Jensen Duette

Jensen Duette "Treasure Chest" [DU300] finished in Mahogany from the mid 50s

Jensen Duette "Treasure Chest" [deluxe version of the one below] interior view, 2-way system with P8RL mid-woofer, RP106 cylindrical metal horn tweeter. The phenolic diaphragmed alnico magnet compression driver is the same motor assembly used in the later RP102 and RP103. Simple 1st order crossover network using a 2uf paper capacitor [beeswax?] and a transformer for attenuation and impedance matching. These came with bare panels and best left alone for ultimate sonics.

Jensen Duette "Reproducer" [DU201]. Essentially the same speaker as above but uses a plastic horn lens and cheaper Fabrikoid finish, from the mid 50s

Heathkit SS1 sold as a kit in the mid 50s until the 60s. Simple 2-way bass reflex utilizing the driver components below. The crossover consist merely of a 2uf paper cap and an L-pad tweeter attenuator. Also no damping inside the plywood box.

Jensen P8RL 8" woofer: frequency response 50 - 3,000 cycles, impedance: 8 ohms, 1/2 lb. alnico magnet, maximum power 20W

Jensen RP103 tweeter: compression horn driver with alnico magnet and phenolic diaphragm frequency response 2,000-15,000, impedance 16 ohms, maximum power 30W

These are the components [or later versions] typically encountered inside the Heathkit SS1 and Eico HFS-1 speaker systems.

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Looking for something a little less exotic and more affordable than WE or Altec. Try these Jensens which sold in great numbers in the 50s-60s. These are JE Labs budget vintage speaker faves. Perhaps a little less transparent over the entire frequency range and less "snappy" in the midrange, but they are pretty damn musical and efficient. I am listening to my Duettes right now driven by my SE71A amp.

If you can find them with the cabinets in good condition, these may have great WAF, especially if she is into modern retro or mid century modern designs like Charles Eames, George Nelson, Noguchi and etc. I am still looking forward to finding a pair of Heathkit SS1 or DU300 in blonde oak finish with matching wooden or metal wire stands at a yard sale or thrift store. So ask around, maybe an old relative has them stored in the basement or attic.

The Jensen P8RL 8" woofer may look cheap [in spite of the fact that people pay $300/pr. for similar the looking and even smaller 6" Diatone PM610 FR] but what sets it apart from modern designs is its natural sounding midrange due to the stiff paper cone that has a built in surround which makes it a pretty quick responding driver [good for the midrange] yet if properly loaded in a proper bass reflex cabinet, there is enough energy below 80 hz to keep you happy just like any modern monitors. I can assure you that the smooth midrange response up to 3,000 is a bit pessimistic because it is almost usable as a full range driver that's why only a 2uf cap in series to the RP103 was used to crossover at 5,000 cycles. The RP103 may not have the silky, detailed and ribbon like response of an Altec 3000, but this phenolic diaphragmed horn tweeter is smooth and sweet with plenty of detail until it rolls off gently above 13 khz. 

Compared to the Jensen Duette, the Heathkit version has slightly bolder presentation because of more energy in the bass frequencies. This maybe due to the heavier cabinet material [5/8" plywood as opposed to 1/2" used in the Jensen] and lower tuning of the bass reflex [that cabinet is a bit deeper]. However the Jensen sounds smoother and slightly more refined. I can live with either one. Unfortunately I do not have a copy of the Heathkit cabinet plan. If someone can send me a jpeg scan, I will upload it in this page.

Take note that these cabinets were designed to be placed horizontally on bookshelves and during that time, people did not pay much attention to mirror image placement of drivers. Stand mounting horizontally is best for pure sound if you have the space. If you crave for that sweet and airy extreme highs which will bring these speakers abreast with the 21st century you may also add the Fostex FT-17H as a super tweeter Just use a .5uf oil cap and adjust levels to taste with an 8 ohm L-pad.

