Assembly Manual • Schematic • Parts List • Tubes and Applications
At the time this manual was written, the Tubelab SPP has only been tested as a stand alone push pull amplifier. This means that a single SPP board is used with one power transformer and two output transformers to make a stereo HiFi amplifier. Other optional components like a filter choke or supplemental power supply capacitor may be added.
I have done preliminary testing on using the board in single channel mono block configuration. This means that a single SPP board is used with one power transformer and one output transformer to make a single channel amplifier with higher the same power output as both channels combined on the stereo version. This may be used for HiFi or MI (musical instrument) applications. This is in the preliminary testing stages, and is not yet included in this manual. It will be added at a later date if all of the testing yield positive results.
I have built and tested two different amplifier configurations that all share the same basic design. It is recommended that you duplicate one of these designs if this is your first amplifier, or you want to copy a known good design. Even if you use one of the standard configurations you should expect some variation from my measured values because of tube and line voltage variation.
It is possible to build this amplifier using two different types of tubes. They are NOT COMPATIBLE with each other. The amp can be set up for use with one tube type OR the other but NOT BOTH. Even though the two tubes may look identical, putting 6CW5's into ANY amp designed for EL84's will result in blown tubes and possible amp damage. I learned this the hard way about 20 years ago! You can put EL84's into an amp designed for 6CW5's without damage, they just won't work very well.
The EL84/6BQ5 Family
This includes the 7189 and the Russian 6P14P. The 7189A is a special tube that has higher ratings. True 7189A's are very hard to find and much of what is being sold today as 7189A's are actually relabeled Russian 6P14P's. The EL84 and the 6BQ5 both have a maximum plate and screen grid voltage spec of 300 volts. Many vintage amplifiers, especially guitar amplifiers routinely violated these specifications, often by considerable amounts. My experience is that many of the 1960's vintage tubes can indeed live at voltages near 400 volts, but almost all current production tubes glow or blow up instantly at these voltage levels. All of these tubes work best at a load impedance of 6.5K to 8K ohms plate to plate.
For this reason I have developed and thoroughly tested the "textbook" EL84 / 6BQ5 amplifier version with dozens of tubes, and recommend its use for most builders and tubes. It "bends" the specs by 10 to 30 volts, operating the tubes with a tube (plate to cathode) voltage of 310 to 330 volts. It works well with every "EL84" tube type that I have tried. The plate and screen grid operate at the same voltage, so triode, UL , or pentode mode can be used. NOTE, the Sovtek SV83 and the Russian 6P15P have a 200 volt maximum rating for the screen grid and are NOT compatible with this amplifier. The Sovtek SV84 and Russian 6P14P will work just fine.
As previously stated some tubes are capable of operating at voltages above the published ratings. For those who want to push things a bit, I have developed a "hot - rod" version of the EL84 / 6BQ5 amp. It is still undergoing my thorough testing procedures and will be added as soon as I have identified all of the necessary components needed. It does require a different power transformer to develop the higher voltages, but I have not yet determined the exact voltages needed. The plate and screen grid operate at different voltages, so only pentode mode can be used.
The 6CW5/EL86
The 6CW5 was developed for low voltage use in line powered television sets and radios. It has a 250 volt maximum plate voltage rating, and a 200 volt maximum screen grid voltage rating. They will melt quickly if fed 300 volts or more! These tubes are often priced under $5 each and can be a good choice for a low budget amp. They require much more current that the EL84 types. This prevents the use of conventional rectifier tubes, so solid state diodes must be used. The optimum load impedance is 3K to 3.6K ohms plate to plate.
I have presented the build information for the 6CW5 amp that I currently use. It operates the tubes at about 220 volts plate to cathode which is slightly over spec. This is not an issue with any decent 6CW5 tube. Another "hot - rod" version is under development, but it is not yet operational.
