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Midwest Audio Fest

It’s that time audio enthusiasts!

Registration for the 2019 Speaker Design Competition is now open! Visit for details and to list your speaker project.
We are excited to see all returning participants, and look forward to meeting some new designers this year, as well!

Be sure your plans include a visit to the Parts Express Tent Sale for the lowest prices of the year, and the Audio Swap Meet where you can buy and trade with other audio fans.

We hope to see you this summer!

Vivian and Jill
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Midwest Audio Fest

It’s that time audio enthusiasts! Registration for the 2019 Speaker Design Competition is now open! Visit for details and to list your speaker project. We are excited to see all returning participants, and look forward to meeting some new designers this year, as well! Be sure your plans include a visit to the Parts Express Tent Sale for the lowest prices of the year, and the Audio Swap Meet where you can buy and trade with other audio fans. We hope to see you this summer! Vivian and Jill
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SET Amplifiers

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  • SET Amplifiers

    Would anyone have suggestions for interesting SET amplifier projects, something you might have built or seen? I'm interested in this more for the fun of building one, than for use. Mono in this case may be preferable. Thank you in advance.

  • #2
    First, understand a few things about the nature of the beast. SET amplifiers are low power, and as a consequence you also need to find hi sensitivity speakers to play with them. Not a whole lot of different designs to choose from, they are just not that popular with the top notch designers we are used to building from here. A two way with a compression driver and a high sensitivity driver is going to be big and large offsets to get crossover slopes in proper relative phase must be dealt with. Secondly, the amplifiers themselves are not for the congenital f**k-up. The most useful triodes require very high voltages for the main power supply. My SET's use 845 tubes for the output stage, which I have biased produce about 18 watts. The power supply required for this starts with about 1,500 volts RMS, which can kill you dead in one heartbeat with the simplest mistake with the multimeter. In fact, you may even need a special multimeter to deal with such voltages. You also need to understand how to do your wiring to avoid arcing. Printed circuits with the high electric field values produced at the edge of a copper trace is destined to arc and cause no end of problems. You'll have to use strings of four 1,000 volt diodes just to get some DC voltages.

    Now, the good part is the distortion they produce: Lots of first order harmonic distortion, which makes the sound to die for. It is as if Emmylou Harris is singing backup vocals for everything. I have done my setup with 845 tube amplifiers for the Radian compression driver tweets and KT-88 based Cary Superamp for the 15" bass driver, crossed at about 1,100 HZ. MiniDSP solves all kinds of intrinsic problems. Just understand that you are doing things the hard way. For some, (c'est moi) it is well worth the effort. If you just want something that that sounds good, this is a high maintenance path to getting there.


    • #3
      If that didn't scare you off, lots of people have built one of the JE Labs amps. You'll find them if you Google. Low power, fairly simple, but fun amps to build and they sound pretty good. Not sure if he ever designed a mono amp, but they could be built that way with a little work. Be warned - most AC Direct Heated triode amps will have a little residual hum - depends on the sensitivity of your speakers. PWRRYD (Craig) built a very nice SE KT88 amp. The KT88 is not a true triode, but you can run it in triode mode. I think the big thread for that amp is on and Blueglow Audio has a whole video series on one variation -
      We enjoy making videos of our repairs and restorations at Blueglow Electronics. We service both tube and solid state equipment. Its a side business we have b...
      Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.


      • #4
        No, Mr. Hill has not scared me off. He has provided a thoughtful, sensible and amusing introduction to these amps. His warning of potentially deadly consequences in the first paragraph, and indication in the second that these amps can produce a sound "to die for", seems nicely balanced.

        I'll keep in mind the high voltages, and prepare judiciously if anticipating Ms. Harris singing backup vocals. I appreciate the cautionary warnings, advice and the explanation of these amp's benefits.

        I'll also look at the linked videos, and I think I recall PWRRYD's posts, I'll try to find these.

        Thank you for the responses.


        • #5
          Yup, scary to consider all the implications of starting the SET addiction. With regard to residual hum, there are a couple of different approaches to dealing with it: Most direct is to build a high powered SET such as 845. Then you have enough power to use less sensitive speakers. An 845 SET sounds as powerful as an ordinary 30-40 watt PP amp and even stronger at the bass end. Speakers in the 85-88 db range are very happy with the 845 driving them. A second approach is to use DC on the filaments. There is an element of the SET crowd that scorns such use but I sure can't hear a difference. The easiest approach is just don't get hooked on vinyl and the problem is very manageable. I have heard triode wired kt88 amps and that are very good. In essence, they offer the performance of 2A3 and 300B amps at a fraction of the cost. If you spend some time shopping, there are bargains on DIY SET amps to be found on Craigslist and e-bay. Biggest considerations for buying such amps are neatness of construction and output transformers. Transformers are a subspecies of the hobby and you can really get involved indulging an interest in them. Still, the biggest factor in your ultimate sound is the speakers. Pi speakers are a good place to start, along with the Econowave speakers that were popular a few years ago.

          BTW, the reason I use my 845 SET's on the tweeters is that the hum from the SET's is attenuated by the hi-pass tweeter crossover slopes. The push-pull kt-88 amp covers the bass and low mid frequencies. With a crossover at 1,100 Hz a lot of the SET magic is still present through the tweeters. Not ideal if you are an SET purist. Such is the nature of compromise.


