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Eddie Current Balancing Act Mod for PX25
Feb 28, 2012 at 2:15 PM Thread Starter Post #1 of 19
purrin

purrin

aka Marvey, purr1n
Member of the Trade: Eddie Current
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This is for the two to three people in the world who are interested in the mod to run PX25 tubes.
 
The Eddie Current BA in its current iteration runs PX4 and 300B tubes. The heater voltage is set to 4V for the PX4 and 5V for the 300B. In addition, there is a switch on the back of the amp. The UP position is for PX4 and the DOWN position is for 300B. After the mod, the UP position will be for PX25 and the DOWN position will be for PX4 (or 300B).
 
The mod requires x2 of this part: http://www.mouser.com/ProductDetail/IRC/GS3-100-5600-JLF/?qs=XjRxj70TXJJMO6iKOGoRcg%3d%3d It's a 3 watt 560ohm resistor.
 
There is a 470ohm resister that sets bias current for the PX4. For the PX25, this resistor should be 250ohms. So basically we parallel it with a 560ohm resistor with the 470ohm to get ~255ohms. Here are the steps:
 
  • Remove all tubes and connectors
  • Flip amp over
  • Unscrew 8 screws on the bottom cover
  • Here is a zoomed out view for some perspective
  •  
  • Here is zoomed in on the 470ohm resistor (in the middle.) It's brown and in series with the black 270ohm resistor. Note that the mod is already applied with a 560ohm resistor to the right next to it and in parallel with the 470ohm. (Excuse the extraneous leads and solder - I had initially tested the mod with RatShack 1k and 2k ohms in parallel.) 
  • Repeat on the other side / channel
 
As an aside, the switch in the DOWN position puts in series an additional ~ 300ohms (or 270ohm in this my particular amp) to run the PX4(modded) / 300B (unmodded) in the circuit. The UP position basically shorts this connection for PX25(modded) or PX4 (unmodded). Craig mentioned that after mods, we should be able to run 300B's in the DOWN position - I guess things were set pretty conservatively.
 
So a recap:
 
 
MODDED (total resistance in circuit)
 
PX25: 255ohms (470 | 560)
PX4 or 300B: 525ohms (255 + 270)
 
Code: 
 -----560-----+ | -----470-----+-----270----- #====PX25====# #====PX4/300B=============#
 
STOCK (total resistance in circuit)
 
PX4: 470ohms 
300B: 740 ohms (470+270)
 
 
Code: 
 -----470-----+-----270----- #====PX4=====# #==========300B===========#
 
 
Don't forget to set voltages for the 4V tubes properly!
 
Next post will be the adapters if you don't have the Yamamoto multi-sockets.
 
Feb 28, 2012 at 4:24 PM Post #2 of 19
reserved for adapters.
 
By popular request, I am resuming this thread. An easier thing that you can do is replace the UX4 teflon socket that currently comes with the Balancing Act with a Yamamoto UX4-BX4 combination socket. I hate those combo things though because it's easy to plug things in the wrong way and make them go boom.
 
STEP ONE: obtain the following materials
 
  • UX4 base to plug in BA: http://www.ebay.com/itm/4Pin-Tube-Base-GOLD-Ceramic-Socket-U4A-300B-2PC-811R-/250733210572?pt=Vintage_Electronics_R2&hash=item3a60dd2bcc
  • BX4 socket to plug in PX25 tube: http://www.ebay.com/itm/1pc-Teflon-5pin-Tube-Socket-Gold-Pin-B4-B5-PX25-PX4-ML4-/320701714621?pt=LH_DefaultDomain_0&hash=item4aab4fd0bd
  • Two part epoxy with resin and hardener
  • Toothpick or something similar
 
STEP TWO: solder wires according to diagram below. This is a bit tricky.
 

  • I soldered all four wires on the BX4 (for PX25) pins first. 
  • Then I threaded and pulled all four wires through the larger pins on the UX4 base.
  • I poured solder down the holes of the pins where the wires come out. Don't over do this step. Then I cut the wires extending past the pins down to the length of the pins.
  • There should be a little bit of slack, a few millimeters, where you can push and pull the BX4 piece in and out of the UX4 base.
  • Now here's the messy part - in a multiple iterations, pour down the resin portion of the epoxy, and then pour down the hardener into the base, mixing with a toothpick as you go. Whatever you do, don't overdo the hardener. The resin will dry no matter what, even if it takes weeks. The hardener will continue to leak for months if you use too much, which is what I did.
 
TIP: use isopropyl alcohol to clean off epoxy
 
 
 
I usually don't mind doing simple stuff like cable terminations for friends on Head-Fi, but please don't ask me to do this for you. It was a serious pain in the arse and a very messy procedure.
 
Feb 28, 2012 at 4:55 PM Post #5 of 19
Thank you for the info. I will give this a try eventually. Did you try the KR px25 by any chance?
 
Feb 28, 2012 at 5:33 PM Post #6 of 19
 
Quote:
kingstyles said:
Thank you for the info. I will give this a try eventually. Did you try the KR px25 by any chance?
Click to expand...

