Actually, there are some considerable advantages- you might try doing some more experimentation.
MOSFET drive is, I agree, the right way to do it; I changed from 6BX7-based cathode followers to IR hexfets about 20-some years ago and never looked back.
When you look at these tubes as zero-bias triodes (which the curves in g2-drive service sure resemble), you can get past the usual approaches to loading. Take a look at the linearity at very low plate currents- you can idle an output stage at a few milliamps and get less crossover distortion than from a conventional AB1 output stage. The distortion is moderate, the power is large, and the overload and recovery characteristics are outstanding. Output transformer design can be greatly simplified. In exchange for using a high plate voltage, power supply currents are relatively low.
These are real-world virtues.
MOSFET drive is, I agree, the right way to do it; I changed from 6BX7-based cathode followers to IR hexfets about 20-some years ago and never looked back.
When you look at these tubes as zero-bias triodes (which the curves in g2-drive service sure resemble), you can get past the usual approaches to loading. Take a look at the linearity at very low plate currents- you can idle an output stage at a few milliamps and get less crossover distortion than from a conventional AB1 output stage. The distortion is moderate, the power is large, and the overload and recovery characteristics are outstanding. Output transformer design can be greatly simplified. In exchange for using a high plate voltage, power supply currents are relatively low.
These are real-world virtues.
Tubes4e4 - you site hasn't shown up in any search I've ever done for triode strapped pentodes. Thanks for the link, though. Thanks also for the heads up on the Svetlana "EL509". When the time comes to use the ones I have, I'll throw one on the bench and plot curves manually just like I did for the 6CD6GA.
SY - the curves I've seen in the sweep tube data sheets for VG1=0 and for varying screen voltage are of limited utility, as they are generally for pretty high screen voltages, outside the regions where I plan to operate. However, they do give you an idea of where the screen current really starts to be a significant factor. I ran my curves for the 6CD6GA at plate voltages from 100V-500V and screen voltages from -20V to =+30 to 40V. Screen current was ~ 0.1 mA worst case for the voltages mentioned, so I felt pretty comfortable with using a fairly modest triode to drive the screen. For this go-round I'm using the 6CY7, which has a small mu=64 triode that can be used for the gain stage, along with a 5W triode that will make a very respectable cathode follower. As it's a little 9-pin miniature, it'll look lost along side of the big guys....
SY - the curves I've seen in the sweep tube data sheets for VG1=0 and for varying screen voltage are of limited utility, as they are generally for pretty high screen voltages, outside the regions where I plan to operate. However, they do give you an idea of where the screen current really starts to be a significant factor. I ran my curves for the 6CD6GA at plate voltages from 100V-500V and screen voltages from -20V to =+30 to 40V. Screen current was ~ 0.1 mA worst case for the voltages mentioned, so I felt pretty comfortable with using a fairly modest triode to drive the screen. For this go-round I'm using the 6CY7, which has a small mu=64 triode that can be used for the gain stage, along with a 5W triode that will make a very respectable cathode follower. As it's a little 9-pin miniature, it'll look lost along side of the big guys....
Re: Re: Re: Re: Horizontal TV tubes used in amplifier?
D'OH!!! Of course 😱
Hi Tom,
Indeed. But was just checking... why go through the steps if a set already existed 😉
Miles Prower said:
D'OH!!! Of course 😱
Tubes4e4 said:
Hi Tom,
Indeed. But was just checking... why go through the steps if a set already existed 😉
Hi SY,
Huh? Take for example this one. Linearity at low plate voltages is worse than early (real, kinky) tetrodes 🙄
Impedance even goes "negative" at certain conditions, so to say. Highly instable and prone to (Barkhausen- ) oscillations in that mode, I would say.
Roughly same situation with Vg1 = 0V, curves are only "shifted" (you can see the effect for the 3 different Vg1 prebias values I used to plot the curves).
BTW, each and every EL509/PL509 I could get hold of (including Svetlana) show those horrible kinks. That is why I say don´t trust the EL509 screen driven curves from ... you probably know.
