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| Bach Chaconne from Partita No. 2 in D minor for Solo Violin |
An important lesson I learned as a classically trained violinist was that music theory evolved from the analysis and study of great masterpieces from Bach to Bartok. Great composers broke theory to reach new heights in musical expression. Just like any artistic pursuit, music continues to evolve through individuals who are not afraid to go against convention for a greater good!
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| Tube load line simulator from Trioda.com |
On the other hand, audio engineering, for the most part, has always been theory driven. Audio design simulators are significant modern conveniences but don't guarantee musical-sounding circuits even if the metrics look great. Creativity in audio design is when human ears are still the final arbiter, at least in my experience and opinion, which is why my hifi system is a hodgepodge of mostly obsolete technology, some of which date back to almost a century ago.
Last year, a Juilliard-trained pianist turned lawyer visited my man cave and listened to various amps + a couple of speakers I built. On his second visit, he brought his amps for comparison. At the end of that session, he commissioned a pair of Altec 753C speakers, the second pair I built in collaboration with John Piro of Hommage Audio, who crafted the speaker cabinets.
The chosen amp was the Radiotron SE2A3 - a topology from the pre-WWII Radio Designer's Handbook, 3rd edition. This circuit was from an era in which calculators were but a dream and engineers computed complex equations aided by a slide rule.
!!!WARNING!!!
The voltages found in the following circuit can be lethal! Build at your own risk!
Although this pentode-driven circuit sounded fine as is, to my ears, its fullest potential was only realized after I adjusted the plate and screen grid voltages to achieve a 2:1 ratio as suggested by my Ukrainian internet buddy, Sergey Didkovsky in the winter of 2007.
"Try please, to increase the cathode resistor (1,2k-1,5k ohm) and reduce anode resistor (to75k-82k). It will allow to increase Ua up to 150V and to receive Ua:Ug2 = 2:1 ratio. The sound should become more 'interesting and old.' That is especially good for jazz. Still mine 5 cents...use good NOS oil capacitors in G2. I use TCC Visconol. I think oil caps in the grid - adds bass. Reduction of the anode resistor - improves the hi range a little."
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| Click here for the 2007 Radiotron SE2A3 article |
I'm pretty sure the 2:1 ratio wasn't a number crunched by a simulator, it was a figure painstakingly chosen by Sergey's ears because of his love for music!
Sergey, wherever you are, I pray that you and your family are safe. Freedom-loving people around the world are very grateful, yet very sorry that Ukraine is all alone in shielding the free world from the atrocities of a megalomaniacal autocrat!
Hello Joseph! I love your amp circuits! Repeated many! but look here and notice the date. This is already a year after the Maidan in Ukraine.
ReplyDeleteYou are being misinformed about this war.
Thank you for your kind words about my circuits!
DeleteAFAIK, the Maidan Revolution you are referencing happened in 2014, in which the Ukrainians ousted their pro-Russian/pro-Putin president, Viktor Yanukovych, who sought exile in Russia.
On February 24, 2022, Putin ordered the invasion of Ukraine, a peaceful and sovereign nation with a democratically elected government. Thousands of people have already died. This kind of violence is absolutely unjustified!
Since I don't know who you are and where you're based, I can't give credibility to your assertion. My source of information came from the free press of the western world - NPR, BBC, DW, etc.
Hello. I built again this amp this weekend (using the old diagram, with 5R4). Now I used a very similar design you show on this photos. But now I get a very disturbing hum. Any ideas how I can fix it or find what is happening?
ReplyDeleteThe sound is great, the voltages are correct, no cables cross at any moment.
The hum is identical with or without the 6C6, so I believe it’s the 6B4 (same 2A3 but octal and 6V - and a lot cheaper here!). The power transformer and choke are 90 degrees out of output transformers.
I would love any tips!
Best,
Martin
If you built a quiet version using 2A3s then you know how to avoid ground loops. Therefore, that's out of the equation.
DeleteIME, any DHT with a filament requirement above 2.5V, like a 71A, 300B, 10/VT25, etc. will need DC filament power for quiet operation.
I’ll try! Well, chance to learn something different. :)
DeleteThe cathode resistors can be just the same? I never made a DC supply for a tube like that (just for preamp tubes like 12AX7 and so on).
Make a 6VDC source that can handle the current of a 6A3/6B4G. Google Herb Reichert's Flesh and Blood SE300B amp schematic. Your 6A3/6B4G 6VDC supply should be wired in a similar way except for the Radiotron SE2A3 cathode bias resistor.
DeleteWorked perfectly! Thanks a lot! And I’m already on a happy listening. :D.
ReplyDeleteBest,
Martin
Great! 👍
DeleteHi, can I use this circuit substituting a PX4 instead of a 2A3. I don’t find many circuits using the PX4 and suggestions are that the PX4 is very much a British 2A3.
ReplyDeleteTIA
I don't have first hand experience with the PX4 but after looking at its data, I'd surely try this circuit if I had a pair.
DeleteGood luck!
Hello Joseph,
ReplyDeleteI am putting the parts together to assemble your version of the 2a3 radiotron amplifier, and I found that my valves do not have a guide to place with the 6-pin base, I only see an arrow under the valve, that indicates the heaters (pin 1 and 6)? Thank you very much for your blog and teachings. I don't have very efficient speakers (JBL A130 86db / 6ohm) but I'm sure that for what I'm looking for, the amplifier will sound excellent. Following this path in the hobby, at some point I will find the right speakers. thanks again
Pins 1 & 6 are the largest pins + all the 6-pin sockets I've used have a round marker between the two largest pins. With those two things in mind, you shouldn't have problems.
DeleteGood luck with the project and happy listening!