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Old 29th October 2002, 05:53 PM   #11
Joel is offline Joel  United States
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I know both halves of the tube are good, because I'm getting a voltage out of 11.25V with an input of .8V - a gain of 14. That seems like what a 6SN7 should be doing.
Gain is not the problem - I need a larger swing.
... probably making this harder than it is - my brain is getting old.

Oh, and the load resistors were 22k. Plate supply 255V.
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Old 29th October 2002, 06:00 PM   #12
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Default 6SN7

Joel,

The only thing I can point a finger at is the plate voltage on the 6SN7.
I've used 6SN7s' with as much as 450V on the plates.
Assuming you're running this from the same supply as the output tubes could you check the value of the dropping resistor on B+?
Carboncomp resistors have a bad rep for going up in value over the years.

Other than that???

Good luck,
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Frank
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Old 29th October 2002, 06:29 PM   #13
SY is offline SY  United States
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Well, you still haven't given all the information needed to diagnose this. It's important to know the voltage on the first stage plate, because that will set the operating point of the split load. And it's probably not a bad idea to see if that stage is swinging all the voltage you need- you can find that out by running it without the split load connected to it.

But let me do a little guessing, and maybe you can fill in the blanks with some real numbers. Let's say that the voltage on the first stage plate is 120. That means the voltage on the split load cathode will be something like 125-127V. That corresponds to about 6 ma through that section. Unfortunately, at a supply voltage of 255, 6 ma would correspond to a plate voltage of 120 or so- that will NOT be a happy tube.

My educated guess is that you have grossly too low a supply voltage. Use something more like 450V, aim for 120-150V on the first stage plate. That'll give you about 6-7 ma going through the split load stage for your 22K resistors. So, you'll end up with about 300V on the split load plate, a plate-to-cathose voltage of about 150 V, and no problem swinging 40-50 volts.

Careful on the power rating of those 22K resistors, though!
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Old 29th October 2002, 06:39 PM   #14
Joel is offline Joel  United States
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Thanks Sy - I will try that out tonight. I couldn't give you too many details because I don't have the amp in front of me (at work)... I can't exactly remember the plate voltage on either tube - but your diagnosis sounds like a reasonable one.
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Old 30th October 2002, 12:10 AM   #15
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If you have a negative bias supply, it will provide a way to increase the driver voltage - return the first stage cathode, and maybe the second to the negative instead of ground. I think this circuit will work out with the right operating point.

My next amp will be similar, but I will use the 6SN7 as a dif amp, with a 6SQ7 or 6SJ7 direct coupled. Probably more gain than I need, but it's always easier to give some up than to find more... circuit is on my audio page - http://www.audiophool.cjb.net/Audio.html
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Old 30th October 2002, 12:21 AM   #16
SHiFTY is offline SHiFTY  New Zealand
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Hey Joel

I don't know why more people dont like the 6V6; it is a very good sounding tube IMHO.

I have been using my PP 6V6 for 8 months now, trying out different circuit mods and tubes. I have found the Electro-Harmonix 6V6EH to be very good, as well as a fair bit more powerful than the old RCAs I have. I have mullard and telefunken preamp tubes, most of which I got free or dirt cheap. Heh just the other day I picked up a telefunken ECC83 smooth plate and a Mullard ECC83 for $10!

I was just wondering, the 6SN7 is the octal equiv of a 12AU7, low gain tube... Have you tried using a 6SL7 instead? You might need a bit more oomph?

Henry
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Old 30th October 2002, 12:38 AM   #17
SY is offline SY  United States
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A 6SL7 (or 6SU7, 5691, or whatever) is a better choice for that first hole- more mu is a good thing. For the split load, the 6SN7 running at a higher current will probably give better bandwidth and better drive capability.

Although, to be frank, I'm not a big proponent of using a split load inverter in the first place. It's a cost saver, but these days, no-one building tube stuff is that particularly cost conscious. You lose gain and drive capability with this sort of phase inverter. For marginally more ducats, you can use a long tail or (if you're a real geek) a cross-coupled inverter.
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Old 30th October 2002, 02:14 PM   #18
Joel is offline Joel  United States
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Default fixed.

Guys, thanks for all the suggestions - but hats off to Sy. The voltage on the 6SN7 was too low. I upped the 5V4 to a 5AR4 and when I got 285V B+, the 6SN7 could swing the 30V Pk-Pk needed for each output tube grid.
Anyway I finally measured my 10 watts.

Since that rectifier is total overkill in this circuit, I think the final fix will be to just have a seperate supply line for the inverter. [Frank, are you reading this??? a seperate PS for each tube!! ]

I feel dumb. This was an easy problem afterall.

Joel
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Old 30th October 2002, 02:21 PM   #19
SY is offline SY  United States
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Don't feel dumb. Easy is GOOD.
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Old 30th October 2002, 02:26 PM   #20
Joel is offline Joel  United States
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Default 6SL7 vs 6SN7

Well, I had considered a 6SL7, but everything I've read on phase inverters says that the higher gain, the harder the circuit is to balance. But, yes, I agree that ideally I would have a high gain tube in the first slot only - although in this case, I had already drilled out the chassis for only a single tube, so I was stuck.

A differential inverter looks like an interesting circuit - although I've never built one. I've seen the "schmitt" in the Radiotron Handbook. I believe it used the octal 6C8-G (very good sounding tube!). Maybe that'll be my next one.
I've also wondered about the performance of a straight-up old school paraphase... but no one seems to like them anymore.

RE: 6V6's. They sound pretty darn good to me too Henry. Better than EL84 amps I've heard. I have no problems advocating their use now in hi-fi. Hmmm.... wonder what the output would be with them triode-strapped....
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