tube fanatic Posted June 11, 2014 Share Posted June 11, 2014 Being a custom designer/builder has the advantage of not having to be locked into a particular tube type or design format which keeps things in the "fun" mode! A while ago I was asked to prototype a SET using 6BX7s and, since I haven't done anything with that tube in many years, I agreed. The tube was originally registered by Sylvania in 1951 to answer the need for a rugged vertical oscillator/amplifier tube in one envelope for use in televisions. Due to its fairly high plate dissipation (10 watts each section, or 12 watts with both sections in use) it attracted the attention of the nascent hi-fi community. The earliest published article using the tube that I'm aware of was in the 1st issue of Popular Electronics in October 1954. The amp used the two sections of the tube as a push-pull pair which could put out around 4 watts. Since I don't like push-pull amps I decided to run this one as a parallel single ended arrangement to bump the power output up slightly vs using only one section. With the 12 watt plate dissipation being split between the two sections, the operating parameters had to be chosen as if each section were a separate tube each having a 6 watt dissipation. To keep the distortion at maximum output reasonable (in this case 2nd harmonic at approx. 1.6 watts out calculates to about 3.8%), the tube is being run at a relatively low plate voltage but at higher current. The voltage amp is a contact biased 12AX7 since the guy wants to be able to play around with rolling all kinds of electrically equivalent types without having to worry about improper bias. As mentioned in previous posts, contact bias allows the tube to totally self regulate and is a terrific way to utilize hi-mu triodes. The total dual mono design necessitated using a separate 12AX7 per channel. I wanted to construct this unit with a separate power supply as well but, having seen the 7 watt parallel SEP that I posted a while back, he decided that he likes the big chassis look better! Also, I included my "locally famous" variable ear-bleed filter although it's not likely to be needed due to the very smooth top end (typical of SETs in general). To me it sounds wonderful with the usual enormous soundstage which SETs excel at. Even the bottom end performance is very potent given the modest output xfmrs (Hammond 125ESE). I think the guy will be very happy! Now I have to get back to the shop and start pondering the next round of insanity (and I'm supposed to be retired)! Maynard 4 Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted June 11, 2014 Share Posted June 11, 2014 Nice looking amp! Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted June 12, 2014 Author Share Posted June 12, 2014 The voltage amp is a contact biased 12AX7 Grounding the cathode with a very large grid leak? What sort of bias do you get on a 12AX then? With the 10 meg grid leak, the bias is around -.8V with the B supply running approx. 240V. And, of course, the input to the tube has to be AC coupled. Will try to address your question in the other thread some time today! Regards--- Maynard Quote Link to comment Share on other sites More sharing options...
minermark Posted June 12, 2014 Share Posted June 12, 2014 Nice. Quote Link to comment Share on other sites More sharing options...
erik2A3 Posted June 14, 2014 Share Posted June 14, 2014 Awesome job Maynard! THIS is true DIY amplifier building! I always appreciate your neat lead dressing and symmetrical layout. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted June 14, 2014 Share Posted June 14, 2014 Awesome job Maynard! THIS is true DIY amplifier building! I always appreciate your neat lead dressing and symmetrical layout. Ditto! Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted June 14, 2014 Share Posted June 14, 2014 Nice clean work Maynard. Cool looking design. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted June 14, 2014 Author Share Posted June 14, 2014 Thanks for the compliments guys! My ultimate goal, even after doing this for so many years, is to emulate the creations of some of the Japanese DIY folks. I've seen some truly breathtaking creations come from that group. If I could get my friend Garrett to motivate himself (a truly impossible feat since he insists on living the life of a "true" retiree- in other words, he sleeps until 10 or 11, takes 2 hours to have his coffee, then gets on the computer for a couple of hours, and so on), I could attack the enclosure design as well. He's an expert cabinet maker/refinisher. I've offered him all kinds of bribes, but he won't budge!!! I'm trying to stay optimistic...... Maynard Quote Link to comment Share on other sites More sharing options...
jorjen Posted June 14, 2014 Share Posted June 14, 2014 Man, that is some beautiful work Maynard! You are a talented individual to say the least. Quote Link to comment Share on other sites More sharing options...
