Hello All,
I hope that someone will be kind enough to help me since I am disappointed with my just completed 2 mono blocks SET 300B amplifiers. Based on a simple schematic: 5842 Raytheon + Mûndorf 0.22 cap + 300B Svetlana + Partridge Xformer (all tubes = matched pairs). All voltages and bias are ok for both channels (180V/22mA) and (410V/70mA) automatic bias. I also used good quality componants: Caddock, Cerafine... Everything is ok except
the sound stage! By exemple, with some Cds that I know very well I can't really locate the singer wich is exactly in the middle at a couple of meters when I use a 'basic' solid state integrated amplifier. The problem is the same with my P610 Diatone and my Harbeth HL-P3. I you have any idea, please let me know.
kind Regards
Jeepy, Switzerland.
I hope that someone will be kind enough to help me since I am disappointed with my just completed 2 mono blocks SET 300B amplifiers. Based on a simple schematic: 5842 Raytheon + Mûndorf 0.22 cap + 300B Svetlana + Partridge Xformer (all tubes = matched pairs). All voltages and bias are ok for both channels (180V/22mA) and (410V/70mA) automatic bias. I also used good quality componants: Caddock, Cerafine... Everything is ok except
the sound stage! By exemple, with some Cds that I know very well I can't really locate the singer wich is exactly in the middle at a couple of meters when I use a 'basic' solid state integrated amplifier. The problem is the same with my P610 Diatone and my Harbeth HL-P3. I you have any idea, please let me know.
kind Regards
Jeepy, Switzerland.
The quality of the cathode bypass capacitors are critical in an auto-bias circuit. What are you using in these locations?
Your description sounds more like a lack of resolution. Careful listening may reveal that a lot of information you have been hearing with the other amplifiers is just not being reproduced.
I've designed and built a lot of 300B SE amplifiers and they generally image pretty well in my experience - no inherent reason with either of the tube types you have chosen to have this problem.
Can you post the schematic or provide a link to it?
Edit: Can you post some pictures of your amplifiers showing internal construction details such as grounding, component placement and anything you think might be relevant.
Your description sounds more like a lack of resolution. Careful listening may reveal that a lot of information you have been hearing with the other amplifiers is just not being reproduced.
I've designed and built a lot of 300B SE amplifiers and they generally image pretty well in my experience - no inherent reason with either of the tube types you have chosen to have this problem.
Can you post the schematic or provide a link to it?
Edit: Can you post some pictures of your amplifiers showing internal construction details such as grounding, component placement and anything you think might be relevant.
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Bypass caps are Cerafine 100mF/100V for 5842 and 220mF/100V for 300B.
Bandwith is ok, signal is clean but I noticed that the amps are cliping for 1VRMS sine wave for almost of the frequencies between 20Hz and 20Khz on input wich means 5.3 V RMS (about 15V peaks on the scope) on output with a 7.5 Ohms resistor load. If I am right, I'm just a hobbyst not an engineer, this means that the max power is U2/R = 3.74 W where is amp is supposed to deliver 7 to 8 W!?!
I put the schematic on the web for you. Many thanks.
300b - image / photo - Image perso - Hiboox
Bandwith is ok, signal is clean but I noticed that the amps are cliping for 1VRMS sine wave for almost of the frequencies between 20Hz and 20Khz on input wich means 5.3 V RMS (about 15V peaks on the scope) on output with a 7.5 Ohms resistor load. If I am right, I'm just a hobbyst not an engineer, this means that the max power is U2/R = 3.74 W where is amp is supposed to deliver 7 to 8 W!?!
I put the schematic on the web for you. Many thanks.
300b - image / photo - Image perso - Hiboox
I looked quickly at the schematic, basically I would recommend taking an oscilloscope and measuring the signal voltages in each stage just prior to the onset of clipping to determine whether the design intent is being met. Are all voltages as shown in the schematic?
You should be able to determine where the problem lies. Incidentally 1Vrms is insufficient based on the available gain to drive the amplifier to full output which would require ~140Vpp on the grid of the 300B, 1Vrms given the basic design would give you about 45Vp (90Vpp) at the grid of the 300B. Something like 1.4Vrms is required and this may exceed the available cathode bias if it is less than the expected value. (Resulting in very poor linearity)
I suspect the bias on the first stage is more than a bit off - some tweaking might be required to get the voltages just right. I've found that high transconductance triodes and pentodes do not always bias that predictably due to the fact that minute changes in effective grid bias can cause very big changes in operating point due to the steep slope of their characteristic curves. IMO conventional cathode bias does not work well with these tubes at all. (I have gone to LED or fixed bias and tweak circuit constants over a small quantity of bogey tubes to get consistent performance.)
