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Old 17th November 2003, 07:32 PM   #21
JDeV is offline JDeV  South Africa
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Default Re: Far too much gain

Quote:
Originally posted by EC8010
All you need is the cathode follower to buffer the volume control. To do that, you need to remove the first stage, and insert a 2k2 resistor between the cathode and the existing 100k cathode resistor, then take a 1M resistor from this junction to the grid. (And add a 1k grid-stopper to the cathode follower to prevent RF oscillation.) Finally, connect the grid to the volume control via a 10n 400V capacitor.

Just like this?
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Old 17th November 2003, 07:47 PM   #22
EC8010 is offline EC8010  United Kingdom
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Very nearly. A grid-stopper goes directly onto the grid and before anything else, so if you move your 1M resistor along to connect at the junction of the 1k and the 10n, it will be correct.
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Old 17th November 2003, 10:56 PM   #23
JDeV is offline JDeV  South Africa
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Now I am smiling
Changed circuit as suggested by EC8010, all problems solved. So I ended up with not an "Improved Audionote M7 Clone" - but still a very good sounding preamp (or rather just a buffer). It does the job and I like it. Thanx again to EC8010, Frank and all others for your advice and assistance. Much appreciated

Now what to build (try) next??

Final circuit:
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Old 17th November 2003, 11:18 PM   #24
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Hi,

Quote:
EC8010, Frank and all others for your advice and assistance. Much appreciated
You can still improve on this one but it kind of proves what people like EC8010, Brett and myself have advocated for the past years...

Glad you like it.

Cheers,
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Old 17th November 2003, 11:27 PM   #25
JDeV is offline JDeV  South Africa
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Last question:
The unused half of 6072, must I just leave all pins open , or should I ground them? Does it matter? Both heaters is still active.
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Old 18th November 2003, 03:25 AM   #26
EC8010 is offline EC8010  United Kingdom
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Default Glad you like it.

I'm impressed. You modified it pretty damn quick. If you were driving a transistor amplifier, I'd say, "parallel the two valves and halve the resistor values," but you're not, so I suspect you will get better quality by grounding all the second valve's electrodes and leaving the first valve as it is.
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Old 18th November 2003, 10:16 AM   #27
DrG is offline DrG  South Africa
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To me it always seems "wasteful" to use only half a tube. Why not use both halves of a single 6072, one for each channel?

Or use the remaining half as an active load for the follower? Your B+ is sufficient and all you'd really need to take into account is the max permissible V(hk)... requiring only an R-R network to float the filaments at an acceptable voltage - say +80v. Best to check the 6072 datasheet for V(hk) limits though...

Another possibility to try would be the so-called "White cascode" configuration. If memory serves the tubecad website has plenty lowdown on these and other configs...
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Old 19th November 2003, 01:55 AM   #28
JDeV is offline JDeV  South Africa
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Quote:
Originally posted by DrG
To me it always seems "wasteful" to use only half a tube. Why not use both halves of a single 6072, one for each channel?

Or use the remaining half as an active load for the follower? Your B+ is sufficient and all you'd really need to take into account is the max permissible V(hk)... requiring only an R-R network to float the filaments at an acceptable voltage - say +80v. Best to check the 6072 datasheet for V(hk) limits though...

Another possibility to try would be the so-called "White cascode" configuration. If memory serves the tubecad website has plenty lowdown on these and other configs...

If not, why not ?
I am catching a like in these glass things and can only see it as a way to learn more and in the process maybe improving my sound system.

So looking at the attached pic. , how do I then calculate Ra and Rk? It IS the White Cathode Follower - just drawn a little bit other wise B+ can be reduced if needed.
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Old 19th November 2003, 12:18 PM   #29
EC8010 is offline EC8010  United Kingdom
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Your fixed bias White cathode follower injects power supply noise directly into the signal, which is not ideal. If you lose that potential divider but add a cathode bias resistor in series with the first cathode, you can take a 1M grid-leak resistor from the grid to its lower end, and this will reject power supply noise. The cathode bias resistor will be the same value for both valves. The anode resistor should be 1/gm for maximum output current.
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Old 19th November 2003, 12:32 PM   #30
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Default SOMETHING LIKE THIS...

Hi,

The formulas can be found here:

TUBECAD

And a pic of a working circuit never hurts:

Cheers,
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