The type 50 is somewhat unique... download some of the PDFs for it here:
http://tdsl.duncanamps.com/show.php?des=50
I would probably recommend transformer input coupling as the grid circuit is clearly stated as no more than 10K resistance, making it a more difficult load for a simpler RC-coupled circuit.
If you opted for cathode biasing, you will need a supply that's easily 600+ volts (450 + 84 +OPT loss +filter choke loss). Fixed bias would reduce this by 85 volts but you would then need a negative supply which is additional components. As you can see, your power supply iron/circuitry gets more expensive and complex, in addition to your driver circuitry. A 300B circuit would only be recommended with the proper changes to the OPT impedance and biasing, and the requirement for the low grid circuit resistance.
I'm able to get 4.5 watts (cleanly) from better NOS 2A3 tubes without pushing them really hard, but the 2A3 is much easier to drive and grid circuit resistance can be much higher, a maximum of 500K.
Regards, KM
http://tdsl.duncanamps.com/show.php?des=50
I would probably recommend transformer input coupling as the grid circuit is clearly stated as no more than 10K resistance, making it a more difficult load for a simpler RC-coupled circuit.
If you opted for cathode biasing, you will need a supply that's easily 600+ volts (450 + 84 +OPT loss +filter choke loss). Fixed bias would reduce this by 85 volts but you would then need a negative supply which is additional components. As you can see, your power supply iron/circuitry gets more expensive and complex, in addition to your driver circuitry. A 300B circuit would only be recommended with the proper changes to the OPT impedance and biasing, and the requirement for the low grid circuit resistance.
I'm able to get 4.5 watts (cleanly) from better NOS 2A3 tubes without pushing them really hard, but the 2A3 is much easier to drive and grid circuit resistance can be much higher, a maximum of 500K.
Regards, KM
kmaier said:
KM,
What is the cathode resistance value on this? Looks like there is hardly any at all. I am not used to working with 4 pin triodes.....but that bugle schem has a 1.5k and the data sheet says 775-OHM for AB2, but in classA is not really listed.
Do you think we really need the hum balance pot?
Thanks
Cathode resistor in schematic is 1.5K 10W. Look at the class A1 specs for the 45 tube, the AB2 specs are for a push-pull output stage, i.e., a pair of 45 triodes. Did you received the PDFs? You can zoom in and all values are clearly marked, plus the BOM list shows each value.
My design uses a split balance technique, the Bugle45 does not. The 1.5K 10W resistor from the filament transformer center-tap goes to ground, this IS the cathode bias resistor and provides a fixed DC balance for the 45.
As for the balance pot... yes, you need it, trust me. The balance pot is configured for AC balance only (DC being fixed via the 1.5K 10W resistor above). Without it, you can not null the hum component contributed by the AC filament.
Regards, KM
My design uses a split balance technique, the Bugle45 does not. The 1.5K 10W resistor from the filament transformer center-tap goes to ground, this IS the cathode bias resistor and provides a fixed DC balance for the 45.
As for the balance pot... yes, you need it, trust me. The balance pot is configured for AC balance only (DC being fixed via the 1.5K 10W resistor above). Without it, you can not null the hum component contributed by the AC filament.
Regards, KM
Look here for a simple but great design with the 50 triode.kmaier said:
In case you're going to build the Bugle, try a power supply with choke input and a real good output transformer. It is soundwise among the best amplifiers I have come across. Very natural, vivid but only 1.5W/ch. Nothing wrong with that 12AY7 tube in SRPP.
Agreed, the 50 is a nice triode, but requires fairly high voltages to get 4 watts output. Also, unless you already own a nice pair, they are expensive... and if one fails, getting replacements will be difficult. I've been looking for some 50s for a while... either in poor shape or just out of he ball-park on pricing... too bad.
The schematic linked appears incomplete... seems they left out a filter cap for the 227 stage which is choke loaded. There should be a filter cap from the top of the plate choke to ground, otherwise you're just running the 227 with a resistive load.
Regards, KM
The schematic linked appears incomplete... seems they left out a filter cap for the 227 stage which is choke loaded. There should be a filter cap from the top of the plate choke to ground, otherwise you're just running the 227 with a resistive load.
