Yeah, still gathering parts. I have all the big expensive stuff bought and ready to go. I still need to buy the caps, resistors etc. My troubles with the Hammond wood case I was going to use set me back to the drawing board. I'm not rushing either, as I would then need to find another hobby.
I still think it's going to be one fabulous amp! Too many guru's had a hand in it for it not to be!
I still think it's going to be one fabulous amp! Too many guru's had a hand in it for it not to be!
Speaking of caps, I'm about to give up on the horrible selection of 500v PS caps. I'm ready to go with doubled up 250V caps instead. Any chance of one last schematic change? I guess "balancing resistors need to be added?
If there's one thing I've learned from participating in diyAudio projects here for 5 years, it is... There's no such thing as "one last schematic change". 😉
I think you are right though, TubeMack, it's a reasonable option to want. I'll try to find a way to fit both variations onto the schematic.
..Todd
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Just stack your lower voltage caps on top of each other, and put 220K/2W (or so) resistors across each cap. You may want to consider 350V-400V caps, as they are slightly more $$ for additional dielectric margin and lower ESR. The composite new cap will have double the voltage rating, half the uf rating, and double the ESR of each cap.
I would look for 500V rated Panasonic TS-UP series at Digi-Key; their stock is very thin these days, but sometimes they have a few in stock. Allied electronics & Newark both sell Cornell Dublier 500V caps also.
I've been studying Taj's excellent schematic (v 0.8b) with an eye to start ordering parts and building. As this will be my first p-2-p build, I'll need all the help I can get!
So, I do have a few questions...
1. Would the turret board construction method be appropriate for this design? Or, what about those phenolic terminal / eyelet strips?
2. On the schematic, R12/C11 and R23/C17 appear to be drawn in faint lines. Is there a reason for this?
3. Would someone please define the values and details of the pots P2, P3 & P4 a little more. This area is a bit vague for my experience, and I want to order the correct parts. Also, once the amp is up and running, just what are these pots supposed to be set at?
4. There seems to be a typo for R32/33. Reads "4K7". I gather it should read 47K?
Thank you to all for your patience and help with these questions.
So, I do have a few questions...
1. Would the turret board construction method be appropriate for this design? Or, what about those phenolic terminal / eyelet strips?
2. On the schematic, R12/C11 and R23/C17 appear to be drawn in faint lines. Is there a reason for this?
3. Would someone please define the values and details of the pots P2, P3 & P4 a little more. This area is a bit vague for my experience, and I want to order the correct parts. Also, once the amp is up and running, just what are these pots supposed to be set at?
4. There seems to be a typo for R32/33. Reads "4K7". I gather it should read 47K?
Thank you to all for your patience and help with these questions.
Hi Neon,
I'm just checking the schematic. (Attached for everyone's reference).
1)Whatever you're comfortable with. You're paving a new path. 🙂 I will probably just use a few terminal strips strategically sprinkled underside as needed.
2) I believe these components are to be used if needed in case of instability.
3) P2 lets you fine tune the tail current of the ECC88 (LTP). P3 and P4 let you adjust the bias of each KT88. I don't think they are critical as to the type. Set P2 to get the current shown on the schematic. I don't recall what the intended KT88 bias current was supposed to be, but I'm sure it's mentioned in this thread somewhere. If you find it, let me know and I'll add it into the schematic. Start all pots 'somewhere in the middle' and measure to adjust.
4) 4K7 for KT88 grid stoppers sounds more reasonable than 48K to me. Typically, it's between 1 and 10K. I think the schematic is correct. [edit...] It just occurred to me that the nomenclature might be the issue here. 4K7 means 4.7K.
..Todd
I'm just checking the schematic. (Attached for everyone's reference).
1)Whatever you're comfortable with. You're paving a new path. 🙂 I will probably just use a few terminal strips strategically sprinkled underside as needed.
2) I believe these components are to be used if needed in case of instability.
3) P2 lets you fine tune the tail current of the ECC88 (LTP). P3 and P4 let you adjust the bias of each KT88. I don't think they are critical as to the type. Set P2 to get the current shown on the schematic. I don't recall what the intended KT88 bias current was supposed to be, but I'm sure it's mentioned in this thread somewhere. If you find it, let me know and I'll add it into the schematic. Start all pots 'somewhere in the middle' and measure to adjust.
4) 4K7 for KT88 grid stoppers sounds more reasonable than 48K to me. Typically, it's between 1 and 10K. I think the schematic is correct. [edit...] It just occurred to me that the nomenclature might be the issue here. 4K7 means 4.7K.
..Todd
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Thanks for the prompt replies!
Taj...I'm sorry I misunderstood the nomenclature of "4K7". Chalk that up to my lack of experience.
Regarding the pots... I built the Millett DCPP. It used small trimmer pots intended for PCB's. I assume the CCS (P2) would use that type. But P4 and P3 probably require something larger that can dissipate more current? Comment?
Continued thanks!
Taj...I'm sorry I misunderstood the nomenclature of "4K7". Chalk that up to my lack of experience.
Regarding the pots... I built the Millett DCPP. It used small trimmer pots intended for PCB's. I assume the CCS (P2) would use that type. But P4 and P3 probably require something larger that can dissipate more current? Comment?
Continued thanks!
Taj: That schematic is looking mighty professional, nice job!