Were it me once I understood the problem I would replace the RC based cathode bias with a ~ 2.5V - 3V led (maybe a green led?) and use a CCS in the plate circuit of the 5842 to set the desired operating conditions. Based on my experience this would allow you to swing enough on the plate of the 5842 to reach full output.
Related issue: I have not yet found a large value electrolytic that sounds (or measures) ok with just a couple of volts of dc on it. I have not tried the very low voltage Black Gate which might be ok - or not.
I would start with just a LED for biasing the 5842 and then tweak the supply dropping resistors (not plate load resistor) to get the plate voltage in line, but a CCS or even a plate choke (Helps linearity with large signal swings and can swing above the quiescent supply voltage) are worthwhile experiments. Note that even working optimally there is not a lot of margin for variations in operating point in that first stage. Another tube with more gain that is worth considering is the D3A - I like it even more for these sorts of tasks than the 5842 which is really more suitable for direct driving types that require less than 100Vpp for full output. (An IT with a mild step up changes the picture considerably.)
Note that output power will be slightly reduced if you are using an OPT with a 3K primary, but you would still be able to achieve at least 7Wrms out with a properly working driver stage.
One other comment is I do not see a grid stopper resistor on the grid of the 5842, this is critically important because without it the tube can oscillate at frequencies well beyond the range of most scopes used for audio work. (VHF typically sometimes > 100MHz) A 221 - 1K resistor mounted right at the socket in series with the grid should be used. I have found that most resistors (even metal films) work just fine in this application despite the sensible recommendation for a carbon composition resistor in this location. (CC are non-inductive into the VHF region, whereas the others generally aren't - IME in most cases any resistor except a wirewound is better than no resistor.)
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This surely explains the bad sound. Stoppers should go on all 4 grids. Stoppers should also probably go on the plate and cathode.
May I momentarily hijack this thread and ask an oscilloscope question?
This is a stupid technical question, but how do I measure the signal voltages at the output of each stage with the oscilloscope. My digital oscilloscope has an input voltage limit of 30V p-p, so if I use the 10x probe, can I just measure up to 300V? That would be fine for the input stage, so would I just need a 100x probe for the output? Or do I have a concept wrong and am going to blow up my oscilloscope?
I would put the 280 ohm power dropping resistor between the rectifier tube and the choke. I don't know if this has any impact on the sound but I would rather get the B+ from the choke and capacitor so that both of them would provide the energey to the 300B when needed.
The sound stage mostly affected by the speakers and the room accoustics. Take your amps to your friend's home that has good sound stage for comparison.
Regarding the output power, it should be about 7 watts RMS with 410V B+. You need to drive the output waveform starts to clip on the top and/or the bottom for the calculation. It may need more than 1V at the input. Just crank it up.
Johnny
The sound stage mostly affected by the speakers and the room accoustics. Take your amps to your friend's home that has good sound stage for comparison.
Regarding the output power, it should be about 7 watts RMS with 410V B+. You need to drive the output waveform starts to clip on the top and/or the bottom for the calculation. It may need more than 1V at the input. Just crank it up.
Johnny
Thanks again to all,
I could not see any oscillation on the driver with the oscilloscope. However I have added a 180 ohms connected to the four grids. But before I can dare to connect any loudspeaker to the amps and check for the nice 3D sound stage I am expected to hear, I must find why the amps are cliping for 1VRMS on the input which means 5.3 VRMS only (measured of course) at the output for a 7.5 ohm resistor load connected to the 2.3k/8 Ohms Xformer, resulting in a max power of 3,7 W where 7-8W is expected.The swing on the plate of the driver is about 120V peak to peak for 1VRMS on the input and something about 150 V for 1.6V. I could probably go further but this is the limit of my generator (Imac Apple + Audacity).So again, the amps start clipping with 1VRMS on the input. So I suspect a problem with the second stage even if Ht, bias seem to be correct! I'am not an engineer and I'am starting to get crazy!
I could not see any oscillation on the driver with the oscilloscope. However I have added a 180 ohms connected to the four grids. But before I can dare to connect any loudspeaker to the amps and check for the nice 3D sound stage I am expected to hear, I must find why the amps are cliping for 1VRMS on the input which means 5.3 VRMS only (measured of course) at the output for a 7.5 ohm resistor load connected to the 2.3k/8 Ohms Xformer, resulting in a max power of 3,7 W where 7-8W is expected.The swing on the plate of the driver is about 120V peak to peak for 1VRMS on the input and something about 150 V for 1.6V. I could probably go further but this is the limit of my generator (Imac Apple + Audacity).So again, the amps start clipping with 1VRMS on the input. So I suspect a problem with the second stage even if Ht, bias seem to be correct! I'am not an engineer and I'am starting to get crazy!
