You are reading the older HTML site

Positive Feedback ISSUE 7
june/july 2003


herbie's audio lab

Hal-O Vacuum Tube Dampers

as reviewed by Roger S. Gordon, CPA (with a few brief comments by Dave Clark)


gordon_nether3_rev_dwr.jpg (28666 bytes)






Balanced Audio Technologies' VK-75SE amplifier, and Herron Audio phono and preamplifier.

VPI Mk IV turntable, Benz Micro Reference cartridge, ET 2.5 tonearm with Mg II arm wand, high pressure air pump, damper trough, and a pressure regulator.

DIY phono cable using Van den Hul mono-crystal silver wire, Harmonic Technologies interconnects, and Audio Magic silver loudspeaker cables.


Vacuum tubes vibrate. They have resonances. They ring. Tap them with a pencil or rub them with a toothbrush with the system on and you will hear the vibrations—the microphonics—translated into sound. Not good! Many mechanisms have been devised over the years to damp tube vibrations. These mechanisms have ranged from the simple—covering the top of the tube with silicone sealant or slipping O-rings over the tubes; to the more elaborate—gluing brass or lead weights on to the top of the tubes. These various tweaks have all affected the sound either by absorbing some of the vibrations (O-rings, silicone sealant, etc.) or by shifting the frequency of the resonances (brass hats, etc.).

All of these tweaks have had drawbacks of various kinds: they're messy, non-reversible, not very effective, materials breakdown over time due to heat from tubes, etc. Because of these problems I have never gotten serious about trying to dampen my tubes. I thought that I would wait until someone came up with a better solution.

While surfing the web, I stumbled across Herbie's Audio Lab ( Being offered for sale on the site were a number of damping products, including CD and turntable mats and damping feet of various kinds. What caught my attention were the vacuum tube dampers. This was a design that I had not seen before—Teflon C-rings of various diameters, with 3 or more little wheels strung on the C-ring. The wheels were made of energy absorbing material. One pushed the C-ring down over the vacuum tube. The little wheels contacted the side of the tube but there was still plenty of room left so air circulation was not restricted. This design looked promising. Simple to implement. Non-messy. Reversible. And covered by a 90 day money back guarantee if not satisfied, and a lifetime warranty to boot. Was this too good to be true? Hmmmmm…

One of the web pages on Herbie's site had a list of glowing testimonials from satisfied customers. One of the testimonials was from someone I knew—a former PF writer who assists each year in one of the manufacturer's rooms at the Las Vegas CES. Each year I chat with him and enjoy the great sound in the room. So, off went an e-mail: "Hey, are these HAL-O's any good?" The answer came back "Yes. I sell them in my store and you heard them in our room at the CES. They make a noticeable difference." A few more e-mails back and forth, and I ended up sending off my check for 23 HAL-O tube dampers.

The HAL-Os range in price from $9.98 for small tubes (12AX7, 6922, etc.) to $24.95 for the humongous (e.g., the Kronzilla T-1610). The C-rings come in two flavors—white or black. Functionally, there is no difference. The black does cost a bit more. The damping wheels also come in two flavors. However, here there is a functional difference. For standard applications the wheels are a translucent milky white. These wheels are rated up to 460 F or 238 C. For higher temperature applications, such as power tubes, the wheels are a bit more expensive and are colored a reddish orange. These higher temperature wheels are rated up to 572 F or 300 C. In the rare event that your tubes really heat up prior to perishing, the wheels can take up to 700 F. After that, they melt. If that happens, you return them for replacement under the lifetime warranty.

The tube dampers that I ordered were for the following:

(for the Balanced Audio Technology VK-75se amplifier)

6 x 6SN7, 4 x 6C33C-B, 2 x 6H30, 2 x 6V6

(for the Herron Audio phono stage)

4 x 12AX7, 1 x 12AT7

(and for the Herron Audio preamp)

4 x 6922

The total cost for these tube dampers was $283.94. However, there was a 10% discount for orders over $200, plus free shipping. See Herbie's website for details.

I had been told that damping the small signal tubes provides the greatest benefit. That stands to reason. The sound caused by microphonics in a tube in the phono stage gets amplified by the preamp and then by the amplifier, while the sound caused by microphonics in an amplifier tube gets multiplied only by the amplifier. Thus, I decided to start with the amplifier to see if I could hear what should be the smallest difference.

Listening Impressions

After the system had been warmed up for a few hours I put on the turntable Classic Records' 45 RPM one-sided pressing of Ravel's Alborada del Gracioso (LSC-2222-45). This is a good recording for doing A-B comparisons because it contains both very quiet passages and very loud passages. There are short solos by castanets, bassoon, and trumpets, all with very different timbres.

