The Ballzz wrote:I could only find those particular diodes at a fairly outrageous price, so decided to wait on them, maybe I'll add them later
Most "belt + suspenders" Si + tube rect installs I've seen use a single 1N4007 on each HV winding, with the rect's perched on the tube socket. I use 2 UF4007 & bridge them from a tie strip, considering the possibility of an arc with those HV connex all squeezed together on a socket.
Either way the rectifiers are dead cheap, available from the usual suspects for pennies apiece when you buy 100 quantities: Mouser, DigiKey, Allied, etc etc. Doesn't hurt to have a small pile of extras, in time you'll use 'em.
Some buy from mysterious Far East sources on Ebay, even cheaper but who knows what you're really getting. They can put any label on anything, if it fails, too bad .
Nice looking build. I had to comment here on three things.
I do use the diodes prior to the rectifier tube, been doing it since I read about it in Dave Funk's book published in 2000 or so. No complaints, no difference in tone that I can hear, no noise either. I use a 1000 volt rated diode, one on either leg of the primary.
As to the use of the diode string on the plates - no, not for me. This idea was presented by Ken in one of his articles that appeared in Vintage Guitar, and subsequently published in the Groove Tubes tube amp book. I think this is a bad idea. I had these in everything I built (Class AB amps only) starting in 2001. Finally a few years back while working on an Express build I heard something I had missed before. I'll call it 'rat tone' for lack of a better word, and it appeared at the speaker. I finally sourced it to the diode strings when I removed them - and then replaced them - and then removed them again. I no longer use this idea and have removed them from everything I have built.
As to the use of the Center tap of the filament supply - no, not for me. I use 1/2 watt metal film resistors. When building a transformer, it seems hopeful that the transformer builder would be able to create the center point of the filament wind. Sure, maybe. But wire being wire, and the manufacturing process being what it might be on a bad day, why trust it? When the 100 ohm resistors are in place, the Center of the wind is found perfectly - no error. The only mistake that could be made here is to use some carbon comp resistors that are misbalanced, or that burn in to a misbalanced state after a few years. That's why metal film is better here.
Lastly, that Mojotone schematic showing the AC connection. It's funny but it had never occurred to me to run the AC hot lead to the fuse and then to the transformer, and then switch the AC return path. Funny how the mind sees things. This is a good idea, so thank you for the post, Ballzz.
Most people stall out when fixing a mistake that they've made. Why?
.