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Old 28th October 2002, 01:42 PM   #1
Joel is offline Joel  United States
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Default 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
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Old 28th October 2002, 01:46 PM   #2
Joel is offline Joel  United States
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"chunck"?
"opterating"?
"inveter"?

I apologize for all the spelling errors! I'm really not retarded.
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Old 28th October 2002, 02:07 PM   #3
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Default 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,
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Old 28th October 2002, 11:19 PM   #4
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Depends on the current and load. Here's a circuit using that same configuration: http://www.space74.freeserve.co.uk/diagrams/a7-e.gif
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Old 28th October 2002, 11:40 PM   #5
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Default 6SN7

Joel,

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

See you,
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Old 29th October 2002, 04:26 AM   #6
SHiFTY is offline SHiFTY  New Zealand
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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...
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Old 29th October 2002, 03:05 PM   #7
Joel is offline Joel  United States
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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?
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Old 29th October 2002, 03:07 PM   #8
Joel is offline Joel  United States
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Default 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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Old 29th October 2002, 04:28 PM   #9
SY is offline SY  United States
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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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Old 29th October 2002, 04:45 PM   #10
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Default 6SN7

Joel,

Quote:
the monoblock amp is 6SN7
I assume you only have one of these?

Have you tested the tubes as well?

Rgds,
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