cfishbowl Posted November 16, 2015 Share Posted November 16, 2015 Hey all. I'm somewhat new here. Ive built a few kit projects, most recently a decware zen kit which I really like. Anyway, I just finished Ed Jurich's book and I am in the middle of building the 6v6 single ended amp found here http://www.ejjamps.com/articles/6V6SE-6L6GC-amplifier-circuits.html The only mod I am doing is putting in a heater switch so I can run 6cg7 in addition to 12au7/12bh7 My question is whether it is worth putting in a triode mode switch so I can run the 6v6 in triode mode. If you guys think it would be worth it (I don't need more than 1-2wpc with my speaks), would someone be willing to draw it for me quick? I'm a rookie. Thanks folks! Corey Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
joessportster Posted November 16, 2015 Share Posted November 16, 2015 Welcome, There are many here that I am sure will chime in, IMHO less is more, and if you dont need it I would leave it out Come back and post often 1 Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 Hey all. I'm somewhat new here. Ive built a few kit projects, most recently a decware zen kit which I really like. Anyway, I just finished Ed Jurich's book and I am in the middle of building the 6v6 single ended amp found here http://www.ejjamps.com/articles/6V6SE-6L6GC-amplifier-circuits.html The only mod I am doing is putting in a heater switch so I can run 6cg7 in addition to 12au7/12bh7 My question is whether it is worth putting in a triode mode switch so I can run the 6v6 in triode mode. If you guys think it would be worth it (I don't need more than 1-2wpc with my speaks), would someone be willing to draw it for me quick? I'm a rookie. Thanks folks! Corey Sent from my iPhone using Tapatalk Hi Corey, How far along on the build are you? Matt. Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 16, 2015 Author Share Posted November 16, 2015 (edited) Hey Matt, not very. Basically wired the fuse, iec, and on off switch. Haven't wired any of the actual circuit yet. Probably won't get to do too much more for a few days. Sent from my iPhone using Tapatalk Edited November 16, 2015 by cfishbowl Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 I've never been a big fan of "global feedback" let me scribble something up for you that will sound much better. Since you already have your cool purple anodized chassis drilled I won't mess with the number of tubes required, just the choice of driver. Matt. Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted November 16, 2015 Share Posted November 16, 2015 Sinice Matt drew you an alternative schematic I won't put in my 2 cents! Just some observations about the original circuit- it looks like he's running the screen voltage well above it's maximum rating of 285V which could affect tube life. Using a feedback resistor (R12) without a small value cap in parallel isn't the best idea (I can explain why if you're wondering). That can be implemented later on (and I agree with Matt about global feedback not being the best way to go). As to switching to triode operation, that's extremely easy to do with a single pole/double throw switch. That would allow you to connect the transformer side of R10 to either the ultralinear tap on the xfmr or to the plate (if you do this, do not hot switch it as it will present an audible pop in the speaker). Of course, I would have to graph the operating point in triode mode to see how the distortion looks, but if the idea appeals to you, install it.Maynard 1 Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 I totally agree with Maynard on his suggestions regarding the original schematic. If for whatever reason you "need" to build that amplifier, I would suggest doing away with the triode "strapping" idea through a switch/or not altogether as I have performed many "blind" listening tests with the 6V6 wired Triode and Pentode and could never justify the drop in power output for this particular tube. On the other hand, if you are going to build either this schematic or the one I attached, AND, you really would like to use the ultralinear tap, I would recommend dropping the voltage from the tap "through" a 21v/3watt Zener "bypassed" with a 0.01uf/350v film cap. This will ensure you keep the potential of G2 below that of the plate. Matt. Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 16, 2015 Author Share Posted November 16, 2015 (edited) Hey thanks a bunch guys. I'll skip the triode switch then. Matt, I really really appreciate your help. I noticed you eliminated the volume pot from the circuit. Not a huge deal since I have a buffer as my volume control but I thought it might be nice to leave in for channel matching? I'd love to hear your take. If I do, I assume I can just shove it in there? I'll look at the two schematics and figure out what I have already and what I would need to buy to make the proposed changes. I have no doubt it'd be worth it. Maynard, Joe, and wdecho, thanks for your input as well. I'll keep ya'll updated. Sent from my iPhone using Tapatalk Edited November 16, 2015 by cfishbowl 1 Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 16, 2015 Author Share Posted November 16, 2015 (edited) Also, Maynard, I am curious... Would you mind learning me? Sent from my iPhone using Tapatalk Edited November 16, 2015 by cfishbowl Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 (edited) What exact output transformers are you using? Are you using the Edcor recommended in the original design? Are you using the original designs power supply design? I have a few concerns with the proposed schematic in post #6. First is the power tube, I estimate the power tube to be idling around 15 watts plate dissipation, you could roast some weenies while you listen to music I suppose. The screen is idling around 1.5 watts which is still under max rating but during max signal it will jump above the max rating. That and the fact the voltage rating is being disregarded. Other than that I have small ideas. The "Schade" feedback resistor could be increased to 180k to 200k in order to give the amp a bit more sensitivity, as it is now 1Vrms won't drive it to full output, you need around 1.5Vrms to drive it to full power. The coupling cap could be made much smaller in order to save on space and $$. With a grid leak of 470k you can use a .047uF, this also helps with LFO's. I will look at the performance of the original circuit. As Maynard pointed out, if you don't have a scope and don't know how to empirically find the compensation cap across the feedback resistor you could have very mixed results even with a good schematic. I haven't calculated how much feedback they are using, if it's a very small amount you can get away usually without any compensation networks. IF you do have a scope it's easy to figure out. Or Matt has proposed an idea which I like which is the use of "Schade" feedback. I know it's a controversial name but calling it what Schade called it is much more of a mouthful to say; "Inverse voltage feedback for adjustment of tube impedance". Edited November 16, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 (edited) Hey thanks a bunch guys. I'll skip the triode switch then. Matt, I really really appreciate your help. I noticed you eliminated the volume pot from the circuit. Not a huge deal since I have a buffer as my volume control but I thought it might be nice to leave in for channel matching? I'd love to hear your take. If I do, I assume I can just shove it in there? I'll look at the two schematics and figure out what I have already and what I would need to buy to make the proposed changes. I have no doubt it'd be worth it. Maynard, Joe, and wdecho, thanks for your input as well. I'll keep ya'll updated. Sent from my iPhone using Tapatalk Ok then, there ARE a couple of things I'd like to point out with the design I sent you. 1) YES, please add a 100K Mono pot between the input and the grid of the 6201, this will function as an input gain/balance control for each mono. 2) Wire the 6201's using the "opposite" triode in each bottle (Pins 1,2,3 on one amp and 6,7,8 on the other amp), this way when the tubes get near the end of their usable life, you can just swap them for each other to have a "fresh" pair of new drivers. Lastly, 3) A bit about "grounding" the amp. Do the following: Ground the IEC directly to it's own point on the chassis, do not use the filament center-tap (cut it off and shrink-wrap the end) then tie a cement 100 Ohm 5 watt resistor to each leg of the filament and land them on the same ground point as the IEC ground. DO NOT GROUND ANYTHING ELSE TO THIS POINT. This is the "Chassis or Earth" ground. Next, tie ONLY the following: HT center-tap, C8, C9, C10 and the "Bleed-Down" resistor R15 (per your PS schematic) to a separate ground point on the chassis. This is the "Power Supply" ground. NOTE: The 10uF smoothing cap ground will become part of the "Signal" ground which contains the rest of the grounds per the circuit INCLUDING the input jack ground. My recommendation is to run a separate "Flying Lead" from each "group". This group should have it's own tie-point (either a turret or lug) from which to solder the flying lead. Group or "Node" 1) 100K Pot, 6201 grid-leak and cathode, Node 2) 470K grid leak, Node3) 100uF by-pass and cathode of 6V6. These 3 Nodes, along with the Input jack flying lead should be "SOLDERED" to a copper ground lug (after they are appropriately "squished" down/secured with the set screw) then attached to the chassis at third location. Note: As I am truly "anal" about sympathetic mechanical vibration, I always use fairly soft durometer rubber washers between the PT, PS Chokes, and OT's and the Chassis, I never use the PT bolts as ground points. I drill separate holes in the chassis for that purpose. Cheers! Matt. Edited November 16, 2015 by ToolShedAmps Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 Thanks JPM, those are great ideas. 