If you have or find the drivers without the cabinet, try the two Jensen plans below. Make sure you use 1/2" plywood as per instructions and make it rigid. No damping material required for best sound and please don't use MDF. The only modification I may consider is using a removable grill covered with silk cloth as suggested by Walt Bender in his "Ask Walt" series. These two drivers also sound good in an open baffle.

Cabinet plans from Jensen

Eico also marketed their own version, the HFS-1, using these two Jensen drivers. I have seen them but never heard them in a system. These are also worth hunting down. Just remember that these were budget designs and if your taste and pocket crave for ultimate transparency and finesse, these may not be suitable. But for apartment dwellers or those on a tight budget who dabble with SE amps these speaker systems may be your ticket to musical pleasure.

I am sure there other sleepers out there, particularly drivers imported from Japan and Germany by radio chain stores during the late 50s and 60s. Actually some choice Japanese models from this era are now being re-exported back to Japan. 

Link to Jensen Ultraflex enclosure

MC step up transformers

Microphone transformers converted to MC step-up devices. Back row, left to right - RCA MI12399A, Altec 4722 and Altec 15095A. Front row, left to right - UTC P-1, Beyer Dynamic TR/BV 351015006 and Dukane 3A55.

Tamura TKS 83

Many SET users are also analog aficionados and have discovered the musical qualities of classic moving coil cartridges like the Denon DL103 series, Ortofon SPU, Fidelity-Research, Supex, Koetsu and etc. There are a few brand new high quality MC transformers available from Tamura, Jensen, Sowter, S&B and Lundahl. But chances are, you may be looking for something classic or have a pair of obscure input transformers in your shelf that may do the job. The Western Electric 618A, B and C input transformers are prized acquisitions in Japan for use as MC step-up devices based on my perusal of MJ and Stereo Sound Tube Kingdom magazines.

For the past couple of years, I have seen prices skyrocket for the Altec 4722 mic transformer [38 or 150:50K] since many have also discovered its merits as a high quality MC step-up device. Others have even resorted to wiring its companion line out transformer [15K:600 or 150] - the 15095 and 15095A - backwards [150:15K] for MC step-up duty. However there are still many sleepers out there like the RCA MI12399A and certain obscure models from Thordarson, UTC and etc. Moving Coil cartridges behave very much like condenser microphones - low impedance and low output - requiring a voltage boost. Step-up transformers are passive devices and as long as they are wired properly, they are virtually noiseless. To me a wide bandwidth input transformer is the most elegant way of boosting MC output to MM phono level.

Back in the 70s and 80s when MC cartridges where at their peak in sales, a lot of high end manufacturers sold black boxes wherein they inserted some sort of step-up ratio transformer inside to boost the gain on an MC cartridge into a typical 47K input phono preamp. Many of these transformers were probably designed originally as microphone input devices. In general any transformer with an impedance ratio from 150:15K [voltage ratio = 1:10], 150 ohms: 50,000 ohms [vr = 1:18] or even 250:50k ohms can work well as an MC step up. If the primary has multiple taps [3-250 ohms] so much the better. Remember that the square root of the impedance ratio = the voltage ratio. Another relevant specification [if provided] is bandwidth, typically a good audio transformer should have a flat response [less than +/- 1 dB] from 20 or 30 hz to 20 khz or greater.

Most mic transformers have the wiring diagram printed on its case. However there are some rare instances wherein they are unmarked. The best way to sleuth a mystery transformer is to take impedance measurements between terminals to determine the primary and secondary taps. After you have written down the terminal arrangement, inject sine waves to the primary and view the output from the secondary on an oscilloscope for proper phasing. Usually a transformer that exhibits minimal phase shift or ringing when injected with a 1khz square wave [secondary loaded by a 47K resistor] will have good bandwidth and generally good sonic performance.

I have learned through the years that loading the primary and/or secondary of an MC step-up device with resistors to suit a cartridge manufacturer's specification does more harm than good because it can induce unwanted ringing in the transformer. The 47K input impedance at the phono preamp is sufficient loading at the secondary.