I have found two examples of tubes called 6CW5 that will not work in this amplifier even when operated well within the published ratings. These are both examples of some "glass stuffing" that happened as the vacuum tube production era was ending. A few manufacturers (especially Sylvania) were known to stick some real strange guts inside the glass to fulfill (often military) contracts. These tubes were ONLY TESTED in the circuit required by the contract. The most odd example of this were the tubes labeled 6B4G or 6B4GA (a DHT) that contained 6AV5GA guts (a TV sweep tube). One tube is an RCA 6CW5 / EL86 "made in Canada". This looks like a 6CW5 but the plate is about 1/4 inch shorter than all of the others. They show mild red plate at "normal" operating ratings. The other is a Phillips ECG 6CW5. My pair have 8713 date codes on them and were made by Sylvania late in the vacuum tube era. They have gray tinted glass so that the guts are hard to see, but the guts do not look anything like a 6CW5. These things glow brightly at 200 volts. Avoid them!
This picture shows several types of 6CW5's. The first six work just fine, the last two do not. From left to right, Amperex - 3 slots in the end of the plate, Amperex - 1 hole in plate, Amperex - 1 slot, GE - 1 slot, unknown Japanese tube - 1 slot, another unknown Japanese tube - 1 slot, RCA "made in Canada" - tiny "guts", Philips ECG - smoked glass - 4 slots.
The "textbook" EL84 / 6BQ5 SPP Design
I expect that most SPP's will be assembled this way. It is a well tested and proven combination. If this is your first amp build, this is the best place to start. The amp runs with a B+ of 320 to 340 volts using 6.6K to 8K ohm output transformers and can be run in triode, pentode or ultralinear mode. EL84's, 6BQ5's, 7189's, 7189A's and Russian substitutes can be used. I have used 6P14P's.
Power Transformer
The primary winding should match the line (mains) voltage available at your location. A universal (multi winding or multi tapped winding) primary can be used if it supports your line voltage.
The high voltage secondary should produce 640 to 650 VCT (325-0-325) and should be rated for at least 175 mA DC output. It should be noted that many transformers produce a voltage that is considerably higher than their published specifications (especially Hammond). For this reason you need to use a transformer that is rated for less than the actual required voltage. I used 600 VCT (300-0-300) Hammond transformers in my amps and I get the correct voltage. This winding MUST have a center tap.
The rectifier filament winding should be 5 VAC at at least 2 amps. If a 5U4 or 5R4 rectifier is used this winding must be at least 3 amps. A center tap on this winding is OK, but it will not be used.
The main filament winding should be 6.3 VAC at at least 4 amps. A center tap on this winding is OK, but it will not be used.
The transformer may have additional windings or a bias tap. These will not be used.
For builders in the USA, Canada, or other 120 volt 60HZ countries the following transformers have been used:
- Hammond 272JX (overkill but works great).
Builders outside the USA will need a transformer with a 240 volt primary. I have used the Hammond 372HX in a SPP. It is a universal primary design and can be used in the US or just about anywhere in the world. Many builders feel that Hammond's 300 series transformers exhibit higher build quality than the 200 series. I must say that this is the first time that I have used a Hammond 300 series transformer, and it does not make any mechanical noise at all and remains slightly warm after several hours of continuous use. There are likely other transformers available to builders in other countries that I don't know about. If it matches the requirements listed above, it should work. Other transformers will be added as they are tested.
Output Transformers
The OPT is the most critical component in any tube amplifier, and one that is not likely to be upgraded later. The ability of an OPT to reproduce low frequencies is directly related to its size. There are plenty of different types of output transformers available for tube amplifiers. The prices go from $15 USD each to well over $500 USD each. How do you choose? First let me state the requirements:
The SPP is a push pull amplifier. The output transformer must be designed to work in a push pull amplifier. The ideal primary impedance for this design is 7.6 K ohms. Impedances from 6.6 K ohms to 8 K ohms have been tested and work well. A center tap is required. The output transformer must have a UL mode tap if you intend to use UL. The secondary impedance should match your speaker impedance. The transformer should be rated to handle the power produced by this amplifier. This could be as low as 5 watts in triode mode, or as high as 20 watts in pentode mode. It is recommended that the output transformer is rated in the 20 to 50 watt range if the budget allows. A transformer that is too small will saturate on strong bass notes, and a grossly oversized transformer will have higher magnetic losses reducing the available output power for the speakers and possibly reducing the details in the sound.
The performance of an output transformer is generally related to its size and therefore cost. It will have the biggest effect on the sound quality in the amplifier. I usually recommend that you start with the best transformers that the budget allows even if it means using cheaper tubes. You will upgrade the tubes later, probably not the transformers.
