Note: I recently started a thread with the same name on Audiokarma.org.
I'm preparing for a Tubelab SE (or SE II) build with 45 tubes. I have a pair of NOS Tamura F-475 OPTs and couple of hunkin' PAECO PT's, though one of those should be plenty. I was getting ready to pull the trigger on a tube order from Brent Jessee, but his stock of 5842/417A tubes seems to have evaporated. I said to myself, "Well, self, we might as well get into some calculations loadlines while we ponder where to get some sweet driver tubes!" I annotated the SE II schematic and plotted up a few loadlines for the 45 tubes. Of course, the loadline for the 5842 is dead simple because the design uses a constant current source (CCS) on the plate, so the loadline is just a horizontal line. I found that the CCS is spec'ed for 12 mA, and the quiescent plate voltage is typically 175 V. Wait, what? Sure, the 5842 can take a max plate voltage of 200 V, but none of the characteristic graphs include lines for anything over 150 V. Moreover, the spec plate current is 26 mA at 150 V. That's an operating point that appears to give the tube some room to breathe, whereas 12 mA at 175 V is crammed down into the lower-right part of the plate characteristics.
Then, I look into necessary signal swings for the 45: 36 Vrms on the plate with mu=3.45 means 10.4 Vrms on the grid, or a swing of +/- 14.8 V. Even with the operating point of the 5842 crammed into the lower right, that should be fine and would require, wait for it, 0.243 Vrms on the grid of the 5842. Quarter-volt sensitivity? Jeez, seems kind of ridiculous, but OK.
Still, might another, more readily available tube do the trick? The Powerdrive stage in the design (i.e., MOSFET source follower between driver and output stages) seems to open up a lot of options. I look into the 6J5, which is the single-triode equivalent of the 6SN7. It has less than half the gain (mu=20 vs. the 5842's mu=43), but all that gain of the 5842 seems a bit over the top. Plus, there seem to be issues with the 5842: It's microphonic, and according to one post, it threatens to oscillate while sitting in the cardboard box.
So why fuss with the 5842? Why not the 6J5? It has low distortion, and mainly 2nd harmonic. With less current and substantially higher, but still reasonable, plate voltage, the operating point can be smack in the middle of the plate characteristics graph. Thoughts?
A summary of some replies to this post on Audiokarma.org:
I'm preparing for a Tubelab SE (or SE II) build with 45 tubes. I have a pair of NOS Tamura F-475 OPTs and couple of hunkin' PAECO PT's, though one of those should be plenty. I was getting ready to pull the trigger on a tube order from Brent Jessee, but his stock of 5842/417A tubes seems to have evaporated. I said to myself, "Well, self, we might as well get into some calculations loadlines while we ponder where to get some sweet driver tubes!" I annotated the SE II schematic and plotted up a few loadlines for the 45 tubes. Of course, the loadline for the 5842 is dead simple because the design uses a constant current source (CCS) on the plate, so the loadline is just a horizontal line. I found that the CCS is spec'ed for 12 mA, and the quiescent plate voltage is typically 175 V. Wait, what? Sure, the 5842 can take a max plate voltage of 200 V, but none of the characteristic graphs include lines for anything over 150 V. Moreover, the spec plate current is 26 mA at 150 V. That's an operating point that appears to give the tube some room to breathe, whereas 12 mA at 175 V is crammed down into the lower-right part of the plate characteristics.
Then, I look into necessary signal swings for the 45: 36 Vrms on the plate with mu=3.45 means 10.4 Vrms on the grid, or a swing of +/- 14.8 V. Even with the operating point of the 5842 crammed into the lower right, that should be fine and would require, wait for it, 0.243 Vrms on the grid of the 5842. Quarter-volt sensitivity? Jeez, seems kind of ridiculous, but OK.
Still, might another, more readily available tube do the trick? The Powerdrive stage in the design (i.e., MOSFET source follower between driver and output stages) seems to open up a lot of options. I look into the 6J5, which is the single-triode equivalent of the 6SN7. It has less than half the gain (mu=20 vs. the 5842's mu=43), but all that gain of the 5842 seems a bit over the top. Plus, there seem to be issues with the 5842: It's microphonic, and according to one post, it threatens to oscillate while sitting in the cardboard box.
So why fuss with the 5842? Why not the 6J5? It has low distortion, and mainly 2nd harmonic. With less current and substantially higher, but still reasonable, plate voltage, the operating point can be smack in the middle of the plate characteristics graph. Thoughts?
A summary of some replies to this post on Audiokarma.org:
- Some recommend ditching the Tubelab design or aspects thereof.
- I could probably use 6J5's, and it might be fine.
- The 6J5 is pretty awesome.
- But the lower gain of the 6J5 would make the amp less dynamic-sounding.
- Turns out the 5842 works really well with the Tubelab design.