6V6's sound GREAT at the above mentioned plate and screen dissipation levels, however, should you want some longevity out of the power tubes, I would recommend reworking your existing PS schematic to drop B+ to about 305v (which is the "sweet spot" for this design). Don't change the value of the feedback resistor (per my schematic), it is actually 1.71v to full output (factoring in the supply voltage @ 305v), but, sounds FANTASTIC. An active line-stage would be appropriate (like the one started in the new thread. On the other hand, any source capable of 1.71v can drive the amp to full power anyway. Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 (edited) Yes if you have the means to drive it fully then leave the feedback resistor where it is, lower the feedback resistor is just an option if he isn't using an active preamp with gain or his source won't drive it to full output. I do recommend lowering B+ to 300 and increasing the 6V6's cathode resistor to 330, with the Edcor you should get an 8V drop across primary leaving you with about 292*.042=12 watts plate dissipation. With the 270 cathode resistor and B+ of 305 you will have the 6V6 around 14 watts plate dissipation, 297*.05=14.8 watts. I don't think you will notice any sound difference and the valve will be a lot happier and last longer. Edited November 16, 2015 by xxJPMxx 1 Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 OK, I thought I would just double check the ORIGINAL PS parameters and guess what? The author may not have verified the values of components prior to publishing it. As it sits, (although I don't know the resistance of the HT winding of this particular PT) I am calculating a B+ of LESS THAN 300v. Can either JPM or Maynard check it as well? Also, it seems that the rise-time could be improved as well. Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 Oh and the original design looks like open loop is around 36db and the closed loop is roughly 26db, so 10db of global feedback. This may or may not need compensation, you would have to look at it with a scope and shoot some square waves through it. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 OK, I thought I would just double check the ORIGINAL PS parameters and guess what? The author may not have verified the values of components prior to publishing it. As it sits, (although I don't know the resistance of the HT winding of this particular PT) I am calculating a B+ of LESS THAN 300v. Can either JPM or Maynard check it as well? Also, it seems that the rise-time could be improved as well. Matt. Yes the B+ is roughly 295. C8 can be adjusted to raise or lower B+ and also R14 as stated by author can be raised or lowered. Raising C8 to 100uF gets you 305 B+. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 (edited) I am not sure if it's the 12AT7 (it wasn't designed for audio) or what but the distortion spectrum doesn't look that great, it's dominated by the third harmonic and lots of higher order harmonics. I haven't built either and listened so this is just from simulations. The original circuit shows mainly 2H. Edited November 16, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 Hi JPM, are you sure you have my feedback loop plugged into your sim correctly? The 12AT7 (actually I prefer the 6201 or 5965) is an EXCELLENT driver in this design. This is the circuit I built for most of my kids and friends. In my opinion, the only driver that can eclipse this performance is another pentode. Either the 6AU6, D3a, or E80F, none of which can be simplified enough for "Corey" to just plug into the circuit and go. FWIW, I use a variation of THIS driver topology in my Renaissance4, tweaked to drive the EL84. Corey will be MUCHO HAPPY with this design. Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 Hey Matt, feel free to look over my schematic maybe I made some errors. I will have to dig out the article and look at the math for inverse voltage feedback but I think it only cancels 2H so if the circuit has higher order harmonics they will now dominate. Quote Link to comment Share on other sites More sharing options...
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