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In Reply to: Best power tube made today for my new project posted by SnaggS on May 1, 2007 at 16:55:37:
The EML 520Bs are a great great tube, you won't do better, but how about a DC-coupled TWO stage ( only ) P-P amp with a frame grid input / driver tube of about mu equals 50 and about 10 mA. idle current to drive the EML outputs ?? Sorta much like a Lofin-White, only as a P-P amp !!NO interstage transformers to compromise the amps bandwidth at the lowest signal level ( the input signal ). Eliminate the C input filter, make it L/C/L/C. Make it Low HY, Low C, Low DCR as I have previously outlined on Forum. If you go three stage you lose transparency. Simple can be better.
Jeff Medwin
Follow Ups:
... your previous advice regarding driver valves - in that they should be low mu????What experiences have you had to change this?
I would hesitate to claim that “The EML 520Bs are a great great tube, you won't do better”. They appear to be an indirectly heated pentode, in the 6550/KT88 family, not a DHT. They will require the use of a negative feedback loop to lower distortion and to increase the damping factor, which adds a whole new set of issues. DHTs do not require the use of negative feedback.Frame grid triodes are generally less linear then conventional triodes, leading to higher distortion. My recommendation would be a 3 stage design (6SL7 DC coupled to a 6SN7 capacitvely coupled to PP 300B’s – Kevin Kennedy’s approach).
an indirectly heated pentode?dave
Sorry. I couldn't find a 520B or EML520B using TDSL (Dunacnamps). I confused it with the EL156 listed below. Sorry!Yes, the EML 520B appears to be a super 300B, at a super price! $629 per piir. Oh well, that's still cheaper than WE 300Bs.
Mikeyb,No, sorry, the Emission Labs IS a DHT tube...with EXCELLENT construction, filament implementation, materials choices, modern / state of the art and far better than vintage DHT tubes, as is the EML 45 and 2A3. Look over their site and browse around, its a worthy tube to know of and use.
When you add the unnecessary third stage you lose transparency, sorry. A two stage, particularly DCed, with even a slightly non linear driver tube will SONICALLY beat the pants offa any three stager you or anyone else may execute, IMHO. KISS, etc.
Respectfully,
Jeff Medwin.
...I have heard some unpleasant rumours about their reliability and response to warranty claims.
I've had business dealings with their main sales outlet in Europe, who I understand is one of EML's main principals - and I refuse to ever do business with him again.Regards, Allen
I have built several DC coupled PP amps . There are various problems to overcome at the design stage with DC coupled amps . The first problem with this is that the input stage now stage controls the balance and operating point of the output stage . Then there is the risk that if the input valve fails , the grids of the output stage will see the full driver B+ ! Also with DHT output stages , there needs to be a way of measuring balance , which even with perfectly matched output valves , can be screwed up by the input stage . A pot between the cathodes of the input stage can set balance but there is also the consideration of the input stage tail current , which also sets the current of the output stage . Also much higher voltages will be used , cost of the power supply would increase dramatically . So I would recommend the following for a safe DC amp : CCS under the output stage , pot between the cathodes of the input stage , output transformer with both sides of the primary brought out to allow balance measurement (instead of a centre tap) . I also prefer valve rectification as the DHT will warm up far more quickly than the input stage .
Hi,Curious, why you post without leaving us your ID??
Anyway, NONE of your suggestions for making a "safer DC coupled amp" are worth a toot, or necessary, or an improvement in implementation sonically, IMHO, except for the use of tube rectification.
Serious Stereo manufactures a modern DC coupled 2A3 amp, ( see their brand new website BTW ), and, to my knowledge, they have never had a single failure since the DCed amp's inception in the 1990's.
Jeff Medwin
DC coupled circuits are difficult to design well and are a saftey nightmare. I know this from personal experience - my 1920s vintage radio uses a Loftin White connection. It blew up a 45 DHT when the detector tube became loose in it's socket, placing 420 volts on the grid of the 45. Nearly took out the power transformer as well. No fuses in this baby! Wouldn't wase money on a 5 cent fuse ...
However a DC coupled two stage PP amp gave me a new experience. I know all the potential problems, but it is worthy of trying for a DIYers. Not a manufacturer's region I think. Overcurrent can be handled with CCS at the DHT's common cathode also making it a differential ouput. The DC balance can be measured even with normal center tapped OPT, like the way Gary Pimm did with 1 ohm resistor before each plate.DC coupled differential PP also 'brings out' the music in old, otherwise untolerable recordings.. from my experience.
Cheers,
C.Y
C.Y,Hello!! Next, you need to figure out how to eliminate the CCS on the output stage's cathodes.
Also, you do not want to place 1 ohm Rs in the plate circuit of the output stage as it decreases the coupling of the tubes to the output trannie.
Why not just measure the DCR of each half of the output trannie's winding, and use Ohm's Law to calculate current for each tube ??? Less is more !!! Zen like.
Jeff Medwin
Hi Jeff,DC coupled differential PP is the one that I remember as something to do in the future. But as pointed by others there are some problems to solve or to live with.
I'm building a DC coupled 300B resistor loaded parafeed, with low DCR power supply separate for driver and 300B. I'm so hooked by the R loaded 2A3 parafeed and I wanted to have that sound with higher power. I will share some pictures later.
BTW in my hobby club more people are interested in low H low R power supply. :-)
Cheers,
Chul
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