After building my first DHT PP2A3 in '93, the logical step was to build a SE 300B. I was involved in the development of the Angela Instrument's WE91 clone project as well as servicing Audio Note Kit 1 amps. The WE91 with 6SJ7 input/driver and Audio Note 3K OPTs sounded very musical to my ears however I found the frequency extremes a bit soft. The Kit 1 on the other hand had typical high-end detail without enough warmth in the midrange which I attribute to the SRPP 5687 driver. While visiting Ding in NYC, I heard a pair of Herb Reichert's "Flesh and Blood" which he had on loan. I was very impressed and since it uses my favorite 6SN7 tube I built a clone that was sonically disappointing because I did not have the proper power transformer to duplicate his operating points. So back to the drawing board.
|JEL Classic SE300B circuit [one channel]|
One night I was reviewing the Williamson circuit amp and realized that maybe I can slightly modify the first two stages so that it can function as an input/driver circuit for my SE300B project. All my free time in the ensuing weeks was spent testing and tweaking the circuit shown above until one day I brought the amps to Steve and he asked me to write about it for his webpage. That was the birth of the classic JEL 300B. The design shows great similarity to the Fi-Primer 300B and Sun Audio SV300B but to those who are aware of how operating points have a significant effect on an amp's sonic character would know that this is a totally different application.
During the initial development of the classic JEL 300B, I was still using Spendor LS3/5A speakers and its lack of efficiency was a limiting factor in my R&D. Eventually I acquired a pair of Altec 755Cs, a little later the Altec 755As and mounted them on open baffles. The greater efficiency and quality of the 755s showed that the classic JEL 300B circuit could be improved. I had a bread board amp at that time wherein I can change power transformers and experiment with various tubes and operating points as well as circuits. I tried a WE417/5842 2-stage design, cap and DC coupled, 3-stage with either a 76/37 or 6J5 or 1/2 6SN7 on the first stage and the second stage with a higher gm tube - 5687, 6BX7, 6BL7 and etc. in place of the 6SN7GTB driver. Technically speaking having a great amount of AC swing and low impedance driver looks ideal on paper. However high transconductance tubes were introduced at the dawn of the solid state era for radio frequency applications and using these tubes in an ultra low impedance input/driver stage in an SE amp sounds like a high end amp to my ears - been there, done that...so I am sticking with my Art Deco era tubes which were developed mainly for audio frequency applications. Nowadays I only listen to efficient speakers that only need 8 watts or less to produce concert hall SPLs and at the same time give delicacy and refinement. Perhaps the brute approach works better with less efficient speakers so let your ears decide.
JEL SE300B Deluxe
The deluxe circuit above is a refinement of the original topology utilizing a 76 or 37 tube for the first stage with a better filtered power supply that maximizes AC voltage swing for the 6SN7GTB driver stage. Having the type 76 at the input stage and an enhanced power supply was a significant step forward. The type 76, 37, as well as its 2.5V filament equivalents - 56, 327, 27 and etc. - were the predecessors of the 6SN7/6J5 and have similar electrical characteristics to the WE 262 used in the WE 86 PP300B amp.
Since it is becoming hard to find good type 76 variants, you can upgrade the classic circuit by utilizing the unused half of the 6SN7GTB in lieu of the 76 for the first stage coupled with the upgraded power supply or add another 2.5V filament transformer and use 56s or 27s. Incidentally the meshplate 227s sound really good but tend to be overly microphonic and susceptible to hum. There is also an octal version of the type 76 - 6P5 which I have not tried and even harder to find. Some early glass ST type 6J5s have sonic characteristics similar to the type 76.
This circuit was primarily designed to match Altec 755 speakers, to my ears and in the context of my system it is still the best combination! Your mileage may vary when driving another speaker but if you use the Angela Universal power transformer it can easliy be converted to an SE2A3 amp which I favor when driving more efficient horn based systems.
Stereo SE 300B and 2A3
|Stereo SE300Bdx with Tamura F2007|
Stereo SE300B and SE2A3 DX versions using just a single Angela Universal power transformer [don't use the 5V, 300B filament taps to keep the power transformer core running cool]. The bi-plate 2A3s are operated at around 17-18W plate dissipation and with this driver circuit I can get 4.5W/ch and they have held out for years. Since the 2A3 is more sensitive, a Type 37 or 27 will lower the input sensitivity.
|Stereo SE2A3dx with James 6123HS outputs|
I do not recommend using vintage 'mono-plates' in this circuit at all unless the operating point is reduced to 15W maximum plate dissipation. The Sovtek and Shuguang 2A3s mono-plates will do just fine and are good modern alternatives!
|JEL SE2A3DX with Tango NY15s|
Building tips and parts recommendation
Notice when using the Angela Universal transformer, conversion to 300B operation is merely a matter of changing the filament supply transformers and disconnecting/insulating the 320 V leads and then soldering the 380V leads.
