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#1
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10 watt PP amplifier
Hi, vacuumlanders on RAT.
I've just about finished my homebrew "Scratch 10" amplifier, so-called because it's based on the bits I had lying around or could scrounge up for next to nothing. Power transformer and chassis from John Bartley (again, many thanks), Hammond "468" OPT from a garage sale, about 50 cents (what a find! This sparked the project.) Tubes from spares bin (but I had to buy one, the EF86.) I've posted the schematic on "alt.binaries.schematics.electronics", plus a photo (under same Subject name.) Here are some "design notes", well, general comments about what happened... g 1. I decided to use 2 x 6V6 O/P tubes as I had them, I could then experiment with some 6K6's and 6F6's also to hand. BTW, OPT is 10,000 ohms CT to 4, 8 & 15 ohms, a Hammond "468", crinkle finish paint. It is marked "4, 8, 15 ohms" at the terminals but in the 1956 Hammond catalogue it's shown as "3.2, 6.4, 10 ohms". 2. Phase splitter is the venerable 6SN7 since: a) I had several of them, and b) an octal tube base was on the scrap chassis used. 3. The first stage is an EF86. Reasons: a) lots of gain, b) low noise and microphony, c) I wanted to use the "Mullard 5-10" design here as I'd used it before very successfully, and d) there was also a B9A tube base on the scrap chassis! g Until then I had been planning a 6SN7 cascode circuit (maybe try later...) 4. Phase splitter: I wanted to use the 6SN7 as a long tailed pair but, since it's a low gain tube, I wanted the highest cathode impedance possible for good balance. However, I also wanted direct coupling from the EF86 plate to g1 of the 6SN7 but I was also a bit "short of some B+" headroom" for a large cathode resistor (no negative bias supply to hand and disinclined to build one!) I finally used a MJE340 power transistor constant current circuit in the cathodes, set to 4.2 mA (2.1 mA per tube.) Seems to work. No lack of PP drive, well balanced and no clipping up to over full power (in a no NFB test.) 5. I chose a choke input P/S since: a) I had one, and b) the power transformer was a bit too high voltage at 425-0-425, so choke input was a Q&D way to get a well regulated 315 VDC or so B+ for the 6V6's. This is well within rating since there's 17 volts of cathode bias. BTW, rectifier is a 5V4 but anything indirectly heated and big enough will do (forget expensive GZ34's!) 6. Directly coupling the 6SN7 puts the 109 VDC or so EF86 plate voltage on the 6SN7 g1's giving about 114 VDC on the cathodes at 2 mA, or so. Recommendations are max 90 VDC heater cathode voltage, didn't want to violate that. So, I connected the heater 6.3 VAC supply CT to a divider from B+ to get about +25 VDC (well decoupled.) Then, I used that as the voltage reference on the constant current power transistor. I had planned to use a zener diode here but did not have the right one to hand, so I just tried it "as is" - it works. The B+ droop under continuous maximum power is very small so the 25 volts does not move much (I may zener it later.) 7. O/P tubes are cathode biased: a) to drop a few B+ volts, b) I could not be bothered to build a bias supply and, anyway, I could not find a 5K balance pot in my spares box! Now I've found one, so I may do this later to allow a better-balanced O/P stage and push the 6V6's a bit harder. 8. NFB. Amplifier works well with no NFB (on the 'scope that is, not listened to!) It clips at just under 10 watts RMS continuous (9.6 watts) and has a good square wave response, a small droop on 60 Hz square wave. I've played with NFB and settled on a 22K f/b resistor from the 15 ohm tap bypassed by 220 pF. 9. Sounds good. Performance (with NFB as stated): a) Max power at 1 KHz = 9.6 watts RMS, continuous (just below clipping) If I up the B+ to 325 (by using 120 VAC supply as opposed to the rated 115 VAC) I can get 10.4 watts but the heaters are then at 6.57 VAC - not so good. b) Distortion: can't measure it (no instrument), can't see it on the scope (below clipping, that is), and, the good news, I can't hear it! (Cloth ears, you say!) c) Power bandwidth: from below 20 Hz to 30,000 KHz +/- 1.5 dB at 1.67 watts RMS. Actually, at 20 Hz the output is barely 1 dB down and around 17 KHz about 2 dB up, but taking it lower, below 16 Hz where it really rolls off, the waveform distorts badly. d) Hum and noise about 20 mV across 15 ohms with input shorted. 10. Replacing the 6V6's with 6K6's reduces the maximum power to around 6 watts RMS, all other figures about the same. I've not tried the 6F6's yet. I suspect the OPT is the wrong ratio for 6L6's. So, what do people think? How might the circuit be improved? All ideas for improvements welcomed. Disclaimer: don't build the "Scratch 10" as shown until the design is finally optimized. Or build it and improve on it. Cheers, Roger |
