Original prototype ca. '95
using PP OPTs hacked from a
Harman Kardon 500 integrated amp
!!!WARNING!!!
The voltages found in this circuit can be lethal, build at your own risk!!!
Mid 2004
The Simple 45/2A3 was 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.
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 primary Z], 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.
Mid 2004 with Hammond 125ESE OPTs
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.
ca. 2005 with James 6113HS OPTs
Simple 45/2A3 FAQs:
1. Power transformer - you are on your own if you decide to use a power transformer other than specified in the above schematic.
2. Output transformers - the Hammond 125ESE output transformer sounds good and offer great value for the money. It is air-gapped and has multi tap secondaries - 2.5K, 5K and 10K - great for experimentation with other circuits or output tubes. No need to scavenge old tube receivers or integrated amps for high quality PP iron.
However for around $200/pair, one can get significant improvement in overall transparency, resolution, dynamics and bass response with the James 6113HS or better yet the 6115HS available at eBay.This unit uses higher grade M3 laminations potted in a cylindrical can with Tango ISO/Tamura level of finish. This output transformer really excels and have used it 2A3 applications at over 60 ma. current and 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 the 6SL7 is not the ideal input/driver tube due to its low transconductance being inadequate to drive the high capacitance of a directly heated triode's grid. Perhaps my naivete and empirical approach to DIY paid off because the anemic driver stage assures pure Class A1 operation and enhances the already well know virtue of soft clipping and even order distortion harmonics inherent in tube devices.
Ding's Simple 45 ca. '96
4. Mods - the only modification I did to this circuit is batter bias. Remove the resistor/cap combo at the cathodes of the 6SL7 and replace with a rechargeable 1.5V AA Ni-Cad or NimH battery like I did on the JEL phono stage schematic. Positive side of the battery to the cathode and negative to ground. To my ears this adds definition and clarity which may be system or taste dependent.
5. Parts - I always use carbon composition or carbon film resistors for plate load, metal oxides in the power supply, paper in oil coupling caps, Sprague Atom electrolytics for power supply decoupling and cathode bypass. It is a very simple circuit, season to taste ;)
6. Primary Z of OPT - some people have expressed concern on the use of 5K load for a 2A3 which only requires 2.5K according to the tube manual. Well folks, as stated the operating point for this amp is for a type 45 and since this is not the most linear region for a 2A3 a higher impedance load reduces distortion and improves damping factor in the low frequencies. I've measured 2.5W output from this circuit with 2A3s inserted, it sounds more dynamic and punchy lacking only the purity and refinement of a type 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 (Ohm's law) which will clarify the matter (DC voltages as per schematic):
- B+ voltage [325V] - cathode voltage [55V] = plate voltage [270V]
- cathode voltage [55V]/cathode resistor [1500 ohms] = plate current [.036 or 36ma.]
- Plate voltage [270V] x plate current [36ma. or .036] = Plate dissipation [9.72W]
As you can see the type 45 DHT is operated well within the recommendations of the tube manual.
Through the years I have received numerous emails from builders of the Simple 45/2A3 praising its musical virtues. It is an amp that appeals more to a hobbyist who listens more to the music than equipment.
Last November 2011 Hi-Fi Show my cohorts and I featured this not so simple 45 behemoth of an amp (weighed over 60 lbs.) built by Joel Villanueva. It is a dual mono block on a Harana Audio chassis made from a 6 mm thick copper plate bolted on a Philippine Mahogany base using 300% over rated power transformers, low DCR chokes, Hashimoto OPTs, silver wires in the signal path, Allen Bradley resistors and paper in oil caps driving 100dB efficient JBL horn speakers!
The first watt rules!