Here are some under the hood pics of stereo versions built using the classic JEL SE300B circuit:
This unit uses Audio Note UK 2.5K OPTs, power transformers is a Thordarson with a 400-0-400 @ 300 ma. HT winding. So I had to stack paralleled power resistors amounting to ~ 200 ohms @ 50W so that the B+ is reduced to the required level under load.
|Stereo SE300B unit using Tango U808s|
Ground loops [hum] can be frustrating in any amp/preamp project. I do not have any strict wiring procedure and basically try and follow the flow of a schematic. Take a look underneath my 300B monoblock:
I don't use a "star ground" and employ a localized grounding scheme [similar to how classic vintage components were wired]. By using terminal strips, I take all ground connections to one point by section - input, driver, output and power supply. The chassis acts as the ground plane for the whole circuit. Sometimes in stereo versions like this Stereo 2A3 amp, the axial filter cap for the input stage is grounded at the same point in that particular stage.
With this wiring technique and quiet tubes, I only get between 3 - 6 millivolts [even 2.5 mV in the SE2A3 version] of hum at the output of the amp with the inputs grounded, which is pretty low since I prefer the sonic result of using AC heaters in all my amplifier projects.
For wiring, I am happy with Kimber TCSS copper even if I have used their expensive silver AGSS version. If you insist on silver and don't have the budget, I've gotten good results with 19 ga. solid core silver [99.99%] with teflon sleeving.
Capacitors and Resistors
It is inevitable to talk about parts and how they affect the sound of the circuit. For resistors, I like the "earthy" sound of carbon film [MCM/Parts Express 1 watters] or carbon composition [Rikens or Allen-Bradleys] 1W or 2W [paralleled values for quieter operation] for plate and cathode loading. Since the early 90s, I've stayed away from the "clinical" sounding metal film and metal oxides, except for power supply duty wherein the additional inductance of metal oxides and wirewounds is an advantage in terms of filtering ripple. I have also shied away from using "plastic" caps [polypropylenes, 'styrenes, mylar] for coupling and almost exclusively use paper in oil with either copper or aluminum foil [Jensen, Facon, Ducati, Icar or Sequa].
Being an apartment dweller, I avoided using paper in oil or motor start capacitors for power supply decoupling only out of practicality. To be able to use them properly require large and heavy chassis and I do not have the real estate space to accomodate them. Anyway, electrolytics also sound good [Cerafine & LCRs] and have had good luck with Sprague "atom" axials [which incidentally are used even in deluxe projects by our Japanese friends]. For cathode bypass, Cerafines, Nichicons and Sprague Atoms do a good job too. Overall Black Gates are nice, but I cannot justify the expense in most applications.
|JEL Stereo SE300B w/ Tango XE60-3.5S|
Output transformers that I've used or heard in the context of this circuit with satisfactory results include, Tango XE60-3.5s, NY15s, U808, Tamura F5002, F7002, F7001, F2007, Audio Note UK, 2.5K and 3K, James 6123HS, Magnequest RS025 and the classic TFA204 [2A3 only]. If budget is tight, the Hammond 125ESE will work in a 2A3 circuit or for a little more cash out and you like more bass performance but less refinement from the midrange to high frequency, the 1627SE at full 80ma. 300B operating point. I prefer not to rank the sonic quality of output transformers since it is a very subjective field.
Power transformer and chokes
I made an effort to specify off the shelf iron that is available from Angela Instruments or other Hammond dealers to guarantee predictable results. You may use similarly specified substitutes but you are on your own in being able to maintain the proper operating points indicated in the schematic. Since not all iron are made equal, I cannot help those who will send queries to adapt a surplus transformer. This can be done as I did in the past through trial and error but it takes time. I will truly appreciate your consideration on this matter!
JEL Classic SE300B monoblocks by Ding
300B amps using Tango and Tamura transformers built by SETUP - Single Ended Triode Users of the Philippines - friends in Manila
Other applications - WE205D, Type 10 and Type 71A
Stereo SE WE205D amp using Tamura F5004 amorphous core opts and a custom wound Tango power transformer built by my friend Joel in Manila.
|Stereo SE300Bdx amp |
used as a
WE205D bread board
For exclusive WE205D use, I recommend a 7K OPT, like a Tamura F5004, F7004, F2005, Tango FW20-7s or Nature Sound NS70SE.
As further reference, here is a schematic I found in a Japanese publication for a WE205D amplifier. I tried this input/driver circuit [with 76s] and sonically prefer DC coupling between the two stages with the slightly higher gm 6SN7GTB as a driver tube.
Lowther users, take note!
During a recent discussion with an experienced Lowther TP1 user for my mono playback article, Brian Clark confirmed that these speakers delight in being driven by a type 10/VT25 because of the relatively high output Z and low damping factor. I have never been a fan of Lowther speakers until I heard this amp drive a pair of vintage Brociner TP-1s loaded with PM6As, I was simply blown away!
Sherwin's SE171A amp - the 790mW wonder!
After several years of amp building I have to admit that this is my favorite specially when fitted with the scarce type 183 DHT because it reinforces the philosophy of the first watt counts!
Have fun and happy listening!