Hello all,
I was just wondering if anyone had schematics for a parafeed SET amp (2A3, 6AS7, etc.). There just doesn't seem to be much of anything out there besides the stuff on the Magnequest site. I'm sure those are some great-sounding amps, but I just don't have that kind of money for iron right now!
I seem to remember a 6EM7 parafeed amp floating around on this forum in the past, and that sounds like it's right up my alley. At any rate, I'd appreciate any sort of help or wisdom anyone could share on this matter...
Thanks!
I was just wondering if anyone had schematics for a parafeed SET amp (2A3, 6AS7, etc.). There just doesn't seem to be much of anything out there besides the stuff on the Magnequest site. I'm sure those are some great-sounding amps, but I just don't have that kind of money for iron right now!
I seem to remember a 6EM7 parafeed amp floating around on this forum in the past, and that sounds like it's right up my alley. At any rate, I'd appreciate any sort of help or wisdom anyone could share on this matter...
Thanks!
zman3 said:
You can take any single ended design and make it parafeed. You just need a choke, cap and output transformer. This site gives some good guidelines for choosing the choke and cap. The OPT just needs to have the right impedance ratio (same as it would be for regular single ended, but no gap needed): http://www.siteswithstyle.com/VoltSecond/Parafeed_fun/Parafeed_fun.html
You can maybe get away with more ripple on the B+, as the choke load provides better isolation that a SE transformer, otherwise everything else can remain exactly the same.
Sheldon
Ryssen said:It was a question,and here´s anotheroneWhy twice the B+?
Ok,posted to late...
Full explanation here: http://members.aol.com/sbench/outstru.html
There are some really good replies here, folks - thanks!
To alexq: The 6AS7/6080 amp sounds like a good candidate. I'd love to see the schematics for that one.
Now for another question: Are any changes to the power supply section necessary when converting a conventional amp to parafeed configuration? I ask because I seem to remember reading somewhere that a higher B+ is needed for parafeed amps...
Thanks again!
To alexq: The 6AS7/6080 amp sounds like a good candidate. I'd love to see the schematics for that one.
Now for another question: Are any changes to the power supply section necessary when converting a conventional amp to parafeed configuration? I ask because I seem to remember reading somewhere that a higher B+ is needed for parafeed amps...
Thanks again!
Sheldon said:
That article makes the point that parafeed has the trouble of that "extra" cap in the signal path. This is true, but it ignores all the caps that are in the signal path with single feed, too. This is a little over simplified, but might prove helpful in comparing topologies: http://nutshellhifi.com/library/ETF.html
I made up a simple Parafeed, with a 300B and 6SL7 gain-stage driving a MOSFET source follower I had a +B of 450V and fixed bias, passing 65mA Ia...
I used a Microwave Tx as the choke load, and a 12V toroidal 50VA Tx as the O/P transformer. I used a 47uF cap, as its all I had spare but this is too high, Should be around 8-10uF apparently Motor-Run caps should be a good choice for the cap...
I was quite impressed by the sound, even though the MOT was a little low in inductance at only 19 Henries, the bass response was reasonably flat down to just under 20Hz.... I think that the Parafeed could sound quite magical if done right...
I have no real idea what the advantages of Parafeed would be, apart from economy of the output transformer not passing any DC current therefore not needing 'gapping' etc....
My config as above really calls for a choke of around 25 Henries @ 70-80mA--They say, the higher the Inductance the better--....Need to get some Tx iron and wind some up-- I have calculated, around 3000 turns of 0.30 wire on an EI96 core, 1.5" stack should give around 30-35H --Sometime I'll get round to it!
I used a Microwave Tx as the choke load, and a 12V toroidal 50VA Tx as the O/P transformer. I used a 47uF cap, as its all I had spare but this is too high, Should be around 8-10uF apparently Motor-Run caps should be a good choice for the cap...
I was quite impressed by the sound, even though the MOT was a little low in inductance at only 19 Henries, the bass response was reasonably flat down to just under 20Hz.... I think that the Parafeed could sound quite magical if done right...
I have no real idea what the advantages of Parafeed would be, apart from economy of the output transformer not passing any DC current therefore not needing 'gapping' etc....
My config as above really calls for a choke of around 25 Henries @ 70-80mA--They say, the higher the Inductance the better--....Need to get some Tx iron and wind some up-- I have calculated, around 3000 turns of 0.30 wire on an EI96 core, 1.5" stack should give around 30-35H --Sometime I'll get round to it!
I built Alex Kitic's RH807 amp with a TT21 valve CCS on the plate and a power toroidal as the output transformer. I used a 4uf parafeed cap. I lived with this for about a year and was very happy with the result. What I didn't like was having a wasteful 700V +B supply. I converted it to choke loaded version with a +B of 350V and used a MOT as well. I never quite liked it in this configuration, but couldn't ever define why. I took it to a DIY meet and it wasn't that well received. There was a suggestion that there might be a phase shift problem in the bass registers. I would put this down to the inadequacies of the MOT (ie not enough inductance).
However I would say that a 6080 parafeed amp using MOT would probably work very well as the lower rp will bite harder into the lower inductance and should push any issues into the subsonic range. With the very low output rp I would guess that the parafeed cap would need to be around 10uf. This should all be very doable as the 6080 works very well at low voltage (100V) and high current (100mA) so parts selection would be a lot easier than for a higher +B. If the parafeed cap is sized to small you will get audiable resonance in the low bass which will manifest as bloomy fat bass.
Questions to ask, can you live with just a few watts of output ??
How big a case are you willing to devote to a low wattage amp ??
You may find the RH84 or RH807 with conventional SE iron a cheaper bet in the end.
Hope that helps.
Shoog
However I would say that a 6080 parafeed amp using MOT would probably work very well as the lower rp will bite harder into the lower inductance and should push any issues into the subsonic range. With the very low output rp I would guess that the parafeed cap would need to be around 10uf. This should all be very doable as the 6080 works very well at low voltage (100V) and high current (100mA) so parts selection would be a lot easier than for a higher +B. If the parafeed cap is sized to small you will get audiable resonance in the low bass which will manifest as bloomy fat bass.
Questions to ask, can you live with just a few watts of output ??
How big a case are you willing to devote to a low wattage amp ??
You may find the RH84 or RH807 with conventional SE iron a cheaper bet in the end.
Hope that helps.
Shoog
alexg said:
Hi,
I'm also interested on 6AS7 SE parafeed amp and would like to use the main toroid power TX as OPT. What's your OPT configuration (for 8 ohm load)?
Recently I have the 50VA main toroid - 2 x 115V pri / 2 x 9V sec. It can be configured either both primary and secondary connecting in parallel or series, it would give the same primary inpedance of 1.3K to 8 ohm load. The question is which connection is best? Thanks!
mus said:
I would wire both the Pri and secs in series....
That way, you have the greatest Inductance. The Toroid I used had a single 240V primary and single 12V secondary, which comes out somewhere around 2K5 to 8 ohm. My toroid had an Inductance of 14H with no DC current.
ANY DC current through the toroid will reduce Inductance and seriously affect performance, but this isnt an issue with Parafeed......
I don't have any software to draw my schematic but here it goes:
B+ is around 260VDC. (both driver and 6080 tube)
Driver tube (12at7) is plate choke loaded and with a 220R cathode resistor bypassed by 470uf Nichicon. Grid stopper is 330R.
6080 is plate choke loaded with plate voltage around 250VDC, cathode resistor is 1K, 25W bypassed with 470uf Nichicon. Grid leak is 470K and grid stopper is 1K.
Coupling cap between the driver and 6080 is a 2.2uf Mundorf MCap.
Parafeed capacitor is 10uf polypropylene bypassed by a 1uf polycarbonate (Wima).
OPT is a 30VA 0-230 primary, 2X12V secondary Encapsulated Talema mains toroidal transformer. Primary is connected to the plate of the 6080 and the other end to the parafeed caps and then into ground. The 2X12V secondaries are paralleled for speaker output.
I will draw the schematic by hand and scan it and post it tomorrow.
This amp sounds excellent!
Look for my post of the pictures of my amp in the gallery:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=71300&perpage=25&pagenumber=75
B+ is around 260VDC. (both driver and 6080 tube)
Driver tube (12at7) is plate choke loaded and with a 220R cathode resistor bypassed by 470uf Nichicon. Grid stopper is 330R.
6080 is plate choke loaded with plate voltage around 250VDC, cathode resistor is 1K, 25W bypassed with 470uf Nichicon. Grid leak is 470K and grid stopper is 1K.
Coupling cap between the driver and 6080 is a 2.2uf Mundorf MCap.
Parafeed capacitor is 10uf polypropylene bypassed by a 1uf polycarbonate (Wima).
OPT is a 30VA 0-230 primary, 2X12V secondary Encapsulated Talema mains toroidal transformer. Primary is connected to the plate of the 6080 and the other end to the parafeed caps and then into ground. The 2X12V secondaries are paralleled for speaker output.
I will draw the schematic by hand and scan it and post it tomorrow.
This amp sounds excellent!
Look for my post of the pictures of my amp in the gallery:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=71300&perpage=25&pagenumber=75
Re: using a mot as choke
I used the MOT with both the sec and pri in series. realistically, the primary has only a henry or so inductance, Its the secondary that has the huge number of turns where the main inductance lies. The heater coil I removed, its only three turns of wire anyway.
I was able to remove the grounded end of the secondary and soldered this to one end of the primary so the turns were in the same direction....
Take care, some MOT's use Aluminium wire which cannot be soldered, Break off the small tag into which its fixed off the stampings and solder to that....
This will then isolate the iron from the coils so that the trans. frame will not be at +B when used....
poynton said:
I used the MOT with both the sec and pri in series. realistically, the primary has only a henry or so inductance, Its the secondary that has the huge number of turns where the main inductance lies. The heater coil I removed, its only three turns of wire anyway.
I was able to remove the grounded end of the secondary and soldered this to one end of the primary so the turns were in the same direction....
Take care, some MOT's use Aluminium wire which cannot be soldered, Break off the small tag into which its fixed off the stampings and solder to that....
This will then isolate the iron from the coils so that the trans. frame will not be at +B when used....
site not working. any other links please? i am planning to do a pcl 86 amplifier. Getting a OPT with airgap is not possible in my place. One has to import from china, and that amounts to large shipping expenses. I like the idea to use mains trafo as opt. Is there any schematics for a pcl 86 parafeed?
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