Hi!
Unfortunately I couldn't make it to the ETF this year and I missed all the good talk and equipment...
I was wondering about two potential issues with this circuit:
To which potential do you bias the heaters? I always disliked large voltage differences between heater and cathode, especially large signal swing. Not necessarily an issue with all circuits and tube types but a potential source of problems which I prefer to avoid.
Second is the signal swing which is imposed on the secondary of the input transformer. Not sure if that might be of any relevance. At least there is a shunt capacitance from grid of U2 due to the capacitance between the windings of the transformer. The impact of this depends on the quality of transformer I guess.
has this been adressed by JC?
Best regards
Thomas
Unfortunately I couldn't make it to the ETF this year and I missed all the good talk and equipment...
I was wondering about two potential issues with this circuit:
To which potential do you bias the heaters? I always disliked large voltage differences between heater and cathode, especially large signal swing. Not necessarily an issue with all circuits and tube types but a potential source of problems which I prefer to avoid.
Second is the signal swing which is imposed on the secondary of the input transformer. Not sure if that might be of any relevance. At least there is a shunt capacitance from grid of U2 due to the capacitance between the windings of the transformer. The impact of this depends on the quality of transformer I guess.
has this been adressed by JC?
Best regards
Thomas
JC's Circuit sounds like Moth Audio's Positive Ground Amp
Interestingly:
In Vacuum Tube Valley Issue 17, a review described Moth Audio's (now out of business) positive ground amplifier. I quote "Instead of supplying B+ to the plate via the output transformer as conventional SE amps do, here power (B+) is supplied to the cathode of the driver and output tube, and the plates are tied to ground." By the way, I think the reviewer should have said B- instead of B+. The driver tube, a 6AN4 used "a reactance drive circuit" (inductor) to drive a 300B output.
Sounds remarkably similar to JC's concept and if you look at the drawing on the blog site of his implementation of a choke load instead of a CCS on the driver, (http://i102.photobucket.com/albums/m83/jonjinkim/ETF 2010/IMAG0322.jpg) you get the Moth amp in concept. Moth Audio reckoned that there was a patent pending but several searches never revealed such. Usual manufacturerer's scare tactic to avoid forking out the money for a patent or perhaps knowing someone else did it before but forgotten in the mists of time.
This post is not meant to dish JC but to increase the knowledge pool on other implementations. JC himself says, it probably has been done before. He and I go way back, some 18 years with much correspondence over a WE437/VT25 amp. I have the greatest respect for the man. It would be a cool project!
Ejam
Interestingly:
In Vacuum Tube Valley Issue 17, a review described Moth Audio's (now out of business) positive ground amplifier. I quote "Instead of supplying B+ to the plate via the output transformer as conventional SE amps do, here power (B+) is supplied to the cathode of the driver and output tube, and the plates are tied to ground." By the way, I think the reviewer should have said B- instead of B+. The driver tube, a 6AN4 used "a reactance drive circuit" (inductor) to drive a 300B output.
Sounds remarkably similar to JC's concept and if you look at the drawing on the blog site of his implementation of a choke load instead of a CCS on the driver, (http://i102.photobucket.com/albums/m83/jonjinkim/ETF 2010/IMAG0322.jpg) you get the Moth amp in concept. Moth Audio reckoned that there was a patent pending but several searches never revealed such. Usual manufacturerer's scare tactic to avoid forking out the money for a patent or perhaps knowing someone else did it before but forgotten in the mists of time.
This post is not meant to dish JC but to increase the knowledge pool on other implementations. JC himself says, it probably has been done before. He and I go way back, some 18 years with much correspondence over a WE437/VT25 amp. I have the greatest respect for the man. It would be a cool project!
Ejam
The schematic that Jaap posted makes much better practical sense than the earlier B- version.
No input xfmr capacitance loading issue, the pentode is actually running as a pentode here, and a CF is used to handle the high Z output from the 1st high gain stage. Only thing missing is a gyrator mod to the Mu follower/CCS, or some type of servo feedback to the pentode screen, to keep the idle output voltage of the 1st stage under control. Looks like the Aikido type CF stage got used for PSRR.
