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Old 11th February 2011, 03:09 PM   #11
HFGuy is offline HFGuy  Canada
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Oops I just noticed I added the snubbing network to the wrong schematic. Should I put the network before or after the 1k screen stopper ?
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Old 11th February 2011, 03:12 PM   #12
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Quote:
Originally Posted by HFGuy View Post
If I am not mistaken both phase and gain margin only refers to the use of NFB, as the feedback becomes positive with the phase shift caused by passive networks. As I mentioned somewhere in my post I have disabled the NFB in my circuit, even going as far as to pull the 6SN7 tubes.
I think you are right. With the 6SN7s pulled there's no NFB loop. The only thing I can think of is that you have got the screen connections crossed which might turn this into a really nice power oscillator.

If the problem exists with the 6SN7s pulled then that is where to start. Pull them then ground the grids of the output tubes and see if it still occurs.

Cheers

Ian

Last edited by ruffrecords; 11th February 2011 at 03:14 PM.
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Old 11th February 2011, 03:36 PM   #13
HFGuy is offline HFGuy  Canada
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Ok update here. I just applied the NFB. It now oscillates at 60Hz ... but that is from the hum problem (one problem at a time). When i leave the NFB (so now there is a reference to ground) but pull the input 6SN7 the oscillation is GONE !!!! I suspect that if i track down the source of my hum I will be good to go.

So this brings up another question, how much 60Hz swing should I see at the output of each of the stages ?

I built this tube amp many many years ago. Building it totally put me off DIY audio and high end audio period. I picked up flying RC helis and let the amps collect dust. I got bored this winter and decided to try and finish the things off. So please don't judge my HORRIBLE layout on my current engineering skills
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Old 11th February 2011, 03:37 PM   #14
DF96 is offline DF96  England
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The snubber goes on the OPT side of the g2 resistor. That means the resistor still does its main function of preventing VHF oscillation.
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Old 11th February 2011, 03:49 PM   #15
HFGuy is offline HFGuy  Canada
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I think I have the parasitic oscillation problem solved. I now have the following issues

- It seems i am getting a lot of 60Hz on my input stages
- Not entirely sure I am applying NFB, might be PFB, i am not sure I have the proper inversions wired up properly
- Not sure about my bias currents. the KT88 are drawing 85mA each, which i was under the impression I was suppose to aim for 50mA.

P.S. I really appreciate all of your FAST responses and interest. As much as this is driving me nut I am starting to remember why i use to love DIY audio.

Ok here is the amp, please be gentile

Click the image to open in full size.

Click the image to open in full size.

BTW Here is my day job (I am a doctrate student, so while I don't get paid I still call it a job).

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Old 11th February 2011, 04:23 PM   #16
kevinkr is offline kevinkr  United States
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This doesn't look too bad at all.

It is quite possible that you are in fact applying positive feedback and the easiest thing to do in this case would be to swap the connections from the final driver stage to the output tubes. Do this only after observing oscillation with the feedback applied and none without.

The 60Hz hum could be coming from your filament wiring or the interface between the filaments and their respective cathodes. Both are easily fixed.
In the case of wiring make sure all filament lines are tightly twisted and placed away or at right angles to sensitive circuitry. Make sure you have not created a large loop with the filament wiring around the sockets, if so rearrange to minimize the loop area as much as possible - best if the wires are twisted right up to and going away from the socket. I am assuming that you are not using the chassis for filament returns - if you are go back and fix that.

Generally applying some dc bias to the filament windings is a good idea to turn off potential diode effects between the filament and cathode. Generally a simple resistive divider capacitively decoupled to ground and connected to a filament transformer center tap or one side of the filament wiring will suffice. You need to select a voltage such that the filament to cathode insulation voltage ratings are not exceeded anywhere. 50 - 60V should suffice in most cases. Use a large enough capacitor to assure that the impedance of the node is reasonably low at 60Hz.. Aim for about 1mA of current in the resistive divider and choose an appropriate capacitor.

85mA bias seems not too unreasonable given your use of cathode bias, however dissipation is pretty close to the maximum tube rating so it could be that 560 - 600 ohm bias resistors would be a better choice. The 50mA value is most appropriate for fixed bias in this design. Once everything is sorted out look for signs of red plating in your KT88s. It's a bit unfortunate that you did not stick with the original fixed bias, but I think this will work well enough.

Fix one issue at a time... Post with further questions and we'll try to help.
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Last edited by kevinkr; 11th February 2011 at 04:31 PM.
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Old 11th February 2011, 04:31 PM   #17
HFGuy is offline HFGuy  Canada
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Ok, I just measured the 60Hz hum on the 2nd 6SN7 stage and the KT88 stage. Both are about 2mV which is small enough I believe. I believe all of the hum is coming from the input stage (I will measure that shortly).

I am concerned about the 85mA bias on the KT88, is that way too much ?


Correction, what I thought was hum isn't 60Hz at all. It is all kind of very low frequency artifacts, my scope frequency measurement is jump all around 5-15Hz. I have a feeling this isn't actually coming from my amp but simply due to my test setup.

Last edited by HFGuy; 11th February 2011 at 04:39 PM.
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Old 11th February 2011, 04:51 PM   #18
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by HFGuy View Post
Ok, I just measured the 60Hz hum on the 2nd 6SN7 stage and the KT88 stage. Both are about 2mV which is small enough I believe. I believe all of the hum is coming from the input stage (I will measure that shortly).

I am concerned about the 85mA bias on the KT88, is that way too much ?


Correction, what I thought was hum isn't 60Hz at all. It is all kind of very low frequency artifacts, my scope frequency measurement is jump all around 5-15Hz. I have a feeling this isn't actually coming from my amp but simply due to my test setup.
The second stage hum is reasonably low and should cancel as long as it is all common mode. (In phase on both phases) Voltage regulation of the mains is imperfect and a lot of what you are seeing in the very low frequency region may be nothing more than slow, long term variations in the line voltage. Probably not an artifact of your measurement setup. I see much the same thing here, but at much lower frequencies of a couple of Hz or less, sure that is not what you are seeing?

The bias is a bit on the high side IMHO, but again look for signs of obvious distress and I would ultimately change those resistors as I remarked.
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Old 11th February 2011, 04:56 PM   #19
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85mA is ballpark for the resistor you're using. Like Kevin said, swap in a 600R if you are worried.

Regarding your work pic: I'd love to have that isolation stand for my turntable.
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Old 11th February 2011, 05:05 PM   #20
HFGuy is offline HFGuy  Canada
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Ok, I am getting 50-70mV of 60Hz hum on the output of the input stage, none of the other stages seem to add any hum.


Hehe yeah the isolation table is great .... but the two pumps for it are super noisy.
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