the McGrath Mini amplifier

  I determined that the lower-than-expected fx loop gain was due to a wiring error: effectively, I was only running through one of the two gain stages on the return, so instead of a gain of 100, I had a gain of 10; given the send attenuation of 20:1, this led to an overall loop gain of 0.5, which was commensurate with the lower level I was hearing when the loop was engaged (-6dB). After fixing the wiring error so that both stages were operational, the gain (as expected) was too high.  Theoretically, the overall loop gain would have been 5, but my ears told me it wasn't quite that high (for complicated math reasons, the gain of these negative-feedback triode stages isn't quite as high as the simple resistor ratios would suggest; akin to how cathode followers have a true gain slightly less than 1).  Nonetheless, I first tried substituting a 1M resistor for the 4.7M feedback resistor in the second stage.  This should have theoretically resulted in a perfect gain of 20 o...

  Here's my basic design for a 555-based audio signal generator.  I will need this for troubleshooting the McGrath Mini amplifier prototype.  In particular, the issue of lower-than-expected gain in the effects loop. I carefully traced my wiring in this section of the circuitry, hoping to find some obvious error, but there was none that I could find.  The circuit appears to match the schematic.  This is only partly reassuring, because the design embodied in the schematic is my own, never before tested, based purely on theory and my speculative understanding of tube circuits with feedback.  However, I researched triode feedback stages in several different trusted sources online, and this seemed to confirm that I'm on the right track and the gain should be approximately as I have calculated. So, my plan is to send a test tone into the input of the amp, using the circuit shown above, and disconnect the top of the master volume pot from the circuit, then use a t...

I have now completed initial work on the electronics of the v2 prototype, and have given it its first audio test: https://youtu.be/cYrF3rC199E On my youtube channel (Industrial Baroque), you'll also find a number of other videos documenting the build process leading up to this point. The photos show the unit in "testing mode", with the electronics assembly hanging out of the top of the chassis, with the upper chassis box stacked on top of the lower chassis (power supply), and the speaker connected via an extension cable.  (Enabling this configuration -- which of course makes it much easier to take measurements, make changes, and debug problems -- is the reason I used tube sockets which mount from the inside of the chassis, rather than from the outside.) My initial impressions: the clean sound is good, and output is strong, seemingly exactly the same as the v1 prototype.  However, each of the circuit sections beyond the basic clean signal path seem to need work.  This is a...

https://youtube.com/shorts/0dfgtBJ0n7o?feature=share https://youtu.be/9qQm6-WrNnc      

I'm posting rough copies of the schematics again (just photos), including I think a few minor changes and corrections over the "nicer" copies posted a while ago.  Also, for the four pages comprising the preamp section, I have now made layout diagrams: i.e., slightly re-factored schematics which more clearly suggest how the circuit board and wiring will be laid out. 

  https://youtu.be/Teu2joRiQj0 This video is, I think, currently the only documentation I have describing the construction of the two-button footswitch for the McGrath Mini. The video shows the footswitch as it was first built, intended for the Version 1 amp.  As of Version 2, it is similar except the buttons are just labeled "1" and "2" (since they now have varying functions), and the second LED is blue.  The first remains orange; in v1, both were orange.  The resistors setting the comparator threshold in the LM339 circuit are adjusted to make the threshold higher, to operate reliably with the blue LED voltage. In the video, I say that the construction is "two thick and one thin" layers of plywood, but that's wrong, it's three identical ("thick") layers.