Very good, thank you.
Quoted from George's site;
"Increase the grid stopper resistors on the 5842 (R31 and R32) from 1K to 4.7K ohm. These resistors should be carbon composition if at all possible. The 5842 will oscillate if given the opportunity. The higher stopper resistor value will help. If you are using a volume pot on the board, make sure that the case of the pot is grounded. The cable from the board to the input jacks should be connected to ground on the PC board, and connected to ground at the input jacks. All ground connections to the chassis should be made at the input jacks. See the wiring diagrams for the Simple SE for details. The oscillation occurs in the 100KHz range which is inaudible, but may be heard as a hum or buzz. It can also cause the output tube current to rise unexpectedly."
This is most likely his problem. High gm tubes can be a pain in the butt sometimes.
Since the better shielding and also the higher grid stopper which in turn w/ Mr. Miller keeps high frequency oscillations at bay I wonder if fine tuning the low end could help clean things up? I am curious if anyone has tried different value cathode bypass caps.
Thanks guys but I just pulled one lead on the R31 and R32 resistors and checked them and they are 4.7K ohm. They are metal film instead of carbon though. I will have to get some carbon ones when I return to this board.
bermtoog/Kevin: I also had -300V bias on the 300Bs after first completing the build. Since the schematic indicates -150V bias, I ended up removing one of the hexfreds and using off-board resistors for R6 (10k 25w) and R7 (6.8K 25W) to bring it down to -150V, which drastically cleaned up the sound and lent it more openness. If you think it would help, you can follow the process in posts #134-#168 in this thread, or just go here for the grand finale.
I also blew the 5V filament regulator for the 300Bs when the amp was hooked up to the oscilloscope with dummy load resistors (it was pretty much distortion free and linear from 30Hz to well above 20kHz, with an input sensitivity of 0.78V). I ended up replacing it and adding a second regulator with a breakout circuit (scraping away the trace on the PCB between the two 300Bs), so that there's one per channel. It halves the current draw on each IC and results in better channel separation, increased dynamics, and more detail. That process is documented here, if you're interested.
In all, I think the TSE's origin as a 45 amp is clear, as a lot of people have challenges with the higher voltages required for 300B operation. Once you get it dialed in, though, it's a very nice amp.
I'm with you there, wdecho. Which project are you doing (or is it a self-design?).
The TSE 300B has the refinement that was a goal for the project (I used 5K:8ohm OPTs and higher B+/bias current for less distortion). It has all the bass performance and coherence of a high quality SS amp without the transistory/feedbacky sound. I mean, the TSE is in TOTAL control of the speakers.
But now I'm pining for the lush decadence of a traditional 300B, and am considering removing the MOSFETs from the TSE (with other modifications to pull it off) to see if it gives more of the tube sweetness. If I don't like it, or if there's no major change, then I've got a pair of 2SK3563 to put back in.
Regardless, I'm planning to make a pair of 300B monoblocks that are more about romance, as it would be nice to have both to listen to. Right now I'm thinking either the JC Morrison Fi Primer or the JE Labs 300B SET.
I also blew the 5V filament regulator for the 300Bs when the amp was hooked up to the oscilloscope with dummy load resistors (it was pretty much distortion free and linear from 30Hz to well above 20kHz, with an input sensitivity of 0.78V). I ended up replacing it and adding a second regulator with a breakout circuit (scraping away the trace on the PCB between the two 300Bs), so that there's one per channel. It halves the current draw on each IC and results in better channel separation, increased dynamics, and more detail. That process is documented here, if you're interested.
In all, I think the TSE's origin as a 45 amp is clear, as a lot of people have challenges with the higher voltages required for 300B operation. Once you get it dialed in, though, it's a very nice amp.
I'm with you there, wdecho. Which project are you doing (or is it a self-design?).
The TSE 300B has the refinement that was a goal for the project (I used 5K:8ohm OPTs and higher B+/bias current for less distortion). It has all the bass performance and coherence of a high quality SS amp without the transistory/feedbacky sound. I mean, the TSE is in TOTAL control of the speakers.