Romy the cat wrote about Lamm phonostage using WE 417 tube and its influence on the sound. (response to Dick Olsher review of the phono stage'
"He quite wrong about 417A. First of all ~85% of 417A are not suitable for the LP2’s first stage. Second this damn 417A after 2 weeks of operating drift with it’s parameters absolutely randomly absolutely hugely. Among perhaps 40 tubes that I did try and that WERE suitable for the first stage none of the pairs remained marched after a week or two of operation (and the tubes WERE burned-in 100H before I matched them!!!). I mean 2-3 weeks of running LP2 and one of your channels has minus 2-3 db relative to another. Good luck with that High End! The 5842 were more stable but they sounded more mechanical then 417A. Let even forget about the huge capacitate of the 417A (and Lamm shunted his MM input with an extra, if I remember correctly, 200pF cap!!!) and go for the most important thing. The most important thing is that 417A/5842 is juts unspeakably CRAPY SOUNDING TUBE. Get any piece of audio equipment that use 417A and you will hear the same tacky 417A sound with a synthetic upper range – or the typical WE sound.""
I would say the LP2 could do with imaging approximately 30% of horizontal/vertical L1/L2’s positioning tricks and minus 10000000% of L1/L2’s depth positioning. This is a huge problem with this corrector – THE LP2 IT IS HORRIBLY FLAT, DEPTH-WISE. Actually the depth presentation is not juts flat (like a brick wall) but it also has a completely skewed harmonic proportion and the images size as decay into reverberation. I EXPERIENCED THE VERY SAME EFFECT at each and single installation where the LP2 was used. This is very very unfortunate and from my perspective THAT is completely terminal flaw of LP2. Vladimir very “suggestively” tried to imply at his web site….
New Page 4
…. suggesting that “LP2 is especially effective in large systems” and hoping that a larger reverberation time of large system would mask out the LP2 depth’s limitations but unfortunately he does not know (or do not care) all truth about LP2. (Here is where some listening would be useful for a supercilious Vladimir but trying to get a better product in the hands of his customer is not a variable in his Formula of Human Hearing). So, even in context of a large systems with a SPECIAL POSITIONING OF LOUDSPEAKER TO DEAL WITH THE LP2 DEPTH LIMITATIONS (and I went to that extend) you can therapy the geometry and positioning of images in depth. However you can’t fix that completely horrible digitalization by LP2 of the images that presented behind of the immediate front of sonic presentation.
Curtesy of GoodsoundClub.com (I hope he won't mind me reposting this excert")
"He quite wrong about 417A. First of all ~85% of 417A are not suitable for the LP2’s first stage. Second this damn 417A after 2 weeks of operating drift with it’s parameters absolutely randomly absolutely hugely. Among perhaps 40 tubes that I did try and that WERE suitable for the first stage none of the pairs remained marched after a week or two of operation (and the tubes WERE burned-in 100H before I matched them!!!). I mean 2-3 weeks of running LP2 and one of your channels has minus 2-3 db relative to another. Good luck with that High End! The 5842 were more stable but they sounded more mechanical then 417A. Let even forget about the huge capacitate of the 417A (and Lamm shunted his MM input with an extra, if I remember correctly, 200pF cap!!!) and go for the most important thing. The most important thing is that 417A/5842 is juts unspeakably CRAPY SOUNDING TUBE. Get any piece of audio equipment that use 417A and you will hear the same tacky 417A sound with a synthetic upper range – or the typical WE sound.""
I would say the LP2 could do with imaging approximately 30% of horizontal/vertical L1/L2’s positioning tricks and minus 10000000% of L1/L2’s depth positioning. This is a huge problem with this corrector – THE LP2 IT IS HORRIBLY FLAT, DEPTH-WISE. Actually the depth presentation is not juts flat (like a brick wall) but it also has a completely skewed harmonic proportion and the images size as decay into reverberation. I EXPERIENCED THE VERY SAME EFFECT at each and single installation where the LP2 was used. This is very very unfortunate and from my perspective THAT is completely terminal flaw of LP2. Vladimir very “suggestively” tried to imply at his web site….
