5f2a with a 6SL7 instead of an 12ax7

chas.wahl

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I'd also be interested in seeing what your resistors around the 6SL7 halves, and the resulting voltages, turned out to be.
 

printer2

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There is a 6SL7 model in the simulator. The "Quiescent Operating Point:" has the mA that is going through the tube and the voltage the plate is sitting at. The "V+" is your supply voltage "Load (Ohm): Resistive" is the plate resistor and "Next stage AC Impedance (Ohm):" is the next stage grid leak resistor or a volume control.

With a 250V supply, a Load of 100k, and entering 1mA it gives a grid bias of -1.4V. It is not quite center biased, that would be a little colder. With 0.8mA it is more centered biased (if that was the goal, it may not be for you) and the plate voltage ends up at 170V. You can adjust the mA and the grid voltage by adjusting the cathode resistor. In this case, 1.9V / 0.8mA = 2,375 ohms. A 2.2k will get you there. Adjusting the cathode resistor and plate resistor will move things around for you. You could add the next stage load, say a 1M pot, it changes the AC load line a little, without it you get the dc load line.

You can figure out the gain by reading off the chart, "Out. headroom (+/-V):" is something you can input. 100V looks a little too wide for the above, 80V seems to work. With the bias at -1.9V and the output voltage of 80V we have, 80V / 1.9V = a gain of 42.

It also gives harmonic distortion, in this case with a "headroom" of 60V, "2nd:1.97 3rd:1.70 4th:0.15 THD:2.60"

With the bias moved over one way or the other you will get a different amount of clean headroom and one side clip before the other. If you reduce the cathode resistor and use a value of 1.2mA you get the following, "2nd:4.91 3rd:2.26 4th:0.03 THD:5.40"

Hours of fun.
 

James Knox

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There is a 6SL7 model in the simulator. The "Quiescent Operating Point:" has the mA that is going through the tube and the voltage the plate is sitting at. The "V+" is your supply voltage "Load (Ohm): Resistive" is the plate resistor and "Next stage AC Impedance (Ohm):" is the next stage grid leak resistor or a volume control.

With a 250V supply, a Load of 100k, and entering 1mA it gives a grid bias of -1.4V. It is not quite center biased, that would be a little colder. With 0.8mA it is more centered biased (if that was the goal, it may not be for you) and the plate voltage ends up at 170V. You can adjust the mA and the grid voltage by adjusting the cathode resistor. In this case, 1.9V / 0.8mA = 2,375 ohms. A 2.2k will get you there. Adjusting the cathode resistor and plate resistor will move things around for you. You could add the next stage load, say a 1M pot, it changes the AC load line a little, without it you get the dc load line.

You can figure out the gain by reading off the chart, "Out. headroom (+/-V):" is something you can input. 100V looks a little too wide for the above, 80V seems to work. With the bias at -1.9V and the output voltage of 80V we have, 80V / 1.9V = a gain of 42.

It also gives harmonic distortion, in this case with a "headroom" of 60V, "2nd:1.97 3rd:1.70 4th:0.15 THD:2.60"

With the bias moved over one way or the other you will get a different amount of clean headroom and one side clip before the other. If you reduce the cathode resistor and use a value of 1.2mA you get the following, "2nd:4.91 3rd:2.26 4th:0.03 THD:5.40"

Hours of fun.
Thank you sir for taking the time to explain a little bit about how to use this amazing tool. Once again I am going to try to wrap my head around it.
 

printer2

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Thank you sir for taking the time to explain a little bit about how to use this amazing tool. Once again I am going to try to wrap my head around it.
It took me a little while to get it (instructions? don't need any instructions). For power tubes the power output does not quite give what the tubes are doing, if you put a high voltage in and low primary impedance it calculates the power using the voltage and the impedance. If you shift over the bias you can have one side of the waveform clean and the other clipped. A lot of second harmonic distortion and it will give you a power out that seems to ignore that the one side is clipped and give you a exagerated power output. But the web page probably assumed you would want to have the tube center biased, something guitar amps like a Champ may not be. Other than that a fun tool.
 