After listening to the short work I put the fourteen HAL-Os onto the amplifier tubes. This was done without shutting off any of the equipment. The instructions said to put the dampers not quite half-way down the tube. For the 6C33C-B tubes, which have shoulders, the damper was placed just down to the shoulder. The HAL-Os went on real easy. It took less than two minutes. I then replayed Alborada del Gracioso. The difference was striking. I then removed the HAL-Os from the amplifier tubes, again without shutting off any of the equipment. Removing the HAL-Os would not have taken any longer than putting them on had the equipment been shut off and the tubes cool. However, not only were the tubes HOT, but I had to be careful not to loosen the tubes in their sockets.

With a few scorched fingers I did remove the HAL-Os. Again I replayed Alborada del Gracioso. Yechhhh! That's what I've been listening to all these years?! Back on went the HAL-Os. Replay Alborada del Gracioso. Ahhhh! A summary of my notes of the effect of the HAL-Os would be "veils removed" and "more natural timbres." Think of the difference between a $400 CD player and a $2,000 CD player. The music from the $2,000 CD player is cleaner. The instruments sound more like real instruments. There is just less grunge between you and the music. These tube dampers really work!

Having had such great success with the amplifier I immediately unscrewed the cover on the Herron Audio preamp. The vacuum tubes in both the Herron Audio preamp and phono stage run much cooler than most. Even after the units have been on for hours, you can touch the tubes for several seconds without discomfort. Thus, getting the HAL-Os on and off was very easy with the Herron Audio units. On went the HAL-O's, and then a replay of Alborada del Gracioso. Not as big a difference as with the amplifier. I removed the HAL-Os from the preamp and re-played Alborada del Gracioso. Put the HAL-O's back on the preamp and again replayed Alborada del Gracioso. The difference was an order of magnitude less than with the amplifier. Think of the difference between a $3000 preamp and a $6000 preamp. The improvement was definite, but more subtle. The notes seemed silkier, smoother. The concert hall, which had been large, was now even larger. Instruments, which had been easily locatable in space, were now locked-in. Each instrument was more clearly delineated.

I then removed the cover on the phono stage and put on the HAL-O's, replayed Alborada del Gracioso, removed the HAL-O's, replayed Alborada del Gracioso, put the HAL-O's back on and played Alborada del Gracioso one final time. The improvement was very small but still noticeable. Again the notes with the HAL-O's installed seemed slightly smoother. There was more inner detail in complex passages. Micro-dynamics were improved; i.e. the small, subtle changes in the volume of individual notes were more apparent.


So, in summary, the $160 I paid for the 14 HAL-O's to dampen the tubes in my amplifier was money very well spent. The $40 for the four HAL-O's for the preamp was also money well spent. The $50 for damping the phono stage I am not so sure about. The improvement was noticeable, but I can buy a lot of used LPs for $50. Still, overall, I am very happy the improvement the $260 worth of HAL-Os brought to my system.

The day after installing the HAL-O's, I put a used Sound Application cryo'ed XE-12 power line conditioner with a used Elrod Power Systems EPS-3 Signature power cord into my system. The twenty-three HAL-Os made a much bigger improvement for a lot less money. So (no surprise here!) I am going to keep the HAL-O tube dampers and not avail myself of the money back guarantee.

Will the HAL-O tube dampers work magic with the sound of your tube gear? That's a good question. The impact of vacuum tube microphonics on sound is dependent on a number of factors including

  1. the circuit design

  2. the type of socket being used to hold the tube

  3. the type of tube being used (some tubes are more subject to microphonics than others)

  4. which brand of tube is being used (some brands of the same type of tube are more subject to microphonics than others, and

  5. the construction of the individual tube being used (there can be considerable variation in the microphonics of the same tube made by the same manufacturer).

Your mileage may vary. However, if you have tube gear and you have a few bucks that you can invest, why not contact Herbie's Audio Lab and order some HAL-O's? If they don't work magic in your system, return them. All you've got riding on the experiment is the return postage.

I would like to add that I have had the opportunity to audition the Hal-O RCA dampers on several cables from different companies. Unfortunately the results appear to be more dependant upon the type of the RCA used than anything else going on—at least as far as I can tell! Using the Hal-Os on any metal RCA of reasonable quality and heft, the results will far exceed those as heard on the RCAs found on my reference Audio Magic Clairvoyants—modified Eichman Bullet plugs. That is, one can expect—though not in terms of being a musical or sonic revelation, but they do make a difference—a cleaner more articulate sound with less smear and blur. Using the Hal-Os on either the DH Lab Revelations (locking RCAs reminiscent of Vampire) or the JPS Superconductor 2s allows one to hear more music with less "visual" artifacts. Musical images simply appear to be more stable and defined. On the Eichmans it is more of a "Do I hear the same, or am I just thinking I hear more of the same?" as the light plastic casing appears to be less susceptible to vibrations and such than those based on an audio jewelry origin. At the price, these are one of the final touches any true tweaker must have in the their system. But like I said the differences are not one that will turn heads. I use them and find their look and application a real tweaker's delight! Dave Clark

halo.jpg (77199 bytes)
Photo courtesy of Herbie's Audio lab

Herbie's Audio Lab
3836 Kentucky Avenue
Riverbank, CA 95367
TEL: 209. 499. 6342

web address: