I concur with Chris. No matching on this end either and as Chris says once the bias and off set smooth out all is good.
A few other comments.
Not surprising I switched the OTL to one of my quad 63's and the sound was very nice. The ESL's in general are more like a 40 to 100 ohm load so the amp really enjoys that.
Heat is a real concern on these amps. With just 2 tubes the amp get amazingly hot. With that said I WILL NOT be building the second amp as I did the first. I will be redesigning the case and go for a NYAL sytle 4U case with fans and put the sockets up on legs to get the 6c33's away from everything else. The heat is so extreme that the other components will suffer a fate worst than death very shortly after the amp is finished. Even if you buy all your components super heavy duty the heat is extreme. At minimum I will remove the sockets from the metal top and place the above the top plate on standards and put in a second top plate at the rear and see if that helps. My current case does not lend itself to a fan elegently so if this does not work then the 4U case it is.
Not sure if I will go with four 6C33's for the output due to the heat issue. Certainly I will not until I feel I have it under control. My NYAL OTL3's do not have heat even vaguely close to this amp and those have 6 - 6LF6's per amp. Also not sure that I need the 4 output tubes. The amp plays pretty load and clear on my DIY 15" Triaxial Tannoy as well as on my 63's which are not nearly as efficent. So I will hold on this for now although I will draw up the modified schematic for 4 output tubes and post it for comments.
As for the sound the amp is nice but I still need more time to give my final opinion. My NYAL/Futtermans are amazing and I do not think this amp is at that stage at this point but the next week or so should tell me how close they get once they break in a bit more. I am running them at the suggested 200ma current draw and the meter does not jump around but it does move a bit hear and there.
Also I cannot stress the need to make sure you deal with the heat issue upfront as it is real and ever present so design your chassis accordingly.
In final I will start to build the Bruce R. Otl ASAP as a side by side comparison before I build the other channel for the Tim Mellows amp. I will test the Bruce R. design out as originally built with the 6AS7's/6080 as I have several hundred NOS NIB military version of those. I will also swap in some 6LF6's to test the updated version of that amp which uses the 509's (The 2 tubes are generally identical except for the sockets). Lastly on this I will test the Bruce R. amp using the 17KV6A tube as the output. Just another comptractor tube that is similar in nature to all other comptractor tubes such as the 6LF6 and the 509/519 tubes.
I will keep you all posted and will touch base with my finals on the Tim M amp shortly as well as the updated chassis and schematic for 4 output tubes.
Thx
Not surprising I switched the OTL to one of my quad 63's and the sound was very nice. The ESL's in general are more like a 40 to 100 ohm load so the amp really enjoys that.
Heat is a real concern on these amps. With just 2 tubes the amp get amazingly hot. With that said I WILL NOT be building the second amp as I did the first. I will be redesigning the case and go for a NYAL sytle 4U case with fans and put the sockets up on legs to get the 6c33's away from everything else. The heat is so extreme that the other components will suffer a fate worst than death very shortly after the amp is finished. Even if you buy all your components super heavy duty the heat is extreme. At minimum I will remove the sockets from the metal top and place the above the top plate on standards and put in a second top plate at the rear and see if that helps. My current case does not lend itself to a fan elegently so if this does not work then the 4U case it is.
Not sure if I will go with four 6C33's for the output due to the heat issue. Certainly I will not until I feel I have it under control. My NYAL OTL3's do not have heat even vaguely close to this amp and those have 6 - 6LF6's per amp. Also not sure that I need the 4 output tubes. The amp plays pretty load and clear on my DIY 15" Triaxial Tannoy as well as on my 63's which are not nearly as efficent. So I will hold on this for now although I will draw up the modified schematic for 4 output tubes and post it for comments.
As for the sound the amp is nice but I still need more time to give my final opinion. My NYAL/Futtermans are amazing and I do not think this amp is at that stage at this point but the next week or so should tell me how close they get once they break in a bit more. I am running them at the suggested 200ma current draw and the meter does not jump around but it does move a bit hear and there.
Also I cannot stress the need to make sure you deal with the heat issue upfront as it is real and ever present so design your chassis accordingly.
