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Old 23rd February 2012, 03:52 PM   #11
SandyG is offline SandyG  United States
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Quote:
Originally Posted by DF96 View Post
Don't run a valve amp without a load, unless you have lots of money to keep buying new OPTs. Screen stoppers prevent the output valves from oscillating at RF.
I am guessing you mean when it is driven? With the input shorted and with a stable circuit I would expect this not to be a problem, but I'm learning as I go!

I'll be hitting the bench with the Amp this weekend and will try the feedback suggestions first, then the screen stoppers to see what affects the problem.

Huge thanks for all the suggestions, doesn't feel that bad now with some ideas to try.

Sandy
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Old 23rd February 2012, 05:39 PM   #12
DF96 is offline DF96  England
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A stable amp may be OK with no input and no load, but you would need to ensure no operator error!
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Old 23rd February 2012, 11:53 PM   #13
SandyG is offline SandyG  United States
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Thanks DF96. Hopefully will get some time this weekend to play with the changes.

For those that have not seen the build thread, here's the link.

EZ260A Grommes 260A Project

Sandy
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Old 24th February 2012, 12:13 AM   #14
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Quote:
Originally Posted by DF96 View Post
A stable amp may be OK with no input and no load, but you would need to ensure no operator error!
Other than the fact that a tube amplifier is calculated based on the OT load, there is no problem. The load is part of the circuit.
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Old 26th February 2012, 06:02 PM   #15
SandyG is offline SandyG  United States
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Got a few minutes to play with some changes on the amp -

1. Pulled the Feedback off the transformer and the amp became unstable at idle (No signal). With some signal it calmed down.

2. Changed the Feedback resistor for 27k to 10k (arbitrary value) and it did not seem to make much difference except on the required drive. Both the 100Hz and 1kHz signals looked as original.

I did not get a chance to pull the board and cut the screen traces to install the screen stoppers, but will try that as soon as I can.

Sandy
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Old 26th February 2012, 07:19 PM   #16
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by SandyG View Post
Got a few minutes to play with some changes on the amp -

1. Pulled the Feedback off the transformer and the amp became unstable at idle (No signal). With some signal it calmed down.

<snip>

Sandy
That indicates that there is a really serious stability issue with your design or layout. Make sure the secondary is loaded when you do your testing, but you need to identify the source. Screen stoppers may help, there may also be some issues with the board layout that make the design marginally unstable.

Be methodical and verify that each stage in succession is not at the root of the problem starting at the input and working your way to the output - unless you are absolutely certain the output stage is oscillating as it may not be.

Check that input stage with the input shorted and unshorted, maybe even plugged into an interconnect that is not connected to anything at the other end - you should see no misbehavior other than some minor hum pick up...

Check that the supplies are adequately decoupled at all frequencies..

Etc.. Others will have a lot to add I'm sure..
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Old 27th February 2012, 03:56 PM   #17
SandyG is offline SandyG  United States
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Thanks Kevin,

I think the first step will be to add the screen stoppers and see if that helps or changes anything.

How critical are the values for the stoppers, I think DF96 recommended to try 1.5k which I have so will give it a shot.

Then do the recheck of component values, could be something as simple as a wrong stuffed part, but that would be too easy a fix

Sandy
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Old 27th February 2012, 06:28 PM   #18
DF96 is offline DF96  England
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The exact value does not matter too much, but a screen stopper should not drop too many volts. About 1k is often used.

The 27k feedback resistor should be increased to reduce the feedback, not reduced.
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Old 27th February 2012, 07:31 PM   #19
SandyG is offline SandyG  United States
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DF96 -

I tried it both ways, when I had the 10k in series with the existing 27K it didn't make much difference to the ugly signal other then sensitivity. So effectivly had 10k in one test, 27k stock initial value, and 37k for feedback.

I will try 1K in the Screens as soon as I can take the board off the chassis.

And all testing was into an 8ohm dummy load, some with a shorting plug and some with just the cable.

I will start to write this stuff down as it is not very accurate of me to try to remember all the test cases.

Hopefully the screen stoppers will clean things up.

Sandy
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Old 3rd March 2012, 07:49 PM   #20
SandyG is offline SandyG  United States
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Did a bit more testing today on the amp. Added 1K screen stoppers to the amp, and didn't see too much different on the 100Hz signal or the 1kHz. I did not try to run the amp without a load to see if it was stable.

Now I decided to look at the driver stage, in the 12AU7A slot I tried a 6189 and 5814 to see if it was a tube issue, but no difference was noticed. I left the GE 5814 in moving forward.

So I started looking at C15 and the 0.47uf cap as a suspect, and while observing the scope as the amp was showing it's odd ringing pattern on part of the waveform, I was probing voltages and as soon as I hit the Pin 7 of the 12AU7A the signal was perfectly clean!

I was probing with a Fluke 189 meter (volts mode) and I'm not sure why but the signal is perfect with the probe on Pin 7 of the tube, but I did a sweep and it looked very good, removed probe, back to unstable.

So it seems the ringing is in the first stage of the amp.

I think DF96 mentioned that it was odd the way it was biased (self) but not exactly sure.

I the Fluke meter specs show a 10meg impedance with 100pf on the voltage measurement inputs so could be a combination of both that cleans up the circuit, but not sure. I'm going to make a tester with 2-4.7M resistors in series and see if that has the similar effect as when the probe is on Pin 7, another words I'll do a 9.4M resistor from Pin 7 to Ground and see if that cleans things up.

Also for C15 I was using a Solen .47uf 630v cap, I swapped it for a low cost Xicon Polyester (I think) cap and noticed a slightly worse ringing.

I wonder if the old time paper capacitor that was used in the 50's was loss-y enough resistance wise to keep things stable?

Ok, I'm going to try the resistors and see what give, but any thoughts always welcome!!

Sandy
P.S. No, I don't want to listed to the stereo with my Fluke Multi-Meter Connected no matter how good it cleans up the signal
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