Erik Mandaville Posted September 30, 2005 Share Posted September 30, 2005 Home from work this week, I've had some time to think about the 300B subject that has prompted some interest here. It suddenly came to me that I have very, very close to what's needed for 300Bs in my Moth amp, which is now set up for the higher operating points of the KR2A3. I have always wondered about the 300B triode, and have never had the opportunity to hear them in our system. So, I can transform the Moth, which now uses two 6SN7 voltage gain stages rather than former paralleled 6SL7, into a stereo 300B amp with very little work. Since the Moth uses regulated DC for the 2.5 volts, I can tap into the dual 5-volt filament supply for the 2A3s, and take the voltage before the regulator in order to get the 5 volts needed to heat the 300B. It's possible that I may be able to use the existing bias resistor on the 2A3 cathode, as well. It always happens! This amp in 2A3 mode is as quiet as a mouse and IMO sounds really great -- and I am thinking of changing it into a 300B amplifier. It's almost begging to be tried though, and would take such a small amount of work to get it there. Erik Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted September 30, 2005 Author Share Posted September 30, 2005 ....But, I just don't think I can hack into what has turned out to be a really fine sounding SET amp. Forget it, I'm not going to do it. I can make one from used power and OPTs, and a simple Hammond chassis. I think that's a smarter way to experiment with 300Bs. Erik Quote Link to comment Share on other sites More sharing options...
3dzapper Posted September 30, 2005 Share Posted September 30, 2005 Erik, Here is a nice sounding, simple 300b SET to build: Rick Quote Link to comment Share on other sites More sharing options...
3dzapper Posted September 30, 2005 Share Posted September 30, 2005 Now I have a question for you. If i wanted to try a choke draining the grid, would I just have to replace the 200K 2W grid stop resistor as is or tie it to the cathode? Rick Quote Link to comment Share on other sites More sharing options...
fini Posted September 30, 2005 Share Posted September 30, 2005 I'll take this one, Eric! (yeah, right...)[] Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted September 30, 2005 Share Posted September 30, 2005 Erik, here's another 300B schematic to ponder over...[H] Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 1, 2005 Author Share Posted October 1, 2005 Rick and Jim: Thanks for that encouragement! Both of those look interesting and very possible. I could really do this with minimal surgery to the Moth, so I'm still thinking about it. Grid chokes: Do try it if you can -- I think they are a great modification for this kind of an amp. The part in question, Rick, is the grid-LEAK resistor, which is connected from the grid of the output tube to ground. The value you provided of approx. 200K is the kind of resistance one would normally see in that position, whereas the grid-STOP resistor you mentioned would be in the range of maybe a few hundred ohms to a few thousand. Grid-stoppers have a different function in a circuit than the grid leak. You are right though about the installation of grid chokes. You simply remove the grid leak resistor (in your case 200k ohms), and install the choke in its place, from grid to ground. You just need some way to physically mount the chokes, but they are small. Check JFL at Pantheon Audio for them -- I'm also going to get in touch with him for the same reason, and maybe another part or two. He was very helpful with the last grid chokes I ordered for the Horus amps. The 300B schematic you attached could work extremely well for this experiment, Rick! There are some small changes I would need to make here and there, but I would still be able to use about 4.98Volts of filtered DC on the 300B heaters. I would bascially just need to remove a dropping resistor across the filter caps in the power supply, and bypass with a length of wire for B+. It's very tempting, but I can't do it without the 300Bs! Any thoughts on some inexpensive 300B triodes? Thanks both again, I'm going to print the schematics! Erik Jim: Is the attachment you included your amplifier? Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 1, 2005 Author Share Posted October 1, 2005 Rick: attached is a picture with chassis plates of my amps upside down. If you look at the one on the right, the grid choke is seen at the lower right-hand side. All iron but the filter chokes is from MagneQuest, and the grid chokes are solid little things! I havent' posted pics with this new format, so I'm hoping this works. If not, I'll send it to you by email. Erik Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 1, 2005 Author Share Posted October 1, 2005 Wow, that was easy enough! You just need to install a lead on each solder lug of the choke, inserting it and soldering, but without bending the lead around the lug (they are a bit delicate where the connection is made). Erik Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted October 1, 2005 Share Posted October 1, 2005 You've been PM'd![] Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 1, 2005 Author Share Posted October 1, 2005 Jim: I'm still amazed at the fair price for that amplifier of yours! It really is an interesting looking design, Let me get to your pm later.....this day has gotten on for me just a bit. Take it easy! Erik Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted October 1, 2005 Share Posted October 1, 2005 Later, Erik... Quote Link to comment Share on other sites More sharing options...
