Greetings, Friends. I was looking at some scans of the old Vacuum Tube Valley magazine and ran across their Octal Line Stage, posted in Issue 11, Spring 1999.
I'll include the entire article at the end of the post.
It's a nice looking circuit, a cathode follower.
Looks like the 100k volume pot is acting as the grid leak resistor, don't love that. I'd add a 1M from Pin 4 to Ground.
And I guess that's a voltage doubler (?) to make 12v to allow 12S_7 tubes, don't think I'll use that. Might run the tubes on AC to start, or use a bridge rectifier with 1N5820 and CRC to make 6.3vDC.
I'd like to use some of those Vishay film caps that are so cheap now. C1 in the power supply is much too large, even with the low current demand. I'd go with 20uF, and this cap from Vishay looks very nice at $8. The 2nd cap in the power supply is spec'd at 100uF 450v, a very affordable EL cap but a rather expensive film cap. Could I use a lower-value film cap in this position without impacting the performance of the preamp?
The cap in the amp circuit is very large, 330uF 450v. The best option here from Mouser is an Audio-grade EL with a rather short lifespan. Could I use another of those 20uF film caps? As there's no film cap option for the bypass cap (330uF 35v) I'd probably use an audio-grade EL for that, and just plan to replace both caps after 2000-ish hours.
Thoughts? Thanks for taking a look!
w
I'll include the entire article at the end of the post.
It's a nice looking circuit, a cathode follower.
Looks like the 100k volume pot is acting as the grid leak resistor, don't love that. I'd add a 1M from Pin 4 to Ground.
And I guess that's a voltage doubler (?) to make 12v to allow 12S_7 tubes, don't think I'll use that. Might run the tubes on AC to start, or use a bridge rectifier with 1N5820 and CRC to make 6.3vDC.
I'd like to use some of those Vishay film caps that are so cheap now. C1 in the power supply is much too large, even with the low current demand. I'd go with 20uF, and this cap from Vishay looks very nice at $8. The 2nd cap in the power supply is spec'd at 100uF 450v, a very affordable EL cap but a rather expensive film cap. Could I use a lower-value film cap in this position without impacting the performance of the preamp?
The cap in the amp circuit is very large, 330uF 450v. The best option here from Mouser is an Audio-grade EL with a rather short lifespan. Could I use another of those 20uF film caps? As there's no film cap option for the bypass cap (330uF 35v) I'd probably use an audio-grade EL for that, and just plan to replace both caps after 2000-ish hours.
Thoughts? Thanks for taking a look!
w
That resistor would just be in parallel with the pot wiper so would have no effect on the circuit, other than upsetting the pot control law.
As far as the caps are concerned, lowering their value will increase the hum on the supply lines.
I think you should be careful here.
And film caps are not indicated here - an elcap has some lossy part which is perfect to absorb some of the noise. A filmcap is a more perfect cap without losses and will just insert the noise into the ground line, which may cause troubles upstream.
Bottom line: f*ck with a fine product only if you are absolutely sure of what you are doing and willing to accept (and debug) the consequences.
Jan
This has some decent gain. Do you need gain? Probably not. At least remove the bypass cap on the gain stage 750 ohm resistor to drop some gain. Add a safety diode (1N4007) between pins 3 and 1, pointed at pin 3, to limit the grid voltage on power up. Yes, film caps are nice. You can use a 50uf - 100uf film cap instead of the 330uf 350v. The DC Link film capacitors can be had in 100uf sizes, they are relatively large but they are good and never expire. Replace the parallel first two 1K power supply resistors with 10H choke and split off after that. This will help lower the hum if you go with lower value caps. If you are not a complete Luddite you can substitute a MOSFET source follower for the cathode follower (No other circuit changes are required to do this, 2 is the Drain, 1 is the Gate and 3 is the source) then you can do this preamp with only 1 tube.
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So what does it mean by "trivial" design? is it not worth pursuing? Can it be improved to offer better sound quality?
I'd defer to Jan's expertise on the Power Supply caps. I was unaware of the Loss issues he presented, I was just thinking of lifespan. For Els, Nichicon CY should do here, maybe a F&T 2x100uF. I still think c1 is too big at 100uF, that strains the 5Y3, but it fills up slowly at start-up and gives a nice high b+ so it must be ok. They wouldn't have published it if it blew up tubes.
I like the idea of adding a choke, say the Hammond 157H, in place of the 1K 2W resistors. Might have to adjust that 100ohm 5w to get to 370v.