NTC

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One of my future projects is going to be based on that early 6SL7 Princeton but with a 6L6 of some sort.
 
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Mongo Park

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Well I found my notes.
I have 151V going into the plates resistors of 240k and 61V on the plate. 4.7K on the Cathodes. This is duplicate for the other side of the tube. I have no idea where I goth this from other than it is not very good. So I am glad you revived this thread to get me to look at this amp. I think now I can get this in better working order. I did this so long ago I had little understanding, I would just copy from other schematics without regard to voltages.
So all you can tell from this is what not to do. The amp does work with clean headroom.
 

James Knox

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After doing a lot of study and experimentation with the load line calculator I’ve decided to go ahead with my build and treat the Octal Dual Triode just like a Noval Triode. For experiment sake i’m going to build 2 amps side by side. They will be 5F2As on Hammond Box’s the same but for the preamp tubes.

I will start the 6SL7 at 100K Screens and 1.5K Cathodes, just like the 12AY7 in the other amp. My reasoning is that I will be able to compare side-by-side the difference in sound between the Octal and Noval Preamp. Probably sub a 5751 for the AY7, since the 6SL7 mu is closer. Then if I want I can bump up the bias to GA5 territory with 220K screens and 2.2 Cathodes.

image.jpg
 

NTC

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I like 12AY7's. 6SL7's seem to have avery rich and pleasing sound. This is a great side-by-side experiment. I bet the octal comes out the winner...
 

James Knox

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69E5A2B3-4F9D-4182-969B-A2EB7678E3D8.jpeg


I finished them today and was able to spend a couple hours playing and comparing. Different volume levels, different pedals, etc.

They both sound really good, but the Octal just brings a little more to the party. I know that some of that might be mind over matter, but it seems a little bigger. Slightly wider frequency range. Slightly more harmonic content.

I ended up biasing the 6SL7 exactly like the 12AY7. 100K/1.5K. Just the right amount of gain for what I like. I had 220K/2.2K standing by, but didn’t feel a need to swap them. After about 30 min of auditioning, I swapped a 5751 for the 12AY7. It was like two brothers from the same family… Similar, yet different, with one brother bigger and more outspoken.

I kept trying to tell myself that the Octal didn’t sound bigger because the tube is bigger. I don’t know though, it’s more than an optical illusion, lol.

I wish I had a good way to record so you guys could hear the difference.

My last thought is that the OCTO preamp had the edge for tactile response. You could feel it in your playing, under your fingers a little more if that makes sense. It made enough difference to me that I would probably only use the Octal preamp in the future for my own personal amps.

I’m gonna take some voltages tomorrow and check back in…
 

James Knox

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I’ve had some more time to play with this today and I really like the way it sounds. I’m impressed with the Octal 6SL7 and convinced that it is significantly different enough than the 12AY7/5751 to do on a regular basis.

My 6V6 is dissipating 90.7% for 12.7 watts with its measured cathode resistor 358K. Plate current 40ma.

On the 6SL7 I’ve got 151/153 on the plate to cathode and voltage drops of 1.44/1.46. That is using 100K on the plates and 1.5K on the cathodes. Nice headroom and natural compression.
 

chas.wahl

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For anybody who's interested in trying this out, using the same anode, cathode resistances and grid supply, there are such things as noval to octal adapters that do the wiring swap between a 12A*7 and a 6S{N|L}7. Search eBay. Adapters for octal to noval are, it seems more plentiful, available from TubesandMore and Tube Depot, for instance.
 

James Knox

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For anybody who's interested in trying this out, using the same anode, cathode resistances and grid supply, there are such things as noval to octal adapters that do the wiring swap between a 12A*7 and a 6S{N|L}7. Search eBay. Adapters for octal to noval are, it seems more plentiful, available from TubesandMore and Tube Depot, for instance.
That’s a cool idea! It would make an easy way to investigate octal goodness without building a new amp.
 




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