Here is my typical wiring configuration for an MC step-up transformer:

*Grounding, hum loops and etc.

The diagram above is my tried and tested arrangement to wire a transformer as an MC step-up device. I discovered it by reverse engineering the wiring of a Denon AU320. As long as the negative phases of the primary and secondary are connected, I never encountered a ground loop. Some transformers have a separate terminal for the shield in which case this should be connected to a binding post wherein the tonearm cable ground lead should also be attached. Another possible scenario is when the case itself is connected internally to the shield. In this situation you cannot use an all plastic chassis since the case has to be "grounded" onto the metal chassis itself as well to ensure proper shielding.

As you can see from the pictures above I mount octal tube sockets and RCA jacks in plastic project cases available from your local Radio Shack or other electronic stores like Parts Express and wire them according to the terminal arrangement of the transformer. If you use a metal case, make sure the barrel of the RCA jacks are properly insulated otherwise excessive hum may result due to a ground loop.

Here is the wiring diagram for the Altec 4722.

Notice the switch on the positive phase of the primary, this was done so that I can avail of the 38 ohm [5 & 6] and 150 ohm [4 & 6] inputs at the primary.

Above is the diagram for exclusive 15095 or 15095A [150 ohm, parallel connection]. To my ears the 15095 and 15095A do not sound exactly the same and cannot recommend their use as a pair.

Sonic Impressions

I have been using the Tamura TKS83 and Altec 4722 as my main step-up devices. My reference combination is the Denon DL103/4722 at 150 ohms and the Ortofon SPU GME/TKS83 at 3 ohms. Sonically, I rate the 4722 at the same level as the TKS83 with the Tamura sounding a bit leaner and detailed [perfect for the softer sounding SPU] while the 4722 a bit warmer [tames the leaner character of the Denon] with more air and decay. Others may find the 4722 a bit euphonic depending on taste and system context, so try the 15095 and 15095A. These I rank very close to the 4722 with just slightly less gain. In certain ways the 15095 have more of that modern TKS83 character, especially the 15095A. The RCA MI12399A also belongs to this same league with a bit more of the 4722 euphony.

The UTC P-1 and Beyer Dynamic are about as good as the Denon AU 320 with the Beyer sounding leaner and detailed while the UTC is classically warmer and with more air. The Dukane 3A55 is a decent performer but outclassed by the others in this survey. My friend Ding in NYC found a pair of Thordarson T-43606-A several years ago at a hamfest and this model actually sounded on the same level as an Altec 4722 and the TKS83 when we did a shoot out at his place.

These are subjective observations gathered from extensive listening through my two systems using the following MC cartridges, Denon DL103R, 103, 103C1 and Ortofon SPU GME and MC10.

Have fun and happy listening!

Garrard 301 + Thorens TD124

Garrard 301

Garrard 301
Ortofon RMG309 + SPU GME
SME 3012 + Denon DL103

I acquired this grey grease bearing model Garrard 301 in the mid 90s. This particular sample is not cosmetically in mint condition but mechanically sound. All I had to do was clean and lubricate all the moving parts. In the past I used Castrol GTX automotive bearing grease, but it proved bit too thick for this application. So I replaced it with 2.5 NGLI multi-purpose grease available at Lowe's or Home Depot. It has given trouble free performance for years.

301 after cleaning, ready for a test drive
Rega RB300 + Denon DL103

Garrard 301
Ortofon RMG309 + SPU GME
Schick 12" + Denon DL103C1

See Garrard 301 - minor restoration

Thorens TD124 Mk. I and II

After several years of using both the TD124 and 301, I cannot say that one is superior to the other. Sonically the 301 is more direct and forward in presentation whereas the TD124 is slightly more laid back with the same amount of detail retrieved. 

I attribute this to the drive mechanism of the TD124 which uses a rubber belt to 'decouple' the motor which drives a pulley that turns the idler.  

