Jul 5, 2017

VT25, VT25A Push Pull rejuvenation part 3

Wiring and components

I started wiring the SRPP driver first.


It is the trickiest part to build as I favor compact, direct to socket implementation whenever possible. I also like to use soldering lugs like SATO's when I have enough room (MC1/60 driver). In my youth I was very impressed by Tektronix wiring method using ceramic soldering tags.
The third resistor (upper anode) is missing, it will be directly soldered on the last decoupling cap when filtering bank will be held in place in chassis.

Star grounding, point to point wiring, short leads, no shielded wire (whenever possible) are the basics of a trouble free construction. I actually make most of my supply and ground connections with solid and find it to work well. Had the chance to find a roll 0,5mm2 heavy silvered OFC wire for that purpose.
Allen Bradley, Corning, Koolohm resistors, ERO MKC 1860, Philips 021, F&T electrolytic and SEL polypropylene decoupling capacitors are the reliable parts of this amp.
Like in all my works I use sub chassis for the HV and filaments filtering caps and it greatly eases construction.


From outside looking inside....clean and clear.


Third SRPP resistor from 5687's upper anode straight to the decoupling capacitor. Very short connection.


Listening

To say it straight, this modified amp sounds really great. LC supply and SRPP driver made a HUGE improvement on sound qualities! In addition to the fact that it is now dead silent this amp drives my La Scala effortless, mainly because of the low damping factor (mandatory on these speakers) and music is rich, full bodied, detailed, involving... Not enough words to say how I am delighted whatever the material. This amp has the kick and speed of the big DHT SE amp with a wider and deeper, almost 3 dimensional sound stage. Its extremely low noise level helps to catch micro details and brings music reproduction to a level of clarity and realism seldom heard.

Obviously the tubes combo changes the amp character and the best to my ears are GB 5687 Sylvania, VT25A Philco (a rare find, even scarcer than the very sought after WE, that need some burn in time to develop their rich texture) and 5R4GYS Philips.

5R4GY rectifier is the tube of choice with 530V AC and the Philips/RTC deserve their reputation of naturalness and accuracy, they are way better than the RCA's I used first. An other choice could have been the EY500A, they need a 6.3V filament supply and provide a HV soft start which have some advantage with oxide coated cathode triodes (unnecessary with thoriated tungsten tubes).

VT25A Philco



5R4GYS RTC Philips



Prefer the VT25 (tried Sylvania, Visseaux and Neotron from my stock) for strings music (excellent on guitar) or as a mid/upper band triode in an active system . They lack, to my opinion, the punchy low end the VT25A have but are unbeatable on voices.

10 / VT25 Visseaux bright light.
These tubes are among the best 10 I have heard, just outperformed by Visseaux A710 big plate triode. These have oxide coated cathode and concentrate the very best of the VT25 and the VT25A in a single tube. From my own experience they better the famous PX4. Years ago I sold a bunch of these (Osram brand) to get a few A710's...



I gave a try to the E182CC. This tube is a good contender in place of the 5687 to get more gain from the driver (not really an issue in facts). Sound is a bit more thinner and it does not extend in the bottom end like the Sylvania Gold Brand's but stays very natural, articulated and vivid.

As I said this amp is very involving…sooo good that when I stopped listening it was three in the morning! Needless to say the MC1/60 went to the attic (for the moment).

Just a few CD's and LP's for this report













1 comment:

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