This amplifier board was designed with flexibility in mind. The board has been successfully been used as a stand alone SE amplifier with several types of output tubes. I have used 45’s, 2A3’s, 300B’s and NX-483’s in the same amplifier without any parts changes. The standard parts that are called out in the parts list (or supplied in the kit) are designed to accommodate these tubes in a SE amplifier configuration with no modifications. The board can also be used as a driver board for a larger amplifier. This and other applications are discussed further on the Tubelab web site.
It is expected that most users of this board will be building an SE amplifier. Most builders already have an idea of which tube they will use before building an amplifier. The output transformer is then ordered to match the tube. Before you can complete your amplifier you should choose the output tube that you want to use, and then choose an output transformer to match that tube. Then you choose a power transformer that can deliver the power needed for your chosen output tubes and load impedance. I will briefly discuss the common tube choices and the transformer requirements for each. A section is included that shows how to build a “universal” amplifier that will work with all of the different tube types. It is by design, somewhat of a compromise though.
The 45 Tube
This amplifier works very well with 45 tubes and other tubes that are similar to 45’s. My Lexan amplifier uses NX-483 tubes which I found in a radio chassis at a flea market. They are like 45’s with a 5 volt filament. The 45 works best with a 5K ohm load impedance, so the output transformers should have a 5 K primary and a secondary to match your speakers. The 45 will make about 1.5 watts per channel when operated at the “typical” value of 275 volts. This would require a power transformer that has a 500 volt center tapped HV winding of at least 125 mA, a 5 volt 2 amp rectifier winding, and a 6.3 volt center tapped filament winding of at least 3 amps. There is no common off the shelf transformer with these specifications, although one can be special ordered from several vendors.
There are several off the shelf transformers that are in the 540 to 550 volt center tapped range. These will provide a B+ voltage in the 325 volt range if used with the power supply components specified in the parts list. The maximum plate voltage rating given in the RCA literature is 300 volts. You will drop 5 to 10 volts across the output transformer. This leaves about 320 volts on the 45 tube. I have operated dozens of 45’s this way without incident, and I get about 2 watts per channel depending on the efficiency of the output transformer.
If you prefer to be more cautious with your 45’s (they are getting scarce) there are a few ways to lower the voltage. The simplest way is to raise the value of R4. This helps reduce power supply ripple, but makes voltage stability worse. Another way is to reduce the value of C4. I prefer a different approach. In all high quality amplifiers, I replace R4 with a choke. Choose a choke that has a relatively high DC resistance. I have been using surplus chokes in my amplifiers that measure 200 ohms. The Hammond 158M will work also. The B+ voltage is then fine tuned to the desired value by changing C4. Start with a very low value for C4 (4.7 uF at 450 volts) and increase if needed.
The 45 should be biased in the 25 to 30 mA range. I use 28 to 30 mA on 280 volts, and 26 or 27mA if operated at 320 volts.
Possible power transformers are the Hammond 270FX, the Allied 5K56VG (made by Hammond). You need 540 or 550 VCT at 125mA, 5 volts at 2 amps, and 6.3 volts CENTER TAPPED at 3 amps.
The 2A3 Tube
The 2A3 is essentially two 45’s in parallel in the same envelope. The filament current is 2.5 amps per tube. The 2A3 also has a 300 volt maximum plate voltage rating, although several new production “2A3 types” have higher ratings. The higher plate current drawn by the 2A3 will bring the plate voltage to about 300 volts when used with a 550 volt transformer and a choke in place of R4. The choke should be rated for 150 to 200 mA. I use the Triad C-14X or the Hammond T193J. The B+ voltage is then fine tuned to the desired value by changing C4. Start with a very low value for C4 (4.7 uF at 450 volts) and increase if needed.
The 2A3 will make about 4 watts with a 2.5 K ohm load, 3.5 watts with a 3 K ohm load and 2.5 watts with a 5 K ohm load. The distortion and damping factor improves with increasing load impedance. Most users use a 3 K ohm transformer with 2A3 tubes.
The filament current for two 2A3 tubes is 5 amps. A power transformer with at least 6 Amp capability on the 6.3 volt winding is required. A center tap on the 6.3 volt winding is required. The filament regulator IC will require a supplemental heat sink with 2A3’s. I use the heat sink from an old Pentium IC. I have used a Hammond 270HX power transformer in a 2A3 only amplifier.
The 2A3 should be biased in the 50 to 60 mA range.