But now I'm pining for the lush decadence of a traditional 300B, and am considering removing the MOSFETs from the TSE (with other modifications to pull it off) to see if it gives more of the tube sweetness. If I don't like it, or if there's no major change, then I've got a pair of 2SK3563 to put back in.
Regardless, I'm planning to make a pair of 300B monoblocks that are more about romance, as it would be nice to have both to listen to. Right now I'm thinking either the JC Morrison Fi Primer or the JE Labs 300B SET.
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I really like my SSE amp I built that George designed. Right now it is my favorite amp of the more than dozen I have built including some Firstwatt clones. I have not gave up on this SE build yet. I built the SSE without a board building point to point without any serious hiccups. Played great right from the get go.
Right now I am working on the JC Morrison design with a few refinements. I am using a regulated heater supply for the 300B's. I also have split the PS by adding some more caps and resistors along with using some motor run caps for better channel separation. Mine is going to be sorta a cross with the JC Morrison and the newer takeoff,
DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier Project
I am using the same PS as the tubelab which is lower V than the specified but right now without a load I have 500V which should be close to my target B+ of 380V or more. To keep the same bias resistors I understand you need at least 380V for the amp to self bias with parts described. I much more enjoy building point to point. Building is as much enjoyment to me as the listening aspect of audio.
Right now I am working on the JC Morrison design with a few refinements. I am using a regulated heater supply for the 300B's. I also have split the PS by adding some more caps and resistors along with using some motor run caps for better channel separation. Mine is going to be sorta a cross with the JC Morrison and the newer takeoff,
DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier Project
I am using the same PS as the tubelab which is lower V than the specified but right now without a load I have 500V which should be close to my target B+ of 380V or more. To keep the same bias resistors I understand you need at least 380V for the amp to self bias with parts described. I much more enjoy building point to point. Building is as much enjoyment to me as the listening aspect of audio.
Wow, thanks so much for all that. That's a good idea. Question though: would just removing one of the freds bring it down enough since it's only half-wave at that point?
So you really noticed a big difference in the sound? I'm also wondering if it would make the bias adjustment less temperamental.
Nope, only drops it about 30V because of the big caps (still around 280V or so after everything comes up). I ordered a few resistors to try out. I'm going to try raising R6's value and see what that does in an attempt to leave the board as stock as possible and not have major power dissipation.
That sounds about right. I remember the B- being about -274V after removing one of the hexfreds. You'll probably also need to change the value of R7 in addition to R6, and use the 25W heatsinked resistors shown earlier in this thread. You're going to burn off A LOT of heat if you do that.
Hello. My TSE with 300B's is working. My B+ is around 370 volts depending on what my AC is doing at the time. But my B- is at about -280 volts.
I posted this once before and was told that the -150 volts on the schematic was for a 45 power tube, and that the -280 was normal for a 300B power tube.
It seems you are saying that this is not true?
You left R6 the same value at 10K, and just used a 25 watt one? And changed R7 to 6.8K, 25 watts. And remove one Hexfred Diode.
Ill be watching to see what you decide on as far as the R6 value.
Did reducing the B- in this manner cause any other measurements to change? Just wondering if I will need to readjust anything else after lowering the B- voltage. Thank you for this great and very useful post.
I posted this once before and was told that the -150 volts on the schematic was for a 45 power tube, and that the -280 was normal for a 300B power tube.
It seems you are saying that this is not true?
You left R6 the same value at 10K, and just used a 25 watt one? And changed R7 to 6.8K, 25 watts. And remove one Hexfred Diode.
Ill be watching to see what you decide on as far as the R6 value.
Did reducing the B- in this manner cause any other measurements to change? Just wondering if I will need to readjust anything else after lowering the B- voltage. Thank you for this great and very useful post.
The B- and B+ rails were pretty independent. I don't recall any changes to other parts of the amp after adjusting B-.
