seriously thinking about building this schematic. Can you find any issues with it? - diyAudio
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Old 28th October 2013, 02:33 AM   #1
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Default seriously thinking about building this schematic. Can you find any issues with it?

This circuit is basically copied from something I found on the net (ballsy simple design), but I'm subbing in 1960's Fender transformers. I just wanted to run this drawing by the forum and see if you notice any deficiencies in the circuit before I start putting it together.

Also, 50V should be a high enough rating for C6 and C13, right?


help and/or comments appreciated


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Old 28th October 2013, 09:20 AM   #2
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I can't say I like the phase splitter arrangement, but presumably high distortion is what you're after?.

The input impedance also seems rather low?, presumably it's for active guitars more than passive ones?
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Old 29th October 2013, 09:52 PM   #3
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Hmmm... yerrrrrr...

I'll be even more direct and ask what you are trying to acheive

I'm all for innovation and such but fail to understand what this is about....

What kind of sound are you trying to acheive with this design and what are you planning to plug into it??? Please don't misunderstand my questions... I just cannot imagine how it'd work with an electric guitar (the instrument traditionally associated with a need for a 'ballsy' sound - unless this is for a Hammond or other electronic keyboard???)

Any road... I'm usually pretty good at spotting where this sort of stuff originates but you've got me stumped... I'm not a great fan of inputs that go through a cap at the start of the audio chain at any rate...

perhaps you can build it and prove us wrong
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Old 29th October 2013, 10:32 PM   #4
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Nor have I ever seen a full wave bridge used with a CTed HT winding... unless I am mistaken it should be a half wave rectifier if the CT is grounded... otherwise you'd wind up with a voltage in excess of the capability of your valves and any single filter caps that I know of...

Another convention that is odd is that the filter stages are lettered backward to how most people would have... ie usually the first filter stage would be denoted as 'A', second 'B' and so on... doesn't make it wrong... there are no rules as such... it's just odd...

These are just a couple of things that make the schematic smell a bit of inexperience... and well... possible danger to anyone who might follow it without questioning it's validity...

Last edited by yankeerob; 29th October 2013 at 10:43 PM.
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Old 29th October 2013, 11:37 PM   #5
Enzo is offline Enzo  United States
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Not only is the full wave bridge not needed, the two diodes that conect to ground are dead across the windings, and will blow fuses. Get rid of them.
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Old 30th October 2013, 05:12 PM   #6
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This schematic has never been built as is. Consider it to be the first draft by someone who has never designed or build a tube amp, or a guitar amp. Sorry if that's the OP.

1) Voltage gain is low and half a 12AX7 is unused.
2) Power supply problems as Enzo mentioned above.
3) Power supply will likely go over 500V at idle so caps need to be rated at 550V or higher, or series stacks need to be used. The voltage is actually too high for a cathode biased 6L6 output stage. A 300-0-300 transformer would be better.
4) Power tubes need screen resistors.
5) Although the 6L6GC data sheet specifies 0.5 meg for grid resistor in cathode bias, a lower value would be prudent.
6) Power tubes' bias will likely be too hot with 200 ohm cathode resistor. That resistor needs to be 10W rated, 250 to 330 ohm. A bypass capacitor of perhaps 100uF at 100V would increase gain and give more tube like distortion.
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Old 30th October 2013, 05:42 PM   #7
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An on-going saga... Just build the darn thing and see if it works already... if not, figure out why.
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Old 31st October 2013, 08:35 PM   #8
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Quote:
Originally Posted by jazbo8 View Post
An on-going saga... Just build the darn thing and see if it works already... if not, figure out why.
Hmmm... I like to know that it's going to be close to working and at least safe when it does... you can call me an old fart... but I like to build something that works and then freak around with it...

This is a mish-mash of bits and pieces that is likely to be more trouble than it's worth...

Good luck... you're going to need it
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Old 31st October 2013, 08:52 PM   #9
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The OP is not interested in copying working circuits, that would too easy, so trial and error it is...
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Old 31st October 2013, 09:46 PM   #10
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OK - so I'm a thick-o... - who hasn't a clue what you mean by 'OP'...

I was merely expressing my opinion - which is what forums are for - that some of the basics (like the rectifier) were completely wrong for a start... and that the more I look at it - the more I think it's going to take a lot of backward engineering to make it stable - which - in my experience will more than likely nullify the whatever innovation the author is trying to acheive... it's a funny thing... but nobody's really come up with anything new... apart from unstable designs... where the use of standard output valves are concerned... and there is a sound and fundamental reason for that.

But OTOH - I did say build it and prove me wrong in post #3...

So knock your socks off ... it's no odds to me... I am just stating an opinion with no desire to get into a flaming match... good luck...
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