More on capacitors

[SoundMods]

Those of you that know me personally and -- even some that don’t -- have witnessed my preference to Electronic Concepts polycarbonate capacitors. For me they definitely have a place in passive speaker crossovers and in many cases a place in electronics’ signal path preferably when bypassed with complementary Teflon capacitors especially those made by Custom, CRC, or Tex-Cap hermetic capacitors.

That said – it was time to re-cap (all power supply electrolytics) my pre-amplifier (a Balanced Audio Technology VK-3i – 6DJ8s or 6922s or 7308s with 6V6 current sources). Since time has passed, the BAT pre-amp was already 23-years-old and yet I had no desire to replace it. Since the work involved full disassembly I also took a hard look at the output coupling caps for upgrade plus some resistor upgrades. Since it is a balanced design we’re talking about four matched output caps. The stock offering was Jensen aluminum paper-in-oil caps that I bypassed years ago with Teflon caps. Those of you that have heard my system recognized that the pre-amp acquitted itself very well.

Before committing to a change -- I performed trials using a DC-coupled pre-amp (Spectral DMC-10) and an adapter that I made to evaluate coupling caps and resistors in the signal path against a straight wire. I was leaning toward my preference until I discovered some Russian caps in my inventory that I long since forgot about. To make a long-story-short these caps were represented by the seller as polycarbonates and they were set aside since they did not perform well in trials for my speaker’s crossover.

As it turns out they were NOT polycarbonates but hybrid capacitors. HUH? They were confirmed by the product number K75-24 and as it turns out have a “cult” following with one manufacturer using them in their hi-end phono pre-amps. (the $12,000 Brinkmann Phono stage “Edison").

These caps are a metalized-paper-in-oil together with a mylar (polyester) dielectric. Products of the Cold War that defy reason or experience when it comes to all things audio. Bypassed with Teflon caps the combination provides an open, grain-less, flesh-on-the bone level of reproduction that far exceeded any expectations I had. WOW!! “See-through transparency" – amazing 3D mid-range performance – together with a smooth-musical character that does not smother. Vocals especially are a treat. Strings do not sound like screaming cats. Keeping in mind that the Jensens were the product to beat. Hell -- my polycarbonate favorites didn’t even make it a contest.

Who knew?!

Attachments

0 Russian Hybrid Capacitor.jpg (41.5 KiB) Viewed 23175 times

[ArthurOtt]

Hello, I also own a BAT VK3i preamp but I don't have it in my system currently. I was actually thinking about selling it. It was recapped in 2019 with the stock Jensen 1uF 400 VDC caps. I see that you've done some mods on your VK3i in regard to the caps. Can you tell me more about what you've done and how it's changed the performance of the preamp? Thx......Arthur

[SoundMods]

Hello, I also own a BAT VK3i preamp but I don't have it in my system currently. I was actually thinking about selling it. It was recapped in 2019 with the stock Jensen 1uF 400 VDC caps. I see that you've done some mods on your VK3i in regard to the caps. Can you tell me more about what you've done and how it's changed the performance of the preamp? Thx......Arthur

I discussed the end result when I announced the discovery of the Russian Hybrid caps. As far as global upgrades are concerned -- the question is -- what didn't I do?

All of the power supply caps were replaced with industrial-grade products having a 105-degree C or better rating and designed for long life and low ESR.

The 1-ufd power-supply bypass caps were replaced with 20-ufd polycarbonate caps bypassed with .27-ufd CRC hermetic Teflon caps

The 1-Kohm grid-stop resistors were replaced with RN-65 Vishay bulk-film resistors. The input resistors were candidates but "buried" under the four volume control boards (disassembly is not recommended).

The Jensen output couplers were replaced with hermetic 2.2-ufd K75-24 Russian hybrid capacitors bypassed with .25-ufd Tex-Cap hermetic Teflon caps.

Output buffer resistors were added as the pre-amp was designed without any. ??? I installed Vishay RN-90 bulk-film resistors.

Past mods included shielding to limit RF contamination from the digital volume level controller. I also added 2-ufd capacitors across the filament supply to the 6V6 current sources

Tube complement is Eastern Block Amperex ECC88s and mil-spec RCA metal jacket 6V6s. Teflon damping rings were installed on the ECC88s and Pearl Coolers on the 6V6s.

Case work included extensive constrained layer damping where-ever possible using ceramic tiles and RTV silicone.

Since I am obsessive (no surprise there) I blinged-up the cosmetics.

Was all that WORK justified and worth it? DAMN RIGHT!!! I had no interest in any other pre-amps either new or used. For me -- this baby is a keeper!

Attachments

[ArthurOtt]

Wow! I thought you just changed out the output caps and that made all the difference. I could handle that. I didn't know you had done extensive mods. I bought the Beckert Labs Rhumba 1.2 tube preamp because I wasn't happy with the VK3i. Some good NOS tubes helped but I just wasn't able to get the transparency and openness from it that I wanted. Sounds like you've been able to do that with the mods you've implemented. It's not a bad preamp but it's not great in its stock configuration. Thx for the info.

[Pelliott321]

he is avid and I mean avid crazy man

[Roscoe Primrose]

he is avid and I mean avid crazy man

Who here isn't

[SoundMods]

Wow! I thought you just changed out the output caps and that made all the difference. I could handle that. I didn't know you had done extensive mods. I bought the Beckert Labs Rhumba 1.2 tube preamp because I wasn't happy with the VK3i. Some good NOS tubes helped but I just wasn't able to get the transparency and openness from it that I wanted. Sounds like you've been able to do that with the mods you've implemented. It's not a bad preamp but it's not great in its stock configuration. Thx for the info.

I've tried numerous tubes in the BAT. What works best are either Eastern Block Amperex ECC88s, Valvo ECC88s, or Tesla Gold-Grid ECC88s. The only 6V6 current-source tubes that really gave the pre-amp the breath of life were the metal 6V6s.

BTW -- you can't change tubes without re-setting the bias and balance -- otherwise it's a fools' errand.

The coupling caps I used provided most of the improvement -- but since I had to do a full disassembly to replace them I went after anything and everything. Like I said, the BAT is a keeper and I wanted to extend its expected useful life.

Attachments

[ArthurOtt]

My VK3i seemed to sound best with a quad of Philips Miniwatt ECC88 tubes and a pair of Mazda 6V6GT from the 1950s. I wasn't aware there were user-accessible bias and balance adjustments on the VK3i.

[SoundMods]

My VK3i seemed to sound best with a quad of Philips Miniwatt ECC88 tubes and a pair of Mazda 6V6GT from the 1950s. I wasn't aware there were user-accessible bias and balance adjustments on the VK3i.

Yep. That's what has to be done when changing tubes. And BTW -- no glass envelope 6V6s work as well. Why? The metal enclosure of the metal tubes is tied to ground and shields the critical current source from the self-generated RFI of the display and volume control electronics.

[DaveR]

he is avid and I mean avid crazy man

Who here isn't

Indeed.