No input xfmr capacitance loading issue, the pentode is actually running as a pentode here, and a CF is used to handle the high Z output from the 1st high gain stage. Only thing missing is a gyrator mod to the Mu follower/CCS, or some type of servo feedback to the pentode screen, to keep the idle output voltage of the 1st stage under control. Looks like the Aikido type CF stage got used for PSRR.
Hi Yves,
Maybe JCM just showed that to excite and implant some "free your mind" type thoughts in the audience. His whole lecture was spiced with such fodder for thought. The usual textbook concept of (signal-) ground and algebraic signed supply voltages is a rather arbitrary one, I am sure you would agree.
Tom
Maybe JCM just showed that to excite and implant some "free your mind" type thoughts in the audience. His whole lecture was spiced with such fodder for thought. The usual textbook concept of (signal-) ground and algebraic signed supply voltages is a rather arbitrary one, I am sure you would agree.
Tom
hey all, thanks for checking this out. thomas asks a fair question and the answer, on the surface, is a bit dodgy. it all depends on the application. i have to say that just asking this question indicates a much better knowledge of tubes than most. i will start by saying one thing: grounding the heater is never a good idea... it should be lifted or dropped, or left floating, always. get it away from ground. the heater draws 10 - 100 times the current of the amplifier stage... you want that circulating on ground?
as to your question, is it a preamp you are talking about? drop the heater to the level of cathode of the first stage... you're done. it will be -150 to -70. the swing is a volt or two, max. you think the parasitic C of the trans is any different floating than fixed?
is it an amp with a large signal swing? you need to think about it. a 6922 is not a good choice for a 300B amp... 50V max heater to cathode V, 100Vpp. you need 180PP to drive the output tube. your question is spot on.
a 12AU7 (and nearly all other commercial small signal tubes) has 100V heater to cathode rating (200Vpp). you're at the hairy edge here. ever consider a directly heated tube? a 2A3? no problem. TV sweep tube? doesn't even notice... etc. 6L6GC? you get the idea.
many people are blissfully unaware of the parasitic diode that exists between the heater and cathode sleeve in an indirectly heated oxide coated vacuum tube. and even fewer know what it does when you exceed this rating. the heater is made from nichrome variants and is smothered in aluminum oxide before it is pressed into the nickel sleeve of the cathode. these chemicals are always good quality, but there are always traces of other metals and semi-metals. its nearly impossible to get it 100%, not commercially anyway. not even WE.
the work function of nichrome is extremely poor, but it is very hot. 1100+ kelvin, roughly. at that temp, even nichrome has an emission. if you get the voltage potential high enough (exceed the heater to cathode voltage rating) , ions are drawn to the cathode, through the "insulating" schmutz. as someone once said, "everything is electrical" and that goes for insulators too. the few (large) positive ions that are freed can "poison" the environment of the vacuum tube with pure evaporated metals. one positive ion can do a lot more damage than years of high voltage acceleration of the negative ones... the common ones are aluminum, molybdenum, and of course, silicon (effing bad). the temp of the interior is not hot enough to make nickel or iron gas, but these other lovely things make their way to the other side of the cathode where they form an "interface layer", a semi conducting or outright insulating layer of stuff on the active surface of the cathode. emission goes down, noise goes up, and the tube dies. this is also quaintly known as "cathode poisoning". makes you wonder about marshall and mesa boogie using cathodyne phase splitters with grounded heaters and 260 volts at the cathode of one section of a 12AX7... hmmm. do they ever read a tube manual? oh yeah, that's a guitar amp. if you ever wondered why the splitter/tone stack tube dies out with one bad section, that's always the same section... oh yeah, its those damned russian 12AX7s.
anyway, the circuit is fun and makes it possible to do some very simple dc circuits. if you do phono, the trans is a great help because you can maximize noise performance, block hum, and get a built in rumble filter with one handy part. of course it has to be good. you want to use bad parts?
thanks for checking it out...
your,
jc
i digress.
as to your question, is it a preamp you are talking about? drop the heater to the level of cathode of the first stage... you're done. it will be -150 to -70. the swing is a volt or two, max. you think the parasitic C of the trans is any different floating than fixed?