In final I will start to build the Bruce R. Otl ASAP as a side by side comparison before I build the other channel for the Tim Mellows amp. I will test the Bruce R. design out as originally built with the 6AS7's/6080 as I have several hundred NOS NIB military version of those. I will also swap in some 6LF6's to test the updated version of that amp which uses the 509's (The 2 tubes are generally identical except for the sockets). Lastly on this I will test the Bruce R. amp using the 17KV6A tube as the output. Just another comptractor tube that is similar in nature to all other comptractor tubes such as the 6LF6 and the 509/519 tubes.
I will keep you all posted and will touch base with my finals on the Tim M amp shortly as well as the updated chassis and schematic for 4 output tubes.
Thx
Using a cooling fan is very helpful, for keeping the temperature under control. I included one in my construction, and I made a few holes in the top of the chassis, so that air can be drawn through and extracted by the fan. I also mounted the 6C33C tube sockets with washers, to provide a little air gap between the sockets and the chassis. There is quite a good draft of warm air extracted by the fan. The chassis never gets above about 140 degrees F.
Chris
Yes I feel Chris is completely correct space matters. On my chassis although it looks like the plate is close to the wood base it actually hovers over it via stand offs about 3/16" (5mm) so air can rise out from under it along all 4 sides. I will use my thermo gun tonight or tomorrow and let you know the readings I get.
I posted this earlier:
I recommend that the socket be stood off at least 1/8 below the bottom side of the chassis. You could see from the photos that the amp in question here was going to have some heat issues. Air holes around the sockets are mandatory! If you think you have too many then you are getting close to where you want to be. If the socket is bolted directly to the chassis, it will easily transfer heat to the chassis. This is OK as long as you plan for it- mounting caps with hot melt glue may not turn out to be the best move. Silver/teflon wire for the sockets, done with SN96 (800 degree eutectic solder) is a very good idea.
I would not put the socket above the chassis, tempting as it is, due to danger to the operator.
If the socket is mounted properly, the amp will be quite reliable over long periods- years in fact.
Took heat measurements
125 to 130 at sockets
85 to 90 at the back of amp
90 to 95 at the front of the amp
Will move sockets off metal as suggested. I hve no issues with putting the sockets above the top plate because plans were already in place to protect against shock. No glue used on any of my amps as it seems silly to me due to heat issues with power amps in general. I will retest temps after I move sockets which I will do this weekend. In generall not as worried now that I took measurements and sorry if I worried anyone. But I do say that my futtermans never get this hot in the surrounding areas as I took measurements of them this evening and I never got over 105 anywhere other then directly on the 6lf6's themselves. Fan on those amps so that I am sure has a lot to do with it
I did finally get a chance to crank the amp today since I got home early enough to do that without waking the kids and I took notice that the mA meter moves around a bit more then Tim seems to suggest in his article. On transistions in the music the current draw does range 50 to 100 mA above the 200mA idle set point. Not actually surprised just thought I would mention it.
125 to 130 at sockets
85 to 90 at the back of amp
90 to 95 at the front of the amp
Will move sockets off metal as suggested. I hve no issues with putting the sockets above the top plate because plans were already in place to protect against shock. No glue used on any of my amps as it seems silly to me due to heat issues with power amps in general. I will retest temps after I move sockets which I will do this weekend. In generall not as worried now that I took measurements and sorry if I worried anyone. But I do say that my futtermans never get this hot in the surrounding areas as I took measurements of them this evening and I never got over 105 anywhere other then directly on the 6lf6's themselves. Fan on those amps so that I am sure has a lot to do with it
I did finally get a chance to crank the amp today since I got home early enough to do that without waking the kids and I took notice that the mA meter moves around a bit more then Tim seems to suggest in his article. On transistions in the music the current draw does range 50 to 100 mA above the 200mA idle set point. Not actually surprised just thought I would mention it.
Still collecting parts for mine. I did have a great stroke of luck though. A friend of mine owns a machine shop and did a job for Harley and Earl which is a huge design firm here in Detroit. The job was to punch 2' x 4' picture frames out of a Brass/Copper 1/8" plate. The shop that did the stampings gave him all the scraps and he gave me two sheets for my amp chassis'. As you may recall, I am building the two tiered amp with PS on the bottom and the signal path on top.
I also went a little crazy and had a friend of mine design a PS using tube rec.....gotta love tubes. 4 3B28 in bridge mode.
Don't know when I will have the money for the tubes and sockets, but.......
I also went a little crazy and had a friend of mine design a PS using tube rec.....gotta love tubes. 4 3B28 in bridge mode.
Don't know when I will have the money for the tubes and sockets, but.......