3dzapper Posted October 1, 2005 Share Posted October 1, 2005 "Any thoughts on some inexpensive 300B triodes?" The EH gold grids $120/pair sound very good once burned in for a few hundred hours. Nice bass and midrange. The Valve Arts $100/pair have a very fluid mid range and shimmering highs (more oriental?), but lack some on the bottom end in comparison. The EH standard for around $100/pair are good, just not quite as good as the gold grids. They will accept 5VDC heater supplies. For some reason I can't fathom, the Gold Grids did not want to play at all with that filament set-up so i went back to AC heaters with the very slight hum involved. Trade-offs!! Rick Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 2, 2005 Author Share Posted October 2, 2005 Jim -- I PM'd you back! Rick -- Thanks for the info on those 300Bs. I can surely check that out later this morning. It's sort of odd, though, that you had some problems with using DC on the filaments. What exactly happened? Was it the sound that suffered, where you just preferred the sound AC? Was this something you experimented with on your own 300B amp? I have two 5 volt (3+amps) windings on the power transformer to work with, and I was mainly interested in using DC because of the hum component you mentioned. That can of course be brought down to a very low level with a hum-null circuit (as seen in the schematic you provided), but I use the amp most often with headphones. The phones seem to really put a microscope on every aspect of perforamance, and that's why I've worked so hard to get residual noise down as far as I can. I've always like using AC on 2A3s, though, and it would be possible to do the same, if need be, with the 300B. Helpful information -- thanks again! Did you see those little grid chokes? They are neat little things, but are costly if compared to the resistors they replace. One way around that is to not compare them![] Erik Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted October 2, 2005 Share Posted October 2, 2005 Erik, I've PM'd you in return...we gotta stop PMing eachother like this. Others will start to talk...[:#] Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 2, 2005 Author Share Posted October 2, 2005 Jim: LOL!!! Hey, I replied back, but I just hit 'send' the way I do with regular email. Will you still receive it, or should I have used the link in the email back here to the forum? Let me know if you get that, ok? I'm ordering cable today! Erik Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted October 2, 2005 Share Posted October 2, 2005 Hmmm...I still haven't received your latest PM yet, but clicking on "Post" should've done that IMO. Getting cable is definately a good thing...we're happy with BrightHouse Digital Cable here! Quote Link to comment Share on other sites More sharing options...
3dzapper Posted October 2, 2005 Share Posted October 2, 2005 The output with DC was just muddled. i put my one remaining non GG in and a Sovtek 2A3 and they played fine. I send the first pair of GGs back thinking them defective. There is only like 1.5mV of hum on the worst channel just audible with the Khorns with no signal. With any music it is inaudible. I might put the rectifiers back in to see if there is a change now that there are some hours on the GGs. It only takes a couple of minutes I built the caps and bridge into a module. I have a pair of chokes I got from 18audioguy on eBay (Hong Kong) I will tack them in this week and report back. Rick Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 2, 2005 Author Share Posted October 2, 2005 Rick: The Sovteks really are excellent tubes, and for their low price, one of the those audio 'steals'. What chokes are your referring to? these are grid chokes or filter chokes? Erik Quote Link to comment Share on other sites More sharing options...
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