is it an amp with a large signal swing? you need to think about it. a 6922 is not a good choice for a 300B amp... 50V max heater to cathode V, 100Vpp. you need 180PP to drive the output tube. your question is spot on.
a 12AU7 (and nearly all other commercial small signal tubes) has 100V heater to cathode rating (200Vpp). you're at the hairy edge here. ever consider a directly heated tube? a 2A3? no problem. TV sweep tube? doesn't even notice... etc. 6L6GC? you get the idea.
many people are blissfully unaware of the parasitic diode that exists between the heater and cathode sleeve in an indirectly heated oxide coated vacuum tube. and even fewer know what it does when you exceed this rating. the heater is made from nichrome variants and is smothered in aluminum oxide before it is pressed into the nickel sleeve of the cathode. these chemicals are always good quality, but there are always traces of other metals and semi-metals. its nearly impossible to get it 100%, not commercially anyway. not even WE.
the work function of nichrome is extremely poor, but it is very hot. 1100+ kelvin, roughly. at that temp, even nichrome has an emission. if you get the voltage potential high enough (exceed the heater to cathode voltage rating) , ions are drawn to the cathode, through the "insulating" schmutz. as someone once said, "everything is electrical" and that goes for insulators too. the few (large) positive ions that are freed can "poison" the environment of the vacuum tube with pure evaporated metals. one positive ion can do a lot more damage than years of high voltage acceleration of the negative ones... the common ones are aluminum, molybdenum, and of course, silicon (effing bad). the temp of the interior is not hot enough to make nickel or iron gas, but these other lovely things make their way to the other side of the cathode where they form an "interface layer", a semi conducting or outright insulating layer of stuff on the active surface of the cathode. emission goes down, noise goes up, and the tube dies. this is also quaintly known as "cathode poisoning". makes you wonder about marshall and mesa boogie using cathodyne phase splitters with grounded heaters and 260 volts at the cathode of one section of a 12AX7... hmmm. do they ever read a tube manual? oh yeah, that's a guitar amp. if you ever wondered why the splitter/tone stack tube dies out with one bad section, that's always the same section... oh yeah, its those damned russian 12AX7s.
anyway, the circuit is fun and makes it possible to do some very simple dc circuits. if you do phono, the trans is a great help because you can maximize noise performance, block hum, and get a built in rumble filter with one handy part. of course it has to be good. you want to use bad parts?
thanks for checking it out...
your,
jc
i digress.
In that circuit I would connect the heater circuit to one of the cathodes, but I'm not certain which one.
U1 cathode would prevent the heater-cathode insulation from forming part of the load for U1 - h-k insulation is never perfect so you don't want a non-linear resistance from the cathode to ground. Normally, grounding a heater circuit is a sensible thing to do but not here. Note that grounding a heater circuit is not the same thing as injecting heater current into the ground, unless you do it wrongly.
U2 cathode would make it harder for the heater circuit to inject noise, as this is a low impedance point. So the choice is: do you prefer noise or distortion?
I might read nanana's post but he seems to have mislaid the shift key.
U1 cathode would prevent the heater-cathode insulation from forming part of the load for U1 - h-k insulation is never perfect so you don't want a non-linear resistance from the cathode to ground. Normally, grounding a heater circuit is a sensible thing to do but not here. Note that grounding a heater circuit is not the same thing as injecting heater current into the ground, unless you do it wrongly.
U2 cathode would make it harder for the heater circuit to inject noise, as this is a low impedance point. So the choice is: do you prefer noise or distortion?
I might read nanana's post but he seems to have mislaid the shift key.
Agreed 1000%
Just remind me some sneering laugh (literally from Harrap's) when i suggested (long ago !) to "ground" the anodes of a 4CX250 Audio Push Pull in order to simplify heat evacuation
Sure, in that situation heaters can no longer be grounded
Found it
Dated January 2004 Yves.
Attachments
Hi JC,
thanks for your reply. I regret that I couldn't make it to the ETF this year.
