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Old 17th April 2004, 07:22 PM   #1
sith is offline sith  Croatia
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Default Yet another valve pre, comments?!

Hi there...

Well, here's another circuit I've found on the Internet, and I wander if it is any good? And if it is, how much gain may I expect from it?

thanx
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Old 17th April 2004, 07:51 PM   #2
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Nothing special about that one.
Looks like another poorly designed preamp.
What's with the 470 ohm resistor in series with the 22k resistor on the cathode of the output section. Is this some kind of attenuator? Why?
You can probably lose both 220 ohm grid resistors as well.
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Old 17th April 2004, 09:56 PM   #3
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The only "bad" thing I can say about this pre is
it is phase-inverting,

if memory doesn't betray, This design is based along
the lines of "THD cancelling" principle by a greate
designer named John-Broskie.
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Old 17th April 2004, 10:25 PM   #4
DougL is offline DougL  United States
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Quote:
I wander if it is any good?
Just for my curiosity, I simulated the grounded cathode with Tube Cad. I got about 6.2 Ma Bias current and 111 Volts across the tube. This represents about 25% of the allowed plate dissipation of the 12au7. These numbers are far too conservative IMHO, especially for an ecc82, which doesn't have a great reputation for linearity.

Quote:
And if it is, how much gain may I expect from it?
About 19 dB, or about 9.3 X.

Quote:
What's with the 470 ohm resistor in series with the 22k resistor on the cathode of the output section. Is this some kind of attenuator? Why?
Here is an explanation of the technique in the Glassware audio design software website.Link . As with all circuit techniques, your milage may vary.


If I were going to build a line amp in this style, I would use 6SN7 or 7N7 tubes, biased for at least 12 ma, and 200 + volts across the tube. Nothing really wrong with this design. Perhaps other designs might be better.

Cheers;

Doug
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Old 18th April 2004, 02:14 AM   #5
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Topologically similar to the Foreplay.

Quote:
Originally posted by Frank Berry
What's with the 470 ohm resistor in series with the 22k resistor on the cathode of the output section. Is this some kind of attenuator? Why?
Isn't that to get the voltages right for the direct coupling?

dave
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Old 18th April 2004, 02:33 AM   #6
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Hi,

Quote:
Isn't that to get the voltages right for the direct coupling?
Actually, it isn't.
At that point the DC coupling is done already as bias is set by the voltage difference between the voltage present at the anode of the first half of the twin triode and the bias set by the sum of both cathode resistors of the second half.
The difference between the two voltages than determines the gridbias of the second triode.

So the 470R is part of the mirror circuit, anode-cathode follower, allowing for some modicum of distortion cancellation.
Provided the two triodes are closely matched this should work.

No doubt you noticed that the anode resistor appears in the cathode of the CF and the cathode resistor of the anode follower is added on top and the output is taken from the joint between them.

Quote:
Topologically similar to the Foreplay.
I don't have a copy of the diagram but that's how I understand it's designed too.

Cheers,
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Old 18th April 2004, 02:46 AM   #7
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The topology is similar to the Foreplay's, but the plate voltage and bias are considerably higher for the first stage.
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Old 18th April 2004, 03:32 AM   #8
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IMO, the design is FINE. The 'U7 and the 'SN7 (plus its electrical relatives) are close enough in behavior to use the all values unchanged. How about an 8CG7 or 14AF7 with DC energizing the heater?

This "pup" or a variation just might meet my needs for a SLIGHTLY euphonic, LOW gain, line stage. Make the O/P coupling cap. 12 muF. Insert a Sowter model 8940 parafeed O/P trafo. Wire the trafo secondary to make the unit non-inverting as a whole. Float the trafo secondary to break up ground loops. Per Brian Sowter, a 10 KOhm resistor gets wired across the O/P RCA jack. Noise control is taken care of by wiring a 10 nF. ceramic cap. from the O/P RCA jack's grounding tab to the chassis. The icing on the cake would be CCS loading the cathode follower with a pentode.
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Old 18th April 2004, 03:43 AM   #9
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Hi,

Quote:
Make the O/P coupling cap. 12 muF. Insert a Sowter model 8940 parafeed O/P trafo. Wire the trafo secondary to make the unit non-inverting as a whole. Float the trafo secondary to break up ground loops. Per Brian Sowter, a 10 KOhm resistor gets wired across the O/P RCA jack. Noise control is taken care of by wiring a 10 nF. ceramic cap. from the O/P RCA jack's grounding tab to the chassis. The icing on the cake would be CCS loading the cathode follower with a pentode.
Do you still need a CF if you're going to use a xformer anyway?

As for the rest of it, I'd personally give the SN7 and family a B+ of 300V or higher and run them at around 15mA...YMMV.

Cheers,
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Old 18th April 2004, 05:13 AM   #10
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>>Do you still need a CF if you're going to use a xformer anyway?<<

Yes, the CF is needed. The Sowter model 8940 is specifically designed to be used with AC coupled cathode followers. In addition, a CCS loaded CF buffering a resistively loaded common cathode triode fits in neatly with the SLIGHTLY euphonic, but essentially linear, voicing I want. This line stage will be used with a H/K Cit. 2 and an AVA FET-Valve. Both power amps are pretty "neutral" and a SMALL dose of euphony wouldn't hurt.

How does a 'CG7 run at a "textbook" 250 V. on the plate, -8 V. on the grid, and Ib = 9 mA "grab" you? Bias would be by a stack of 4X red LEDs and a 15 KOhm load resistor would be used. I figure the B+ rail has to be 393 V.
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