#2
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10 watt PP amplifier
Engineer wrote:
Hi, vacuumlanders on RAT. I've just about finished my homebrew "Scratch 10" amplifier, so-called because it's based on the bits I had lying around or could scrounge up for next to nothing. Power transformer and chassis from John Bartley (again, many thanks), Hammond "468" OPT from a garage sale, about 50 cents (what a find! This sparked the project.) Tubes from spares bin (but I had to buy one, the EF86.) I've posted the schematic on "alt.binaries.schematics.electronics", plus a photo (under same Subject name.) Here are some "design notes", well, general comments about what happened... g 1. I decided to use 2 x 6V6 O/P tubes as I had them, I could then experiment with some 6K6's and 6F6's also to hand. BTW, OPT is 10,000 ohms CT to 4, 8 & 15 ohms, a Hammond "468", crinkle finish paint. It is marked "4, 8, 15 ohms" at the terminals but in the 1956 Hammond catalogue it's shown as "3.2, 6.4, 10 ohms". 2. Phase splitter is the venerable 6SN7 since: a) I had several of them, and b) an octal tube base was on the scrap chassis used. 3. The first stage is an EF86. Reasons: a) lots of gain, b) low noise and microphony, c) I wanted to use the "Mullard 5-10" design here as I'd used it before very successfully, and d) there was also a B9A tube base on the scrap chassis! g Until then I had been planning a 6SN7 cascode circuit (maybe try later...) 4. Phase splitter: I wanted to use the 6SN7 as a long tailed pair but, since it's a low gain tube, I wanted the highest cathode impedance possible for good balance. However, I also wanted direct coupling from the EF86 plate to g1 of the 6SN7 but I was also a bit "short of some B+" headroom" for a large cathode resistor (no negative bias supply to hand and disinclined to build one!) I finally used a MJE340 power transistor constant current circuit in the cathodes, set to 4.2 mA (2.1 mA per tube.) Seems to work. No lack of PP drive, well balanced and no clipping up to over full power (in a no NFB test.) 5. I chose a choke input P/S since: a) I had one, and b) the power transformer was a bit too high voltage at 425-0-425, so choke input was a Q&D way to get a well regulated 315 VDC or so B+ for the 6V6's. This is well within rating since there's 17 volts of cathode bias. BTW, rectifier is a 5V4 but anything indirectly heated and big enough will do (forget expensive GZ34's!) 6. Directly coupling the 6SN7 puts the 109 VDC or so EF86 plate voltage on the 6SN7 g1's giving about 114 VDC on the cathodes at 2 mA, or so. Recommendations are max 90 VDC heater cathode voltage, didn't want to violate that. So, I connected the heater 6.3 VAC supply CT to a divider from B+ to get about +25 VDC (well decoupled.) Then, I used that as the voltage reference on the constant current power transistor. I had planned to use a zener diode here but did not have the right one to hand, so I just tried it "as is" - it works. The B+ droop under continuous maximum power is very small so the 25 volts does not move much (I may zener it later.) 7. O/P tubes are cathode biased: a) to drop a few B+ volts, b) I could not be bothered to build a bias supply and, anyway, I could not find a 5K balance pot in my spares box! Now I've found one, so I may do this later to allow a better-balanced O/P stage and push the 6V6's a bit harder. 8. NFB. Amplifier works well with no NFB (on the 'scope that is, not listened to!) It clips at just under 10 watts RMS continuous (9.6 watts) and has a good square wave response, a small droop on 60 Hz square wave. I've played with NFB and settled on a 22K f/b resistor from the 15 ohm tap bypassed by 220 pF. 9. Sounds good. Performance (with NFB as stated): a) Max power at 1 KHz = 9.6 watts RMS, continuous (just below clipping) If I up the B+ to 325 (by using 120 VAC supply as opposed to the rated 115 VAC) I can get 10.4 watts but the heaters are then at 6.57 VAC - not so good. b) Distortion: can't measure it (no instrument), can't see it on the scope (below clipping, that is), and, the good news, I can't hear it! (Cloth ears, you say!) c) Power bandwidth: from below 20 Hz to 30,000 KHz +/- 1.5 dB at 1.67 watts RMS. Actually, at 20 Hz the output is barely 1 dB down and around 17 KHz about 2 dB up, but taking it lower, below 16 Hz where it really rolls off, the waveform distorts badly. d) Hum and noise about 20 mV across 15 ohms with input shorted. 10. Replacing the 6V6's with 6K6's reduces the maximum power to around 6 watts RMS, all other figures about the same. I've not tried the 6F6's yet. I suspect the OPT is the wrong ratio for 6L6's. So, what do people think? How might the circuit be improved? All ideas for improvements welcomed. Disclaimer: don't build the "Scratch 10" as shown until the design is finally optimized. Or build it and improve on it. Cheers, Roger Can't see your circuit yet on ABSE even though I'm sure you are only a few miles from here. But the description makes a lot of sense in consideration of what you had on hand! I like that kind of design. Cheers, John Stewart |
