Simple 45/2A3
The original "prototype" Simple 45/2A3 amp ca. '95
JE Labs Simple 45/2A3 with James 6113HS OPTs
Click here to print the schematic
JE Labs Simple 45/2A3 "revisited" summer of 2004
The Simple 45/2A3 [schematic] is perhaps my most successful design effort for an entry level DIY SE amp project at low cost. The design is literally taken out of the RCA tube R-C amp chart. Nonetheless, I firmly believe that its musical performance is hard to beat when driving suitable high efficiency speakers at around 90 dB/1W sensitivity or greater. My original concept for this project was to quickly whip out an SE amp using parts in my bin in order to evaluate the sonic merit of the type 45 DHTs I acquired at a hamfest in the mid 90s. Since my 2.5V filament supplies had enough current ability to light up 2A3s, I dropped them in place and heard decent sound thus sparing my stash of 45s for everyday use. I took it to Steve's shop one day and he was impressed by what he heard and asked me to write an article for the DIY section in his website. It has since been replaced by the 2001 edition but deem it worthy enough to include in my webpage with minor updates and revisions due to its continued popularity among first time DIYers.
Let's review the design topology of this circuit. It is a 2-stage resistance coupled amplifier using a 6SL7 - a hi-gain [mu], lo-transconductance [gm] dual triode as the input/driver tube capacitor coupled to the grid of either a 45 or 2A3 power triode operated in cathode bias. The input impedance of the amplifier is set by the 100K Radio Shack/ALPs volume control. It can be replaced by a 100K, 1/2-1W resistor if you don't need an input level control. If you refer to the 6SL7 R-C amplifier chart, with a 300V supply, 100K plate load (Rp) and 2.7K cathode bias resistor (Rk) a single section of a 6SL7 is able to swing 63V to a grid load resistor (Rg) value of 470K. Since I had about 330V B+ supply to play with, by interpolation I can theoretically expect 10-15% greater voltage swing. However in real world applications this margin of voltage is wasted as heat [exacerbated by the lack of gm] dissipated by Rp when loaded by the input capacitance of the output tube and its grid resistor (Rg). Assuming that I have about 60 "real world" volts, this is still adequate to drive a 45 to full power since it is greater than the bias voltage [56V] of the output tube. To make sure that I maximized the amount of current drive, I decreased the cathode bias resistor to a 1K, bypassed with a 100uf/25V electrolytic to avoid degenerative feedback [more gain] or use a rechargeable 1.5V AA battery [see Mods below].
According to the specifications for a type 45, operated as Class A single ended amplifier, with 275V (max.) at the plate and - 56V grid bias driving a 4600 ohm load [output transformer], output power is 2W. Each tube drawing around 35 ma. of current. Since I sonically prefer cathode bias over fixed bias, I needed a B+ supply of around 330V. When The Type 45 is operated in cathode bias, a grid leak resistor of no greater than 1M should be inserted from the input grid to ground, 470K fits the bill as recommended by the R-C chart. In a directly heated triode [DHT] like the type 45, the filament itself is the cathode. Cathode bias is achieved by inserting the proper value resistor either on the filament transformer center tap or at the center terminal of a hum bucking pot. Since AC filament hum can be a problem, I used 25-100 ohm, 2W hum bucking pot with 1.5K, 12W bias resistor bypassed by a 100uf/100V electrolytic cap [connected in parallel; watch for polarity "+" goes to the center tap] to the center tap of the pot. The filament supply calls for a pair of 2.5V @ 2.5A transformers like the easily obtainable Hammond 166L2. I get 2 mv. or less of residual hum from this arrangement.
Even if this is a cheap design, I did not skimp on the power supply. I always believe that good sounding tube amplifiers and preamplifiers should be tube rectified with at least a choke along the initial stage of the B+ line. I used a 5Y3 tube rectifier in a pi filter topology [C-L-C] with a 10H, 150 ma. choke [L] between the first two stages of filter capacitors [C1] and C2] and then a split rail [R-C1 and R-C2] B+ line for the input/driver stage [6SL7] to minimize crosstalk between channels. The Hammond 272DX power transformer has a 600VCT HV winding @ 125 ma., 5V @ 3A and 6.3V @ 3A filament windings. This stereo circuit draws about 73ma. of current so there is a safety margin of 50 ma. from the HV winding. The 50 ohm, 10W resistor inserted between the rectifier and the first 10uf cap serves to fine tune the B+ supply so that the 45 output triodes will be well within its maximum allowable operating conditions. As a measure of safety, I always insert a 100K-250K, 2W metal oxide [see the PS section of the schematic] "bleeder resistor" to discharge the power supply caps while the amp is turned off. This will prevent electrical shock when the unit needs to be serviced since capacitors can store high voltages for a long time.