Tom
Huh? Take for example this one. Linearity at low plate voltages is worse than early (real, kinky) tetrodes 🙄
Impedance even goes "negative" at certain conditions, so to say. Highly instable and prone to (Barkhausen- ) oscillations in that mode, I would say.
Roughly same situation with Vg1 = 0V, curves are only "shifted" (you can see the effect for the 3 different Vg1 prebias values I used to plot the curves).
BTW, each and every EL509/PL509 I could get hold of (including Svetlana) show those horrible kinks. That is why I say don´t trust the EL509 screen driven curves from ... you probably know.
Tom
The curves are interesting. I didn't see any tendancy for that sort of behavior in my setup using the 6CD6GA, though I also didn't push below 100V on the plate. The plate in my setup was connected directly to the HV rail, if that makes any difference, and the control grid tied to the cathode through a 220 ohm resistor. The screen grid was driven by a MOSFET source follower via a 100 ohm resistor, and I used this resistor to monitor screen current. When I have some spare time, it would be interesting to push the plate voltage a little lower to see what happens. I still have 20V or so of extra grid drive in my setup. It was my intention to stay away from operating near the knee in my actual amplifier, but it would be good to find out what happens if I get there.
Hi wrenchone,
for the normal application - usually you want to get some Po Watts out of the circuit you design - the massive secondary emission kink is not much to worry about because it happens in an operating area you normaly wouldn´t chose (just like with kinky tetrodes). It might cause instability with grossly mismatched reflected loads, though (like by badly designed speakers with extreme impedance peaks).
Screen drive or not, kinky tetrodes, kinkless (beam power-) tetrodes and pentodes generally like closely defined reflected loads for best results regarding distortion and Po.
Ultralinear mode and triode mode are much more forgiving in this context.
Tom
EDIT: When measuring such curves, be aware that the screen grid even may source current under certain conditions. A generic pass regulated PSU used for g2 feed cannot deal with such a situation correctly.
for the normal application - usually you want to get some Po Watts out of the circuit you design - the massive secondary emission kink is not much to worry about because it happens in an operating area you normaly wouldn´t chose (just like with kinky tetrodes). It might cause instability with grossly mismatched reflected loads, though (like by badly designed speakers with extreme impedance peaks).
Screen drive or not, kinky tetrodes, kinkless (beam power-) tetrodes and pentodes generally like closely defined reflected loads for best results regarding distortion and Po.
Ultralinear mode and triode mode are much more forgiving in this context.
Tom
EDIT: When measuring such curves, be aware that the screen grid even may source current under certain conditions. A generic pass regulated PSU used for g2 feed cannot deal with such a situation correctly.
Miles Prower said:
Shush! It's our little secret. 😉
Don't blow it!
No kidding!
I have a list of conspiracy theories on this....
Hey Tom, remember what happened with the EL36 and the 6AH4GT's availability and prices right after your curve sets and our project designs? Someone even walked into a certain store and scooped 200 6FD7's after baby Amp's success
Strange indeed...
tubes4e4 -
I like power as much as the next guy - I also like linearity, which is why I tried to stay away from operating too near the knee of the triode characteristic. With the 6CD6GA, driving the screen to +35V gets me all the current I need and then some, without hitting the knee. I'm using Hammond 125ESE output transformers, which have an 80ma current rating. These will be the limiting factor in my design more than anything else.
The current effort is to see how much I can wring out of a design with minimum cash outlay. The output transformers were about $30 each, the tubes $2 each. The chassis and power transformer were surplus junk, and the rest of the parts came from my lab stock or were purchased surplus. More ambitious designs are in the works for later - this one is for fun/speculation/getting my feet wet with a single ended design. The chassis will also be a good test bed for other experiments. Since it looks functional at best, I won't mind hacking around with it too much. It all started with seeing some 6CD6GAs on Ebay for $2 apiece. I grabbed the output transformers on a whim as well, and one thing led to another...
I like power as much as the next guy - I also like linearity, which is why I tried to stay away from operating too near the knee of the triode characteristic. With the 6CD6GA, driving the screen to +35V gets me all the current I need and then some, without hitting the knee. I'm using Hammond 125ESE output transformers, which have an 80ma current rating. These will be the limiting factor in my design more than anything else.