#3
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10 watt PP amplifier
"John Stewart" wrote in message
... Engineer wrote: Hi, vacuumlanders on RAT. I've just about finished my homebrew "Scratch 10" amplifier, so-called because it's based on the bits I had lying around or could scrounge up for next to nothing. Power transformer and chassis from John Bartley (again, many thanks), Hammond "468" OPT from a garage sale, about 50 cents (what a find! This sparked the project.) Tubes from spares bin (but I had to buy one, the EF86.) I've posted the schematic on "alt.binaries.schematics.electronics", plus a photo (under same Subject name.) (snip) Cheers, Roger Can't see your circuit yet on ABSE even though I'm sure you are only a few miles from here. But the description makes a lot of sense in consideration of what you had on hand! I like that kind of design. Cheers, John Stewart Hi, John, thanks for reading the long spiel! I checked my access to ABSE and saw the schematic along with one photo. I am based in Thornhill (just north of Toronto, Canada) but was not aware that there was any distance issue with Internet NG access. Anyway, if anyone wants the schematic just let me know at "analogdino 'at' rogers 'dot" com" and I'll email it as an attachment. Cheers, Roger |
#4
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10 watt PP amplifier
"Engineer" wrote in message ... : I checked my access to ABSE and saw the schematic along with one : photo. : I am based in Thornhill (just north of Toronto, Canada) but was not : aware that there was any distance issue with Internet NG access. : Anyway, if anyone wants the schematic just let me know at "analogdino : 'at' rogers 'dot" com" and I'll email it as an attachment. : Cheers, : Roger : Heh. Not a distance issue, but many ISP's use proxy servers for (some of the) newsgroups. Depending on the refreshing policy at the ISP, NG's may 'lag' a bit :-) R. |
#5
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10 watt PP amplifier
Hi,
I got your ABSE post .... here in France, far away from Toronto ;) Yves "Ruud Broens" a écrit dans le message de news: ... "Engineer" wrote in message ... : I checked my access to ABSE and saw the schematic along with one : photo. : I am based in Thornhill (just north of Toronto, Canada) but was not : aware that there was any distance issue with Internet NG access. : Anyway, if anyone wants the schematic just let me know at "analogdino : 'at' rogers 'dot" com" and I'll email it as an attachment. : Cheers, : Roger : Heh. Not a distance issue, but many ISP's use proxy servers for (some of the) newsgroups. Depending on the refreshing policy at the ISP, NG's may 'lag' a bit :-) R. |
#6
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10 watt PP amplifier
"Yves Monmagnon" wrote in message
... Hi, I got your ABSE post .... here in France, far away from Toronto ;) Yves Salut, mon ami! Roger le "analogdino" (couper au plus court) |
#7
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10 watt PP amplifier
Engineer wrote:
Hi, vacuumlanders on RAT. I've just about finished my homebrew "Scratch 10" amplifier, so-called because it's based on the bits I had lying around or could scrounge up for next to nothing. Power transformer and chassis from John Bartley (again, many thanks), Hammond "468" OPT from a garage sale, about 50 cents (what a find! This sparked the project.) Tubes from spares bin (but I had to buy one, the EF86.) I've posted the schematic on "alt.binaries.schematics.electronics", plus a photo (under same Subject name.) Here are some "design notes", well, general comments about what happened... g 1. I decided to use 2 x 6V6 O/P tubes as I had them, I could then experiment with some 6K6's and 6F6's also to hand. BTW, OPT is 10,000 ohms CT to 4, 8 & 15 ohms, a Hammond "468", crinkle finish paint. It is marked "4, 8, 15 ohms" at the terminals but in the 1956 Hammond catalogue it's shown as "3.2, 6.4, 10 ohms". 2. Phase splitter is the venerable 6SN7 since: a) I had several of them, and b) an octal tube base was on the scrap chassis used. 3. The first stage is an EF86. Reasons: a) lots of gain, b) low noise and microphony, c) I wanted to use the "Mullard 5-10" design here as I'd used it before very successfully, and d) there was also a B9A tube base on the scrap chassis! g Until then I had been planning a 6SN7 cascode circuit (maybe try later...) 