CW's recent Dumbleator II build (amazingly clean) and all of the recent chatter inspired me to finish up my standard Dumbleator build. I started working on this project about 2 years ago and right before we moved, so I packed it up and then life happened. First, I just want to thank everyone here who has contributed to the files section and knowledge base. It made this project very easy and fired up the first time and within spec of the target voltages. I am going to post pics I took throughout the build process below.
I used the transformer and power supply from Martin's reinterpretation and the main board is traditional. But either board can be used build a fully traditional circuit or fully updated circuit (silk screen may not match exactly). I do have some extra boards and stickers for the front and back...if anyone wants to trade for some vintage parts my PMs are open
After a few fails with my cricut to cut a negative decal and then paint in the text, I just adapted my file and ordered some bumper stickers online. They came out great, but I did need to spray some additional adhesive since the Hammond chassis has a bit of a texture to it.
I rolled a few tubes and ended up with an RCA short grey plate.
For the sake of posterity, I forgot to mention I used isolation washers on the 1/4 inch jacks and ran a ground wire from the star to the jack bus bar and also to the pot bus bar. While testing I did not have any ground loop problems.
For the sake of posterity, I forgot to mention I used isolation washers on the 1/4 inch jacks and ran a ground wire from the star to the jack bus bar and also to the pot bus bar. While testing I did not have any ground loop problems.
For the sake of posterity, I forgot to mention I used isolation washers on the 1/4 inch jacks and ran a ground wire from the star to the jack bus bar and also to the pot bus bar. While testing I did not have any ground loop problems.
Really not sure if there would be any problems without them but I have a similar star-ground implementation and zero noise from the grounding.
I am curious, does the bright switch on the send side do anything for you?
The bright switch on the drive pot was actually a little too much for me when testing initially. I am waiting on some cables to arrive and once I get everything connected in a more permanent state, I may end up changing it. I think it is a combination of the 1000pf value and running the drive pot pretty low. I also need to dial that in a little bit more to find the clipping point of my effects in the loop. I am using 2 500pf ceramic caps in parallel that I got from Jelle a few years ago. I may go to just 1 500pf. I should have clipped these in to test before soldering...
I did test a few different caps on the output pot. A modern vishay 220pf ceramic (one of the dark orange ones), a TDK 220pf ceramic, and a CDE mica 270pf (all were in my parts stash already). I preferred the vishay 220pf and perceived it to be the most neutral sounding. This bright switch is much more subtle and more to my liking. It does brighten things up but not as aggressively as the drive switch.
The TDK cap was interesting and I may use that in specific spots in an amp circuit. I perceived this one as having a more gritty timbre.
What have you noticed when testing your loop? Yours is built into your amp, right?
My memory of the tests I ran last year is a bit hazy now but when I ran the loop into a Boss DD-200 digital delay I couldn't hear any difference with either a 1000p or a 560p cap on a 3 position switch. It could have had something to do with the pedal having a tone control and that, compensating for any high end losses but I can't be sure now. Not saying there is no difference, just that if there were, it was too subtle for my ears. So, when I built my next amp and installed the loop, I omitted the send-side bright switch altogether. I had already made some other changes to the circuit values and topology by that stage as well. Such as, increasing the .47uF cathode coupling cap to 1uF and reducing the 250k drive pot to 25k. I think the brand of component choices make a difference as well and it is increasingly to stay clear of the NOS parts and go for modern equivalents. For example, my form factors are much more compact using DME caps instead of 6PSs and the results are great... a very low noise loop, you have to really crank up the drive and output pots to to get even a small amount of hiss and because of the buffer side mods I used there's a subtle augmentation of tone with nice even order harmonics. Or at least, that's my best educated guess as to what is happening. It's not what you would call a totally transparent circuit. There is also a fair amount of latitude with the tone control depending on how you balance the send and return controls. I provided a more detailed description of my loops here: https://ampgarage.com/forum/viewtopic.p ... 83#p444383
Next level for me is to look into replacing the 12AX7 with a 12AT7.
PS. I'm using a Vishay ceramic 270p send as well. Not a huge difference (in or out of circuit) but about right I would say for longer cables.