Neon: The bias pots flow very little current.....mostly the -80V to ground through the string of R's..no current flows to the grid unless the grid voltage goes positive (which it shouldn't since this is a class AB1 design), so you can use trimmer pots (Tubelab's SE uses trimmers for bias....) or fancy panel mount ones. I would use panel mount ones so that you can adjust the bias without flipping the amp over. You could also use trimmers and a hole in the top deck for your screwdriver; you just have to line the hole up with the pot.
Edit: The bias pots see 80v across 150K ohms, so I=E/R & the current through them is .53ma P=I squared times R (.00053*.00053*50000), so the pot is dissipating .014W.
If you want the convenience of adjusting bias from up top, you also need to put a tip jack or banana jack on the deck on the non-grounded side of each 10R cathode resistor (TP-1 &TP-2). It will be at less than 1V during operation, so you can use anything you want for that.
Also, before sticking the output tubes in, it's wise to adjust the bias pots for max negative voltage, that will assure that the output tubes are not flowing any current (they are in cutoff) when you first fire it up.
A far as current through the output tubes, I would start at 65-75 ma or so, just don't exceed 40W dissipation through the tubes (B+ times current). 65ma=.65V across the 10 ohm R, 75ma=.75V, etc.
Neon: The bias pots flow very little current.....mostly the -80V to ground through the string of R's..no current flows to the grid unless the grid voltage goes positive (which it shouldn't since this is a class AB1 design), so you can use trimmer pots (Tubelab's SE uses trimmers for bias....) or fancy panel mount ones. I would use panel mount ones so that you can adjust the bias without flipping the amp over. You could also use trimmers and a hole in the top deck for your screwdriver; you just have to line the hole up with the pot.
Edit: The bias pots see 80v across 150K ohms, so I=E/R & the current through them is .53ma P=I squared times R (.00053*.00053*50000), so the pot is dissipating .014W.
If you want the convenience of adjusting bias from up top, you also need to put a tip jack or banana jack on the deck on the non-grounded side of each 10R cathode resistor (TP-1 &TP-2). It will be at less than 1V during operation, so you can use anything you want for that.
Also, before sticking the output tubes in, it's wise to adjust the bias pots for max negative voltage, that will assure that the output tubes are not flowing any current (they are in cutoff) when you first fire it up.
A far as current through the output tubes, I would start at 65-75 ma or so, just don't exceed 40W dissipation through the tubes (B+ times current). 65ma=.65V across the 10 ohm R, 75ma=.75V, etc.
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Chokes?
I have just about all the parts I need to get started, but have a question about L2. I have a couple of 5H / 200ma chokes on hand. Would these be ok instead of the spec'd 2.5 / 300ma ?
TIA
I have just about all the parts I need to get started, but have a question about L2. I have a couple of 5H / 200ma chokes on hand. Would these be ok instead of the spec'd 2.5 / 300ma ?
TIA
Good question. All *I* know is that our total idle current is about 150mA, but I don't know how to figure in the AC (signal) current draw. I seem to recall a figure of 250mA as a minimum from our previous discussions about that choke.
I hope someone can pitch in and explain how to estimate or calculate that.
..Todd
I hope someone can pitch in and explain how to estimate or calculate that.
..Todd
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Neon,
The 5H /200mA choke you have will be fine for this amp. You base it off of idle current. That is all mine is rated for and I run AB2 [not often ;-)> ]
Remember the caps are your there for the peaks.
Neon, if you are doing this p2p I proposed a way to lay layout the parts earlier in this thread that should help. It is just one of many ways to do it, but I know the parts fit that way
The 5H /200mA choke you have will be fine for this amp. You base it off of idle current. That is all mine is rated for and I run AB2 [not often ;-)> ]
Remember the caps are your there for the peaks.
Neon, if you are doing this p2p I proposed a way to lay layout the parts earlier in this thread that should help. It is just one of many ways to do it, but I know the parts fit that way
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OK...I think that should do the questions, at least for a while. Thank you all for the prompt and courteous replies.
Neon: Referring to MJ VA 3rd ed. Page 314, the chokes need to handle ripple current plus the idle current, and the formula to calculate ripple current is Vin(rms)/1386L for 60hz mains.
So, using the above we get 465/(1386*(5))=67ma for ripple current using 5H chokes.
adding that to the idle current (assume 75ma X 2 for the output tubes +16.5ma for the ECC88 & EF86)=67+150+16.5=234ma.
...at 65ma idle current,t the total current is 214ma...
That's why 300ma chokes were spec'd.
Although the ones you have are a little light on current rating, there is no harm using them; if the ripple voltage/hum is unacceptable, you can always procure larger chokes later. Since Sgregory is using 200ma rated chokes without issue I would use what you got........Are they Triad C14x chokes? I have a pair of these and they appear to be quite beefy.
Boywonder, I could be wrong but that calculation is for a choke loaded (LC) power supply. In a cap loaded (CLC), the choke isn't as loaded by the ripple current and has a much easier job.
It never hurts to have a little more capacity but since the chokes are on hand I would certainly try them. If they get hot or buzz, then you know they are too small.
P.S. I should mention that while I can run 75mA bias it is at the limit of my PT so I typically run 65mA. IN either case I have no issue with my chokes ( Electra Print )
It never hurts to have a little more capacity but since the chokes are on hand I would certainly try them. If they get hot or buzz, then you know they are too small.
P.S. I should mention that while I can run 75mA bias it is at the limit of my PT so I typically run 65mA. IN either case I have no issue with my chokes ( Electra Print )
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