The 300B Tube
This amplifier works well with the 300B tube. The “normal” 300B tube has a maximum plate voltage rating of 400 volts. There are some “super 300B tubes” that carry a higher voltage rating. The Sovtek 300B claims a 450 volt “MAX” rating, don’t believe it, some of them will go into a runaway condition if operated over 400 volts. I am assuming a 400 volt maximum rating, if you have a higher voltage tube, and wish to go over 400 volts, increase the voltage rating of C4, C5 to 500 volts, and C6, and C7 to 400 volts.
Most 300B amplifiers operate at 350 to 370 volts and use a 2.5 K, 3 K or 3.6 K ohm load. This is the sweet spot for most 300B tubes, and provides about 7 to 9 watts. It is also possible to use a 5K ohm load for reduced power with better distortion and damping factor. 6 or 7 watts will result. Most users go for a 3K ohm OPT and about 360 volts of B+. Some also use a 5K ohm OPT and a B+ nearing 400 volts, this is my preference.
The 300B tube requires 5 volts and 1.2 amps for its filament. Some new production “300B tubes” require more filament current. The filament regulator IC can provide up to 2.5 amps per tube.
The power transformer needs to provide about 640 or 650 volts center tapped at 150 mA or more, 5 volts at 2 amps, and 6.3 volts at 4 amps. A center tap on the 6.3 volt winding is NOT required. The Hammond 276X will work. I have been using some surplus power transformers that are 660 VCT. They give me about 390 volts of B+.
The 300B tubes should be biased at 60 to 75 mA.
The Universal Amplifier
It is possible to build an amplifier that can be configured to use several different tube types. This amplifier requires a few compromises, however it does work well and that is how my amplifier is wired. This option is discussed in the following section.
With this board you can build an amplifier that is optimized for 45’s OR 2A3’s OR 300B’s. Since these tubes have different characteristics, it is not possible to build ONE amplifier that is COMPLETELY OPTIMIZED for all three tube types. The primary differences between the requirements of these tubes is load impedance, filament voltage and supply voltage. This means that the transformer set is usually chosen for each tube. It is possible to build ONE amplifier that will work well with all three tube types. To do this a few compromises must be made.
The first compromise is in the power transformer. A 300B has a max plate voltage rating of 400 Volts. The 45 has a max plate voltage rating of 300 Volts. I used a variable power supply to find a happy medium for the three tubes. It turns out to be 300 to 320 Volts. This is above the maximum limit for all of the tubes except the 300B. I have built 5 amplifiers with this configuration and ran about 20 different tubes through all of them, without issue. I have not tried any globe 45’s. Understand that you are risking a possible tube failure if you operate 45’s or 2A3’s at 320 volts. 320 Volts is slightly low for 300B’s. You will only get 6 or 7 watts with 300B tubes.
The Allied Electronics 6K56VG transformer that is seen in my Lexan amplifier is good for 45’s but does not have enough filament current capability for use with 2A3’s or enough B+ current capability for 300B’s. In spite of this I have used it in my amplifier without any problems for 2 years. I do not use 2A3’s very often though. You will get up to 320 volts of B+ with this transformer at the currents required for 45’s. With 300B’s the supply voltage drops to 305 or 310 volts depending on the bias current. There are other transformers that can be used for this purpose. The Hammond 270HX would be a better choice.
The second compromise that is required involves the filament voltage choice. 300B’s and NX483’s require 5 volts while 45’s and 2A3’s require 2.5 volts. It is possible to use jumpers or a switch so that you could easily change the filament voltage between two different values. I DON’T RECOMMEND DOING THIS! WHY? Because it makes it too easy to put the wrong voltage on your expensive tubes. I will show you how, but I take no responsibility for your actions. I already have my own set of vintage 45’s with BLOWN FILAMENTS from being fed 5 Volts. You have been warned. If you do this, try the amplifier with all of your different tubes, find out which set you like the best, and then hardwire the board for that tube only, or limit your choices to tubes that work on either voltage. I now use either 300B’s or NX483’s in my amp. Both have 5 volt filaments, but no one else seems to have heard of a NX-483.