Other builders have said -280 and more is fine. In my case, though, there was a definite sound improvement in bringing it down to -150.
The reason for going to a 25W for R6 is that the original resistor kept getting too hot and giving off a bad odor. Even a 10W in that position got too hot. The 25W is warm but doesn't cook itself.
Other builders have said -280 and more is fine. In my case, though, there was a definite sound improvement in bringing it down to -150.
The reason for going to a 25W for R6 is that the original resistor kept getting too hot and giving off a bad odor. Even a 10W in that position got too hot. The 25W is warm but doesn't cook itself.
If removing the Hexfred only dropped B- about 25 volts, Could the B- voltage be adjusted without removing the hexfred and bringing R6 value up and R7 value down? Or dose the hexfred have to be removed? And the resistor values adjusted also?
I'm going to put the fred back in because the only thing removing it can do is introduce more ripple in the supply. The purest DC signal is always best.
Interesting. I know that a few components are different and of course the power transformer is certainly different, but the schematic shows 250-380V for B+ but only says -150 for B-. I would like to know if there is an engineering rationale for trying to maintain -150V if if you are able to consistently bias the tubes (of course, mine seem to drift very easily over short periods of time).
Gee, I really wish George would chime in on this topic. He designed the circuit, and should be the one to know about this topic. I would like to have this amp dialed to for top performance.
Please George, If you are out there. -280 volts or -150 volts? And how to lower it, if need be? thank you.😕
Please George, If you are out there. -280 volts or -150 volts? And how to lower it, if need be? thank you.😕
When George designed this circuit years ago I'm sure he used his design skills to get the optimum best from the circuit
and it appears that most builders over the years have been pleased with the design as is.
If a builder thinks he can improve on this or other Tubelab designs then he is free to do his own modification to suit his taste.
However, George may or may not decide to comment on your question.
I mean no disrespect to George or anyone. When I read Georges web page I decided to build the TSE because George seemed like a really great engineer, and a very nice person also. I thought his design was superior.
I would just like to know if the -150 volts on the schematic applies to all output tubes used with this circuit?
I won't remove any parts, or change anything in his design. But if I need to change some resistor values I would do that.
I really just want it to work as George intended it to.
Thank you all.😀
There should've been no disrespect taken. This is a perfectly legitimate question and one that I think this forum is suited for. After all, why even have a forum if we can't draw from the knowledge of others.
It really kind of depends on the FET, I guess. Raising B+ without lowering B- proportionally will operate the FET in a different part of its curve. It may not matter.
That's a good thought, although I don't really know what I'm looking for:
https://www.fairchildsemi.com/datasheets/FD/FDP5N50NZ.pdf
I do see that the drain to source voltage is 500V, so does that mean that the B+ and B- should be closer in order to avoid component failure?
FWIW, square wave output looks identical to what I've seen on the web for my output transformers and sine waves look very smooth, but that's not telling the whole story when it comes to audible noise, distortion, and longevity of components.
I did several things actually, including ruining a 5842 somehow (plate is red hot compared to the other one... wasn't that way before). I blew a MOSFET on the same channel that the 5842 was on. I'm waiting on a replacement to confirm everything's working correctly.
The Hammond transformer I'm using has a 125V and 115V tap but my house voltage is ~120. I originally had it on the 115V tap but I changed it to 125V to lower the voltage. That brought the B+ down to around 380-390 under load (depending on bias current).
Now to your question, the fred needs to be in there. Without it, you're only getting half the wave and, by extension, increasing the ripple in the DC. No-brainer, put it back in.
In addition to lowering B+, I also changed R6 from 10K to 40K. This lowered B- to around 140-150V under load and decreased total dissipation. JP seemed to imply what I've been wondering... is there a reason to lower B-? ---
#2 is a pretty good reason since, as I said above, the drain to source voltage of the MOSFET I'm using is 500V maximum. Other than that, I don't think the B+/B- voltages are critical enough to worry about 10 or 20 volts.