As for referencing the heater: I routinely bias the heater slightly positive above the cathode, around 20V or so. Exactly for the purpose of biasing the parasitic diode from cathode to heater, so that it doesn't conduct. I avoid larger differences in the order of 100V, even if it is within tube ratings. Might work out for some tubes, but many develop some problems after some time with such voltages between heater an cathode.
I was not sure if the circuit shown was for preamp levels or if it shows a driver and power output stage. In the latter case, the signal swing imposed on the sdecondary of the input transformer could be in the order of 50 to 100V RMS, depending on the voltage swing required on the grid of the second tube. I agree, if we're talking a Volt or so, I wouldn't worry about this.
Hope to meet you again soon, I guess latest for next years high end show in Munich?
Best regards
Thomas
thanks for your reply. I regret that I couldn't make it to the ETF this year.
As for referencing the heater: I routinely bias the heater slightly positive above the cathode, around 20V or so. Exactly for the purpose of biasing the parasitic diode from cathode to heater, so that it doesn't conduct. I avoid larger differences in the order of 100V, even if it is within tube ratings. Might work out for some tubes, but many develop some problems after some time with such voltages between heater an cathode.
I was not sure if the circuit shown was for preamp levels or if it shows a driver and power output stage. In the latter case, the signal swing imposed on the sdecondary of the input transformer could be in the order of 50 to 100V RMS, depending on the voltage swing required on the grid of the second tube. I agree, if we're talking a Volt or so, I wouldn't worry about this.
Hope to meet you again soon, I guess latest for next years high end show in Munich?
Best regards
Thomas
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Jean-C,
you planning to stop by here more often, or sticking to the 6-year Houdini act ?
www.diyaudio.com/forums/introductions/47809-i-am-new-here-2.html#post542199
Eer, i've got an idea :
JC could do his next talk from up in the trapeze bar, and the audience could be totally naked.
Voila, no more barriers, i'm game.
Naah, me and my follies.
hehhehhehh... jacco, you crack me up! i have a 4 year old, which means my life is very small, my family's life is very big, and the world is, well, the world. i amount to (as we say in new york) diddly squat, which ain't a lot. we have moved to sweden to get some of that killer socialized healthcare and child education... nearly went homeless in nyc. things are good. the last thing i need is wading through a herd of trolls in my spare time. i dropped in because jan didden emailed me that folks were talking about me behind my back, very nicely for a change. i made a fool of myself at etf, but had a great time. met a lot of new people and ate some weird french cafeteria food. it was like a school trip with no parents.
i can't say i'll be a regular, cause i won't be, but i'll drop in here and there.
god jul! as they say here this time of year...
i can't say i'll be a regular, cause i won't be, but i'll drop in here and there.
god jul! as they say here this time of year...
Jean-Christophe,
i could say the same about you, the "jerker in the bathroom" almost floored me.
(think i'm blacklisted around here for rudeness, crudeness, and flashing naked body parts, aristocat genes are worth JS nowadays)
Already knew you switched to Swedish cooking before ETF, even familiar with the photo shoots of the pretty Miss LL. As my Serb pal indicated: i Spy for the IRS.
Just a quicky and i'll bugger off : your preamp shiner is all relay switched, I suppose that's not just for the demo model only.
I take it both you and Mr Chung are OK with HQ sealed clickers ?
My comment on the speech : tio punkter
=> http://www.diyaudio.com/forums/lounge/178225-you-really-going-dead-6.html#post2383791
i could say the same about you, the "jerker in the bathroom" almost floored me.
(think i'm blacklisted around here for rudeness, crudeness, and flashing naked body parts, aristocat genes are worth JS nowadays)
Already knew you switched to Swedish cooking before ETF, even familiar with the photo shoots of the pretty Miss LL. As my Serb pal indicated: i Spy for the IRS.
Just a quicky and i'll bugger off : your preamp shiner is all relay switched, I suppose that's not just for the demo model only.
I take it both you and Mr Chung are OK with HQ sealed clickers ?
My comment on the speech : tio punkter
=> http://www.diyaudio.com/forums/lounge/178225-you-really-going-dead-6.html#post2383791
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