There is another compromise that must be made for a multi output tube amp. This involves finding the best load impedance for all of the possible output tubes. There are two transformer impedances that would work with 45’s, 2A3’s, 300B’s and NX483’s I have built a few amps with each type of transformer. The first amp like this that I built I used a 5K ohm transformer because I had one, The transformer was made for a 45 but it was a mismatch for the other tubes. Operating a tube with a higher load impedance than it wants generally results in lower power output and lower distortion. It also improves the amplifiers damping factor. The second such amp used a 3K ohm transformer made for 300B tubes. This allows greater power output, but also creates slightly more distortion and slightly weaker bass. You can also use a multi – tapped transformer like the Hammond 125CSE and try all of the combinations. The 5K transformer offers the best sound quality but costs power in the 2A3 and 300B amps. You will get about 2 watts with the 2A3 and 6 watts with the 300B. The 3K transformer causes a slightly higher distortion with the 45 and doesn’t seem to gain any power. You will get 2.5 watts with a 2A3 and 7 watts with a 300B.
If you are not comfortable with these compromises, pick your choice of output tube and then order the output transformer that is made for that tube. Most builders will have their tube choice in mind before starting construction. A few have not made up their mind yet. I have included my observations on the tubes that I have tried. Keep in mind that these are just my opinions, your opinions are likely to be different depending upon speakers, choice of music, personal preferences, and room acoustics.
The choice of output transformers will have the biggest influence on the sound that a single ended amplifier produces. I believe that this is where most of your budget should be spent. Get the best set of transformers that you can afford, you are not likely to upgrade them later.
I have built amplifiers using several different output transformers. I can report my experiences with these. I list these in order of my preference from best to worst. I listed a few budget transformers because I have tested them. I would expect that most builders would use a better quality transformer for this amplifier since it was not designed to be a low dollar amplifier, and uses relatively expensive tubes.
Quality Output Transformers
Electra Print (www.electra-print.com) had two transformers that are useful in this amp, BE5KB (5.3K ohm impedance for 45 type tubes) and TM3KB (3.4K ohm impedance for 2A3’s and 300b’s). They were about $145 each, though Jack’s catalog has changed over the years. Jack can also wind you a custom transformer to your specifications.
Close behind the Electra-Prints are the One Electron’s. Use the UBT-2 with 45’s and the UBT-3 with 2A3’s or 300B’s. These are just under $100 each. Also an excellent choice.
I got a pair of “SE Output transformers for 300B amplifiers” off of Ebay for about $120 for the pair. They were made by a company called Transcendar. These sound excellent and are a good transformer for the money. I liked them well enough to buy 20 of them for amplifiers that I build. At the time this was written they are still being sold on Ebay.
Budget Output Transformers
The Edcor XSE-15-8-5K. This transformer is an absolute bargain at $18 each. These are not meant for high end amplifiers nor are they priced for high end. Someone asked about these in the DIYaudio forums, so I bought a pair to test. That was a month ago and they are still “temporarily” connected into my amp in place of the Electra-Prints. If you use a subwoofer that does not get its signal from the speaker outputs, then this transformer can deliver high end sound. These are available in a single output impedance configuration only. In a different amplifier design (UL wired pentode with CFB) these can rival the sound of much more expensive transformers. Expect a new amplifier board design in the near future to use these.
Hammond 125CSE. The Hammonds are $29 each and are good budget transformers. These sound (and test) very similar to the Edcors. They just cost $11 more. If you need multiple output impedances in a budget transformer, use this one.
I ordered two “Single Ended Champ Transformers” #TF103-48 from Triode Electronics ($14 each). These will really scream in a SE guitar amp, and were never meant for HiFi. They are tiny, 2 inches tall. I didn’t expect serious high fidelity from these, but I tried them just to see what would happen. They don’t sound half bad, but bass is kind of weak. Good for a guitar amp, but for a budget HiFi, spend the extra $8 for the Edcors.
Other Output Transformers
Transformers made for push pull amplifiers normally will not work well in single ended applications because they were not made to handle unbalanced DC current through the windings. They will often work OK if they are severly oversized. I have also used 80 watt Schumaker push – pull guitar amp transformers (found on Ebay) with good results in 45 applications where the current is 30 mA, but they sound better if you take them apart and rearrange the laminations with an air gap. After this is done these actually sound better than the Hammond 125CSE.
I built an amp using transformers from HandwoundTransformers.com which sound quite good (I also used their transformers in the 845 SE), but their delivery and customer service sucks, so I can not recommend them (now out of business).
If price is no object brands like Magnequest, Tango, and Audio Note come to mind, but all of my amplifiers are built on a pretty strict budget so I have not tried any of these. There are plenty of other transformers out there but I have not tried them in this amplifier.