The small OPT's in the 1 to 2 pound (weight) range will generally exhibit distortion on heavy bass notes. They may be OK if your speakers do not reproduce these frequencies, OR you use an active subwoofer. I tried the 10 watt Edcor XPP 8-10-8K's and they sounded very good at low volume, but exhibited noticeable distortion on strong bass when played loud.
The 6CW5 / EL86 SPP Design
The 6CW5 is an audio output tube that has not been "discovered" by the audiophile community. It is often available for considerably less money than the EL84. The lower impedance of the 6CW5 may allow the use of unconventional output transformers which can result in a good sounding amplifier for a low total cost. This may be a valid choice for those looking for a low budget HiFi amp.
The 6CW5 is similar in construction to the EL84. It is NOT directly compatible with the EL84 and will have a very short unhappy life if plugged into an amp set up for an EL84. Instant red plate, red screen grid death will occur and there is a high possibility of amp damage. The 6CW5 is designed for lower voltages than the EL84 but can deliver similar output power. To get the same power from a lower voltage, you must run more current. A lot more current. The amp runs with a B+ of around 240 volts using 3.3K ohm output transformers and can be run in triode, pentode or ultralinear mode. The current demands for both channels runs about 400 mA and can easily go over 500 mA if the amp is overdriven. None of the conventional rectifier tubes that will fit the socket can supply this current, so a solid state rectifier must be used. The board was not laid out for solid state diodes, but they may be soldered into the tube socket holes. Do not install the rectifier tube socket if building the board for use with 6CW5's.
Power Transformer
The primary winding should match the line (mains) voltage available at your location. A universal (multi winding or multi tapped winding) primary can be used if it supports your line voltage.
The high voltage secondary should produce 400 VCT (200-0-200) and should be rated for at least 400 mA DC output. It should be noted that many transformers produce a voltage that is considerably higher than their published specifications (especially Hammond). For this reason you need to use a transformer that is rated for less than the actual required voltage. This winding MUST have a center tap. I used an Antek 1T200 toroid that has two 200 volt windings. They were connected in series to create a 400 VCT winding.
The rectifier tube is not used, so no 5 volt winding is needed.
The main filament winding should be 6.3 VAC at at least 4 amps. A center tap on this winding is OK, but it will not be used. The Antek toroid has two 6.3 volt 2 amp windings, they are connected in parallel to create a 6.3 volt 4 amp winding
Builders outside the USA will need a transformer with a 240 volt primary. The Antek toroid has two 120 volt primary windings that can be connected in series for 240 volt operation.
There are likely other transformers available to builders in other countries that I don't know about. If it matches the requirements listed above, it should work. Other transformers will be added as they are tested.
Output Transformers
The Tubelab SPP is a push pull amplifier. The output transformer must be designed to work in a push pull amplifier. The ideal primary impedance for the 6CW5's is 3.3 K ohms with a center tap. The secondary impedance should match your speaker impedance. The transformer should be rated to handle the power produced by this amplifier. This could be as low as 5 watts in triode mode, or as high as 20 watts in pentode mode. The output transformer must have a UL mode tap if you intend to use UL. It is recommended that the output transformer is rated in the 20 to 50 watt range if the budget allows. A transformer that is too small will saturate on strong bass notes, and an oversized transformer will have higher magnetic losses reducing the available output power for the speakers and possibly reducing the details in the sound.
The low output impedance of the 6CW5 may allow the use of power (mains) toroids as OPT's. Many mains toroids do not have enough inductance in the primary winding to accurately reproduce low frequencies without excessive distortion. Some will not pass the high frequencies, or have excessive imbalance between the two primary windings due to sloppy winding techniques. A few however will work very well as OPT's, but the only way to tell is to try them. Toroidal transformers have less iron in them than a conventional "EI" transformer, so they will be more susceptible to core saturation due to a difference in current between the two output tubes. Some tube swapping may improve the results on a given transformer. The ideal mains transformer candidate for use here would have two 120 volt windings and one 12.6 volt winding (or two 6.3 volt windings in series). It should be in the 100 to 300 VA range. I have not had good luck with conventional "EI" transformers for OPT use in a HiFi amp. The high frequency response is usually bad. I used them in guitar amps in my teenage years and some people have reported reasonable sound quality with some of the smaller sized transformers.