The current effort is to see how much I can wring out of a design with minimum cash outlay. The output transformers were about $30 each, the tubes $2 each. The chassis and power transformer were surplus junk, and the rest of the parts came from my lab stock or were purchased surplus. More ambitious designs are in the works for later - this one is for fun/speculation/getting my feet wet with a single ended design. The chassis will also be a good test bed for other experiments. Since it looks functional at best, I won't mind hacking around with it too much. It all started with seeing some 6CD6GAs on Ebay for $2 apiece. I grabbed the output transformers on a whim as well, and one thing led to another...
After SY mentioned the power he was getting from some sweep tubes in screen driven mode, I decided to torture (oh I mean test) some of my 6AV5's. I built a prototype test amp with 2 6AV5's in push pull. 6BQ7 LTP phase splitter, follower by twin gain stages (one per channel) using 5965's and a mosfet follower to drive each screen. There is a seperate power supply the plate (up to 555 volts), the screen mosfets (400 volts) and the control grid (+ or - 0 to 25 volts). The screen grid voltage is variable from -100 to + 250 volts DC with drive capabilities of over 200 volts P-P. At this time there is no local or global feedback applied.
I spent a day turning knobs, while watching the FFT analyzer, and doing listening tests. This thing can run close to class B (idle current around 10 mA) make 80 watts RMS at 5% and be under 1% at 10 watts. It sounds good too. Crossover distortion is not an issue. Operation at full power for an hour did not produce any red glow with wimpy RCA tubes. I can't do this with a conventional circuit using 6L6's.
I am at work now, so I don't remember the voltages at which the magic happened, but I remember that the plate supply was maxed out at 555 volts. The control grid was a few volts negative. Lots of drive signal was required like 200 volts P-P. Screen was swinging up to +250 or so. I am sure it was drawing considerable current because the mosfets were hot (no heat sink).
I have not touched this prototype amp since I got back from vacation, but I plan to make one, and maybe a bigger one with 6LW6's.
I spent a day turning knobs, while watching the FFT analyzer, and doing listening tests. This thing can run close to class B (idle current around 10 mA) make 80 watts RMS at 5% and be under 1% at 10 watts. It sounds good too. Crossover distortion is not an issue. Operation at full power for an hour did not produce any red glow with wimpy RCA tubes. I can't do this with a conventional circuit using 6L6's.
I am at work now, so I don't remember the voltages at which the magic happened, but I remember that the plate supply was maxed out at 555 volts. The control grid was a few volts negative. Lots of drive signal was required like 200 volts P-P. Screen was swinging up to +250 or so. I am sure it was drawing considerable current because the mosfets were hot (no heat sink).
I have not touched this prototype amp since I got back from vacation, but I plan to make one, and maybe a bigger one with 6LW6's.
AudioNote made an stereo amplifier (2 push pull 10w + 10w) using only 4 6EM7
Each one has 2 triodes one similar to 6SL7 and the other similar to 2A3.
best regards Jaime
Each one has 2 triodes one similar to 6SL7 and the other similar to 2A3.
best regards Jaime
Jaime said:
How did they manage to do that?
I've got 10W rms (at ~3% distortion) out of my PPP 6EM7, 4 tubes/ch.
I could get 10W with only 2 tubes/ch, but then I have to increase B+ to a level that I suppose will reduce the tube's lifetime.
An externally hosted image should be here but it was not working when we last tested it.
Me like jane. 🙂
tubelab.com said:
Have you seen this:
http://webpages.charter.net/dawill/tmoranwms/Elec_TubeChlng.html
😕
Like you say:
"Too much power is almost enough! Turn it up till it explodes - then back up just a little."
http://webpages.charter.net/dawill/tmoranwms/Elec_TubeChlng.html
Yes I saw it when it appeared. I thought about it for a few minutes, and then decided that it would consume too much time to create a monster that served no purpose. I don't have enough time to build all of my ideas as it is.
