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Old 27th July 2009, 03:03 AM   #1
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Default Line Level tube

I have made the following circuit:
http://www.captain-foldback.com/Hamm...atics/AO10.gif

I built from the point where it says "scan" input from vibrato scanner, to the point where it says "output to tone cab." I built this from scratch so all parts are brand new and my voltages are all OK. I am having trouble sending a keyboard, line-level signal into it. I start to get distortion at just under .5VAC RMS. Any ideas on what's going on? Do I need to match impedances? If I put a pot at the input, I can dial it down to get rid of the distortion, but then I don't the output I need. Help!
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Old 27th July 2009, 04:39 AM   #2
rman is offline rman  Canada
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Hi.

That's a pretty confusing circuit. Did you include an input capacitor?
Does the keyboard output have any dc offset? You say voltages are ok, you have the 1.5 volts on the cathodes? How did you bias the tubes?

That's about all I can think of.

Rolf.
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Old 27th July 2009, 12:35 PM   #3
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I calculate a stage gain for the 6SC7 stages at about 38.
BUT
How did you connect the cathodes of V4 ? Note that in the original circuit several cathodes are connected together at V2, but your circuit is only starting at V4. Dont forget to use a common cathode resistor, value 1.6K by my calculation, to set the dc bias AND provide feedback. Otherwise you are going to have a total gain of 1440, which as you have discovered, is WAY too much for a .5 volt input.
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Old 28th July 2009, 01:39 AM   #4
kevinkr is offline kevinkr  United States
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V2 (source of +1.5V bias to other tubes) has a 30uF cap across the cathode resistor so there is not a whole lot of ac feedback going on here. (It was probably done for no other reason than to save some money.)

A 6SC7 with 1.5V of cathode bias is going to start to have linearity issues at something around 1Vp on the grid which is not far removed from the 700mVp (500mVrms) you are hitting it with.

There is also a lot of gain in the following stage and that expression control appears to be pair of opposing concentric capacitors so the signals are probably pretty attenuated at that point.

I have not taken the time to be sure I understand exactly how this works, but you do know that the scan circuit probably had a pretty high source impedance - I suspect you need a series input resistor to make that (now) input stage happy - note the local feedback loop? I would start with about 39K..
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