Regards, KM
Hello Bryant,
Designing on paper and implementing are two different things. A closer look at the posted 50 SET reveals some other problems as well. I'm somewhat reluctant to think that this circuit was actually built... and if it was it's not final or probably doesn't perform very well.
All bias points and calculated values are lifted verbatim from the typical operating section of the 27 and 50 respectively and just joined together. The only variation is the OPT impedance... it looks to be a 5K load while the particular operating point published recommended 3.6K. I would suspect this design (as is) would barely output 3 watts.
Addtional problems, beyond number 0 below:
0- As noted previously... you would need to add a filter capacitor from the top of the plate load choke on the 27 to ground. Something in the neighborhood of 20uF would be fine.
1- AC input wiring is flawed. A 5amp fuse is far too large and only (seemingly) fuses the plate transformer. It's a moot point however as both switches need to be closed to get power to both transformers. Closing both switches shorts out the fuse... If the intended goal was to fuse both transformers and provide a "standby" switch, two fuses should be used (one for each transformer) and they should be properly calculated for values against the actual load.
2- The DC filament supply looks to be marginal... not for filtering but for output voltage. The voltage drop between the diode bridge and choke will probably result in an output voltage less than 7 volts.
3- The 27 input tube has low mu... about 9. Despite being choke loaded, you won't see a gain of 9 volts/volt... probably closer to 7 volts/volt. As a result, driving the input grid to 18 volts peak-to-peak will only output 126 volts peak-to-peak. It's also a bad idea to think you can drive the input grid to zero without having linearity problems. As a result, you will not be able to fully drive the 50 triode. End result is lack of full power due to insufficient drive and higher distortion due to the input stage being "on the edge".
Perhaps if the original source of the schematic was located, they may have done a prototype and changed the design in the process. I'm not a huge fan of direct-coupled designs... and the 50 is a difficult tube to drive, not due to it's input capacitance but due to the grid leakage. The original specification for the 50 triode was clear that RC-coupling was NOT recommended, hence transformer coupling. Later (revised) specifications show RC-coupling is okay provided the overall grid circuit resistance does not exceed 10K... which is pretty low and having a proper driver stage means something fairly beefy. In any case, you might be able to tweak the circuit a bit and get satisfactory results, but it will require higher B+ voltage, a shift in the input/driver stage operating points and you'll need to ensure you don't have any grid leakage which would result in a runaway condition on the 50 which would eventually burn it out.
Regards, KM
Designing on paper and implementing are two different things. A closer look at the posted 50 SET reveals some other problems as well. I'm somewhat reluctant to think that this circuit was actually built... and if it was it's not final or probably doesn't perform very well.
All bias points and calculated values are lifted verbatim from the typical operating section of the 27 and 50 respectively and just joined together. The only variation is the OPT impedance... it looks to be a 5K load while the particular operating point published recommended 3.6K. I would suspect this design (as is) would barely output 3 watts.
Addtional problems, beyond number 0 below:
0- As noted previously... you would need to add a filter capacitor from the top of the plate load choke on the 27 to ground. Something in the neighborhood of 20uF would be fine.
1- AC input wiring is flawed. A 5amp fuse is far too large and only (seemingly) fuses the plate transformer. It's a moot point however as both switches need to be closed to get power to both transformers. Closing both switches shorts out the fuse... If the intended goal was to fuse both transformers and provide a "standby" switch, two fuses should be used (one for each transformer) and they should be properly calculated for values against the actual load.
2- The DC filament supply looks to be marginal... not for filtering but for output voltage. The voltage drop between the diode bridge and choke will probably result in an output voltage less than 7 volts.
3- The 27 input tube has low mu... about 9. Despite being choke loaded, you won't see a gain of 9 volts/volt... probably closer to 7 volts/volt. As a result, driving the input grid to 18 volts peak-to-peak will only output 126 volts peak-to-peak. It's also a bad idea to think you can drive the input grid to zero without having linearity problems. As a result, you will not be able to fully drive the 50 triode. End result is lack of full power due to insufficient drive and higher distortion due to the input stage being "on the edge".
