randy@matrixq.com
New member
PB - here is how I interpret your proposed changes to the 6L6 wiring. Seems reasonable but I would like to know more of the "why".
Scratch Build (First for me)
- Thread starter randy@matrixq.com
- Start date
There is an assumption in the pentode characteristic curves that the screen grid is bypassed with respect to cathode. This is taken for granted in fixed bias amplifiers, as the cathode is generally a couple tenths of an ohm from ground to begin with.
Give it a shot both ways and let us know what you find.
-PB
Give it a shot both ways and let us know what you find.
-PB
randy@matrixq.com
New member
Ok, I had time to make the cathode bypass change tonight.
Started with one channel only so I could A-B the results. After a while, I could tell the difference and the change was quite positive. More detail overall, but especially in the low end where the bass was much more life-like and the kick bass more completely separated.
I also noticed about one-tenth of an amp lower current draw from the wall. I haven't measured it yet but I'm pretty sure I didn't lose any power at the outputs. Sounds more like I might have gained some.
I want to come back to this amp in a few days and plot some load lines for the 6SL7. That's my next learning target anyway. In the meantime, I have the following projects to occupy the next few days:
1. An update to my Stereomour "45" that I have been following. I understand there was an error in the manual and want to get that corrected. Really love that amp.
2. I have a tape path kit for my 1500 that is crying for my attention. When I do that, I want to run some direct out lines for my Eros tape repro that I recently picked up. I want to go through the resistance and voltage checks on the Eros before pulling it off the bench.
I have some busy days ahead.
Started with one channel only so I could A-B the results. After a while, I could tell the difference and the change was quite positive. More detail overall, but especially in the low end where the bass was much more life-like and the kick bass more completely separated.
I also noticed about one-tenth of an amp lower current draw from the wall. I haven't measured it yet but I'm pretty sure I didn't lose any power at the outputs. Sounds more like I might have gained some.
I want to come back to this amp in a few days and plot some load lines for the 6SL7. That's my next learning target anyway. In the meantime, I have the following projects to occupy the next few days:
1. An update to my Stereomour "45" that I have been following. I understand there was an error in the manual and want to get that corrected. Really love that amp.
2. I have a tape path kit for my 1500 that is crying for my attention. When I do that, I want to run some direct out lines for my Eros tape repro that I recently picked up. I want to go through the resistance and voltage checks on the Eros before pulling it off the bench.
I have some busy days ahead.
If you want to mess with the circuit some more, do some light reading on the purpose of the cathode follower, then Miller capacitance, then Miller capacitance of a pentode. (This will explain my snickering at the cathode follower)
-PB
-PB
randy@matrixq.com
New member
That was some interesting reading. May need to do some more but here is what I'm taking away:
Miller capacitance happens. More prevalent in triodes and almost negligible in pentodes. This makes a tone of sense since you essentially have two plates with just a little space between and a voltage difference between them. That is pretty much textbook definition for a capacitor. I think of the adjustable capacitors in old radios.
OK, so on to the cathode follower: Low impedance output and mostly designed as a buffer and/or feeder into a tone circuit. Well suited for high capacitance follow-on loads which the 6L6 is not. The cathode follower will generally lower gain to about 70% of the original. Bright spot is very low distortion without negative feedback.
One previous builder of this circuit cited the low distortion as the main reason for choosing the cathode follower. But it seems to me that total distortion would be the sum (or maybe product) of the distortion of the two 6SL7 stages together. If he wanted to eliminate as much distortion as possible, why not skip the second stage altogether and get some extra gain in the process?
I will have to look at previous posts but I remember someone suggesting that I go with just the one stage. Now that I've done some research I can see the logic in that approach.
I'm starting to get the joke.
Miller capacitance happens. More prevalent in triodes and almost negligible in pentodes. This makes a tone of sense since you essentially have two plates with just a little space between and a voltage difference between them. That is pretty much textbook definition for a capacitor. I think of the adjustable capacitors in old radios.