- Yeah, the operating point is a bit weird, but don't worry about it.
- If you find you don't like the original operating point, the design has a lot of adjustability built in, and you can change the operating point in conjunction with testing.
- 5842 tubes are available at reasonable prices. (Subtext: You don't have to go with the WE's or Amperex gold pins.)
- The PCB schematic is annoying to some curmudgeons, but I'll have to live with it. Or make my own. Whatever. And get off my lawn!
- I should probably put my question to a couple of other forums, including DIY-audio, where I'd likely get a response from the designer, George.
The original TSE was designed over 20 years ago. At that time 5842's could be found for under $5 each. During the initial design I did real world testing on lots of different tubes that I had or could get a decent sample size of. The design was always intended for the 45 output tube, but I also tested the 2A3 and 300B and wanted the design to be flexible enough to use any of them. After weeding out lots of tubes for the usual reasons some serious bench testing and listening test left me with the CCS loaded 5842 as the best choice. Any of the tubes with a Mu of less than 30 did not have enough gain to drive a 45 tube into clipping with a cell phone as the source. Yeah, people still do this. and even the 5842 falls short with a 300B output tube. If you have a preamp or other source with enough output and will not use a 300B tube, you might get away with a low gain input tube, but I doubt it.
I have not seen Microphonic issues with my Raytheon 5842 tubes. Yes, the tube will oscillate if it is used in a layout with long or poorly routed wiring. Any tube with a high Gm will oscillate if the conditions are right.
Several hundred TSE's and TSE-II's have been built in 20 years. The user response has been great, and the most common complaint was regarding not enough overall gain especially with 300B tubes.
The TSE became the TSE-II back in 2019 after the Sharp filament regulator chip went extinct. At that time I asked the existing user base what they would do differently for the TSE-II. If you are making your own amp, you might want to read through that thread;
https://www.diyaudio.com/community/threads/after-a-14-year-run-the-tse-must-die.331038/
During the testing done long ago the 6J4 tube was the number two choice. Most of the tubes I tested were the mil-spec version, the 8532 / 6J4WA.
I am looking at a driver circuit for an even more demanding amp design than the TSE. This one uses a TV sweep tube, or a paralleled pair for 15 to 35 watts of SE power. The output stage uses UNSET technology to make triode curves on a TV sweep tube. The driver stage uses a pentode in UNSET combined with a circuit I designed for squeezing maximum gain (over 1000 V/V) from a pentode in a guitar amp application to generate large drive voltages at near zero distortion. See posts #110 to 119 here:
https://www.diyaudio.com/community/...he-way-i-always-wanted-to-do-it.410333/page-6
I have not seen Microphonic issues with my Raytheon 5842 tubes. Yes, the tube will oscillate if it is used in a layout with long or poorly routed wiring. Any tube with a high Gm will oscillate if the conditions are right.
Several hundred TSE's and TSE-II's have been built in 20 years. The user response has been great, and the most common complaint was regarding not enough overall gain especially with 300B tubes.
The TSE became the TSE-II back in 2019 after the Sharp filament regulator chip went extinct. At that time I asked the existing user base what they would do differently for the TSE-II. If you are making your own amp, you might want to read through that thread;
https://www.diyaudio.com/community/threads/after-a-14-year-run-the-tse-must-die.331038/
During the testing done long ago the 6J4 tube was the number two choice. Most of the tubes I tested were the mil-spec version, the 8532 / 6J4WA.
I am looking at a driver circuit for an even more demanding amp design than the TSE. This one uses a TV sweep tube, or a paralleled pair for 15 to 35 watts of SE power. The output stage uses UNSET technology to make triode curves on a TV sweep tube. The driver stage uses a pentode in UNSET combined with a circuit I designed for squeezing maximum gain (over 1000 V/V) from a pentode in a guitar amp application to generate large drive voltages at near zero distortion. See posts #110 to 119 here:
https://www.diyaudio.com/community/...he-way-i-always-wanted-to-do-it.410333/page-6
Sob! But perhaps the fact that the WE versions of this tube are now going for $100-200 each is some testament to the 5842's utility in your design.
It's hard to argue with actual bench and listening tests, and I'm happy to benefit, as so many apparently have already, from your hard work.
I get it. I could be a purist and insist that a more proper sensitivity is ~2 Vrms, but all else being equal, it's nice to be able to use just about any preamp or receiver's or integrated amp's pre-outs to drive the power amp. Without the need for GNFB, then as long as the driver makes beautiful music, more gain is more better!
I've read some of it and will give it a complete and careful read before building. May I ask, though, does the thread suggest any changes from the TSE-II BOM you've posted? (attached here for reference is TSE-II_V1.2_BOM_7-26-19.pdf) Last I checked, all of the solid state components listed there are still available.
