When designed*, the 6LF6 horizontal sweep tubes used for the output stage were plentiful and inexpensive. The SA-4 was intended to be used with matched "octets" (sets of eight) tubes and individual tube biasing was not considered necessary. Unfortunately, this tube has become quite rare and expensive, and many, many tubes would need to be purchased in order to create matched octets.
    To further complicate matters, there are no substitutes for the 6LF6. All other horizontal sweep-type tubes require much more filament current (which would overheat the SA-4's transformer); different filament voltage (which means some way of powering them would need to be sorted out); have different tube basing, so they cannot be plugged into the existing tube sockets; have different numbers of pins, so they wouldn't fit anyway, or are not rated to handle the power that the SA-4 demands. And in any case, hundreds would still need to be purchased in order to come up with matched sets.

Described here is a modification to allow individual tube biasing so unmatched 6LF6 tubes can be used in the SA-4.

Note that it is recommended that all eight tubes in the SA-4 be from the same manufacturer. The most common 6LF6 is from GE. They appear to have made two types, a "tall bottle" type and one that is about 1cm shorter. Other than the size of the tube, there does not appear to be any other difference so I think they can be interchanged and mixed in the same amplifier. If you find some other types of 6LF6's and have find they work well, please contact me and I can add this information for other SA-4 owners.

For hobbyists that wish to make this modification themselves, I am giving the basic information below. This is not intended to be a set of "how-to" step-by-step instructions, but rather all the information that a skilled hobbyist or engineer or technician with a working knowledge of electronics needs in order to design his or her own version of the modification. Modification of the output stage printed circuit board is required, and skill with a soldering iron is needed.

In short, if you are qualified to do the work, the following information is more than enough. If you don't find the following information sufficient, then you're not qualified to do the work.

I cannot offer free construction or troubleshooting advice, so if you wish to tackle the project yourself, consider that you will need to fix it yourself, or pay us to put it right, if something does not work.

Here's How It Works:

1. A simplified schematic of the SA-4's output stage is here.
   

   NOTE: These schematic files are in .PDF format. .PDF files are read by the Adobe Acrobat Reader program. If you do not currently have this program installed on your computer, you can download it for FREE here.

   
   
2. As shown, the output stage has four tubes in parallel as cathode-followers for the top half of the signal, and four in parallel for the signal bottom half-cycle. As wired, DC grid bias and AC audio signal are applied to all the tubes equally, through the TOP GRID IN and BOTTOM GRID IN connections. As a consequence, matched tubes must be used to assure equal current-sharing while in idle.
   
   
3. By modifying the grid circuits, it is possible to make adjustment to each tube's DC grid bias, allowing use of unmatched tubes. Click here to view the circuit modification. Individual coupling capacitors allow AC signals to bypass the DC adjustment circuitry, so each tube continues to get the same AC signal drive, but different DC bias levels can be applied to each tube as needed to obtain matching.

Please note the following:

• Each tube must get its own bias network, consisting of a bias potentiometer, a coupling capacitor and a shunt resistor, for a total of eight of each part per amplifier.
  
  
• The networks for the upper four tubes (V1 - V4) are all to be tied to the TOP GRID IN line, and the networks for the lower four tubes (V5 - V8) are all to be tied to the BOTTOM GRID IN line.
  
  
• Plate, Screen Grid 2 (G2), and Cathode (K) connections remain unchanged.
  
  
• Use care when modifying the output stage board: the resistors for tubes V1 - V4 are oriented different than the ones for V5 - V8. Use the tube basing diagram to doublecheck the connections to the tubes.
  
  
• For the bias potentiometers, we use the 750K linear taper Clarostat JA1N056S754UA parts, available from Newark Electronics (www.newark.com) as their catalog number 01F6644. Other brands should work.
  
  
• For the coupling capacitors, we use 0.01uF Dynamicaps, but other brands will work. This part does affect sonics, so use a good-sounding part here, such as Auricaps (www.audience-av.com), or TRT Dynamicaps. Voltage is not critical as it is unlikely this part will see more than a few tens of volts across it.
  
  
• For the shunt resistors, we use a 1.5 megohm Roederstein MK-3 (1/2W) part. Other brands will work here, too.
  
  
• The output tubes in the SA-4 get quite hot, so heat-sensitive parts, like the bias potentiometer and coupling capacitor don't want to be located where they can get hot.
  
  
• The shunt resistors and coupling capacitors should be mounted under the SA-4's output stage circuit board or amplifier stability may be affected.
  
  
• At Alta Vista Audio we mount the controls (potentiometers) outside the amplifiers in a separate chassis because we don't think think that it is a real good idea to have the user fish around in the amplifier with a screwdriver to make adjustments while the amps are on. Hot tubes and high voltages within could lead to messy and embarassing accidents. You may choose to follow our example (see the upgrades page), or you might mount the controls on the rear panel, or come up with your own design.
  
  
• A full data sheet for the 6LF6 is available here.

Modify Amplifier's "Master Bias" Control

1. The SA-4's existing overall bias control must be modified so the entire output stage, with its modified grid circuits, can be biased properly. This is done by replacing the front-panel BIAS control, originally 10K ohm, with a 200Kohm part. We use the Clarostat JA1N056S204UA part, also available from Newark Electronics as their catalog number 01F6283. Other parts may work. The suggested Clarostat part has a plain 3/8 inch long bushing, and a 7/8 inch long, 1/4 inch diameter slotted shaft, which is similar to the original part except for being about 1/2 inch longer.
2. On the board with the three small input tubes, set bias controls VR2 and VR3 to fully clockwise.
3. Locate resistor R202, a 47K 2 watt resistor, to the right of VR3 and replace it with a wire jumper.
4. The front panel bias control will now have a very wide range, able to set the bank of tubes anywhere from nearly off to a very high amount of current. It is not recommended that the amplifer be run at more than 0.25A per tube for extended periods.

Suggested Bias Procedure

1. Set all the individual tube bias pots to minimum resistance (lowest bias).
2. Adjust the front panel bias control so most of the tubes are not too high or low -- average around 0.15A
3. Use the front panel tube select switch to find the tube with the highest reading.
4. Adjust the Master Bias (front panel) and set that reference tube to 0.15A
5. Adjust the controls for the other tubes to bring them up to match the reference tube.

*SA-4's original circuit design by Roger Modjeski, rights to develop and manufacture an amplifier purchased by Counterpoint circa 1984.