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Old 13th October 2003, 12:24 PM   #1
morfeas is offline morfeas  Greece
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Cool 6c33 Se

Any comments welcome.
Schematic
Simulation

Will drive a FE208ESigma based horn speaker
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Old 13th October 2003, 07:34 PM   #2
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Hi,

I don't know what the input and driver tubes are you're using but it seems a little complex just to drive a single 6C33C.

Than there's the feedback loop; not easy to implement correctly in SE amps and it kind of defeats the purpose for choosing the SE road in the first place IMHO.
Also the driver uses paralelled sections of a twin triode, this too isn't easy to make work correctly and any current hogging here will have a heavy penalty on detail resolution.

I don't want to discourage you at all but maybe it's worth to look for a simpler circuit.

Just trying ot help.

Cheers,
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Old 13th October 2003, 08:01 PM   #3
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Quote:
Originally posted by fdegrove
I don't know what the input and driver tubes are you're using but it seems a little complex just to drive a single 6C33C.
Yes, I'd like to see the types too. What's complicated about it? You need 80V into 150k, a 12AX7 isn't going to do it both due to low drive and that stigma on it, same for a pentode (although a 12BY7 would probably work, dunno about gain). And a single medium triode (6SN7 et al) doesn't have enough gain. That leaves using two stages.

Quote:
Than there's the feedback loop; not easy to implement correctly in SE amps and it kind of defeats the purpose for choosing the SE road in the first place IMHO.


Quote:
Also the driver uses paralelled sections of a twin triode, this too isn't easy to make work correctly and any current hogging here will have a heavy penalty on detail resolution.
So put some 100 to 470 ohm resistors in the cathodes. I don't see what's wrong with paralleling tubes, same thing goes on in the tubes themselves in an infinite way.

Tim
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Old 13th October 2003, 08:03 PM   #4
PRR is offline PRR  United States
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> Any comments welcome

You asked....

How much distortion does that second stage make? The penciled "114Vpk" appears to be "at clipping", if this stage runs on the +245V supply. I suspect it will exceed 5%THD at 50Vpk, which is less than your output stage needs.

In general, driving a triode with Mu of less than 5 is "not possible" when the driver stage runs on the same B+ as the output stage. The grid-swing of a low-Mu tube is too large to be driven from the plate swing of any reasonable R-C coupled driver working on the same supply voltage.

That's maybe not a problem. I don't think you will get 20 clean watts, but the FE208 series speaker will make a hell of a noise with a lot less than the full 20 watts.

The only reason I can see for 10K and 7.7K plate resistors (and paralled tubes) in voltage amplifiers is to get very wide-band response so you can put lots of negative feedback around it, up to 26dB NFB in the notes. I like NFB too. But with 26dB NFB around an amp, "all amps sound the same"; why bother with the size and heat of an SET?
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Old 13th October 2003, 08:23 PM   #5
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Hi,

Quote:
What's complicated about it?
I said "complex" (it's overly complex in fact) which doesn't mean complicated.

In fact you could easily kick that big tube tube with a dissimilar triode, a 6EM7 or similar.
There are at least a dozen alternative including trioded penthodes to drive the powerstage.

Gain > Power > Power.

And I agree with PRR, this kind of amp is better off with a dedicated PS for input and driver.

I see you're a little puzzled by my rejection of global NFB...
Why use it when you don't need it?
When designed correctly I'm convinced you won't need it here and with the Fostex the amp will barely need to ever put out more than a few Watts.

Cheers,
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Old 13th October 2003, 08:54 PM   #6
DrG is offline DrG  South Africa
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Sch3mat1c is right. The circuit is about as "simple" as it could be without resorting to pentodes, "wasting" half a dual triode or using a more exotic second tube. Remember morfeas' is probably designing around what he has at hand, as many of us do...

Global NFB has it's applications. SET afficionados and tube purists frown on it. Hell even I steer clear whenever possible. But it has it's merits in certain circuits and can even improve sound in selected cases...
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Old 13th October 2003, 09:06 PM   #7
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Hi,

Quote:
Sch3mat1c is right. The circuit is about as "simple" as it could be without resorting to pentodes, "wasting" half a dual triode or using a more exotic second tube. Remember morfeas' is probably designing around what he has at hand, as many of us do...
Now you have me frowning upon you here...

Wouldn't you agree it's a little early to make final judgements on this?

Quote:
Global NFB has it's applications. SET afficionados and tube purists frown on it. Hell even I steer clear whenever possible. But it has it's merits in certain circuits and can even improve sound in selected cases...
Can't make up your mind?

Cheers,
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Old 14th October 2003, 07:05 PM   #8
DrG is offline DrG  South Africa
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fdegrove:
Quote:
Now you have me frowning upon you here...
What have I given you to frown about? Try smiling.
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Wouldn't you agree it's a little early to make final judgements on this?
Which judgement are you referring to?
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Can't make up your mind?
Can too!

Please be more specific. What are you frowning about? Which judgement? What should I make my mind up about? Is the circuit complex, overly complex or complicated in your opinion? What is the difference between these three?
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Old 14th October 2003, 07:22 PM   #9
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Hi,

Quote:
Please be more specific. What are you frowning about? Which judgement? What should I make my mind up about?
That people jump to conclusions without so little as a circuit diagram not even mentioning the tubes used bar the powertube or any other reason for the design choices.

When you say " he's probably using stuff he's got in his junkbox", how can you know?

How can you say Schematic3 is right with so little data to go on? I sure can't.

You actually say he's right and then say in the same sentence you say you try to stay away from global NFB yourself??

When I say in post #5 that the whole shebang can be reduced to the mininum, doesn't need GNFB, doesn't need // triode sections with all their potential problems, yes, I do frown.

Not that I want to start a flame war on this, mind you.

See the smile?
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Old 14th October 2003, 07:51 PM   #10
panos29 is offline panos29  Greece
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Talking feedback flames!

Bad bad boy Morfeas ! You are causing a flame war around here! Time to remove the feedback!
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