Simple 45/2A3 FAQs:
1. Power transformer
You are on your own if you decide to use a power transformer with greater voltage/current ratings. A google search will return on-line or downloadable software programs that simulate power supply design which can ease the burden of a trial and error approach.
2. Output transformers
The Hammond 125ESE output transformer is good sounding and offer great value for the money. No need to scavenge old tube receivers and integrateds for high quality heavy PP iron like I did in the past. It is airgapped and has a multi-tap [2.5K, 5K and 10K] secondary. I highly recommended it for this project and experimentation with other circuits and DHTs.
However for just about twice the price, one can get significant improvement in overall transparency, resolution, dynamics and bass response with the James 6113HS available from Euphonia Audio. I paid approximately $140/pair promptly shipped within the USA, great service. This unit uses higher grade M3 laminations potted in a cylindrical can with Tango ISO/Tamura level of finish. I used the 2.5K primary tap so that the secondary will effectively reflect a 5K load when connecting an 8 ohm load to the 4 ohm tap and 16 ohm load to the 8 ohm tap. This output transformer really excels and can be used in other 2A3 applications at full 60 ma. current. I've tried it already in my prototype chassis using the SE2A3dx circuit and it was not put to shame pitted against the more expensive and beefier Tango U808 and NY15s. This is an unequivocal best buy in my book because of its synergy with this circuit!
For those with even bigger budgets, I've also had success with this circuit using the Tamura F475, Tango U708 [discontinued], H-5S [discontinued], Tango ISO U808 & Magnequest TFA204 [3k, 2A3s, use the 4 ohm tap to reflect a 6k primary Z for 45s] but the added expense may not be justified in this simple application.
3. Miller Effect
I have been warned in the past by more experienced DIYers as well as criticized by techies who think that the 6SL7 is not the ideal input/driver tube due to Miller Effect - its lo-gm is seemingly inadequate to drive the highish capacitance of a typical directly heated power triode's input grid, especially a bi-plate 2A3. On hindsight, my naivete and empirical approach to DIY paid off because the anemic input/driver stage assures pure Class A1 operation and enhances the already well known virtue of soft clipping and even order distortion harmonics inherent in tube devices. These factors actually became advantageous in this flea power application.
4. Mods
The only modification I did to this circuit is battery bias. All you have to do is remove the resistor/cap combo at the cathodes of the input/driver [6SL7] stage replace them with a rechargeable 1.5 AA Ni-Cad or NiMh battery. For a similar implimentation of battery bias check out the JEL phono stage schematic and take note of the cathodes. Use some double adhesive tape to secure a pair of single AA battery holders within proximity of the spot vacated by the 1K/100uf cathode resistors/bypass caps. Solder the red [+] wires to the cathode and the black wires [-] to ground, and then install the batteries. To my ears the mod added definition and clarity which may or may not be synergistic with your system or taste. So try it and hear for yourself.
5. Parts
In my latest effort I used carbon composition resistors for plate loads, carbon films are fine too, metal oxides for the power supply, paper in oil coupling caps, Sprague "Atom" electrolytics for the power supply and cathode bypasses. Since this a simple circuit, you may even use it as breadboard to audition boutique parts to see if they are worth the extra expense.
6. Primary Z of OPT
Some people have expressed concern on the use of 5k or greater load for a 2A3 which only requires 2.5K according to the tube manual. Well folks, the operating point as stated is for a 45 and since the 2A3 is not at its most linear region under these conditions, a higher impedance load reduces distortion and improves damping factor in the low frequencies. I've measured 2.5W output from this circuit using 2A3s and it sounds more dynamic and punchy which some may prefer over the mellower and more refined 2W of a 45.
7. Plate vs. B+ voltage
Steve at Angela Instruments and I have received several inquiries as to why the type 45 is being taxed over its limit. That is because many people get confused between plate voltage and B+ voltage. Here are some equations to calculate for plate voltage, plate current and plate dissipation that will clarify the matter [DC voltages as presented in the schematic]:
As you can see the type 45 DHT is operated well within the recommendations of the tube manual.
Ding's Simple 45 amp
Through the years I have received numerous emails from builders of the Simple 45/2A3 praising its musical virtues. Since Steve took down the original article, I've surfed a few websites that uploaded the original schematic along with notes of their experience in building the amp. A builder's Simple 45 amp was featured in Enjoy the Music's coverage of James Melhuish [of Single Driver Website and Fullranger.com fame] hosting the Boston Bash '99.
After successfully building the unit and fully understanding how it works, one can move on to more ambitious but not necessarily better sounding designs like the "2001 edition", SRPP 45 or 2A3 and JEL 76/6SN7dx input/driver stage adapted for use with various DH power triodes.
This simple amp design would appeal more to a hobbyist who listens more to music than equipment. It was refreshing to revisit a design that led me to believe that the first watt rules.....
Updated: 11/08/04, all rights reserved by JE Labs.
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