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I am currently running a Sophia Royal Princess 300B in my Eddie Current Balancing Act. This amp will also accept a PX-4 at the flip of a switch or a PX-25 with the addition of a couple of resistors.
I understand the Osram globe PX-25 to have silky highs, beautiful mids, and dynamic bass. The tube is somewhat difficult to source, however, and is expensive for something with no guarantee. Some have tried the KR PX25 in this amp and found it to be lean and nothing special. Does anyone have experience with the Full Music or Sophia PX25?
Alternatively, I understand the KR PX-4 works well with this amp but the tube has serious quality control issues. There is an AVVT PX4 but I don't know how it sounds compared to the KR tube. Has anyone heard the 2 and be willing to share an opinion?
In general, what can I expect the sonic differences to be between the Px4 and the PX25? I am looking for realistic highs, beautiful mids and strong dynamic bass. I value soundstage and instrument separation. Most of all, I look for micro dynamics which make the music sound real.
Follow Ups:
I would steer well clear of Osram globe PX4 , early versions such as these have lower dissipation than ST shaped PX4 . Genuine PX4 and PX25 have a B4 base so I don't know how these will fit a 300B amp which uses a UX4 socket . PX25 also requires a much higher impedence output transformer than 300B . Both PX4 and PX25 also have 4 volt filaments . It seems unlikely your amp can accept either unless a major rebuild/rebias is carried out or these modern 'PX4' and 'PX25' are derived from a 300B and fitted with a UX4 base and different filament .
Al
My ECBA has a Yamamoto base that accepts both UX4 and B4 tubes. I can also select 4 or 5 volts. A switch in the back allows me to use either a PX4 or 300B. To use a PX25 I would have to add a couple of resistors. The amp can definitely use 300B/PX4 or PX25 tubes.
In that case happy hunting although I would question the use of PX25 . This type has higher Ra than 300B/PX4/2A3 and requires not only a rebias but a higher impedence output transformer . I have one low mu PX25 called the AC/PX25 , which bias up similarly to 300B but these are extremely rare . There are others such as DO26 but again very rare . I don't know if modern PX25 are based on the originals with mu of 10 or upon 300B with mu of 4 . Osram PX4 are very nice . I have several sets of these which I'm keeping for a rainy day . KR PX4 are supposed to have a slight edge on the originals , although I have never tried these
Al
Well, you're looking at throwing serious money at these tubes. And of course, they'll deteriorate over the years and that's it. Part of the reason my amp is a PSE 4P1L - 4P1L> Interstage> PSE4P1L. I have a lifetime supply at the very least. Buying is over.
Bear in mind one thing - these tubes have more gain than the 300b. You may be able to drop a stage and have a 2 stage setup. That could be a serious advantage. I get that with the 4P1L - gain of 10.
But it's a lot of money to drop on tubes......
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