Perhaps if the original source of the schematic was located, they may have done a prototype and changed the design in the process. I'm not a huge fan of direct-coupled designs... and the 50 is a difficult tube to drive, not due to it's input capacitance but due to the grid leakage. The original specification for the 50 triode was clear that RC-coupling was NOT recommended, hence transformer coupling. Later (revised) specifications show RC-coupling is okay provided the overall grid circuit resistance does not exceed 10K... which is pretty low and having a proper driver stage means something fairly beefy. In any case, you might be able to tweak the circuit a bit and get satisfactory results, but it will require higher B+ voltage, a shift in the input/driver stage operating points and you'll need to ensure you don't have any grid leakage which would result in a runaway condition on the 50 which would eventually burn it out.
Regards, KM
* Current(ish) type 50 production:
The Shuguang type 50 replica is supposed to be pretty decent at a not so crazy price.
Have not heard much re the TJ and not sure of their approach to constrution (I have read that TJ supposedly have two main approaches, with one generally preferred over the other - but that could be total BS).
Regarding the 50 in general - seems like a pain in the butt to implement and run for only 3.5W out. That direct-coupled circuit at ~620V even more so. There are likely options that present music as well ad that (flawed) circuit with much less hastle and waste...
Cheers
* I also belive the current 50 production does not have the grid leakage issue. However, I would defer to KM's far superior knowledge / advice in this...
The Shuguang type 50 replica is supposed to be pretty decent at a not so crazy price.
Have not heard much re the TJ and not sure of their approach to constrution (I have read that TJ supposedly have two main approaches, with one generally preferred over the other - but that could be total BS).
Regarding the 50 in general - seems like a pain in the butt to implement and run for only 3.5W out. That direct-coupled circuit at ~620V even more so. There are likely options that present music as well ad that (flawed) circuit with much less hastle and waste...
Cheers
* I also belive the current 50 production does not have the grid leakage issue. However, I would defer to KM's far superior knowledge / advice in this...
I won't pretend for a second to be an expert on the 50.... I've tried to collect some to try over the recent years but they are simply too expensive and too rare such that getting a dud or having one fail becomes an even more expensive proposition down the road. The VT52 (aka 45 special) is in the same boat here.
I do have a fairly deep amount of experience with the 45 and 2A3 having over 100 45s and over 50 2A3s. I've tested every single one multiple times... not with a tube tester but in actual operating amplifiers with both standard tests and real-world listening. As a result I'm fairly confident in the designs I've done that use the 45 and 2A3 and also confident that my initial "paper design" for a 50 triode would require prototyping, measurements, testing and swapping multiple tubes thru the circuit to establish just how stable and consistent it is. Unfortunately this is not a reasonable quest based on pricing and availability.
As for new manufactured tubes... I have little direct experience with them. IMHO it's almost impossible to achieve what was the "status quo" back in the 50's. The chemical processes, materials, and techniques used were optimized over time. Some manufacturers had their own proprietary methods as well. In any case, with current federal regulations (which are now expanding into other countries) like the EPA and such, the costs to make tubes the same way would most likely be prohibitively expensive for the small quantities that are consumed by the number of manufacturers and DIYers who want them. Everyone has a price point to design, engineer and manufacture the product and in the end they have to be affordable or they won't sell and you're dead from a business view.
I do have direct experience with the EML 45 solid-plate. I've tested these extensively on the bench and listening. Here's my view on them (and always remember what "free advice" is worth):
From a build quality, they are over the top... rugged and built to withstand a nuclear blast most likely. Visually I personally think they are "butt-ugly". The ST glass shape is not visually appealing and they are so huge that they dwarf any NOS 45 and 2A3. Internal construction is always important... as rugged as they appear to be, they have nothing in common with ANY 45 I've ever seen... absolutely nothing. The filament structure is different, the grid is different and the plate is different. All of these pieces are much larger than any 45 and could not fit inside the envelop of a balloon style 45 much less the later ST style.
I have found that operating points can be similar but don't expect the same performance numbers at similar operating points. Even at 10-watts dissipation, the glass bulb barely gets warm. Listening to them in circuits designed and optimized for NOS 45 tubes, they sound somewhat pale, like they are starved for power. Increasing the current from my normal 34ma to 40ma made a big improvement but they never sounded as neutral or as clean as my better NOS 45. You can easily get 3.5 watts from them if you crank up the voltage a bit (don't go over 40ma of current). Hum balance is about on par with NOS 45s using AC filaments but again, not all are the same and variations are common.