 
I did hear the KR PX25. Extremely clean and detailed sounding, particularly in the treble - doesn't have the KR PX4's harmonic richness. More sterile.
 
 
 
Feb 29, 2012 at 3:57 AM Post #7 of 19
Thanks for the valuable and nicely presented info.
Great idea easy to mod the BA without having to send it back.
 
Purrin out of the KR PX4, KR PX25 and 300B which is your favorite?
 
Feb 29, 2012 at 12:43 PM Post #8 of 19
As you may know, I am running vintage Osram PX25s, but I'll take the KR PX4 out of the above. Every time I have the inclination to get them, Craig seems to be out of stock. The KR PX25 was too sterile (would rather get a solid-state amp). I haven't heard the KR 300B, but I haven't heard a 300B yet that I really loved in most aspects. 
 
One of these days I would like to try the current production EML AD1.
 
Feb 29, 2012 at 12:53 PM Post #9 of 19
How do the Osram PX25 compare with the KR PX25?
 
Is the AD1 directly compatible with the PX4?
 
Feb 29, 2012 at 1:34 PM Post #10 of 19


Quote:
rosgr63 said:
How do the Osram PX25 compare with the KR PX25?
 
Is the AD1 directly compatible with the PX4?
Click to expand...

 
The Osram kicks the PX25's and PX4's ass. The sound is different though. You have to like the vintage USA tube sound, and even then there's more. Silky treble, stable and precise imaging, explosive dynamics, particularly in the low bass. Never heard anything quite like it. A little bloom, so not the clearest sounding, but not nearly as much bloom as some 300Bs I've heard. KR25 = no bloom, no richness (not in a bad way), lots of treble detail and articulation (but not in a nasty etched way.)
 
I believe the AD1 is essentially a 4 volt 300B. The plate resistance and gain is the same as the 300B. The socket for it is really funky though. Keitijai is running vintage Telefunken AD1s I believe. He may know more.
 
 
Feb 29, 2012 at 1:53 PM Post #11 of 19
Yes I know Kiertijai is running the Telefunken AD1.
 
Did you know that EML can rebase to UX4 these tubes?
 
Feb 29, 2012 at 1:54 PM Post #12 of 19
 
Quote:
rosgr63 said:
Yes I know Kiertijai is running the Telefunken AD1.
 
Did you know that EML can rebase to UX4 these tubes?
Click to expand...


Really - I had no idea. Would make things much easier. You'll understand why when I fill in the second post of this thread.
 
 
Feb 29, 2012 at 4:48 PM Post #13 of 19
 Keitijai is running vintage Telefunken AD1s I believe. He may know more.
    I do run the NOS Telefunken AD1 at 4V Filament voltage but I use the PX4 setting, with the PX25 mod
it will be the same between the PX4 and 300B setting.  They were really good and according to the AD1 literature
it was claimed that they have incomparable clean and clear sound. I could not say they are incomparable because my limited
experience.   They have AD1 socket and need the AD1/UX4 adapter.  I have the seller built the adapter for me with high price.
    I found that the PX25's, PX4's NOS and equivalents are hot on ebay right now:  I saw the Mazda PP3 250 (=PX4) used strong
pairs went over 1400$/pair and Mazda PP5 400 (=PX25) two single tubes are going over 335$ while there are still 7 days left.
I also saw two pairs of Tesla RD27AS (=PX25) and I thought I could wait a while because there were two pairs only to find out
that both two pairs were gone after 4 hours later.  There were many private bidders so I think they may be Chinese.
 
 
Mar 5, 2012 at 2:56 PM Post #14 of 19


Quote:
purrin said:
 
The Osram kicks the PX25's and PX4's ass. The sound is different though. You have to like the vintage USA tube sound, and even then there's more. Silky treble, stable and precise imaging, explosive dynamics, particularly in the low bass. Never heard anything quite like it. A little bloom, so not the clearest sounding, but not nearly as much bloom as some 300Bs I've heard. KR25 = no bloom, no richness (not in a bad way), lots of treble detail and articulation (but not in a nasty etched way.)
 
I believe the AD1 is essentially a 4 volt 300B. The plate resistance and gain is the same as the 300B. The socket for it is really funky though. Keitijai is running vintage Telefunken AD1s I believe. He may know more.
 
Click to expand...


Wow those Osrams are pretty pricey!
 
Have you heard Western Electric 300Bs? The Osrams aren't quite in that category price wise, but it looks like they approach them. From what I've heard tube rolling my friend's monoblocks, the WE were clearly better than any other 300B we tried (EML mesh, Sophia, KR)
 
 
Mar 5, 2012 at 7:25 PM Post #15 of 19
I have no idea how the WE 300Bs sound in a headphone application. I've heard them many years ago powering some fairly high-efficiency speakers. Now that I think of it, I wasn't a big fan of them (personal preference thing.) A lot of the overpriced tubes are over-rated and have developed a certain mystique because of their un-obtainium nature. It's probably easier to tweak a circuit to take similar tubes of better or equivalent sound.
 
For the record, I paid about the same for PX25s that I got as I would have the EML or KR tubes. I've seen them around for $1500 or so, not too long ago for $800. Things could have changed by now.
 
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