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craigtone 11th February 2019 01:22 PM

Tube Prototyping Noisy!
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I am breadboarding Thorsten's parallel 6SN7 line stage before committing to a chassis and building it up. When I power it up, it sounds like it is oscillating. I put a 0.1uF across the feedback resistor (circled in red below) and the noise went away. Obviously I don't want to add a cap in the feedback path but does anyone have any tips to keep this node from oscillating?

It does not appear to be a ground loop issue as I have connected an alligator clip to the star ground at the left and touched it to all input, output and tube grounds and no change in the sound. Likewise it is not heaters or wiring since I can touch and move wiring and no change. It is NOT volume dependent and stays the same magnitude regardless of volume setting.

Osvaldo de Banfield 11th February 2019 01:42 PM

Try momentarily removing one of the plate 100Ω resistor, to deenergize one of the triodes. If oscillation continues, then try the other. More than surely, there is an interaction between them.

Alllensoncanon 11th February 2019 04:17 PM

That 100K on the grid along with two set of Miller and input capacitance, may introduce too much phase shift. Try reduce it (as a test) and see if oscillation stop.

rayma 11th February 2019 05:03 PM


Originally Posted by craigtone (
it sounds like it is oscillating.

Try 5k resistors to each grid from the nfb junction.

6A3sUMMER 11th February 2019 05:05 PM

Why are there no Grid stopper resistors?

AC and RF wise:
Cathodes are tied together (through two bypass caps to ground).
Grids are tied together
Plates are tied together through two 100 Ohm resistors.
All lead wires are inductors.
It looks a little like a Butler oscillator.

craigtone 11th February 2019 06:17 PM

I added 5.1K grid stoppers to each grid and still the same noise.

I thought several people had built this circuit with favorable results?

6A3sUMMER 11th February 2019 06:57 PM

craigtone, Same part values? Same wiring lead dress? Same grounding techniques? Same load at the output? i.e. a long shielded cable (lots of capacitive load to ground). It could be the circuit is doing Bursted RF oscillations. Acts and sounds like audio frequency, but is actually RF bursts. Do you have a high frequency scope? Try a new old stock 6SN7. Tung-sol did not used to be Russian. And yes, I have used lots of good Russian tubes.

craigtone 11th February 2019 07:26 PM

Everything else the same. I have a 3' BJC RCA to RCA running to a Tubelab SE amplifier. I have an old Kenwood 40MHz scope.

Interesting comment about the NOS 6SN7. I tried some old GE's and the first time they were quiet but the sound was very rolled-off compared to the Tung-Sols. I have swapped back and forth a few times now and now the same set of GE's that were quiet are doing the same thing.

6A3sUMMER 11th February 2019 07:47 PM


Perhaps you moved some wires around. Or perhaps a ground connection is intermittent.

Also the loop gain is dependent on the setting of the 100k potentiometer. And so is the stability of that loop. It may have been on the verge of oscillation and now is beyond that all the time. Once you turn the pot up and down causing the oscillation to start, the oscillation usually swings full scale no matter where the pot is turned.

These are tough to troubleshoot (I am just preaching to the choir).

Having nothing to do with the oscillation: The 4.7uF cap can drive a 3400 Ohm load and only be -1dB at 20 Hz (and 26 degrees phase shift versus at 1 kHz with the 3400 Ohm load). The capacitance seems like it is larger than necessary, since a parallel 6SN7 driving a 3400 Ohm load will be very distorted, and very low gain.

maton00 11th February 2019 07:56 PM

Hello where are you taking the input signal off?
Maybe the input signal is a little bit noisy, almost all mobile and common output audio outputs develop high frecuency noise because all this things have class D amplifier outputs and they can be poorly decoupled, also the filament supply are you using DC?, some switchers develop lots of noise and end throwing high frequency bursts with some loads.

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