Comments are moderated before they will appear on the website, this is a manual process and may take some time. Please be patient.
Author Name is a required field.
Email Address is optional, but without one I won't be able to contact you back. It is never shown or linked on the website. You can always just email me if you'd rather not post a public comment. I generally reply in-line with a comment rather than email you back, unless I want to discuss something in private or off topic. Please check back to see when I reply.
Website URL is optional, if supplied the Author Name will be hyperlinked to this URL.
You may use wikitext in the body, preview may be handy here. Don't worry if you can't figure them out, just give me a hint what you want linked to what and I'll do it during moderation. Wikitext is not BBcode!
Spammers: Please don't bother wasting your time scripting up posts to this form. Everything is moderated, your post will never be seen on the web even transiently, there is no way to even view it by its internal ID, it will never be indexed. I will simply delete your post in the moderation interface. If I'm your target audience you're really on the wrong track; I'll never click on a URL in your garbage. The post content is not emailed to me (and I don't use a Win32 mail client anyway), I view the posts in plain text in the moderation interface so no clever tricks of any kind will make anything you type be interpreted by anything other than me, a human. Just give up and go elsewhere please!
26th March 2012 01:08
Hi Alan!Nice project!i was looking for something like it for ages,since I found two battery pentodes!I have a question-How many turns should the tickler coil be and how should I space it from the tuning coil and is the ferrite core a must have or I can go without t?Thanks!
21st June 2008 22:31
Alan, in terms of transconductance the valve is indeed similar to a FET, but in this circuit the grid and cathode behave like a diode which is how it demodulates AM. A FET does not have the same diode characteristics so it is usually used as a class B detector, operating almost at cutoff by means of a source resistor. You certainly don't want any bias with a valve grid leak detector as it will effectively reverse bias the 'diode'. With voltages above about 45, the plate load resistor is the only thing limiting the current so it can't be too low. As far as the plate circuit goes, it functions like any other RC amplifier; higher values of plate resistor give higher gain, but practically speaking you can't go beyond a certain point due to the input impedance of the following stage and/or the output voltage swing being limited by the plate voltage. In practice, for this kind of circuit, plate resistors are usually between 47K and 270K. Incidentally, the grid resistor is higher than usual; normally around 1 to 3.3M is used here. However, it is worth experimenting to find the optimum values for these two resistors. They are by no means critical. You may also have to increase the feedback winding turns to compensate for lower plate current.
To work out the optimum plate resistor theoretically for a grid leak detector is not something I've seen done as curves are normally given for class A operation.
Hope this helps; John.
13th June 2008 11:15
John,
How do you calculate the correct value?
I assume it is a lot like a JFET? Normally I'd have a source/cathode resistor in there to set the bias that way, then calculate the drain/plate load needed for the load line I wanted.
Regards,
Alan
12th June 2008 19:54
The 2.2K plate load resistor is way too low for this circuit. It might be ok for a transistor, but you'll get higher output by increasing it. I would recommend something between 47 and 100K. Of course the higher output resistance means the following audio stage needs a high input impedance.
8th December 2014 17:09
Kim VK5FJ wrote...