Well, it will be interesting to hear how that PS works out! I'm not familiar with the 3B28 rectifier, but looking at the data sheet it seems that it's really designed for high-voltage applications (like 5KV or more). I would have thought that trying to get several amps, with very little "sag," at a lowish voltage like 150V could be a bit beyond their capability...
Chris
I just googled again, and this time I found it...
"Easy" OTL 40 watts with 6C33C Output Tubes • DIY Audio Projects Forum
I've been doing totempole OTLs for about 10years now, and although they work great, I have come to regard the circlotron a better design. Look at Atmasphere's M60.
(If simplicity is part of the goal, Take away the tube CCS in the LPT, take away the cascode, take away the cathode follower driver, and u have a circuit much simpler than even Tim Mellow's design.)
I am finishing up my first circlotron (with my odd mods of course) so cannot speak loudly yet of which I prefer. Looking forward to compare.
"Easy" OTL 40 watts with 6C33C Output Tubes • DIY Audio Projects Forum
I've been doing totempole OTLs for about 10years now, and although they work great, I have come to regard the circlotron a better design. Look at Atmasphere's M60.
(If simplicity is part of the goal, Take away the tube CCS in the LPT, take away the cascode, take away the cathode follower driver, and u have a circuit much simpler than even Tim Mellow's design.)
I am finishing up my first circlotron (with my odd mods of course) so cannot speak loudly yet of which I prefer. Looking forward to compare.
I am working on a circlotron also. Trying out different drivers and CCS configs.
Also looking to put in a fixed bias protection for the B+. So it will switch off the B+ if the bias falls too low or fails.
Speaker protection from DC offset is also on my mind.
Have you incorporated any protection circuits?
Well Chris, we are just not that smart around here
I just happen to have a gagle of 3b28 and sockets so I thought I'd go with them. I also have a gagle of 6D22s that might work better if the 3b28 don't work. Honestly, I don't know what you said. I can build anything but have to relie on the brains of others for that kind of stuff.Well Chris, we are just not that smart around here
Chill mate! Not sure Chris ment to be patronizing, just pointing to the fact that often high voltage devices are not always usable for high current outputs. Nothing wrong in that, he may simply be trying to help...
Tho, he states he has no experience with those tubes, neither do I, so he might be better off not saying anything. But then again, this is a discussion forum????
Ooops! I'm very sorry if what I said sounded impolite or patronizing; it was absolutely not intended! http://www.diyaudio.com/forums/images/smilies/redface.gif
It's just that my feeling is that an OTL puts pretty heavy demands on the HT supplies, which in turn puts heavy demands on the rectifiers. The supplies need of the order of 2000uF of smoothing capacitors, they are relatively low voltage, and there shouldn't be too much additional voltage drop when the current draw increases from idle to a value of a couple of amps or so. Rightly or wrongly, I imagined that this could be difficult with tube rectifiers.
But then, if we only ever did things that were easy, we probably wouldn't be in this game at all!
Chris
No problem SemperFi. I was not offended. An I really do relie on other peoples thinking for this stuff. As much as I try, I just don't get a lot of what is say. I am glad he brought it up so now I have something to research.
If you had a set of Futtermans that ran this tube, they would be hot too. The 6C33 runs a very hot filament- even with no plate current, the tube is too hot to touch.
The changes in plate current sound quite normal to me!
No problem Chris. This is all new to me, and I appreciate all this input. I am not to much in the understanding department, I just like the looks of big gas tubes. I have been a large metal sculpture for many years now and sometimes the looks of something that I like will end up not working so well.
With judicious selection of the B+ power fuse, speaker protection should be OK. I would put a fuse in series with each power tube though. 1.5A seems to be adequate.
If the bias goes towards 0V then that is when the protection, if any, should kick in, but that also tends to be when the fuse blows if it was selected right. A protection circuit might need a timer as an audio signal could put the grid voltage into the same window.
BTW, if multiple power tubes are used (4 or more 6C33s) I recommend a 2 ohm 10 watt resistor in the cathode circuit of each power tube, regardless of whether the circuit is totem-pole or Circlotron.
The rectifiers need to be good for several amperes. In addition, the B+ is low as far as tube amps go, so the usual voltage drop that tube rectifiers have is unacceptable.
Now if you had enough of them (more of them than power tubes) you might be able to get away with it. That seems like it would be a prodigious project!