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Now a buzz problem
Posted:
Tue Apr 15, 2014 4:42 pmby DeathRex
I decided to put the pentode back into Kenwood 1100U, but I didn't want to put in a 6AN8. So I wired it for a 6GH8/6U8/6BL8. I've never made an amp with a pentode front end, only worked on them. I've got lots of gain, and a buzz. The buzz comes on with about 20 volts PP at the speaker terminals with a 6BL8(pictured). With a 6GH8 or 6U8 I'm getting it at the bottom at about 30 volts PP.
Here's the schematic, except the 6AN8 is no more:
Re: Now a buzz problem
Posted:
Tue Apr 15, 2014 6:15 pmby DeathRex
If I remove the feedback I only get a oscillation of 29Khz at 42VPP.
Re: Now a buzz problem
Posted:
Thu Apr 17, 2014 5:42 amby Shannon Parks
Since you have a HF instability, put a 100pF cap in parallel with that 20pF cap at C322. Just tack it in place for starters.
Shannon
Re: Now a buzz problem
Posted:
Thu Apr 17, 2014 7:45 amby DeathRex
I did actually add a 27, 18 and 200pf one at a time, and even changed the resistor, but it didn't change the buzz at all. I also have, with a 6BL8 ,a HF (29KHz) oscillation, as the tube warms up. If I increase, or decrease feedback, the oscillation is always there. Might be a bad tube, because the 6GH8 and 6U8 don't oscillate, except at higher power. I found that if I remove C321, the screen grid capacitor, the HF oscillation goes away, but the sinewave doesn't look right.
I'm tired of pentodes, they suck. I put the 12AX7 back in, but I'm going to need another tube to get enough gain. I guess a LTP will get me plenty of gain.
Re: Now a buzz problem
Posted:
Thu Apr 17, 2014 10:52 amby Geek
Hrm... give C326 and 327 a look over. If they failed, drifted or have bad solder joints, that can cause oscillation on peaks.
Re: Now a buzz problem
Posted:
Mon Apr 21, 2014 6:39 amby Brik
I like to second the possibility that C326 and C327 might be misbehaving.
I wonder what would happen to the asymmetrical waveform if you swap V306 and V307, or disconnect C326/C327.
Re: Now a buzz problem
Posted:
Wed Apr 23, 2014 12:06 pmby Impmon
Don't see any grid or screen stoppers on that schemo. I've done projects with high gain pents (active voltage regulators) and always include grid stoppers (4.7K) and screen stoppers (1.0K) as a matter of course. So far, I haven't seen any instability issues. I don't know why they overlook this so much, but there ya go.
Mount the stoppers as close to the socket pins as possible; C-comp resistors are preferable here, but C-film may work. The latter may not be recommended, but they do seem to work.