Tubes have the second biggest influence on the amplifiers sound quality, however they are easily changed. I would recommend getting a reasonably priced set of tubes when you first build this (or any) amplifier. Even if money is not an issue don’t put a set of vintage Western Electric 300B’s into ANY amplifier until you are absolutely sure everything is working perfectly. I use a set of used 45’s and a Chinese 5AR4 for testing a new amp. I will use those for 4 or 5 hours listening time before installing valuable tubes. I will explain my personal tube choices below.
Keep in mind that your speakers and musical tastes have a very large influence on your decision. I have Yamaha NS-10M studio monitor speakers which are inefficient (86db). My listening room, which is also the lab, is small (10 by 11 feet). I sit 3 feet in front of the speakers. None of these are ideal conditions, and are likely to be different than yours. Even in this small room I tend to turn the volume up all the way to compensate for the inefficient speakers. Therefore I tend to favor the 300B’s for dynamic music. If I had more efficient speakers I would probably use the 45 for most music. If you get the Single Ended Fever you are going to start your own collection of tubes anyway so, for what it is worth, here are my experiences:
I am using a NOS RCA 5AR4 in the amp shown here, because I had one in my collection. In other amplifiers, I use the Sovtek 5AR4 or the Chinese ones (Westinghouse branded), they are about $10 each. I have also tried International branded 5AR4’s I don’t notice much difference between the different 5AR4’s in this amplifier, but in my 845SE the Sovteks actually sound the best. I found a used Amperex Bugle Boy 5AR4 in a box in my warehouse. Maybe I’m deaf, but I don’t hear much difference between it and the RCA. The amp will work with other rectifier tubes (5U4, 5Y3 etc) but I like the 5AR4 because it is very slow to warm up, which gives the other tubes plenty of time to get hot before the B+ comes up.
The 5842’s are about $10 each from a tube dealer, about $6 on Ebay. I have over 50 Raytheon 5842’s and two WE 5842’s. They all sound the same to me. I tried a borrowed pair of WE417’s they were slightly lower in gain, but sounded similar. I know that the price of 5842’s has gone up several fold since I started using them. I used to get them for $3 to $4 USD each. Several people have e-mailed asking if there is a suitable substitute. Unfortunately in this board there is no substitute other than the WE417 which is usually even more expensive. I have been searching for a different tube that is just as linear, with good gain, for use in my next design, and I have not found it yet.
I have used several types of output tubes. When I started down the single ended path I wanted a very clean 1 or 2 watt amp as the starting point for a hybrid (tubes and transistors combined) amp. The 45 seemed like the best choice. This amp was conceived with 45’s in mind. I got all of my 45’s ( I now have over 20 of them) on Ebay. If you are patient you can get them for under $20 each. I got 3 for $13 once because they were “weak”, two of those are still being used in my 845 SE amp. All of these 45’s are of the ST type. I don’t have any globe type 45’s, and have not tried any of them since they tend to be more expensive.
The 45 is the cleanest sounding tube that I have tried. These definitely don’t all sound the same. There are different sound characteristics between two tubes of identical construction and brand. I am sure these differences are also quite dependent upon personal preference and the type of music being played. My favorite pair of 45’s are “Westinghouse Radio tubes” that were made in Canada. These have a “close your eyes and you are there” realism that the others can’t match. I have a pair of Tung Sol’s that come close.
I found an old Sparton radio at a flea market for $20. It had a pair of National Union globe type NX-483 triodes, which found their way into one of my amps. These sound almost as detailed and clean as the 45’s but have better bass and a little more power. A good tradeoff considering my inefficient speakers and penchant for rock music. It is interesting to note that both of the tube dealers that I called have never heard of NX-483’s.
I have tried the amplifier with RCA 2A3’s (dual plate) and Shuguang 2A3’s. The 2A3 tubes are electrically similar to two 45’s in parallel. The 2A3’s have more bass and slightly more power than the NX-483’s. They also are not as detailed as the 45’s or NX-483’s. I have tried playing all types of music through 2A3’s and I have not been impressed. The audiophile world refers to the 2A3’s sound as “sweet, silky, smooth” etc. I guess that I just don’t like that sound. I don’t dislike it, I just prefer the sound of all of the other tubes that I have tried more. I have not tried any of the expensive types of 2A3’s. All of them were purchased from a tube dealer (ESRC 407-826-5008) for $30 each. It should be noted that 2A3’s require 2.5 amps of filament current per tube. These will require a good heat sink on the regulator IC.