New Page 4
…. suggesting that “LP2 is especially effective in large systems” and hoping that a larger reverberation time of large system would mask out the LP2 depth’s limitations but unfortunately he does not know (or do not care) all truth about LP2. (Here is where some listening would be useful for a supercilious Vladimir but trying to get a better product in the hands of his customer is not a variable in his Formula of Human Hearing). So, even in context of a large systems with a SPECIAL POSITIONING OF LOUDSPEAKER TO DEAL WITH THE LP2 DEPTH LIMITATIONS (and I went to that extend) you can therapy the geometry and positioning of images in depth. However you can’t fix that completely horrible digitalization by LP2 of the images that presented behind of the immediate front of sonic presentation.
Curtesy of GoodsoundClub.com (I hope he won't mind me reposting this excert")
I'm currently working on a similar amp. I'm using a circuit much like George Anderson's TubelabSE. I'm using 375 V B+ and 60 mA fixed bias on the 300B. This works out to Vgk = -75 V.
I have tried a couple of different topologies. The one I keep reverting back to is the TubelabSE, except, I use LED bias on the 5842 (12 mA anode current, Vgk = -3.6 V). You don't have to use the source follower if you don't like it. Just short gate to source, pull the 22k resistor, and you'll have your regular common cathode driver for the 300B. You'll get slightly worse transient response (muddy bass transients in particular) at higher output power if you disable the source follower. This can be backed up by measurements.
To drive the 300B into saturation under this bias point, you need about 75 Vpeak (53 V RMS). With CCS load, LED bias, the 5842 gets about 44 V/V gain. So you'll need 1.20 V RMS on the input to get full output power. The 5842 sounds a bit harsh in my opinion. Almost like a sand amp. But the overall sound seems very detailed and open to me.
I've tried other triodes - most notably a 6J5 (6SN7-family). The 6J5 sounds slightly slower than the 5842 - though, I have not been able to measure any significant difference between the two. More importantly, the 6J5 gives less gain - about 20 V/V. So it requires about 2.50 V RMS on the input to get the amp into clipping.
I've tried a couple of different 2-stage topologies, but they sound more muffled than I'd like. I prefer the responsiveness of the single stage triode driver.
~Tom
I have tried a couple of different topologies. The one I keep reverting back to is the TubelabSE, except, I use LED bias on the 5842 (12 mA anode current, Vgk = -3.6 V). You don't have to use the source follower if you don't like it. Just short gate to source, pull the 22k resistor, and you'll have your regular common cathode driver for the 300B. You'll get slightly worse transient response (muddy bass transients in particular) at higher output power if you disable the source follower. This can be backed up by measurements.
To drive the 300B into saturation under this bias point, you need about 75 Vpeak (53 V RMS). With CCS load, LED bias, the 5842 gets about 44 V/V gain. So you'll need 1.20 V RMS on the input to get full output power. The 5842 sounds a bit harsh in my opinion. Almost like a sand amp. But the overall sound seems very detailed and open to me.
I've tried other triodes - most notably a 6J5 (6SN7-family). The 6J5 sounds slightly slower than the 5842 - though, I have not been able to measure any significant difference between the two. More importantly, the 6J5 gives less gain - about 20 V/V. So it requires about 2.50 V RMS on the input to get the amp into clipping.
I've tried a couple of different 2-stage topologies, but they sound more muffled than I'd like. I prefer the responsiveness of the single stage triode driver.
~Tom
Thanks to All of you,
I learnt a lot and it seems that 'Keep It Simple & Stupid' does not work every day. I am a bit fed up with the controversed 417/5842 so I plan to redesign the amps based on the less conventional schematic proposed by YvesM with the penthode & some optional feed back (if he agrees)!
This is my actual config test using a Meridian CDP, a Kaneda or passive preamp, Harbeth HL-P3 and Diatone P610.
Instal - image / photo - Image perso - Hiboox
I learnt a lot and it seems that 'Keep It Simple & Stupid' does not work every day. I am a bit fed up with the controversed 417/5842 so I plan to redesign the amps based on the less conventional schematic proposed by YvesM with the penthode & some optional feed back (if he agrees)!
This is my actual config test using a Meridian CDP, a Kaneda or passive preamp, Harbeth HL-P3 and Diatone P610.
Instal - image / photo - Image perso - Hiboox
First, buy a REAL oscilloscope....
Practically, you need to create some 10:1 or 100:1 voltage dividers, preferably protected by zeners or somesuch.
Or, if you are measuring AC, some stepdown transformers.
It does bring us back to the original problem, which is that 30V PP is a significant limit to the usefulness of your CRO.
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