Deang Posted June 14, 2014 Share Posted June 14, 2014 Lol. I actually like the industrial look. Quote Link to comment Share on other sites More sharing options...
jason str Posted June 14, 2014 Share Posted June 14, 2014 Another spectacular looking project, well done Maynard. Quote Link to comment Share on other sites More sharing options...
erik2A3 Posted June 14, 2014 Share Posted June 14, 2014 Dean: a Totally agree with you on the appearance -- entirely functional without anything superfluous. And Maynard, this NOT to discourage anything you might have in mind for another approach. My dad used to build old tube radios in tin bread and cake pans. I loved it! Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted June 15, 2014 Author Share Posted June 15, 2014 Also, I included my "locally famous" variable ear-bleed filter although it's not likely to be needed due to the very smooth top end What's that? Chassis and such can be easily had in very custom form from: http://www.frontpanelexpress.com Yes, I've looked at Front Panel's site a few times. Seems intriguing, but I haven't looked at their pricing yet. Regarding the "ear bleed" filter, it is just a variable RC low pass filter placed between the input jack and the volume pot. I started out building these as an external unit when people began complaining that so many modern recordings, because of their often encountered high frequency boost (I've heard this referred to as the 10-10 philosophy- i.e. 10 db boost at 10 kHz), were making their "ears bleed." For the few dollars in parts, I decided to simply include it in the amps themselves. The switch on the pot allows it to be totally bypassed if desired. It's quite useful and allows many irritating recordings to be enjoyed. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted June 15, 2014 Share Posted June 15, 2014 Another beautiful piece of construction and design Maynard, as I know from personal experience. But more of that anon, in a separate thread. I have a couple of questions for you. Is there some specific reason you used solid-state rectification, rather then a tube rectifier on this build. Any do you ever try hexfreds for rectification? Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted June 15, 2014 Author Share Posted June 15, 2014 Another beautiful piece of construction and design Maynard, as I know from personal experience. But more of that anon, in a separate thread. I have a couple of questions for you. Is there some specific reason you used solid-state rectification, rather then a tube rectifier on this build. Any do you ever try hexfreds for rectification? Thanks Marty! I generally prefer solid state rectifiers for a couple of reasons. One is that they eliminate the need for 2 filament runs in the case of 5 volt filament rectifiers, or additional current draw for the 6.3 volt types which would heat the power xfmr more. The other is that I like to get the ripple at the input filter cap (I very rarely use a choke input filter) to be in the range of .25-.3% which necessitates using a fairly large value. Peak current limitations of tube type rectifiers preclude doing that. Regarding hexfreds or other fast acting diodes, I have never heard a difference, and standard silicon diodes work just fine. Given how little the dc current draw of single ended amps varies from zero signal to full output, the latter has no problem delivering what's needed. Anyway, that's my take on the situation although others may disagree. Maynard Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted June 15, 2014 Author Share Posted June 15, 2014 I was asked to prototype a SET using 6BX7s Aside from the beautiful look, how do you compare the sound of this to the more typical SET amp? Did you find anything unusual or unexpected about the sound of the 6BX7? Well, if by a more typical SET you're referring to a triode strapped pentode/beam power tube, I can honestly say that I hear virtually no difference in sound. Over the years I've built many SETs around a triode strapped 6V6 (and many of its variants), 6L6, 6W6, 6K6, EL84/6GK6/6BQ5, and other beam power and pentode types and the sound of all is quite similar when used with the same output xfmrs. The only real differences are those of power output. Using a pentode voltage amp vs. a triode, I usually find the former to be a bit brighter. I've never built any using DHTs and have only heard one amp of that type (using a 300B) and didn't find anything particularly remarkable about it when driving a pair of CWs. Maynard Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted June 17, 2014 Share Posted June 17, 2014 Mark used a smiley!! Quote Link to comment Share on other sites More sharing options...
Thaddeus Smith Posted June 17, 2014 Share Posted June 17, 2014 stop that. you're on a slippery slope.. Quote Link to comment Share on other sites More sharing options...
Thaddeus Smith Posted June 18, 2014 Share Posted June 18, 2014 i dont think the forum can handle such a paradigm shift. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted June 18, 2014 Share Posted June 18, 2014 Next thing you know I will be posting animated GIFS! Where's Boxx when you need him??? Quote Link to comment Share on other sites More sharing options...
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