My one main flaw with them is their susceptibility to mechanical excitation. At idle, even a gentle tap with your fingernail on the chassis makes quite a stir on the scope trace. Whistling near them also proves they are frequency resonant. A hand clap makes a noticeable impact and playing a trumpet within 8 feet threw them over the edge... very highly resonant at many notes. Needless to say, I think they are unable to present a neutral amplification unless they are in another location which is simply unrealistic. Damping the glass envelope and/or isolating their mounting is not going to have a huge affect... and you shouldn't need to go to such great lengths and expense to use a pair of $375 tubes versus some NOS ones which perform better (IMHO) at a fraction of the cost without any resonant issues.
As for continuing on with a 50 SET project, it's no longer on my list. It becomes a very expensive 5 watts and difficult to maintain going forward based on the availability. I for one am not counting on new manufactured tubes to get me thru my audio days, however many more remain. My next SET project is based on the new issue WE300B... (don't shoot me yet)... enough reading on these would seem to indicate that they are as close to the original as possible, and are even based on the original filament wire from the original smelt... they found NOS of this wire... how nice is that? I have a pair and have done some initial prototyping and testing, but completion is a ways off. Anyway, enough rambling for now... (real) work calls. For those who have some 50 triodes... don't let the above disenchant you from building an amp with them... just make sure you have some backups just in case.
Regards, KM
I do have a fairly deep amount of experience with the 45 and 2A3 having over 100 45s and over 50 2A3s. I've tested every single one multiple times... not with a tube tester but in actual operating amplifiers with both standard tests and real-world listening. As a result I'm fairly confident in the designs I've done that use the 45 and 2A3 and also confident that my initial "paper design" for a 50 triode would require prototyping, measurements, testing and swapping multiple tubes thru the circuit to establish just how stable and consistent it is. Unfortunately this is not a reasonable quest based on pricing and availability.
As for new manufactured tubes... I have little direct experience with them. IMHO it's almost impossible to achieve what was the "status quo" back in the 50's. The chemical processes, materials, and techniques used were optimized over time. Some manufacturers had their own proprietary methods as well. In any case, with current federal regulations (which are now expanding into other countries) like the EPA and such, the costs to make tubes the same way would most likely be prohibitively expensive for the small quantities that are consumed by the number of manufacturers and DIYers who want them. Everyone has a price point to design, engineer and manufacture the product and in the end they have to be affordable or they won't sell and you're dead from a business view.
I do have direct experience with the EML 45 solid-plate. I've tested these extensively on the bench and listening. Here's my view on them (and always remember what "free advice" is worth):
From a build quality, they are over the top... rugged and built to withstand a nuclear blast most likely. Visually I personally think they are "butt-ugly". The ST glass shape is not visually appealing and they are so huge that they dwarf any NOS 45 and 2A3. Internal construction is always important... as rugged as they appear to be, they have nothing in common with ANY 45 I've ever seen... absolutely nothing. The filament structure is different, the grid is different and the plate is different. All of these pieces are much larger than any 45 and could not fit inside the envelop of a balloon style 45 much less the later ST style.
I have found that operating points can be similar but don't expect the same performance numbers at similar operating points. Even at 10-watts dissipation, the glass bulb barely gets warm. Listening to them in circuits designed and optimized for NOS 45 tubes, they sound somewhat pale, like they are starved for power. Increasing the current from my normal 34ma to 40ma made a big improvement but they never sounded as neutral or as clean as my better NOS 45. You can easily get 3.5 watts from them if you crank up the voltage a bit (don't go over 40ma of current). Hum balance is about on par with NOS 45s using AC filaments but again, not all are the same and variations are common.
My one main flaw with them is their susceptibility to mechanical excitation. At idle, even a gentle tap with your fingernail on the chassis makes quite a stir on the scope trace. Whistling near them also proves they are frequency resonant. A hand clap makes a noticeable impact and playing a trumpet within 8 feet threw them over the edge... very highly resonant at many notes. Needless to say, I think they are unable to present a neutral amplification unless they are in another location which is simply unrealistic. Damping the glass envelope and/or isolating their mounting is not going to have a huge affect... and you shouldn't need to go to such great lengths and expense to use a pair of $375 tubes versus some NOS ones which perform better (IMHO) at a fraction of the cost without any resonant issues.
