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Old 9th December 2006, 08:19 PM   #1
kff322 is offline kff322  United States
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Default Citation II oscillating, Uh oh

Hey I am nearly finished building a "modern" (and modified) citation II Amplifier.
BUT...
The rig oscillates at a VERY high frequency. This problem has been keeping this thing from getting off my bench and onto my audio rack.
A little Background
- Its not the tubes.
- Unlikely to be the power-supply
- Oscillations can be heard from both sides which makes it difficult to tell where its coming from in the circuit.
- I checked for bad caps and wiring errors.
- Moving wires around makes it sound like your "Tuning" a radio.

I modified its design a little bit.
Schematic from McShane's Site
-The output is in Triode mode with 330 ohm resistors between pins 3 and 4
-Removed Feed-Back loop consisting of R8, R17, R43, and R52.
- 12BY7A has DC Filaments
- .1 uf 600V caps on all the wipers of all the POTS.
Got these mods from
this site.

And I have a scope, the oscillation is detectable from any point in the amp.

Thanks
Kff322
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Old 9th December 2006, 08:31 PM   #2
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I suspect the removal of the feedback loop is what's making the amp unstable. Put the feedback back in temporarily and see if the feedback goes away. If it is, then you can play with reducing the amount of feedback little by little until it pleases your ear and it no longer oscillates.

BTW, do not plug in your speakers while it is oscillating - it will blow your tweeters!

-- josť k.
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Old 9th December 2006, 08:39 PM   #3
kff322 is offline kff322  United States
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Sounds like a plan but.
What do you think the minimum values would be for putting the feedback loop back in should be?
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Old 9th December 2006, 09:37 PM   #4
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Default 0 NFB causing oscillation?

I never heard of taking OUT NFB and causing oscillations. It's the other way around. TOO MUCH NFB will cause problems. It's probably the lead dressing on your grid resistors or something like that. Check your wiring. Take a pencil or wood stick and gently move your wiring around as it's powered up. Do the oscillations stop when you touch a particular wire? Also there's a possibilty your OPT primary is wired out of phase. Switch the two primary wires around and it should go away.

~~~Johnny~~~
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Old 9th December 2006, 09:51 PM   #5
Giaime is offline Giaime  Italy
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Default Re: 0 NFB causing oscillation?

Quote:
Originally posted by TubeHead Johnny
I never heard of taking OUT NFB and causing oscillations.
Try to remove feedback from a big and bad SS amp: instant blowing


That happens because amps have exagerate open loop gain (to get high NFB factors).
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Old 9th December 2006, 10:18 PM   #6
kff322 is offline kff322  United States
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Its not the OPT polarity, I tried switching the leads and I made the floor rumbling sound that these reversed OPTs make. Moving wires around will vary the pitch of the very high pitched oscillations but not stop it.
Although I made an observation.
Adjusting the bias pot changes the oscillations the most and sometimes even stops it, but it starts again when music plays.
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Old 9th December 2006, 11:32 PM   #7
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I had another idea, the 330 ohm resistor between pins 3 and 4 seems kind of low - for triode-strapped pentodes, the value is usually about 1k. Also, make sure you disconnected the ultralinear connection when you triode strapped it.

Keep us posted,

-- josť k.
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Old 9th December 2006, 11:54 PM   #8
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Time out please!

The H/K Cit. 2 uses a sophisticated set of nested NFB loops. By eliminating an inner loop, the net gain was raised. That increased the otherwise minimal amout of global NFB significantly and oscillation was the result.

Triode strapping the "finals" is fine. Put everything else back the way Stu Hegeman designed. None of us on this site, STARTING WITH ME, can dare to think lifting Stu's jockstrap is possible. In this day and age, we have the advantage of better passive parts and SS stuff to support tube circuitry. The mind boggles at the thought of what Hegeman, Recklinghausen, et. al. would have achieved with access to the modern parts inventory.

Please guys, show some respect for the GIANTS of the past. Their memory is revered for good reason.
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Old 10th December 2006, 12:04 AM   #9
kff322 is offline kff322  United States
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I only removed the feed-back because I thought it was part of triode strapping the power tubes. You can talk to the man who goes by kevinkr on this site for having that recommendation on his site. (www.Kta-hifi.net under OTS Articles)
Increasing the resistor values to 1.2K on the Powertubes just make the sound worse. Tomorrow I am gonna see if I can put those Feed-Back loops back in and see what happens.
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Old 10th December 2006, 12:13 AM   #10
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Quote:
Originally posted by kff322
I only removed the feed-back because I thought it was part of triode strapping the power tubes. You can talk to the man who goes by kevinkr on this site for having that recommendation on his site. (www.Kta-hifi.net under OTS Articles)
Increasing the resistor values to 1.2K on the Powertubes just make the sound worse. Tomorrow I am gonna see if I can put those Feed-Back loops back in and see what happens.

Dude,

When it comes to the "care and feeding" of a Cit. 2, Jim McShane is THE man.
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