Testing and Miscellaneous info....

Preamp tube testing and batch power tube testing.

We took possession of our first Amplitrex AT1000 on 06/16/08 and our second Amplitrex in 2012. We simply call these machines the last word in tube testing. It is the most well thought out and precise tester we have ever used. In short, a thoroughly amazing piece of gear! In the past we had found the VTV Characterizer very helpful in allowing us thru experimentation to find certain grades and characteristics that worked very well with different amps but the extraordinary accuracy and available data that the AT1000 offers has taken our in house testing and grading to new levels. 

As you can see in the second picture we are actually bumping up the internal audio level with an outboard amp and then literally placing a headphone speaker right on both sides of each tube to check for noise and microphonics. Most will consider this excessive but that's the way we are!

In the third pic you can see Michael using an old Sencore where we pre-heat each tube before running them thru the AT1000 which runs them at real world voltages.

So major kudos' to Chris Terraneau for brining the AT1000 to market! If you would like to check out his site here is a link:  Amplitrex

Power tube matching.

April 10th 2012, 

When matching power tubes, it all comes down to two things: Regulation and consistency. If this cannot be maintained, a proper match can be difficult to obtain and a repeatable grading scale can be very difficult to develop.  

The standard piece of equipment in today’s tube resale industry for testing/matching power tubes is the Maxi-Matcher. Built by Maxi Test in Seattle Washington. We’ve used these units since day one (1998) and up until recently maintained what we would call a small fleet of them to keep up with our matching needs. They have served us well but as any mechanic will tell you, there’s nothing like having that perfect tool for the job. The Maxi-Matcher is a nice piece of kit but it doesn’t take long before any user starts to get a sense for its quirks and cumbersome temperament. After using these units for years we often discussed one day having matchers that would be the definitive “last word” in supplying all necessary data and ultimate accuracy.  

Enter Matt Kamna. Matt has been present in the local audio community in and around the Portland Oregon area for quite some time. His passion is HIFI as well as most things tube related with a particular knack for “myth-busting”. His background gives him a unique insight into our industry. After spending years working in some of the largest US based Electronics firms he went rogue.  

Matt has done some free-lance design work for us in the past and with dreams of the perfect power tube matcher in sight, we knew he was the best man for the job.  

The criteria for our new matcher was pretty simple, but not easily obtainable. Unlike the Maxi-Matcher which has two options for plate voltage (325VDC and 400VDC) we wanted the plate voltage to be continuously adjustable from 300VDC to 500VDC. This setting also needed to be regulated to within a tenth of a volt under normal line conditions (115-130VAC).  

This aspect alone would be one the biggest hurdles but one of the most crucial, as the Maxi-Matcher features no active regulation (which is its biggest pitfall). This means that even a 1VAC change in line voltage had a measurable effect on the test result of the Maxi-Matcher. Since we started using Maxi-Matcher’s in 1998 we’ve had to regulate our line voltage to less than half a volt to hold consistency in our grades.  

Although the Maxi-Matcher heats up four tubes at a time, it only applies high DC plate voltage to the tube that’s been selected to test, leaving the remaining three tubes in “stand-by”. This design allows Maxi Test to get by with a very small power supply since they’re only effectively running one tube at a time aside from the filaments. But this power supply is in fact so small that even when running a single higher output power tube (like a KT88) the B+ rail sags under normal test conditions. We had to compensate for all of these variables to create a solid matching system and to insure a consistent grading scale.  

The new matcher features individual readouts for each socket and each socket is fully active during the duration of the matching process which includes testing for current, transconductance and individual audio tests (covered later). This means that a very stout power supply was needed and the regulation circuits had to be capable of dissipating some pretty serious wattage under severe circumstances. To get an idea of what this means you’re looking at a 5lb Maxi Matcher vs. 40lbs for the new matcher. Almost all of this difference comes from the power supply where we had to use separate transformers in each aspect (bias voltage, B+ and filaments).  

The next aspect was the bias circuit. The Maxi-Matcher is pretty versatile in this regard. They give you a bias voltage range that’s stepped from -12VDC to -60VDC in five pre-set increments. Once again we wanted this aspect to be continuously adjustable and with a slightly wider range of about -10VDC to -60VDC. The point of making the plate voltage and bias voltage continuously variable is that we can now replicate real world situations. If you have a fixed bias amp that runs a quad of EL34’s with 455VDC on the plates and the grid voltage is fixed at -37VDC we can replicate that. This allows us to approach things in a completely different manner than any other piece of test equipment would allow, and we can actually find the perfect grade of tube for a fixed bias amp if these parameters are known.  

During the prototyping process we observed how much a small variance in any aspect of the power supply affected the tubes performance and thus the quality of the match we were able to achieve. One thing that caught us by surprise was the filament supply. We discovered that a 5% change in filament voltage changed the tubes plate current output by at least 1.5mA. We ended up using a separate transformer for the filament supply along with a 10 amp regulator so we know whether we’re running EL84’s or KT88’s, the filament supply will stay at exactly 6.3 Volts.  

The last twist we had to throw in was an audio circuit. It almost seemed silly to go through all of this trouble and not whip out a stethoscope during the physical… Matt got straight to the point by employing a solid state amplification circuit that we can insert directly into the cathode connection for performing audio tests on each tube separately. Now all power tubes undergo the same little “hammer test” and microphonic test that our preamp tubes go through.  

We refer to our new addition as the “Kamna Matcher”. It is a one off “one of a kind” machine, with about 75% of its chassis being massive heat sinks. However underneath its industrial and “prototypish” exterior it is pure “state of the art” precision, and our new standard.  

To finalize, we wish to acknowledge that we have depended on a fleet of Maxi-Matcher’s for fourteen years, and with skilled men at the wheel and highly regulated line voltage, we gained a stellar reputation for critical matching.  

Now we would like to extend our profound thanks to Engineer “Par Excellence” Matt Kamna for making our dream machine matcher a reality!  We can now say that the matching supplied by Eurotubes is unsurpassed and unequaled!  

Click to enlarge

All power tubes have a 24 hour burn and we grade all of our power tubes right down to the milliamp so that we can supply future orders to customers that will exactly match the originals. 

We still occasionally use our Hickok TV2 for certain tubes and we also have two of the VTV Preamp tube Characterizers that we can use if you're looking for a specific grade of tube that you have used in the past that was spec'ed out on a VTV. Otherwise we will be using our Amplitrex AT1000 for all preamp tubes as we have found it to be a far superior unit in every way.

We also offer graded, balanced and matched preamp tubes for an additional charge listed in the online store which is very nice for HiFi applications and phase inverters in guitar amps. We also offer hand picked high gain preamp tubes, especially for you players who can never get quite enough gain! The high gain tubes are an increase of between 10 to 15 percent.