that case.
Just take a Sharpie to those screw heads... BTW, how's it sound ?
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post #1046 of 2203
4/30/09 at 11:13am
Quote:
Originally Posted by smeggy View Post
Until I get more time, this $4.50 rat shack plastic box will have to do
I agree... It *does* have a simple, low-budget appealing look.
All it needs is a nice, small, cheap plastic push-on knob w/white indicator line.
Being so specific because I've an extra that I think fits the bill that I've no need for.... I could 1st class mail to you if you want. Just PM. [Edit: Holland's idea below vvvvvv is better & cheaper than 1st class mail]
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post #1047 of 2203
4/30/09 at 1:57pm
Quote:
Originally Posted by adamus View Post
unfortunately this points the finger at a bad component, so tonight i will replace the crd with a resistor and go from there.
Can you pull the anode of the CRD and put an ammeter in series? I'm curious to know what is going on. You don't have to of course, but I figure it's on the way.

Quote:
Originally Posted by cfcubed View Post
All it needs is a nice, small, cheap plastic push-on knob w/white indicator line.
Ratshack has one that's, to be honest, not bad looking (to me). I hate to admit it, but I do use this on cheap builds, now and then. It's a screw on, single screw. Some heatshrink around the pot shaft, and this would fit fine.

Hexagonal Control Knob - RadioShack.com
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post #1048 of 2203
4/30/09 at 2:23pm
how do crds work, in my simple mind it will limit the current is over 2ma, myvoltages suggest there is less than 2ma going through it..... bad science from me there!
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post #1049 of 2203
4/30/09 at 2:54pm
volatge diagram attached if that helps
LL
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post #1050 of 2203
4/30/09 at 3:13pm
Thanks guys.

The thing sounds great. I really enjoyed my time with Sachus CTH and this one sounds just as good. I'm extremely impressed with these amps.

I think the best thing about the plastic project box is that about 10 minutes with a step drill bit and all the casework is done
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