Hey guys, as my first majorish valve project, I'm planning an stereo class AB2 amp using 6P3S's in the output stage. Most of you guys probably know this but they're pretty much the same as the 6L6GB; a lower rated 6L6GC. So far I've been playing around with Spice trying to get the open loop distortion down as much as possible without using the ultralinear transformer in the simulations.
When I decided to use the ultralinear OPT, I came across a problem. In the application notes for the 6L6GB and 6P3S (which is in russian, but the values all seem to be the same), the B+ is 390V, and the screen voltage is 270V.
How can I set the ultralinear trafo up so the DC on the screens is 270V rather than the full B+ voltage?
Any help or advice would be appreciated.
When I decided to use the ultralinear OPT, I came across a problem. In the application notes for the 6L6GB and 6P3S (which is in russian, but the values all seem to be the same), the B+ is 390V, and the screen voltage is 270V.
How can I set the ultralinear trafo up so the DC on the screens is 270V rather than the full B+ voltage?
Any help or advice would be appreciated.
Transformer with tertiary UL windings. Custom from Edcor, and possibly others.
I thought about a separate 270V supply resistivly driving the screen with capacitive coupling off the plate, but the phase shift would probably screw up the UL behavior.
Possibly a transistor driven off the plate using a resistor divider to maintain phase, and drive the transistor off the screen 270V supply.
I thought about a separate 270V supply resistivly driving the screen with capacitive coupling off the plate, but the phase shift would probably screw up the UL behavior.
Possibly a transistor driven off the plate using a resistor divider to maintain phase, and drive the transistor off the screen 270V supply.
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Yeah those were pretty much the solutions I came up with when I thought about how to do this; didn't think of the transistor solution though.
I'm winding the xformers myself so tertiary UL windings wouldn't be a problem. I'm assuming I'd just set the tertiary winding to have the signal AC voltage swing appropriate for whatever tap % I choose and give it a centre tap attached to a 270V supply?
Know of any popular or commercial amp designs running ultralinear with different plate and screen voltages? I wouldn't mind seeing how others have done this, alothough the tertiary winding sounds like the way to go.
Thanks for the response TheGimp.
I'm winding the xformers myself so tertiary UL windings wouldn't be a problem. I'm assuming I'd just set the tertiary winding to have the signal AC voltage swing appropriate for whatever tap % I choose and give it a centre tap attached to a 270V supply?
Know of any popular or commercial amp designs running ultralinear with different plate and screen voltages? I wouldn't mind seeing how others have done this, alothough the tertiary winding sounds like the way to go.
Thanks for the response TheGimp.
The Zener would scale the dc bias point but would give 100% feedback rather than 40% (or whatever UL percentage you wanted).
I think the transistor will have the problem with it's output being clamped by the cap that helps set the %ul.
I may try this just for S&G.
I think the transistor will have the problem with it's output being clamped by the cap that helps set the %ul.
I may try this just for S&G.
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I came across an RCA data sheet for the 6L6GC that has a note for using a screen tap which permits the max V. to be 500v., even though normal ops. was 350/270v. Perhaps under UL conditions there is a higher max for your tube also. At least 390v. when used with a screen tap. You might look a little deeper into the possible exceptions before engineering an unecessary workaround.
Look at the CCS. The first tube will have to drive M1's gate capacitance through the 2N2222's emitter, loading it at high frequencies. Also the Zener will have a high parasitic capacitance, up to 100nF and very nonlinear and will slow down the CCS a great deal. I suggest to use a PNP and P-MOS instead. I can illustrate if you want.
For other CCS options, you may look at this thread:
http://www.diyaudio.com/forums/tubes-valves/167903-ss-ccs-distortion.html#post2205691
- keantoken
For other CCS options, you may look at this thread:
http://www.diyaudio.com/forums/tubes-valves/167903-ss-ccs-distortion.html#post2205691
- keantoken
I have also been mulling over the issue of pentode screen voltage. There is some conventional wisdom out there (I can't find the specific thread at the moment) that the screen voltage limit is strictly relevant ony for fixed screen voltage operation. The idea is to avoid the plate voltage dipping below the screen for too much of the AC cycle, and hence making the screen absorb some or all the power usually deposited on the plate.
The claim is in UL the screen voltage can not end up too far above the plate, and hence the voltage rating isn't nearly as relevant. It is really the power rating of the screen which counts. In a typical UL connection, however, the screen voltage is in fact slightly higher than the plate, so there might still be some cause for concern. Dropping resistors may help here.
I have used Zener diodes to drop 30 V from the UL tap to the screens on a different pentode, but still applying a voltage on the screens well above their rated values. So far I have had no problems, but as always your mileage might vary. I would actually love to hear a reply from somebody with more experience and/or knowledge on this topic.
The claim is in UL the screen voltage can not end up too far above the plate, and hence the voltage rating isn't nearly as relevant. It is really the power rating of the screen which counts. In a typical UL connection, however, the screen voltage is in fact slightly higher than the plate, so there might still be some cause for concern. Dropping resistors may help here.
I have used Zener diodes to drop 30 V from the UL tap to the screens on a different pentode, but still applying a voltage on the screens well above their rated values. So far I have had no problems, but as always your mileage might vary. I would actually love to hear a reply from somebody with more experience and/or knowledge on this topic.