As for continuing on with a 50 SET project, it's no longer on my list. It becomes a very expensive 5 watts and difficult to maintain going forward based on the availability. I for one am not counting on new manufactured tubes to get me thru my audio days, however many more remain. My next SET project is based on the new issue WE300B... (don't shoot me yet)... enough reading on these would seem to indicate that they are as close to the original as possible, and are even based on the original filament wire from the original smelt... they found NOS of this wire... how nice is that? I have a pair and have done some initial prototyping and testing, but completion is a ways off. Anyway, enough rambling for now... (real) work calls. For those who have some 50 triodes... don't let the above disenchant you from building an amp with them... just make sure you have some backups just in case.
Regards, KM
Hello kmaier,
Thanks a lot.
50 tube is one of audio tube history. First time when I looked at 50 Balloon tube. I were fascinated with it immediately. It is very sexy tube. Very big balloon. It's expensive 5 watts. It's easy to get 8 watts from 300B and 3.5 watts from 2A3. But I still want to build it Because it's very good experience, only a few people ever listened from 50 tube.
Bryant
Thanks a lot.
50 tube is one of audio tube history. First time when I looked at 50 Balloon tube. I were fascinated with it immediately. It is very sexy tube. Very big balloon. It's expensive 5 watts. It's easy to get 8 watts from 300B and 3.5 watts from 2A3. But I still want to build it Because it's very good experience, only a few people ever listened from 50 tube.
Bryant
Kevin, et al,
At the risk of getting run out town for hijacking this thread, here's a preliminary design for an all-DHT 50 amp. The tubes were purposely chosen to voice the amp and balance out the darker sound of the 50.
Note that the power voltages are not optimized - this is a "paper design" little more than a cleaned-up sketch.
Enjoy,
-- josé k.
At the risk of getting run out town for hijacking this thread, here's a preliminary design for an all-DHT 50 amp. The tubes were purposely chosen to voice the amp and balance out the darker sound of the 50.
Note that the power voltages are not optimized - this is a "paper design" little more than a cleaned-up sketch.
Enjoy,
-- josé k.
Attachments
Hi Jose,
No hijacking worries.... looks like an interesting design... and transformer-coupled to the 50... very nice. Are you building this?? That means you have some 50 triodes, I'm envious! if you do build it, would love to hook up and take a look-see and a listen of course ;-)
Bryant,
Again, if you have the tubes by all means... build an amp. If had some, I would be inclined as well. My current 2A3 amp delivers 4.5-watts output with decent NOS 2A3s... my preference being Sylvania dual-section. But I agree, it's a pretty tube.
Vlauga,
For hum balance, I generally use a single-turn 100-ohm wire-wound potentiometer... I get a NOS Clarostat from Surplus Sales which comes with mounting hardware... these are slot adjutable with the lock collar. At $5 more than reasonable. Also, for 2.5V filaments I pad the pot with a pair of 12-15 ohm resistors (one from each end to the wiper). For 5.0v - 7.5V filaments I use a 30-ohm pad resistor. The does two things, it widens the adjustment range so it's not a "knife-edge" and lowers the effective resistance by a fair margin, down to 5 ohms with the 12 ohm resistors.
Regards, KM
No hijacking worries.... looks like an interesting design... and transformer-coupled to the 50... very nice. Are you building this?? That means you have some 50 triodes, I'm envious! if you do build it, would love to hook up and take a look-see and a listen of course ;-)
Bryant,
Again, if you have the tubes by all means... build an amp. If had some, I would be inclined as well. My current 2A3 amp delivers 4.5-watts output with decent NOS 2A3s... my preference being Sylvania dual-section. But I agree, it's a pretty tube.
Vlauga,
For hum balance, I generally use a single-turn 100-ohm wire-wound potentiometer... I get a NOS Clarostat from Surplus Sales which comes with mounting hardware... these are slot adjutable with the lock collar. At $5 more than reasonable. Also, for 2.5V filaments I pad the pot with a pair of 12-15 ohm resistors (one from each end to the wiper). For 5.0v - 7.5V filaments I use a 30-ohm pad resistor. The does two things, it widens the adjustment range so it's not a "knife-edge" and lowers the effective resistance by a fair margin, down to 5 ohms with the 12 ohm resistors.
Regards, KM
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.