Thorens TD124-II +
SME 3012 + Audio Technica ATP12T

I have a special affection for the Thorens TD124 having bought, sold and traded over a dozen since the late 80s. I admit that I was in denial of its virtues then. I spent so much time modifying and upgrading an original AR turntable using Merrill components and even toyed with a Thorens TD150. No matter what I did to the AR/Merrill and the TD150 the 3 point suspended belt drive could never equal the musically authoritative presentation of the TD124 even mounted on a lightweight box type plinth particularly in the area of pitch stability. So I always kept a couple (or more ;) as back up and reference.

TD124 + Rega RB300 + Denon DL103R
lightweight box type plinth

By the early 90s my system was evolving into a SET/High-Efficiency speaker set up and this was the period the TD124/Rega RB300/DL103R combination became my reference analog front end and gave up on 3 point suspended belt drive. 

I like the convenience of removable head shells and have used quite a few gimbal bearing tonearms like the Fidelity Research FR64fx.

Thorens TD124 II on 6-layer 3/4" plywood plinth
Fidelity Research FR64fx

Excel ES801 which is not quite as analytic in sound as the FR64fx.

Thorens TD124II + Excel ES801

However classic tonearms like this unimproved SME 3009 with metal bearings will perform just as well if not better.

TD124 + SME 3009 + DL103

In my experience, there is no significant difference in sound between the earlier ivory white chassis commonly referred as Mk. I with the heavier iron platter vs. the later Mk. II 'non-magnetic' but lighter platter which has a grey colored chassis.* I also never had issues with the magnetic platter giving trouble with any of the cartridges I use. 

*See June 2021 update!

Butcher block type plinth for a TD124

All three of my TD124s are running on original idler wheels and only replaced a belt on one unit.** I also advocate the use of rubber mushrooms (or replacement rubber bushings), direct mounting will not make a TD124 sound like a Garrard 301. They are very different in design.

**See March 2018 TD124 updates!

Click here for plinth building tips!

British + Swiss

Nice to have two flavors ;)

Have fun and happy listening!

JE Labs phono and line preamp

!!!WARNING!!! 

The voltages found in this circuit can be lethal, build at your own risk!!!

Phono stage

For severaI years I have been hacking the Dyna PAS chassis with various cascoded and cascaded phono circuits all employing negative feedback RIAA EQ in search for a circuit that will satisfy my musical requirements. My research ended when I discovered this unglamorous 2-stage cascaded passive RIAA EQ phono preamp circuit at the back page of an RCA tube manual. 

The most important lesson I learned in this excercise is negative feedback chokes the sound! I did not alter any component values except for using Ni-mH batteries for cathode bias which I thought improved detail and definition. I strongly advice against the use of a cathode follower and make sure the circuit drives a minimum load of 220K as originally specified otherwise you will lose bandwidth at both frequency extremes. I use an Alps 250K but have also tried the cheap Alpha 250K dual log taper carbon pot sold by AES. It does not have the tracking precision of Alps but at normal volume settings the channels are balanced enough.

I have been using this phono circuit since the early 90s starting with a PAS chassis with Telefunken ECC83/12AX7. Through the years I developed fondness for octals and nowadays I almost exclusively use 6SL7s/5691s which further improved the bandwidth and depth due to its slightly greater transconductance, vis-a-vis, the 12AX7. 

JE Labs phono stage adapted from the RCA circuit

There are people who swear by using high transconductance tubes like WE417A or 6DJ8. To hear what this was all about, I bread boarded a famous circuit using these tubes but did not find the sound as appealing, must have been just a matter of taste...I find the superior linearity of true audio tubes like a 12AX7 or even better yet 6SL7 octals play a lot more LPs.

MC transformer

For MC cartridge use a transformer step up device like this Tamura TKS-83 or mic transformer  to boost the signal to MM level. I am not a fan of active MC stages and rather not go there...

76>6SN7 cathode follower line stage

Another very simple topology with a single gain stage direct coupled to a cathode follower. Originally I was just using a 6SN7 dual triode for both stages but was inspired by the Berman article in SP 13 to try a type 76 as the gain stage. I use a lower B+ supply and run the tube at lower plate dissipation and also prefer the sound with the 2.2K cathode bias bypassed with a 100uf/16V cap [in this case I can justify using Black Gate]. 

