HMV130, I think the schematic by Bates is limited by the maximum voltage swing of the input tube. Changing from 6V6 to 6L6 will not change this.
Here's a thread that I started on exactly this topic (6L6 cathode follower):
http://www.diyaudio.com/forums/tubes-valves/301133-transformer-coupled-cathode-follower-power.html
There are a couple of schematics based on the same idea (roughly same output, just different tube options). That would put out more power than the Bates schematic, but I haven't built it. I may still build it (or something similar) eventually, but my current project has some chassis and power supply limitations and so I'm working on something smaller at the moment.
You could take the advice offered by several in that thread to use a fixed bias for the output tube (and probably a pentode connection). That will allow for larger voltage swings without complicating the power supply too much. This would also mean redesigning the input and most likely using a different choke (maybe something like a 12AT7 input and a 12B4 choke loaded driver).
Here's a thread that I started on exactly this topic (6L6 cathode follower):
http://www.diyaudio.com/forums/tubes-valves/301133-transformer-coupled-cathode-follower-power.html
There are a couple of schematics based on the same idea (roughly same output, just different tube options). That would put out more power than the Bates schematic, but I haven't built it. I may still build it (or something similar) eventually, but my current project has some chassis and power supply limitations and so I'm working on something smaller at the moment.
You could take the advice offered by several in that thread to use a fixed bias for the output tube (and probably a pentode connection). That will allow for larger voltage swings without complicating the power supply too much. This would also mean redesigning the input and most likely using a different choke (maybe something like a 12AT7 input and a 12B4 choke loaded driver).
In my RH type amp i use a simple switch of the rectifier tube to allow the use of 6v6 or 6L6. I have set the amp up to have a plate voltage of 250 with a 5y3 for the 6v6. Then i switch to a 5ar4 and that yields about 280 to 290 volts and use the 6l6 power tube. It would be better if you have switchable bias resistor for the output tube. like 200R for the 6l6 and 250R or higher for the 6v6. Installing a fixed bias supply is better but a lot more work.
Thank you for the suggestion but the problem is that I do not like use a pentode as input stage for some reasons including the fact that I have already a stock of 6sn7, 6sl7, 6sj7, 6j5 etc. I'm also not able to find another suitable schematic that includes specifications for output transformer and power transformer.
I understand the fundamentals of what you propose but unfortunately I have not the needed theoretical knowledge to do this.
Hi HMV130,
The schematic on the first page of that link uses two 6SN7 stages and a triode-strapped 6L6GC. It is all triode. I have not built it, but I think it will work fine (for maybe 3-4W). The reason I was thinking about pentodes instead was just for more power.
One note if you build it: make sure to use 6SN7GTB because the anode will be above 300V (regular 6SN7 is rated for 300V max anode). If you do not have 6SN7GTB, you can also use some series resistance to drop the anode below 300V on that stage.
Here's a concept to think about and possibly try out on a breadboard. I rearranged Bates' circuit to place the OPT in the 6V6 anode circuit and use its voltage gain to achieve maximum output power, which should be near 4W. The OPT is uncritical in this arrangement, including its DC resistance.
> I rearranged Bates' circuit
You lost the "Cathode Follower" aspect and its mythical virtues.
All my tinkerings agree with your thought. CF's advantages are not large in a Power Stage. OTOH a CF output means shunting some Heavy Lifting to the driver stage. A CF with 300V supply may need over 500V p-p to make good use of the costly(?) power bottle. That means we need a small supply of much greater voltage than the big tube's supply, an annoying design. (Or a high-impedance step-up driver transformer, no longer a common part and a very tricky design/wind.)
You lost the "Cathode Follower" aspect and its mythical virtues.
All my tinkerings agree with your thought. CF's advantages are not large in a Power Stage. OTOH a CF output means shunting some Heavy Lifting to the driver stage. A CF with 300V supply may need over 500V p-p to make good use of the costly(?) power bottle. That means we need a small supply of much greater voltage than the big tube's supply, an annoying design. (Or a high-impedance step-up driver transformer, no longer a common part and a very tricky design/wind.)
I was checking your math on the 4W output and noticed that -12V of bias on a triode-connected 6V6 @ 335V B+ puts you way into red-plate territory. I don't think your idle conditions are going to work. Did you perhaps mean to run as pentode?
I worked up a design over the weekend and will share when I get home where I can take a picture of my notes. It shares some similarities with this one.
I think it is a bit extreme to say that the cathode follower's virtues are mythical but they do come at pretty great cost. PSRR is pretty great and so is transformer driving impedance in a follower output stage. Those are actual advantages but they may just not be worth the additional driver complexity when you can have a local feedback arrangement that is much simpler and gets you most of the way there.
Since we're all here talking about direct coupling and pentodes, here's an ultralinear design I have been working on (tweak to the 6V6 one I posted earlier):
It uses an easy to find Hammond 156C choke (measured 3k25 DCR) and 5k output transformer. I am little skittish of the operating point for ultralinear though. I could find only one graph for the 6L6GC to work from:
The 6L6 / 5881 Tube
It looks like about 5W to me.
Anyone have comments on this idea or bias experience for the 6L6GC in UL?
edit: slight tweaks and looks more like 5W output:
It uses an easy to find Hammond 156C choke (measured 3k25 DCR) and 5k output transformer. I am little skittish of the operating point for ultralinear though. I could find only one graph for the 6L6GC to work from:
The 6L6 / 5881 Tube
It looks like about 5W to me.
Anyone have comments on this idea or bias experience for the 6L6GC in UL?
edit: slight tweaks and looks more like 5W output:
Last edited:
This seems interesting. Do you have also a PSU schematic for this? And is the gc version of 6l6 a must? (are other versions of 6l6 fine?). Moreover are the 1M resistances volume and tone potentiometer? And finally can you provide me the values of the bypass capacitor in the output stage?
Last edited:
I built a regulated PSU for it and it probably isn't for the faint of heart. It evolved out of this thread:
http://www.diyaudio.com/forums/tubes-valves/294797-can-we-talk-about-series-pass-regulators-3.html
If you play around a bit with PSUD2, you should be able to easily come up with something simpler. The cathode follower output has nice PSRR and you can add a little extra filtering between stages if desired.
The limiting factor I ran into with the power supply I built was the Vhk of the 6L6GC. I'm not sure what other versions of the 6L6 are rated for and I don't see it listed on the old RCA 6L6 datasheet. As a couple people mentioned, there are ways to get around this, but with the power supply I built I decided to explore other designs instead.
The 1M at the input and second stage are the grid leak resistors (they reference the grid to ground). I don't have a volume pot on the schematic, but it's as easy as sticking one on the input. There aren't any tone controls as I drew it up.
About a 200uf cap should be good to bypass the 400 ohm resistor.
http://www.diyaudio.com/forums/tubes-valves/294797-can-we-talk-about-series-pass-regulators-3.html
If you play around a bit with PSUD2, you should be able to easily come up with something simpler. The cathode follower output has nice PSRR and you can add a little extra filtering between stages if desired.
The limiting factor I ran into with the power supply I built was the Vhk of the 6L6GC. I'm not sure what other versions of the 6L6 are rated for and I don't see it listed on the old RCA 6L6 datasheet. As a couple people mentioned, there are ways to get around this, but with the power supply I built I decided to explore other designs instead.
The 1M at the input and second stage are the grid leak resistors (they reference the grid to ground). I don't have a volume pot on the schematic, but it's as easy as sticking one on the input. There aren't any tone controls as I drew it up.
About a 200uf cap should be good to bypass the 400 ohm resistor.
The 6V6 sees only 250V in this lashup, which puts anode dissipation around 12.5W -- well within the 14W design maximum. We would need class A2 operation to get full output in triode mode, so yes, I must have been thinking to connect as pentode instead. Thanks for a most diplomatic callout!
That is a 6V6 version of the RH807 amp I was referring to earlier in this thread that could be improved by using a pentode in the input stage. When I post the schematic of what I have worked out, you will notice that it will be very similar but uses a pentode first stage.
That amp is also very similar to BinaryMike's concept, but BinaryMike's has also been tweaked to allow for direct-coupling and a freebie screen supply in the big cathode resistor. Of course, BinaryMike's concept creates a bunch of heat in that cathode resistor but that makes the direct-coupling possible.
Tone controls are not that common. If you want one it is easy enough to find a schematic of a tone control and put it before the amplifier stages. Adding a volume control is also very easy. Some people put the volume control in an external preamplifier.
They are there, often times they are just not shown in the schematic. Look at the picture of the amp in the link you posted. Notice the volume control on the front panel along with a power switch.
The cathode resistance dissipates 4.5W altogether. Taking 6SJ7 screen voltage at the tap has another advantage: It's a DC feedback path that acts to stabilize bias conditions.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.