KT66 SE amp NFB loop oscillation problem.

[FranciscoPerez]

Hi!!


Recently built a Single Ended amp around an ECC35 and a KT66. It's a very simple circuit I designed myself, please see attached schematic.
It sounds great, (strongly recommended!!) but I'm having a hard time designing the NFB loop. Thought it'd be great to have a presence control to shape the tone of the amp a little, so I installed a 47K NFB resistor and a 50K lin pot. The amp squealed even at moderate volume and presence settings, so I added another 22K resistor in series. It helped, but the amp still oscillated at max settings, so I increased the 22K resistor to 47K. Now the amp does not oscillate, but I reduced the NFB so much that the presence control has little effect. Any ideas to help stabilise the amp and thus being able to increase the NFB avoiding oscillation?


Thank you very much guys!!
Your help is much appreciated


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[sluckey]

Reverse the primary leads of the output transformer. Does that help?

[FranciscoPerez]

Thanks Sluckey! Will do as soon as I get home and post the result.

[PRR]

What Sluckey said, of course, but....

94K:1.5K is a severe NFB ratio for an amp with open-loop gain of only about 5. I suspect this NFB is doing nothing, positive or negative.

Get your OT primary wires WAY away from all preamp stuff.

While squealing, poke a wood/"lead" pencil in the tone-pot leads. If just putting the "lead" *between* two wires makes it squeal harder, move them apart. I had such a problem in my Champy-thing and lead placement was critical, and apparently a total cure.

[FranciscoPerez]

Thanks guys,

I reversed the OT primary wires without luck, it oscillated even more.
They are away from the preamp circuit, but I moved them farther away with a chopstick without any difference.
On the other hand, I tried as you suggested PRR, and moved the tone pot leads , but the squeal never stopped.I took a couple of quick pictures of the amp guts. Maybe  you see something odd there.

Another option if I can't get rid of the squealing , would be to leave the NFB disconnected , and add another kind of tone shaping control, any ideas??Thank you!!!

[shooter]

I installed a 47K NFB resistor and a 50K lin pot. The amp squealed even at moderate volume

So it didn't squeal before the NFB?
 

[FranciscoPerez]

No, it didn't.
Well, to be honest, the first version of the circuit did not use a grid stopper in the second half of the preamp tube, and it made a little weird noise at max volume settings. Adding the 68K grid stopper solved it, so I moved forward to adding the NFB. Then the squeal appeared.

[sluckey]

Your NFB loop is not going to act as a presence control since there is no capacitor in the circuit.

I don't think you have enough NFB with the resistor values you have now. A 5F1 Champ is a very similar circuit to what you have and the NFB loop uses a 22K to 1.5K voltage divider. Remove one of the 47K resistors from your pot and change the other 47K to 10K. Does it behave now?

[FranciscoPerez]

Solved!! I finally removed the board to change the lead placement beneath it. NFB wires were far from any preamp stuff, but close together underneath the board. Moving them apart totally removed the squeal. Now the amp is totally stable.


Yes sorry Sluckey, it's definitely not a presence control. I expected some change in the frequency response though. With the 47k series resistor, the pot is of almost no use. I changed it to 10K as you suggested, and now I can hear a subtle change in tone between settings, and a not so subtle change in gain and headroom, which is great.


However, it'd be great to further control the frequency response of the amp. Any ideas??


Thank you guys!!!

[tubeswell]

However, it'd be great to further control the frequency response of the amp. Any ideas??

Vibro-champ style NFB with variable frequency boost attached - Freq response for the 2nd gain stage with the 0.47uF bypass cap attached (assuming the vol control is at maximum)


Muck around with the pot and bypass cap values to suit your desired goal

[FranciscoPerez]

Thanks tubeswell!!

I tried it, and played a lot with the smaller bypass cap to get full gain at high frequencies.
Also tried different values for the boost limiting resistor and the other bypass cap.
However, it doesn't seem to offer big enough scope for changing the frequency response of the amp.

I think maybe I should leave the second stage unbypassed, and a fixed NFB, and focus on interstage coupling. Maybe a tone control like the one attached, seen on the 5f2-A amp, would work better here.

I'll let you know!
Thank you!

[FranciscoPerez]

Hi again,


I finally added the tone control and I'm really happy with it! The volume and tone controls are quite interactive, but that's not a problem for me, and now the scope for changing the tone of the amp is much wider.


Thank you!!!

[tubeswell]

Yep the 5F2A tone control is simple but very effective