OK, so on to the cathode follower: Low impedance output and mostly designed as a buffer and/or feeder into a tone circuit. Well suited for high capacitance follow-on loads which the 6L6 is not. The cathode follower will generally lower gain to about 70% of the original. Bright spot is very low distortion without negative feedback.
One previous builder of this circuit cited the low distortion as the main reason for choosing the cathode follower. But it seems to me that total distortion would be the sum (or maybe product) of the distortion of the two 6SL7 stages together. If he wanted to eliminate as much distortion as possible, why not skip the second stage altogether and get some extra gain in the process?
I will have to look at previous posts but I remember someone suggesting that I go with just the one stage. Now that I've done some research I can see the logic in that approach.
I'm starting to get the joke.
Yeah, the funny takeaway is that the first 6SL7 stage (the voltage amp) is actually harder to drive than the 6L6.
-PB
-PB
2wo said:
With the 6L6 wired as a pentode, the high output impedance of the previous stage is inconsequential (6L6 capacitance will be around 10pF). The theoretical negative impact of the high Rp of the 6SL7 driving the 6L6 would be bandwidth limited to several hundred kilohertz.
The 6SL7's Miller capacitance will be more like 130pF., which is right on the borderline of being so high that a 6SL7 shouldn't drive another 6SL7 in an audio frequency power amp.
-PB
randy@matrixq.com
New member
So, should I try eliminating the second stage of the 6SL7 and take the output from pin 2 on to the 6L6? It would seem that there is nothing to lose here.
randy@matrixq.com
New member
OK, I just went for it.
It now has a single input stage feeding the 6L6 from pin 2. A small but noticeable increase in gain. Overall, the amp seems to sound the same however there is more quiescent hum. My workbench puts me in a very near-field listening position and I'm using fairly efficient 4 ohm speakers. The hum would probably not be noticeable in other situations.
Going to do more listening now.
It now has a single input stage feeding the 6L6 from pin 2. A small but noticeable increase in gain. Overall, the amp seems to sound the same however there is more quiescent hum. My workbench puts me in a very near-field listening position and I'm using fairly efficient 4 ohm speakers. The hum would probably not be noticeable in other situations.
Going to do more listening now.
randy@matrixq.com
New member
Spent way too much time listening to the new changes. Good thing I'm on spring break tomorrow.
Listened to Pink Floyd's "Dark Side of the Moon" and Steve Hackett's "Voyage of the Acolyte", Two albums that I'm way too familiar with and they sounds quite awesome.
I'm pretty sure I can reduce the hum issue. It seems to be mostly input related and while the input lines are shielded, I have not grounded the shields yet.
Also, haven't optimized the input stage. Next step is to measure the voltage on the plate of the 6SL7 and plot a load line. Next big learning challenge.
Listened to Pink Floyd's "Dark Side of the Moon" and Steve Hackett's "Voyage of the Acolyte", Two albums that I'm way too familiar with and they sounds quite awesome.
I'm pretty sure I can reduce the hum issue. It seems to be mostly input related and while the input lines are shielded, I have not grounded the shields yet.
Also, haven't optimized the input stage. Next step is to measure the voltage on the plate of the 6SL7 and plot a load line. Next big learning challenge.
I can't quite remember, but did you ground the 6.3V heater winding?
randy@matrixq.com
New member
PB - Finished the tape path mod to my 1500 and have been busy listening. Recorded Pink Floyd's "Endless River" from vinyl so I could listen from end to end. Absolutely mesmerizing.
Anyway, I'll be getting the 6L6 back on the bench tomorrow and plan on grounding the heaters and playing with the plate voltage on the 6SL7.
BTW - I'm currently listening to my Stereomour configured with 45's. Just how much different are your SR-45's? I like the build work. Nice job.
Anyway, I'll be getting the 6L6 back on the bench tomorrow and plan on grounding the heaters and playing with the plate voltage on the 6SL7.
BTW - I'm currently listening to my Stereomour configured with 45's. Just how much different are your SR-45's? I like the build work. Nice job.
