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In Reply to: Re: Legacy 300b with CCS posted by Paul Joppa on February 8, 2007 at 10:00:47:
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?
Follow Ups:
- Re: Legacy 300b with CCS - Paul Joppa 17:55:45 02/08/07 (2)
In Reply to: Re: Legacy 300b with CCS posted by red on February 8, 2007 at 11:03:39:
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.
- Re: Legacy 300b with CCS - red 03:02:06 02/09/07 (1)
In Reply to: Re: Legacy 300b with CCS posted by Paul Joppa on February 8, 2007 at 17:55:45:
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)
- Re: Legacy 300b with CCS - Paul Joppa 16:21:30 02/09/07 (0)
In Reply to: Re: Legacy 300b with CCS posted by red on February 9, 2007 at 03:02:06:
> > "...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?
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