6SN7 troubles!! Help.

Joel · 28th October 2002, 01:42 PM

Does anyone have any experience with half a 6SN7 as a gain stage direct-coupled to the other half as a split load inverter? Specifically, what is the bias point for the inveter? I have -2.5V currently, and I can't get more than 10V peak out of the stage.

The monoblock amp is 6SN7 gain --> 6SN7 inverter --> (2x)6V6GT in class AB push pull. B+ is 250V on the output tubes, and they have -15V on their grids.

I can only get 3.7W at the output before it clips, which seems about half what I should be getting. Is this tube just not capaple of swinging two 6V6's with the opterating point I have???

Help!

I spent a good chunck of this weekend troubleshooting and I seem to be at a loss.

Joel

Joel · 28th October 2002, 01:46 PM

"chunck"?
"opterating"?
"inveter"?

I apologize for all the spelling errors! I'm really not retarded.

fdegrove · 28th October 2002, 02:07 PM

Joel,

With VA at 250VDC vg- can be set at -8 VDC,Ra would then be around 7k7.
This should give enough headroom so it will not go into clipping.

Just my 2 cents worth.

Good luck,

Tom Bavis · 28th October 2002, 11:19 PM

Depends on the current and load. Here's a circuit using that same configuration: http://www.space74.freeserve.co.uk/diagrams/a7-e.gif

fdegrove · 28th October 2002, 11:40 PM

Joel,

Check voltages,I suspect some carbons have gone haywire.
Just a hunch.

See you,

SHiFTY · 29th October 2002, 04:26 AM

Are you sure thats not just the point where it reaches class B? Are you using inverse feedback to cancel the crossover distortion?

I have a 6V6 PP amp using 375V on the plates but only a plate dissipation of 9W per tube... Gets into class B real fast, but with a little NFB it sounds awesome, and it has power to burn...

Joel · 29th October 2002, 03:05 PM

Thanks for the input so far guys!

Frank, I checked the voltages - seems like everything is fine.

I increased the bias to -17V on the 6V6's, and put a 5V4 rectifier in, increasing the B+ to 275V. Wattage output went up to 4.2W.... still really low!
I measured the input voltage, while putting my scope on the output of the first gain tube - at .8V input it starts to clip the top of the waveform.
This tube is biased currently at -6V on its grid.

What am I missing here??? Why would .8V be clipping the amp?

Joel · 29th October 2002, 03:07 PM

Did I also mention that the output stage is in ultralinear mode? Using the Hammond 1609 output tranny, with 40% screen taps. Although, this should be acheiving the same (or just under) wattage output as a normal pentode scheme.

SY · 29th October 2002, 04:28 PM

What's the voltage on the first stage's plate? And what's its supply voltage?

What size are the split-load resistors? Ditto, what's the supply voltage?

fdegrove · 29th October 2002, 04:45 PM

Joel,

Quote:

the monoblock amp is 6SN7

I assume you only have one of these?

Have you tested the tubes as well?

Rgds,

28th October 2002, 01:42 PM	#1
Joel diyAudio Member Join Date: Oct 2002 Location: Brooklyn, NY	6SN7 troubles!! Help. Does anyone have any experience with half a 6SN7 as a gain stage direct-coupled to the other half as a split load inverter? Specifically, what is the bias point for the inveter? I have -2.5V currently, and I can't get more than 10V peak out of the stage. The monoblock amp is 6SN7 gain --> 6SN7 inverter --> (2x)6V6GT in class AB push pull. B+ is 250V on the output tubes, and they have -15V on their grids. I can only get 3.7W at the output before it clips, which seems about half what I should be getting. Is this tube just not capaple of swinging two 6V6's with the opterating point I have??? Help! I spent a good chunck of this weekend troubleshooting and I seem to be at a loss. Joel

28th October 2002, 02:07 PM	#3
fdegrove diyAudio Senior Member Join Date: Aug 2002 Location: Belgium	6SN7 Joel, With VA at 250VDC vg- can be set at -8 VDC,Ra would then be around 7k7. This should give enough headroom so it will not go into clipping. Just my 2 cents worth. Good luck, __________________ Frank

28th October 2002, 11:40 PM	#5
fdegrove diyAudio Senior Member Join Date: Aug 2002 Location: Belgium	6SN7 Joel, Check voltages,I suspect some carbons have gone haywire. Just a hunch. See you, __________________ Frank

29th October 2002, 03:07 PM	#8
Joel diyAudio Member Join Date: Oct 2002 Location: Brooklyn, NY	another detail Did I also mention that the output stage is in ultralinear mode? Using the Hammond 1609 output tranny, with 40% screen taps. Although, this should be acheiving the same (or just under) wattage output as a normal pentode scheme.

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28th October 2002, 01:46 PM	#2
Joel diyAudio Member Join Date: Oct 2002 Location: Brooklyn, NY	"chunck"? "opterating"? "inveter"? I apologize for all the spelling errors! I'm really not retarded.

28th October 2002, 11:19 PM	#4
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	Depends on the current and load. Here's a circuit using that same configuration: http://www.space74.freeserve.co.uk/diagrams/a7-e.gif

29th October 2002, 04:26 AM	#6
SHiFTY diyAudio Member Join Date: Jul 2002 Location: New Zealand	Are you sure thats not just the point where it reaches class B? Are you using inverse feedback to cancel the crossover distortion? I have a 6V6 PP amp using 375V on the plates but only a plate dissipation of 9W per tube... Gets into class B real fast, but with a little NFB it sounds awesome, and it has power to burn...

29th October 2002, 03:05 PM	#7
Joel diyAudio Member Join Date: Oct 2002 Location: Brooklyn, NY	Thanks for the input so far guys! Frank, I checked the voltages - seems like everything is fine. I increased the bias to -17V on the 6V6's, and put a 5V4 rectifier in, increasing the B+ to 275V. Wattage output went up to 4.2W.... still really low! I measured the input voltage, while putting my scope on the output of the first gain tube - at .8V input it starts to clip the top of the waveform. This tube is biased currently at -6V on its grid. What am I missing here??? Why would .8V be clipping the amp?

29th October 2002, 04:28 PM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	What's the voltage on the first stage's plate? And what's its supply voltage? What size are the split-load resistors? Ditto, what's the supply voltage?

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