I don't follow this.
The zener is connected between the UL tap on the transformer and the screen, and just means that V_screen = V_UL - V_zener. The AC signal at the screen will be equal to the AC signal at the UL tap, and will still be 40% of the AC signal on the plate.
Since the plate and screen are no longer at the same voltage, the actual feedback characteristics will be slightly different, but I don't see how this would give 100% feedback.
I don't think he understood you were advocating connecting the zener between screen grid and UL tap.. This will work, but as pointed out in a previous post is probably unnecessary in most UL applications..
I have run 6L6G/6L6GB in MC-30s without blowing them up (look at the circuit of mac 30 and you will see exactly what I am getting at) so I think the 6L6GB will fine run straight off of the UL screen tap at or slightly above the plate voltage. Note that I also used 6P3S in these amps and they worked and sounded fine, but the bluish pulsing glow was a bit alarming.. 😉
I have run 6L6G/6L6GB in MC-30s without blowing them up (look at the circuit of mac 30 and you will see exactly what I am getting at) so I think the 6L6GB will fine run straight off of the UL screen tap at or slightly above the plate voltage. Note that I also used 6P3S in these amps and they worked and sounded fine, but the bluish pulsing glow was a bit alarming.. 😉
Hi kevinkr
Yes, you are right it wasn't really specified where the diode would go. As I said, I have been thinking a lot about this recently, so the diode topology seemed obvious to me...
Yes, you are right it wasn't really specified where the diode would go. As I said, I have been thinking a lot about this recently, so the diode topology seemed obvious to me...
Thats what worries me; in simulation the average screen dissipation when connected directly the UL tap was around 6W, and from memory (don't have the datasheet in front of me) the maximum rating in AB2 was 3W. I don't mind pushing the tubes a bit but doubling the screen dissipation seems a bit dangerous?
Haha, now I kinda want to try it just because it sounds like it looks cool...
I've run a 50V zener diode (from UL tap to screen) with no undue effects on a SE UL amplifier. There was an article in Sound Practices where the author measured slightly lower THD with this type of connection in a PP EL34 amplifier. I haven't played with this concept since... but I do remember Tubelab mentioning this except he used VR tubes.
Yes, I was thinking of another configuration. Series from tap to screen would work. Do you use a bypass cap to help with zener noise?
Interesting idea.
Interesting idea.
Who meeee????? Would I be crazy enough to wire a VR tube in series with the screen grid jusst so I could crank up the voltage some more? YOU KNOW IT! The photo shows another of my "built with clip leads" amplifiers. I was running tests on several different tubes in triode and UL mode. After finding that I could violate the ratings on the 6AV5GA by several hundred volts without letting the smoke out, I tried other tubes. The 6CD6GA is the polar opposite of the 6AV5GA. It starts to get really upset even before you hit the screen grid spec. So I got this wild idea to install a voltage dropping device in series with the screen grid. The first thing that came to mind was a VR tube. Does it work? Well, sort of. There is an issue, and the results would be different with a different tube. What happened?
The VR tube lowers the screen grid voltage about 100 volts. This allows me to crank the power supply up 100 volts more leading to a whole bunch more power, BUT. The whole thing sounded nice until you approach the point where the screen voltage dips to near zero on current peaks. The VR tube will go out momentarily leading to some of the ugliest sounding distortion that I have ever heard.
The 6CD6 is probably worse case since the plate can eat more voltage than any power supply that I have can make, but the screen grid is rated at 175 volts. This imposes serious limitations on any sort of application where the screen grid voltage is related to the plate voltage (triode or UL).
I think that this would be viable on a 6L6 type tube. I also think that it would not be needed either. People have run 6L6GB's and 807's in UL at 400 volts or more. Some people report success with them in a Simple SE which runs at about 450 volts of B+. I have not tried the Russian tubes myself though.
There was an extensive thread discussing several methods of operating the screen grid at reduced voltages using mosfets or other circuits. You must remember that the plate voltage excursion can swing from near zero to twice the B+ voltage. This must be considered when adding silicon based circuits in the plate or screen path.
http://www.diyaudio.com/forums/tube...on.html?highlight=adjustable+distributed+load
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Add a bias resistor to insure that the regulator tube stays on longer (it will still go out if you get the screen low enough).
It is called hysteresis. 😀
VR tube needs more voltage to start conducting than when it is active.
I would think there would be no issue with a 6L6 type and a 51 volt or so 5 watt zener. I have not actually tried this yet, but I got some zeners to play with in my Simple P-P in the quest for maximum power out of a little tube (EL84). I have version that is close to stock producing 30WPC, There is a bugger brother that cranks out about 60 WPC.
Tubelab has the link to the thread I couldn't find. There is a lot of good discussion there with several other ideas.
I didn't notice the noise floor change significantly when I added the Zener, but then I was more worried about distortion. Are you thinking of a bypass from screen to cathode, or just a 'snubber' across the Zener?
This old thread seems to argue (and even has some data) that higher-voltage Zeners aren't that noisy, it is the ones around 6V that you have to avoid:
http://www.diyaudio.com/forums/parts/35821-some-noise-measurements-leds-zener-diodes.html
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