I have also used Sovtek and Shuguang 300B tubes. I can not recommend buying Sovtek 300B tubes due to reliability issues (see the 300Beast page). But I borrowed a pair from the 300Beast and tried them out. Definitely the loudest tubes with the best bass that I have tried, not near as clean and detailed as 45’s but better than 2A3’s (to my ears). These got me interested enough to purchase a pair of Shuguang (metal plate) 300B’s from ESRC for $50 each. The new mesh plate 300B’s were too expensive. I put these in the amp and I was seriously impressed. The same power and bass as the Sovteks but almost as realistic as 45’s. These have been in the amp since I got them (3 weeks). I can recommend these for someone looking for a (relatively) low cost 300B tube. I am tempted to get a set (4) for the 300Beast. I have not tried any other 300B tube, since they are beyond my budget at this time. Western Electric 300B’s go for over $300 each. ( I have since purchased about 5 pairs of the Shuguang tubes for SE amps, there have been no problems).
I was asked about 50’s. Unfortunately 50’s will not work in this amplifier without modification. They require 7.5 volts for the filament, which is above the maximum for the filament regulator IC. Unlike tubes silicon really does not like being fed too much voltage. It would be possible to use an external power supply if you really wanted to use 50’s, but these tubes are well out of my price range, so I have not tried them.
Modifications to Improve the Sound
The board is designed to be built using a capacitor input filter with a choke or a resistor in between the two capacitors. If a choke is desired it may be connected in place of R4. This is a HIGHLY recommended modification. Many early users found that a choke was needed when using high efficiency speakers. I have since added a choke to my amplifiers, and found improved bass and dynamics even though I had no hum. People have asked ” what kind of choke do I use?” I use the Triad C-14X because it has 150 ohms of DC resistance. It is UGLY, so you will want to hide it under the chassis. If you need a better looking choke, use one that has nearly 150 ohms of resistance, and has the same or higher current rating as the power transformer. The Hammond 193J will work, but you will gain a few volts of B+ because it is only 82 ohms. Remove R4 and connect the choke wires in its place.
The second most common modification is to add a supplemental power supply capacitor. Most users have added a motor RUN (not start) capacitor in parallel with C5. You want a non polarized polypropylene capacitor with a low ESR characteristic. There are capacitors made for this purpose such as the Solen “fast cap”. This improves the transient response and reduces hum. I added a large 100 uF, 370 volt AC polypropylene motor RUN cap to my 300B amp, and added a smaller 20 uF polypropylene 400 volt DC ASC leaded capacitor to my Lexan universal amp. They did make noticeable improvements. This is a recommended modification. Leave C5 in place, and run wires to the supplemental capacitor, connecting it in parallel with C5.
A choke input (LCRC or LCLC) filter can be implemented by cutting the PC board runner between the 5AR4 and C4 and wiring an external choke in between these two points. The second choke (if desired) is connected in place of R4. I have not noticed any benefit from this modification however it may be useful to lower the supply voltage in some cases.
Modifications to Use This Board in Ways it Wasn’t Intended to be Used
I have used this board with at least 10 indirectly heated tubes with great success. I have used 6L6, 6AV5, 6CD6, 6LW6 (not enough drive for this one), EL-34, KT-88, 6V6, 6AQ5, 6N7, 807, and similar tubes. They can be triode wired, UL, or pentode connected. Since the cathode is not tied to the filament, cathode feedback can be used. The 6AV5 in UL with CFB sounds awesome with the cheap Edcor transformer. This requires external sockets for the tubes as well as some external wiring. The filament regulator is not needed for this application. This modification was used to develop the Tubelab SSE amplifier.
The filament circuit is designed to operate off of a 6.3 volt center tapped transformer winding. The filament rectifiers can be jumpered for full wave center tapped or full wave bridge operation. There are jumpers for 2 different voltage settings for the filament regulator. The settings using the standard component values are for 2.5 volts (45’s and 2A3’s) and 5.0 volts (300B’s and NX-483’s). 2.0 volt tubes can be accommodated by changing the value of R35. The filament regulator is capable of providing any voltage from 1.5 to 5.0 volts at up to 5 amps. A serious heat sink is required on the regulator to supply 5 amps. The regulator is not rated for operation above 5 volts therefore type 50 tubes can not be used in this board without connecting an external filament supply.