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Help: Pentode current sink for output triode

36.224.249.184
Posted on August 6, 2023 at 09:23:30
Single_Ended
Audiophile

Posts: 53
Location: LA
Joined: November 13, 2004
 Hi folks, I've encountered a high frequency oscillation issue with an integrated amp project and hope experts who have successfully implemented pentode current sinks can help me debug.

The circuit is CCS loaded LED bias 6EJ7 triode DC coupled to 300B with 6L6GC for CCS cathode bias. 6L6GC is set up with the grid grounded, CCS 0A2 for screen regulation (0A2 has 0.1uf bypass), with plate connected to the center tap resistors of the 300B cathode. Current of 50ma is set with 6L6GC cathode resistor to ground. Bypass cap is 150uf. Power supply is 5H choke input to 156uf to 5H to 156uf for output stage and then 100 ohm to 20uf for driver stage.

Issue: With volume control turned approximately 1/3 suddenly there is high frequency oscillation (Picoscope indicated frequency to be @100kHz+) at the output, with or without input signal - even with the input grounded. The magnitude gets larger with further turn. Both channels have similar oscillation.

1. 6EJ7 running by itself is clean, clean 1K sine and square wave output to max volume setting, no oscillation. 1K grid stopper.
2. 300B has 110 ohm grid stopper, increased to 1K with no effect.
3. 6L6GC has 1K grid stopper to ground and no screen stopper. Tried 6K grid stopper and 110 ohm screen stopper with no effect. Tried 56 ohm plate stopper with no effect.
4. Changed to triode mode for 6L6GC with no effect.
5. Grounding is star ground that I have used effectively in the past. This is the first time with pentode current sink, though have used solid state current sinks with no oscillation issues in the past.
6. Not sure if a cold solder join can cause this type of issue, although WBT low temp solder is used to minimize potential cold solder.

Any debugging advice is greatly appreciated! Many thanks.

 

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Just a guess, posted on August 6, 2023 at 16:17:31
garymuffley
Dealer

Posts: 447
Location: Maryland
Joined: March 29, 2006
 You are at the maximum value bypass cap for the 0A2. Try reducing the .1 mfd to .047mfd.

 

RE: Just a guess, posted on August 6, 2023 at 19:44:13
Single_Ended
Audiophile

Posts: 53
Location: LA
Joined: November 13, 2004
 Ah yes, forgot to write that, had tried removing the 0.1uf bypass to no effect :-( This is turning into a real stumper. Thanks

 

RE: Help: Pentode current sink for output triode, posted on August 7, 2023 at 04:49:56
Triode_Kingdom
Audiophile

Posts: 10173
Location: Central Texas
Joined: September 24, 2006
 Have you tried the "chopstick" method? That might tell you which areas of the circuit are sensitive. It would also be helpful to post a schematic and photo of the layout. No one can help much without that.


 

RE: Help: Pentode current sink for output triode, posted on August 7, 2023 at 08:01:47
Single_Ended
Audiophile

Posts: 53
Location: LA
Joined: November 13, 2004
 Solved but not sure what is happening. By good fortune I put a scope lead at the 6EJ7 input and the oscillation disappeared. Therefore, I increased the grid stopper to 10K (as a first test) and the oscillation at the output is gone. The curious thing is that when the 6EJ7 stage was measured without output tubes connected, there was no oscillation with a 1K stopper, all the way to max output. Does anyone know what issue is at work here, could it be something particular to direct coupling circuits? Cheers.

 

"Chopstick method"?, posted on August 7, 2023 at 08:03:02
richardl
Audiophile

Posts: 3565
Joined: September 5, 2002
 What is it? I tried searching the archives and only your post came up.

 

RE: "Chopstick method"?, posted on August 7, 2023 at 12:33:24
Triode_Kingdom
Audiophile

Posts: 10173
Location: Central Texas
Joined: September 24, 2006
 People who build high-gain guitar amplifiers are probably more familiar with this terminology. Essentially, it describes the method of poking and prodding wiring and components in the circuit with an insulated tool. A chopstick is cheap, convenient, and long enough to keep your fingers out of trouble. It's a great way to locate cold solder joints, as well as wiring or components that are involved in an oscillation.

 

RE: Help: Pentode current sink for output triode, posted on August 7, 2023 at 12:38:51
Triode_Kingdom
Audiophile

Posts: 10173
Location: Central Texas
Joined: September 24, 2006
 That's a very high transconductance tube with response into the low VHF region. I'm guessing your layout is encouraging RF feedback when the additional circuitry is connected. Simply having more conducting area might create more coupling back to the 6EJ7.

 

RE: Help: Pentode current sink for output triode, posted on August 8, 2023 at 03:50:31
Single_Ended
Audiophile

Posts: 53
Location: LA
Joined: November 13, 2004
 Thank you. Yes, with the pentode CCS the wiring is not clean and tidy as with SS CCS, for which I made PC boards. Construction ideas to keep in mind for future projects.

 

Thank you!, posted on August 8, 2023 at 09:27:11
richardl
Audiophile

Posts: 3565
Joined: September 5, 2002
 a

 

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