I have Soviet tubes I would like to use on this project; the 6H8C is reportedly equivalent to the 6SN7, and the 5C4M is a copy of the 5Y3. You can find them on eBay if you'd like to send a couple bucks to Ukraine.
I like the idea of adding a choke, say the Hammond 157H, in place of the 1K 2W resistors. Might have to adjust that 100ohm 5w to get to 370v.
I have Soviet tubes I would like to use on this project; the 6H8C is reportedly equivalent to the 6SN7, and the 5C4M is a copy of the 5Y3. You can find them on eBay if you'd like to send a couple bucks to Ukraine.
Agree about value of C1. Spec sheets usually call out 47uF max. I've used 68uF in that place without issues, but never stretched it to 100uF. Best would be to add a 47uF or 68uF before the 5W resistor (or choke). That's the more common design. Also, 100R resistors before pins 4 and 6 are also commonly used to help with surge.
Check out my recent post if you're willing to use a board instead of point-to-point.
Check out my recent post if you're willing to use a board instead of point-to-point.
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This is the oft used CC>CF circuit. This one DC copled, 1 less pole than if cap coupled. Frank’s preamp is a good example, the firt one we tried was the Bottlehead Foreplay.
We scratch built a 6922/6DJ8 variation. PS with choke input and all poly caps. Benefits from a couple CCSs.
Foreplay inpired circuot in a Dynaco FM-3 chassis. Suggested names were Dynaplay and Fornaco. The latter won out.
dave
We scratch built a 6922/6DJ8 variation. PS with choke input and all poly caps. Benefits from a couple CCSs.
Foreplay inpired circuot in a Dynaco FM-3 chassis. Suggested names were Dynaplay and Fornaco. The latter won out.
dave
Those resistors would be used to increase the Series Resistance that the rectifier sees from the power transformer, but they aren't necessary here.
If we look at the 5Y3 Datasheet we can see that the Total Effect. Plate-Supply Impedance Per Plate is 50 ohms with a Plate-to-Plate Supply Voltage of 700v and rises to 140ohms when the plates are supplied with 1Kv. As this rectifier is supplied with 600vAC from the transformer, we can assume the necessary Series Resistance for the rectifier tube is lower, around 40 ohms.
The Hammond 270 BX has a Secondary side DCR of 412.6 ohms, so that's 206 ohms per leg, well above the 40 ohm requirement.
Tubes such as the 5AR4 and 6CA4 have much higher Series Requirements. You'll see arcing in the tube if you don't have those resistors on the plates. Perhaps it has something to do with the indirectly heated cathodes.
w
You should refer to the rectifier's chart for required supply impedance based on the circuit's input cap. Yours is 100uF, not 10uF.
I mean very simple, and tolerant of changes. Simple circuits have simple shortcomings, so you can't really go wrong!
Care to elaborate? I've been trying to figure out how to use those charts.
If we're on page 5, the Cap Input Characteristics chart, and we assume 390v at Input to Filter, with 40mA drawn by the circuit, that puts us near Line 5. How do we determine Series Resistance as determined by the size of the filter cap at thia point?
Thanks!
w
This datasheet quotes 20uF with 50 ohms per plate at 350V, so for 100uF this suggests about five times more, or 250 ohms per plate at full load. http://www.r-type.org/pdfs/5y3gt.pdf
However, the load current is quite small for this circuit so only the start up conditions matter (which are guesswork really), and the voltage is lower. The circuit already has a 100R resistor plus transformer resistance per plate, so I'm gonna guess (wet finger in the air) that it's probably OK as designed.
However, the load current is quite small for this circuit so only the start up conditions matter (which are guesswork really), and the voltage is lower. The circuit already has a 100R resistor plus transformer resistance per plate, so I'm gonna guess (wet finger in the air) that it's probably OK as designed.
Before this thread goes off into the weeds, would anyone care to explain the heater circuit?
I haven't seen this diode-and-cap arrangement (bridge?) before. From the wiring of the tubes it seems he's getting 12.6vDC from a 6.3vAC source. Am I reading that correctly? I'll be using 6v tubes so no need for that.
I see the circuit is elevated by the 68k and 10k divider to the (-) leg of the heater supply. What is the purpose of the 22uF cap?
thanks
w
I haven't seen this diode-and-cap arrangement (bridge?) before. From the wiring of the tubes it seems he's getting 12.6vDC from a 6.3vAC source. Am I reading that correctly? I'll be using 6v tubes so no need for that.
I see the circuit is elevated by the 68k and 10k divider to the (-) leg of the heater supply. What is the purpose of the 22uF cap?
thanks
w