One of the perks of listening to vinyl records is analog audio equipment. Unlike digital sources and integrated circuit amplifiers, analog gear allows a certain degree of tinkering and easy improvements for anyone adept with a soldering iron or the ability to replace op-amps or tubes in their sockets.
The Search for a Vacuum Tube Phono Preamp
Recently I decided to retire my 19 year old Rotel RQ-970BX phono preamp and treat myself to something better. I wanted to try out a modern phono stage based on vacuum tubes (valves). It’s not secret that well-engineered tube equipment can sound fantastic with a huge sound stage and a warm, non-fatiguing tone. Unlike many vintage designs, modern tube equipment does not suffer from excessive noise, hum or distortion. On paper, they feature very low distortion, low noise and a wide frequency response. What is hard to capture through measured specifications is how they sound, that’s the subjective part. However, it’s the overall subjective sound quality that can make or break good analog gear. One of the advantages of tube based designs is that one can try out different tubes which will improve and tailor the sound to one’s preference. The challenge was to get something significantly better than the Rotel, but under $500. A tall order since many top “audiophile recommended” phono stages cost over $1000.
I was going to try my hand at building a high quality valve based phono preamp and looked at a number of kits on from various online vendors including Ebay. There are some excellent tube based kits for DIY hobbyists, however, they do not come cheap (over $500). Moreover, with the anemic Canadian dollar, purchasing such a kit from the USA adds about 35% to the cost, not to mention shipping fees.
On the other hand, one can find budget tube kits on Ebay. There’s a rub, though, most of them are Chinese products of dubious quality. They often use garden variety Chinese valves that are as common as muck and quality control is not consistent. Many of these kits are based on old 1960’s and ’70s designs which were copied from vintage equipment and that means noise is an issue. The Chinese Ebay kits cost between $100 and $300 shipped, not to mention a significant amount of time to build one. After reading a number of less than stellar reviews on hi-fi sites, I decided against it.
After a fair bit of research and reading several online reviews, I finally decided on the German designed Pro-Ject Tube Box S. Luckily, I found one for sale at Whitby Audio. I was also fortunate enough to purchase it at the same price before the Canadian dollar took a nose dive and that saved me $200. I expect it was older stock that was sourced when the Canadian dollar was still strong.
As soon as I was home, I connected my turntable, let the tubes warm up for an hour and spun a record. It sounded fantastic, compared the solid state Rotel RQ-970BX. The Tube Box S has a wider soundstage, greater dynamics, better bass and an overall fullness that really surpasses Rotel. Moreover, there are dip switch adjustments to meet the exact electrical requirements of just about any moving magnet or moving coil phono cartridge. Money well spent and it was within my $500 budget.
Tube Rolling on the Pro-Ject Tube Box S
The factory installed tubes on the Pro-Ject Tube Box S are generic ECC88’s made in Czechoslovakia. They are decent enough, better than many cheap Chinese tubes, but there is room for improvement. I noticed they tended to be a bit noisy, when the amp is running at high volumes – the tubes were prone to picking up a small amount of hum and hiss. I was able to minimize this by plugging the power supply into another outlet and installing a ferrite coil on the power supply wire. As good as this unit is, I want silent valves and better sound.
Knowing full well the sound of the Tube Box S could be improved with better valves, I visited a number of audio enthusiast web sites and made a list of potential replacements. Many of the best valves cost as much, if not more, than the purchase price of the Tube Box S. A single vintage audiophile class tube could cost anywhere between $150 and $300. The reason is that these tubes were made by now defunct valve manufacturers such as G.E., Telefunken, Mullard, Philips, etc. They are referred to as New Old Stock (NOS), meaning they are unused, still in original boxes and culled from old unsold stock. These historic valves are bought and sold like rare vintage wines. On the other hand, one can purchase brand new valves made in Russia, China or Czechoslovakia. Generally, Russian tubes are favoured and the prices are quite reasonable. A good Russian preamp tube costs about $12 and better ones start at $25.
Mullard 12AX7 – Russian Recreation of a British Classic
My criteria was to find a valve known for very low noise, excellent hi-fi performance, musical warmth, energetic bass response and a long life. Each valve had to be under $30, so based on my research, I bought a pair of Mullard 12AX7s. These are Russian copies of famous Mullard tubes made by the legendary and now defunct British manufacturer bearing the same name.
Among audiophiles, original Mullard tubes have a cult status and now command a wallet draining price. The Russian copies get you about 90% of the performance, but at 1/9 to 1/10 the cost. Back in the day, Mullard manufacturing maintained strict quality control and engineering excellence. There is a historic video on youtube that shows how their valves were made with the greatest care and meticulous craftsmanship. The original version of the Mullard 12AX7’s were known for very low noise and excellent audio performance. Since the Russian copies are based on the same design, I expected a similar result.
Replacing the original tubes in my Pro-Ject Tube Box S took about 30 minutes. I had to remove the fiddly cages around the stock tubes and then carefully pull out the ECC88’s, replace them with the Mullards and reassemble the cages. I was anxious to try them out, however, brand new tubes can sound harsh until they have warmed up for a few hours.
I tested the Tube Box S to make sure the new valves worked and played a record. Naturally it sounded a bit harsh, but after the tubes warmed up for a two hours, they sounded great. Much better than the stock tubes and the valve noise is gone. There is virtually no hum or hiss, even at the highest listening volumes.
Bass is better, with more punch and better decay on the notes. The sound stage is outstanding, you can close your eyes and easily visualize the position of each instrument as if they were right in front of you. The sound stage takes on a whole new dimension and invites deep listening. Vocals sound more natural, sibilants have improved and there is more detail on the higher frequencies without any audible harshness or distortion. In other words, it sounds more musical.
The Russian Mullards have had several hours play to break in and sound even better. I expect they will continue to improve slightly after a few more weeks use. It was a worthwhile and modest investment to improve an already excellent phono pre-amp. Tubes rule and records have never sounded better on my system.