4. Phase splitter: I wanted to use the 6SN7 as a long tailed pair but, since it's a low gain tube, I wanted the highest cathode impedance possible for good balance. However, I also wanted direct coupling from the EF86 plate to g1 of the 6SN7 but I was also a bit "short of some B+" headroom" for a large cathode resistor (no negative bias supply to hand and disinclined to build one!) I finally used a MJE340 power transistor constant current circuit in the cathodes, set to 4.2 mA (2.1 mA per tube.) Seems to work. No lack of PP drive, well balanced and no clipping up to over full power (in a no NFB test.) 5. I chose a choke input P/S since: a) I had one, and b) the power transformer was a bit too high voltage at 425-0-425, so choke input was a Q&D way to get a well regulated 315 VDC or so B+ for the 6V6's. This is well within rating since there's 17 volts of cathode bias. BTW, rectifier is a 5V4 but anything indirectly heated and big enough will do (forget expensive GZ34's!) 6. Directly coupling the 6SN7 puts the 109 VDC or so EF86 plate voltage on the 6SN7 g1's giving about 114 VDC on the cathodes at 2 mA, or so. Recommendations are max 90 VDC heater cathode voltage, didn't want to violate that. So, I connected the heater 6.3 VAC supply CT to a divider from B+ to get about +25 VDC (well decoupled.) Then, I used that as the voltage reference on the constant current power transistor. I had planned to use a zener diode here but did not have the right one to hand, so I just tried it "as is" - it works. The B+ droop under continuous maximum power is very small so the 25 volts does not move much (I may zener it later.) 7. O/P tubes are cathode biased: a) to drop a few B+ volts, b) I could not be bothered to build a bias supply and, anyway, I could not find a 5K balance pot in my spares box! Now I've found one, so I may do this later to allow a better-balanced O/P stage and push the 6V6's a bit harder. 8. NFB. Amplifier works well with no NFB (on the 'scope that is, not listened to!) It clips at just under 10 watts RMS continuous (9.6 watts) and has a good square wave response, a small droop on 60 Hz square wave. I've played with NFB and settled on a 22K f/b resistor from the 15 ohm tap bypassed by 220 pF. 9. Sounds good. Performance (with NFB as stated): a) Max power at 1 KHz = 9.6 watts RMS, continuous (just below clipping) If I up the B+ to 325 (by using 120 VAC supply as opposed to the rated 115 VAC) I can get 10.4 watts but the heaters are then at 6.57 VAC - not so good. b) Distortion: can't measure it (no instrument), can't see it on the scope (below clipping, that is), and, the good news, I can't hear it! (Cloth ears, you say!) c) Power bandwidth: from below 20 Hz to 30,000 KHz +/- 1.5 dB at 1.67 watts RMS. Actually, at 20 Hz the output is barely 1 dB down and around 17 KHz about 2 dB up, but taking it lower, below 16 Hz where it really rolls off, the waveform distorts badly. d) Hum and noise about 20 mV across 15 ohms with input shorted. 10. Replacing the 6V6's with 6K6's reduces the maximum power to around 6 watts RMS, all other figures about the same. I've not tried the 6F6's yet. I suspect the OPT is the wrong ratio for 6L6's. So, what do people think? How might the circuit be improved? All ideas for improvements welcomed. Disclaimer: don't build the "Scratch 10" as shown until the design is finally optimized. Or build it and improve on it. Cheers, Roger is there a possibillity that you could post the schematic diagram & picture on a web page instead of the binaries group. i dont have access to any binaries groups. -- "The philosophy behind Free, Open Source Software has parallels in several cultures which practice ethical cooperaton. Perhaps that's why it threatens unethical business practices which feed on the unknowing." -- Bassam A. Hassan. CoFounder of the ILUG, Vice president of the former IOSO, |
#8
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10 watt PP amplifier
"fatboy" wrote in message
... Engineer wrote: Hi, vacuumlanders on RAT. I've just about finished my homebrew "Scratch 10" amplifier, so-called because it's based on the bits I had lying around or could scrounge up for next to nothing. Power transformer and chassis from John Bartley (again, many thanks), Hammond "468" OPT from a garage sale, about 50 cents (what a find! This sparked the project.) Tubes from spares bin (but I had to buy one, the EF86.) (snip) Cheers, Roger is there a possibillity that you could post the schematic diagram & picture on a web page instead of the binaries group. i dont have access to any binaries groups. Not at this time, but I'm planning to do a page later. I'll send it to you as an email attachment if you like. Your " sounds fake! Send your email address to me at "analogdino 'at' rogers 'dot' com". Cheers, Roger |
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