Since the criteria does not require a listenable output (low distortion) or any longevity requirement, the winning circuit would not serve any useful purpose. My guess for the winning circuit would be some type of square wave generator that avoids operation in the linear region. Perhaps cross coupled thyratrons. You could get ridiculous power with very little dissipation in the tubes themselves.
Make this a real contest, with a maximum distortion of 5 or even 10 % at full power and a requirement that the amp be capable of operating at full power for an hour without failure. The amp should then still pas distortion tests. Frequency response should be "listenable", maybe 100 Hz to 15 KHz at 3 db.
Want to compare it to drag racing, look at Hot Rod magazine's Pump Gas Drags. All cars must be street legal, drive on a public road for 1/2 hour in rush hour traffic (that test took out several cars), run on gas that you could buy at the gas station (not race gas), then after only a 1/2 hour cool down period race each other on a dragstrip.
Hi Jane
Lamentably Kondo died months ago. We will not be able to ask to him.
http://www.sibatech.co.jp/KONDO/KaiEn.htm
Your amplifier is very interesting.
It is hurts your Web is not in a Christian language.
There are an translator Norway - English online?
Best regards Jaime
Lamentably Kondo died months ago. We will not be able to ask to him.
http://www.sibatech.co.jp/KONDO/KaiEn.htm
Your amplifier is very interesting.
It is hurts your Web is not in a Christian language.
There are an translator Norway - English online?
Best regards Jaime
I'm sad to hear that.Jaime said:
One thing with 6EM7/6EA7 is that they are not easy to find in match pairs. I suppose that is true for many sweep tubes. (I also got problems to find matched pairs for my 6LU8 based PP amplifier.)
My local tube pusher lend me a dozen of 6EM7 so I could pick pairs for myself. The tested tubes where all Sylvania.
Here are the results:
6EM7/EA7 Va=305V, Vg=-62V
Tube[#] : Ia[mA]
01 : 14,1
02 : 4,0
03 : 21,7
04 : 9,2
05 : 4,2
06 : 4,3
07 : 8,1
08 : 8,7
09 : 9,4
10 : 13,5
11 : 3,9
12 : 7,0
Avg : 9,0
Stddev : 5,1
jane, problem is you're running them at the point where the curves are already bunching up, changing slope, and at much lower current and higher voltage than what the tube wants to see. Since the perveance is high, you can swing down to a pretty low voltage. This implies using a lower B+.
Try matching at 250V and -45V. I bet they look a lot more consistent.
Try matching at 250V and -45V. I bet they look a lot more consistent.
SY said:
Well, it's still quite high spreading at Va=250V for the 6EM7s. I did that exercise when I was working on this project, changed the power supply into choke input (the Hammonds 158Q was not too happy with that, they made a lot of buzz). Today I'm running with 5uF capacitor input and B+=~290V
Sorry that I don’t have the measures for 250V around, but my experience is that there are hard to find "matched pairs" of 6EM7.
Re: Re: Horizontal TV tubes used in amplifier?
Impressive!
Can you post schematics?
Miles Prower said:
Impressive!
Can you post schematics?
Re: Re: Re: Horizontal TV tubes used in amplifier?
I don't have any since I dropped this particular project. With bias stabilization, this whole thingy grew ridiculously complex for just five watts. I'd rather use the design effort, not to mention the junk box pickin's, for other projects.
Furthermore, I have a pretty good reason to suspect that screen drive is not all it's supposed to be.
6BQ6GTBs still look like they'd make excellent audio finals, but in a comventional common cathode, grid driven, screen voltage regulated, topology with fixed bias. If I do anything with them, then that's how I'll go with these.
Wavebourn said:
I don't have any since I dropped this particular project. With bias stabilization, this whole thingy grew ridiculously complex for just five watts. I'd rather use the design effort, not to mention the junk box pickin's, for other projects.
Furthermore, I have a pretty good reason to suspect that screen drive is not all it's supposed to be.
6BQ6GTBs still look like they'd make excellent audio finals, but in a comventional common cathode, grid driven, screen voltage regulated, topology with fixed bias. If I do anything with them, then that's how I'll go with these.
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