|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
70.117.113.124
In Reply to: RE: Driver tube suggetions for 300B amp posted by danlaudionut on February 11, 2016 at 07:34:21
Dan, the Full Music, TJ, and Sophia tubes are made in the same factory, just a name game. I know, years ago I imported a bunch, I had the option of putting my own name on them, but thought it BS, so went with the generic factory name (Full Music). I didn't want to further confuse the situation, and wasn't buying into the deceptive marketing scheme. Here in Texas, bidness is a contact sport, in China it's war.
twystd
Follow Ups:
Not from what I got off searching the web.
Maybe from the same factory but not the same tube.
Not like that was never done before.
TJ is the only PX25 with real PX25 specs.
The others have a mu of 6.5 and 1800 rp.
DanL
Have you seen actual curve traces of the TJ? The data they claim is identical to the printed data of the PX25 and the only curves I have ever seen are a direct copy of an original datasheet.
The Sophia Electric curves are the only actual traced curves I have seen and I have a hunch that they might be representative of all the PX25's.
This hunch also holds true for the WE 101 and 205 replicas. Never do you see actual traced curves, all that can be found is a cut and paste from the original data sheet which doesn't convince me of actual measured behavior.
dave
The 4V filament TJ PX25 was closer to the traditional PX25A or some such. The 5V filament TJ PX25 was a little different and its curves/ ideal operating points did not resemble any particular tube. Still, it was used in the DIY HiFi LadyDay amps with only minor mods (cathode R change to lower plate current) and 5K load. IIRC, apparently it sounded similar to the TJ 300B run at cool operating points and 5K load.All this was going on about 10 years ago on the DIY HiFi forum, back when Brian Cherry owned it and Thorsten was in the mix. Could probably be interweb searched.
Edit, this form 2006:
"I have just seen a reply by Brian Cherry claiming that the 4V TJ PX25 does meet the PX25 specs... I've seen the measurements on a 5V one and it quite a way off mu=9, Gm=2.5 instead of 7.5mA/V..."
Cheers,
91
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 02/11/16
The basic pattern of behavior that I have seen from the chinese tubes is that the bias point and filament voltage determines the tube type. If it has a 2.5V filament and biases up at 60ma with 250V P-K and -45V bias it is a 2A3 and the transconductance never comes into the picture.
The sophia electric site has a number of actual traced curves and here is how they compare to the "original"
The 45
A real 45 biases up at 250V P-K -50V on the grid and runs 34ma of current with a mu of 3.5 and an rP of 1750 which translates to a transconductance of 2000.
The above tube nails the operating point but has a higher Mu 4 and a lower Rp 1260 which equates to a higher transconductance of 3190
Now for the mesh 300B.
Again it biases up really close to where a 300B should but at that point a "real" 300B should have an Rp of 750 ohms a mu of 3.9 which translates to a transconductance of 5300u and this tube has the correct Rp but the mu and Gm are higher.
Both of these cases are within the realm of sample to sample tube variation so things aren't that bad (yet)
Now lets look at the PX25 that got this all started.
Again the bias point is almost nuts on however instead of a mu of 9.5 and an rp of 1256 ((gm of 7500) we get a mu of 6.9 and an Rp of 1880 for a Gm of 3670 which isn't close.
Now lets look at the 50.
Yup the bias points are spot on but instead of a mu of 3.8 and a Rp of 1800 ohms (Gm of 2100) we get a Mu of 5.11 and an Rp of 740 ohms for a Gm of 6880.
To me it sure looks like the 50 and the 300B are the same tube with a different filament voltage and the PX25 actually looks more like a 50.
I do want to give credit to Richard at Sophia for actually publishing this info and while it could be that just his versions of the tubes behave this way, it would take compelling info from someone who does not make or sell the tubes to make me believe it.
dave
Well spotted Dave - thanks for posting.
As a tube manufacturer it would be tempting to rationalise product lines, tooling etc. to minimise costs - maybe that is what is going on. Problem is, although the tubes will "work", they won't really be what the amp was designed for. Seems a "profits first" business model.
Interesting.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: