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I have built a version of the Legacy 300b amplifier (coutesy of Thorsten Loesch) using a conventional power supply using miletary surplus transformers and chokes (oil filled c-core), Cap 10mF - 15H choke - 200mF cap. I have used cheap Audi Note Trans 144 (the experimenters version.It works very well and is essentially quite detailed and musical albiet soft on the bass with a slight electronic haze? - probably due to the regulated filament supply - the schematic for which comes sraight out of the AN Conquest.
So a bit of a mix and match amp really.
Has anyone tried using a constant current source as a load to the C3M instead of the resistor?
My fear is that it is 420 volts and I know that the CCS cannot withstand this - so would need some kind of protection.
Follow Ups:
Hi,Some comments.
In a DHT the heater is part of the signal circuit. Different regulator chips are readily audible, the linear tech LT1085 is recommended out of those tried, over more generic.
Also, for some strange and counter intuitive reason using the POSITIVE pole of a DC DHT filament as AC return and the negative pole as DC return sounds better than the other way around or using a humbalancer pair of resistors or pot.
IIRC the original Legacy Article recommends such a setup, not the one from ANUK.
Given that all the PSU capacitors (screen grid on driver, anode on driver, anode on 300b) are part of signal path and that (of course) the cathode bypass capacitors are just as well, avoiding all electrolytic capacitors in these positions and selecting "non-electronic" sounding types there will have a big impact as well.
Finally, as others mention, a CCS with a pethode operated as pentode is not the best idea.
Thanks - I take on board your recommendation over the regulator - will order one.Also I have tried the positive cap return on the filament - it does work - for whatever reason - not sure even Thorsten could explain that one?
I really need to getrid of those Philips electrolytic caps!
I read a few very positive remarks on those , also Thorsten seemed to recommend them as well , DIY Supply have them along with some nice nickel amorphus OPTS.
but if you must have something more comfortably expensive,
you can send the Philips to me --okay?
I agree the Philips are not bad caps - tried them in another amp and they sound pretty good - I think that the opt is the part that lets the side down.
The gain and operating line (which sets the distortion character) are determined by the plate load resistance for a pentode. A CCS improves distortion for triodes, but generally not pentodes.Without a CCS, the driver stage has little power supply rejection, so a quiet power supply is necessary. Sounds like yours is excellent, especially when you improve the 200uF cap.
It will, in my experience, take something like 50 hours of music before the capacitors and output transformer settle down - if you have only a few hours on the unit, that may be the source of the "haze" you hear. Normally I hear a significant change after the first 10 hours, but that depends on the particular components and on the signal level so it's not an exact science (!)
Posted by Paul Joppa (P) on February 08, 2007 at 10:00:47
In Reply to: Legacy 300b with CCS posted by Red on February 8, 2007 at 03:13:47:
The gain and operating line (which sets the distortion character) are determined by the plate load resistance for a pentode. A CCS improves distortion for triodes, but generally not pentodes.
Without a CCS, the driver stage has little power supply rejection, so a quiet power supply is necessary. Sounds like yours is excellent, especially when you improve the 200uF cap.> > > Ahhh! I did read this somewhere before, that the CCS is of no use in a pentode stage - you have just reminded me - also you are right the power stage is really good in that there is abosultely no hum or noise at all even on my Oris 'style' horn with lowther driver and ear right up against it - kinda eery!!
It will, in my experience, take something like 50 hours of music before the capacitors and output transformer settle down - if you have only a few hours on the unit, that may be the source of the "haze" you hear. Normally I hear a significant change after the first 10 hours, but that depends on the particular components and on the signal level so it's not an exact science (!)> > > > > > I think the components are burned in - have had it on for the best part of 2 months now - also been swapping components in and out - just can't stop that tweaking - have settled on most of Thorstens suggested components (AB's for grid stoppers and grid leak, Mills WW for 300b Cathode and power resistors, Ansar most everywhere - but need to find some goodies to replace the 200mf philips last power sup cap)
I have as stated used the schematic for the filament supply from the AN Quest i.e. regulated but have put in a CM choke after the first caps and just before the reg with two .01mf caps to ground. tip form Morgan Jones to get rid of common mode noise.
The sound is generally clear and dynamic but it is not as refined as I like i.e. the mids are not quite liquid - the Lowthers do have a habit of showing up poor performing components rather ruthlessley!!! - I have attributed this to power supply but it could really be the o/p transformer I guess?
My bet would be it's the power supply cap. In a series feed circuit, it's definitely in the signal current loop.I have often found that less expensive transformers are more compromised in bandwidth than in sonic qualities, as long as they are operated within their proper working range.
It is funny just HOW sensitive SET amps are to every single component and wire - its fustrating and yet perfect because it does enable you change to your liking - loverley!I think you are right - a large part ofthe answer lies in the regulator and power supply cap and then subtle tuning elsewhere.
I mean it is a great sounding amp - its just a little less refined than I would like - when I get the refinement then I will do something about the o/p transformer - any recommendations? (I know this is a contraversial one)
> > "...I will do something about the o/p transformer - any recommendations? "Haha! Well, it's no secret I like Magnequest, and I like parafeed. But parafeed is a fairly big step away from what Thorsten was trying to do in this design, so I would expect some experimenting might be needed to fine-tune it for the different topology.
I have not done series feed for probably 8 years, so I'm kind of out of date. What kind of impedance and plate current is needed?
I don't understand what did you take out of Legacy?? As I understand legacy is two stage with pentode driver (el 84) and conquest is three stage with input & driver 6SN7 cap coupled to 2x 300B. I guess you meant Quest right?? Trans 144 is their cheapest so bass problem my be related to trans , also original choke is 5H and probably lot less DCR than your 15H which can also have an influence .You could try kit one SRPP stage which I heard is more dynamic than 6Sn7 cascade. Funny ,years ago I read interview with Arcam chief designer and he mentioned that their cheap stuff is designed to sound open and dynamic but expensive one should sound refined .Regards, L
disregard my post but my modest experience tells me that unless you use exact major parts (OPTs , caps) the sonic result may not be that great with published circuits . RgRds. W
Hi,I am using the Legacy circuit intact (i.e. C3m driver and 300b o/p tube) but I have chosen to use the regulated filament supply as shown in the Quest - the power transformer I use is very good as is the choke - also I use a mullard GZ34 in a normal CLC mode - the voltages all work out correct.
I agree that psrt can and do make a major difference - I have used Ansar caps and Allen Bradley and Mills resistors with teh exception of some philips caps in the second C of the power supply (until I can find some more exotic caps that fit the chassis).
I guess I wanted to know if anyone has used a CCS on the C3m?
Rgds
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