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Old 19th November 2003, 11:44 AM   #31
JDeV is offline JDeV  South Africa
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While on the subject of changing gain, what must I change and to which value to decrease sensitivity on Mullard5-20 to about 1V ?

Here is circuit of my current amp:
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Old 19th November 2003, 11:55 AM   #32
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With the EF86 triode-strapped, you probably already have a sensitivity of about 1V. There's nothing more you can do without a complete redesign (daft) or adding a potential divider at the input (also daft).
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Old 19th November 2003, 03:56 PM   #33
JDeV is offline JDeV  South Africa
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Quote:
Originally posted by EC8010
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.
O.K. So I basically use Frank's circuit with differant Ra and Rk.

Can this be correct for Ra and Rk?
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Old 19th November 2003, 05:03 PM   #34
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Well, I can't imagine how a number for gm can just be plucked out of the air without considering the operating point, and I certainly don't agree that a value for a DC bias component can be derived from an AC equation.

You have 300V to spend, across a pair of identically biased valves, so each valve receives 150V. A 6072 typically only needs a few volts across the cathode bias resistor, so we can approximate, and say Va = 150V. Looking at the curves, choosing 2V of bias would result in 2.65mA of anode current. By Ohm's law, 2V/2.65mA = 750R, so that's your cathode bias resistor. A different choice of operating point would give a completely different value for Rk...

Drawing a tangent to the 2V grid curve at the operating point gives ra = 20k. Drawing a horizontal line at the operating point to find mu, gives mu = 46.5. gm = mu/ra = 46.5/20 = 2.3mA/V, so 1/gm = 430R.
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Old 19th November 2003, 05:15 PM   #35
JDeV is offline JDeV  South Africa
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I had a look at Tubecad articles and tried to figure out how to derive a value, obviously got it completely wrong But thanx for the corrections I will definately read more about all to get a bigger , better view of the picture.
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Old 19th November 2003, 05:19 PM   #36
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And there you have it... simple
Good luck!
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Old 19th November 2003, 05:42 PM   #37
JDeV is offline JDeV  South Africa
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Quote:
Originally posted by EC8010
A 6072 typically only needs a few volts across the cathode bias resistor, so we can approximate, and say Va = 150V. Looking at the curves, choosing 2V of bias would result in 2.65mA of anode current. By Ohm's law, 2V/2.65mA = 750R, so that's your cathode bias resistor. A different choice of operating point would give a completely different value for Rk...

Drawing a tangent to the 2V grid curve at the operating point gives ra = 20k. Drawing a horizontal line at the operating point to find mu, gives mu = 46.5. gm = mu/ra = 46.5/20 = 2.3mA/V, so 1/gm = 430R.

Is it possible to sent me these curves/graphs on the 6072A - please ? I really want to get to know this thermionic emision stuff better.
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Old 19th November 2003, 06:00 PM   #38
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Just go to Frank's site, and you will find all the valve data you will ever need:

http://home.wxs.nl/~frank.philipse/frank/frank.html
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Old 19th November 2003, 06:23 PM   #39
JDeV is offline JDeV  South Africa
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Quote:
Originally posted by EC8010
Just go to Frank's site, and you will find all the valve data you will ever need:

http://home.wxs.nl/~frank.philipse/frank/frank.html

This 1 works

Frank's Site
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Old 4th August 2004, 05:23 PM   #40
JDeV is offline JDeV  South Africa
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Default Follow up

After a couple of months now, I decided to see if I can get rid completely of my LF oscillation problem (in my 1st post) and still use the org. circuit with my Mullards. I again looked at all the advice form our fellow tube guys, and reconstructed the org. circuit. The LF problem was still there as in the beginning. Then I got the idee of grounding the top plate, on which my valves are mounted, to the star ground point. This is the only thing I did not try previously. What an improvement. I don't know why I did not try it right in the start
The LF oscillation is still there but at about 1/4 amplitude of what it was.
In the mean time I also build a subwoofer with own amp and controler (filter with volume) . The way I use it with rest of system is to T-off from just after pre-amp to subcontroller. So I did same now with tube pre-amp / Mullard combo.
Now this is where it gets very interesting (to me anyway) Since I connected sub controller to output of pre-amp. LF oscillation is virtually gone. With naked eye I can see no movement what so ever on cones. I am not sure what the downside of my subcontroller hookup to pre-out is, but in the sound of the system, it is just fantastic. This is the sound I expected from this pre-amp when I decided to build it.

Now I am happy and can really reccomend this pre-amp, easy to build - if you don't have LF oscilations , really very good sound.

Attached is circuit to show my pre-amp / sub controller / Mullard's interfacing. Please advice on better way to do this, if needed.

Thanx again to all who helped / still helping
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File Type: gif amps.gif (56.8 KB, 132 views)
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