At this signal level the direct coupled 6SN7 GTA or GTB cathode follower is sonically beneficial due to its low output impedance. Thus, it can drive longer interconnect cables without any detriment to the frequency extremes. [Cable geeks take note: many times the "difference in cable sound" you hear is more a function of load/capacitance especially if your preamp does not have low output Z].

Transformer coupled line stage using Tango NP216N

I briefly flirted with this design as a linestage and in spite of the fact that all my SETUP friends liked the sound [whose ears I respect], I eventually went back to the above linestage. The Tango NP216N is a wonderful piece of iron although I have to admit that I needed greater bandwidth than it has to offer. However this is a circuit that offers a solid 600 ohm [as used in studios] output impedance and can even be wired in balanced mode.

At some point I may try this topology again at the output of a tube microphone preamp project. Please do not take my seeming lack of enthusiasm as a dismissal of this topology. I included this schematic because it may well be appealing to other ears and/or applications.

Joel's preamp

12SL7 phono + 12SN7/Tango NP216N line stage + external PS

Nonong's preamp

5691 phono + 76>Tango NP216N line + external PS

Power Supply

During my Dynaco hacking days, I experimented with regulated HV using solid state devices which was 'in tune' to the high end sensibilities of the time. I also chanced upon a Heath tube regulated [6L6GC] Lab PS and hooked this to my preamp, there was a slight improvement towards my goal but my ears longed for a more natural sound.

When I acquired my first copy of the Radiotron 3rd edition, I regressed to 1930s technology. This is the PS circuit I've been using for the past couple of years for deluxe versions of my preamp. A full choke input followed by another choke/capacitor stage in order to get a very well filtered B+ supply.

I use a bridge rectified and highly filtered DC filament supply for the heaters without having to use silicon regulators. If you pay close attention to the schematic you will notice that there is a voltage divider network [120K series connected to a 47K and 68uf/160V combo shunted to ground] at the output of the B+. This network is mounted on the preamp chassis and keeps the filament floated above ground to further minimize hum and noise and also to keep the heater to cathode voltage on the 6SN7 cathode follower below maximum limit.

Note: the numbers in the schematic pertain to the terminals of a 4-prong connector/umbilical cord to connect the preamp to the power supply.

If your power transformer has a 5V, 3A rectifier winding, you can experiment with rectifiers. My favorites are the 5V4G and 5Y3, but have also used GZ34/5AR4 and 5U4. If you know how to use a DVM, you will note that different rectifiers will produce higher or lower B+ voltage which alter the operating point of the circuit. In general, the different voltage is what changes the sound not the rectifier itself.

Prototype

Built on a PAS chassis
12AX7 phono + 6SN7 line + external PS

Octal prototype
6SL7 phono + 6SN7 line + external PS

5691 phono + 76/6SN7GTB line
Gen Rad PS

under the hood

Variations on a Theme

5691 phono + 6SN7GTB line
External PS

6SL7 phono + 6SN7 line
External PS

Simple version 

6SL7 phono + 6SN7 (single gain) line stage
no cathode follower +
simple CLC PS

6SL7 phono + 6SN7 (single gain) line stage
no cathode follower +
simple CLC PS

76>6SN7 line stage + CLC PS, ca. 2006

6SL7 phono + 6SN7 (single gain) line stage +
PS in one chassis

My last project in the USA - a single chassis preamp in 2007 for my brother's system. With careful attention to wiring to avoid ground loops, it is just as quiet as the 2-chassis version.

76>6SN7 line stage preamp
with remote
volume control

JE Labs goes high tech - built by Joel Villanueva for this system and featured in the November 2011 Hi-Fi Show. It is a minimal line stage preamp with remote volume control on one chassis from Harana Audio.

Have fun and happy listening!

JE Labs Simple 45/2A3

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

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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!