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
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
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.
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
Electra Print (www.electra-print.com)
has 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 are about $145 each. Jack will wind you a custom
transformer, with only a single output impedance winding for a little
over $100 each. Ask for the "Albert Pagan design". These are the
transformers that are shown in the amplifier above. They have the best
sound of any transformer that I have tried in this design. An excellent
choice for a top notch amp.
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.
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
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
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
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
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 Simple SE
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.