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SP3

Page history last edited by kstlfido 2 years, 7 months ago Saved with comment


Specifications

 

  • Frequency Response: 20Hz to 20KHz
  • Class AB
  • Power Rating: 38W x 2
  • Manually adjustable bias: 1.15V
  • Input Impedance: 250K Ohms
  • Output Impedance: 4 or 8 Ohms
  • Input Sensitivity: 380 mV
  • S/N Ratio: >90dB / T.H.D - 1% (standard tube spec)
  • Dimensions (WxHxD): 11.9" x 8.1" x 13.25"
  • Weight: 56 lbs.
  • Tube Complement:

    4 X 5881

    2 X 12AX7

    2 X 6922

    2 X 12AU7

     

 

Tubes

About Tubes

Care And Handling

When you first turn on your tube amplifier, let it warm up for several minutes before playing any music. It is also recommended that you turn down the volume or turn off music when you power down the amplifier because it takes the power supply capacitor a few seconds to completely discharge.

 

How Long Do Tubes Last?

A host of factors influence the life of your tubes. The more frequently you use your tubes, the harder you drive them, and the farther they are from the correct bias all detracts from the life of your tubes. A conservative estimate, if you use your amplifier frequently, is 1-2 years.

 

How Do I Remove A Tube?

  1. Make sure the amplifier is off, and that you've given the tubes time to cool (at least two minutes).
  2. Using a clean, dry cloth, shirt, or glove, grasp the tube firmly (near the base) and pull straight up.
  3. Tubes are much less fragile than you might think, but do not apply excessive force.

 

How Do I Know If A Tube Has Gone Bad?

When a tube starts to go "bad" it becomes noisy and microphonic.

 

To test if a tube is microphonic you can tap on it with something hard (usually metalic) while the amp is powered on. Microphonic tubes can make a louder noise than other tubes or make ringing/squealing sounds when you tap them. Note, however, that doing so can result in further damage to your tube - listening for noise, growling, and feedback is the safest way. You can find the bad tube by swapping the tubes one pair at a time to the opposite channel until you discover which tube(s) the noise follows.

 

If shortly after turning the on the amp you see/hear an arc in one of the power tubes, this may be a "grid short" that doesn't cause any damage to your amp or the tubes. If everything sounds normal after the occurance, there is likely no need for concern. The small piece of metal that caused the short will no longer be there to cause problems.

 

It is recommended to completely re-tube every few years based on amp use. An amp that is infrequently used can be re-tubed every 4 years, while one that sees daily use should be re-tubed every two years.

 

Tube Types

Biasing Tutorial

Visit this site for more information on biasing.

 

You should check your bias when your receive your SP3, and check it every 3 months. If you replace the tubes, or even swap the two channels the bias will need to be adjusted.

  1. Before adjusting the bias let your amp warm up for a few minutes.
  2. When adjusting the bias the amp should be at "idle" meaning no music playing.
  3. There are four bias pots (two on each side) with a test point for each one.
  4. Put the black lead of your voltmeter (set to DC) in the ground/ref test point between the two bias pots, and the red lead of your voltmeter in one of the test points.
  5. Adjust the pot closest to your red lead until the meter reads 1.15V.
  6. Switch to the other test point with the red lead while keeping the black lead in the middle ground point, and adjust the pot closest to your red lead.
  7. When you have adjusted all four pots, you are finished.

 

Replacing Tubes

Eventually all tubes need to be replaced. To maximize tube life, it is always best to turn the amplifier off if it will be idle for longer than several hours. Leaving it on all the time generates unneeded heat, wastes electricity, and shortens tube life. The following diagram will assist you in identifying the various types of tubes utilized by your sp3.

 

Where to Get Tubes

Forum members have had good experiences at the following vendors:

Cryoset

KCA NOS Tubes

Triode Electronics

Upscale Audio

Vintage Tube Services

Tube Rolling

Recommended order for upgrading:

  1. Preamp: 12AX7
  2. Phase Inverter: 6922
  3. Driver: 12AU7
  4. Power: 5881

 

Tweaks and Fixes

 

Amp Too Loud?

The stepped attenuator on the SP3 is terrific, but even at the lowest setting it may be too loud in small spaces or during late-night listening sessions. If you find the first few steps overpowering, you might want to consider in-line attenuators (12db has done the trick for some) - they plug into the analog port on the amp and you plug your interconnects into them. Search on Ebay.

 

Another alternative to the volume issue is to substitute 5751 tubes for 12AX7 tubes. This is a direct replacement, but the 5751 only has 70% of the gain of the 12AX7.

 

Component Replacement Recommendations

12AX7 - Several people have been very pleased with the Groove Tubes 12AX7M "Mullard Reissue" as a replacement for the stock Melody re-branded 12AX7 tubes. If you are new to tube rolling, this is a solid recommendation for your first try at improving the sound of your amplifier.

Another good alternative is the Tung-Sol 12AX7 Reissue from New Sensor. Other than its name, this tube has nothing else in common with the original Tung-Sol 12AX7 made in the United States in the 1950's and 60's. This tube has a different sound characteristic compared to the Groove Tubes 12AX7M and has been regarded as one of the best current production 12AX7 tubes available. For a New Old Stock (NOS) replacement tube, many SP3 owners have reported very good results with RCA triple mica black plate 5751 tubes. These tubes are no longer in production and are typically more expensive than current production tubes like the Groove Tubes or Tung-Sol reissues.

 

6922 - A good current production replacement is the Electro-Harmonix 6922 from New Sensor. These are sold as high fidelity replacement tubes and are often supplied as stock tubes in many high end vacuum tube audio amplifiers.

 

5881 - The Chinese-manufactured stock Melody tubes in the unit are quite acceptable for starters. Some SP3 owners have replaced these stock tubes with SED "Flying C" 6L6GC power tubes. These are not direct replacements for the stock Melody 5881 power tubes, as the bias voltage must be adjusted to compensate for the different tube specifications. More information on bias level adjustments for the 6L6GC tubes can be found at the AV123.com forum.

 

Schematics

 

 

This circuit appears to be of an earlier version of the SP3 since it only shows one bias adjustment pot per channel, but we know that there are 2 pots/channel--one per output tube. So, they must have reconfigured the biasing circuit. to allow for biasing each output tube individually. The circuit was further modified so that the cathode of each output tube is now grounded via a 30 ohm resistor (in the diagram, the cathode is connected directly to ground).

 

For Power Supply schematic click HERE

 

Awards and Reviews

  1. Secrets of Hometheater and Hi-FI Budget Tube Amp of 2005  award - 9/20/05 review
  2. 6 moons Best of 2005 - 3/15/05 review - follow-up
  3. Good Sound Great Buy award - 9/1/05 review
  4. Enjoy The Music review - 3/07

 

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