Can you comment on what made the 5842 the ultimate winner? I see that the 6J5 has even higher gain, but maybe the CCS cuts a better loadline through the characteristic curves of the 5842.
That sounds wicked cool. I have a smidgen of experience with TV sweep tubes; I got a Sansui 1000 last summer. It uses 25E5's. They're pretty cheap when you can find them!
I greatly appreciate your response to my post. I will proceed with Plan A, which is to follow your design and use the 5842. I have an acquaintance who is an SET purist sort of guy, and he pointed me to your Simple45 pages a few years ago. I recently received, free of charge, a pair of NOS Tamura F-475 OPTs from a local "collective" of tube enthusiasts. These OPTs were originally purchased new in 1998 but were not used before the original owner passed away and left the OPTs to the collective. I feel a certain burden, or at least obligation, to do these OPTs justice and build the best possible amp with them. I've been building and modding tube amps just long enough to recognize and appreciate the utility of your Powerdrive design. It's an elegant solution and reminiscent of Alexander's cutting of the Gordian Knot. Make the best driver stage. Make the best output stage. Problem coupling the two? No problem, because we have MOSFETs!
If you're at all interested and do the YouTube thing, I've made a video about your design:
Thanks again,
Stephen Lancaster
There is another tube that appears to be part of the same family as 5842/417A and that is 5847/404A. It is a pentode but is high gain with nice triode curves. They are still plentiful and cheap - is it a like a 5842 when triode wired?
The image comes from VT52.com, but I cannot navigate it on my PC.
The curves for the 5842 are not quite so steep, but if you ignore the half volt curves and match the axis, aren't so far away.
The image comes from VT52.com, but I cannot navigate it on my PC.
The curves for the 5842 are not quite so steep, but if you ignore the half volt curves and match the axis, aren't so far away.
I picked 8 NOS 5847's with the intention of using them as a cheap substute. There was quite a spread in the test results on my curve tracer. Not had the need to fit them yet, but the pin connections are somewhat different so not any easy swap being pcb mount.
Are we talking about the same 6J5? The only 6J5 tube that I know of is the old octal tube available in metal or glass envelope that is essentially half of a 6SN7 with a Mu of 20.
I just discovered that there is a Russian tube often translated to 6J5 that is a miniature glass tetrode. The original TSE and the TSE-II evolved from an amp I designed over 20 years ago. At that time Russian tubes were not so common in the US other than what could be sourced from the Ukraine on Ebay. I have never seen one.
The typical operating conditions for a 45 show 2 watts of power output with a maximum of 275 volts on the plate into 4600 ohm load in class A1. Nearly 100 volts peak to peak of drive are required to make those 2 watts. There are several possible sets of operating conditions for class AB2 push pull with power outputs up to 19.1 watts, so operation in the positive grid voltage region is acceptable with the 45 tube. When this data was published transistors of any kind did not exist. Pushing drive current into a grid generally required a hefty driver tube and a transformer. Most of the literature surrounding amps whose output stage drew grid current discussed push pull operation, primarily for second harmonic distortion cancellation.
The PowerDrive configuration used in the TSE can source plenty of grid current without distortion. The 45 can handle an unspecified amount of grid current, but a DC current of 4.0 mA and a peak current of 10.6 mA are listed in the 18 and 19 watt push pull amp's operating conditions. This convinced me to explore the 45 tube in A2 operation. All of my 45's were collected 20+ years ago when you could buy used tubes on Ebay and at hamfests for $5 to $10 each. All were tested for max power output at 3% THD in a TSE breadboard running on 320 volts of B+ (about 310 at the plate due to OPT DCR). This is essentially a test of emission current capability. I have 5 old tubes that make 2.9 to 3.2 watts. One set were in my original Lexan TSE for about a year with no degradation during that time. A chance purchase of a rusty old Sparton radio chassis at an outdoor flea market yielded a pair of NX-483 tubes. Once I figured out that they were 45's with a 5 volt filament, they went into the TSE where they still live nearly 20 years later. These tubes had the usual stickers with an installation date in 1929. Will they live to see 100? Will I live long enough to see it happen?
Pushing a 45 to 3 watts, or a 300B to 10 watts requires about 150 volts peak to peak of clean drive. The choice of driver tube was basically try a handful of every tube I had and pick the tube and operating conditions that gave me the 150 volts peak to peak at the lowest THD consistently from tube to tube. The 5842 and others with the required gain all showed variability from tube to tube. That's why there is a pot in the cathode to set the plate voltage. The current is set by the CCS chip, but I had a pot there too. 10 to 12 mA was the best point for THD on all 5842's and WE417's. The optimum voltage varies from 150 to 175 volts. The original TSE was a well respected amp, so I chose not to touch the audio path when I had to replace the filament regulator circuit.