          • #6
            Thanks for all the good information to think about. Regarding hi sensitivity speakers, I still have a set of EV 12TRXBs I bought in 1969, perhaps these would be suitable?

            I looked at the PWRRYD SET amp build thread again, what a good thread. As I was looking I vaguely recalled the Dynaco Mark lll might be run in Triode mode. I checked online and find Bob Latino on the Dynaco Forum explains how to do this. I have been working on a Mark lll project off and on for a few years, it's nearly finished. I can make the suggested changes for Triode. Although this won't be single ended, it will provide a quick opportunity to listen to the KT 88s in Triode mode before starting a new "addiction". I'll continue looking for a new SET project.


            • #7
              There is somewhat of a line in the sand between push-pull triode mode and SET. Generally, push-pull triode gives low distortion. SET, not so much. Of course, you may like the SET distortion.


              • #8
                I've never heard of "first" order harmonic distortion. I think you meant to say "second" order. SET amps don't have to run 1500 V B+ supplies. That is a product of choosing the 845 tube. My KT88 amp sounds great and produces 8-12 wpc with only a 400V B+. I can change my amp from quasi-Triode mode to Ultra-linear mode with a flip of a switch.


                • #9
                  Looking around online, I cane across the folliwing as an interesting potential project:


                  Using Hammond, unfortunately no longer available from PE, rather than more expensive transformers, and moderately priced tubes(NOS 300B Western Elec. cost nearly $20K), and various other components, it appears $500-$600 would be required to start. Fortunately I can mill a chasis. This does seem to have benign addictive possibilities.


                  • #10
                    Hi Craig:

                    You're a nitpicker for language and absolutely correct ;-). Could you provide a link to the schematic you used? By first order I meant the the amp mixes in a noise signal that is at the "first" octave above the undistorted signal frequency. Yes, the very high starting voltage for the B+ in my amps is due to the choice of the 845 tubes. The 845 tubes require a large voltage swing to drive them, my amps use a triode wired EL34 to drive the 845's. (In effect, I'm using your SE kt88 as the driver for my output tubes). A voltage of around a kiloVolt for the actual B+ (after filtering) is required in order to get something around 20 watts. The good news about the 845 is that the headroom on the amp is massive. Dynamics easily exceed a 40 watt PP kt-88 amp and there is a huge advantage to the 845 in the strength of the bass. It sounds like it must be 100+ watts compared to a PP kt88, simply because (even when overdriven) the distortion isn't noticeable. I've compared the sound of my amps to a very good pair of 2A3 amps and there is really no comparison in my mind.

                    TN: Start with a SE triode wired amp (EL34, kt88, etc.) in preference to any exotic low power DHT such as 300B, 2A3, etc. You'll be much happier. Any excess funds should be allocated to quality output transformers and speakers. The chinese tubes are excellent. Tube rolling with NOS triodes gets very expensive very fast. Whatever you do, don't drop a DHT, even onto carpet! The glass will probably survive but the insides won't. We 77 year old tube DIYer's with shaky hands have problems you can't yet imagine.


                    • #11
                      I will look for my schematic and post it.

                      I have used Edcor transformers and find them to be very high quality and sound nice. They are not super expensive either. The only down side is they stock nothing, so your order takes about 5 weeks to get. Oh and I don't personally care for their powered coated "blue" end bells. So I scuff those with some fine grit sandpaper and paint them my perferred "satin black".


                      • #12
                        I'll look at the Edcor transformers, time is not a problem, I have a few other projects to finish, including the Mark III. Do you recall where you found a suitable switch? I started looking for a 400VDC dpdt switch as recommended on the Dynaco Forum, but need to look futher. I also have a Dynaco ST70 I updated a few years ago, that may be a quick way to try stereo quasi-triode mode, as I recall the Dynaco Forum modification is possible for it.


                        • #13
                          TN: don't spend much effort adding triode mode to your Dynaco amp, you probably won't hear a difference. The Dynaco circuit is a push-pull circuit,where one tube amplifies the positive portion of the signal and the other tube handles the negative portion of the signal. With push-pull, the two tubes each have some uncertainty as to where in the signal to start conducting and the other to cutoff. This is called crossover distortion and applies at very low signal levels. The magic of the SET occurs because this is eliminated. It is simple enough to triode wire the tubes, usually just a jumper between one pair of tube terminals and and a 100 ohm resistor between a different pair. These two new circuit paths carry virtually no current and any cheap microswitch will be adequate. But that wiring doesn't change the push pull nature of the circuit.



                          • #14
                            Mr. Hill,

                            Here is the link to the Dynaco Forum thread:


                            Adding dpdt switches and resistors isn't much trouble, and seems a simple way to try triode mode before building another amplifier. Which is to say that I try to go about "new addictions" judiciously.

                            The Mark III is still easily modified as it remains incomplete. I built the chassis, it's not a standard Mark III, and adding a few new components isn't a problem.

                            Drilling a few holes in the ST 70 chassis is easy enough as well, as is adding the 2 dpdt switches and resistors. I don't use the ST 70 at present, perhaps the possible change in sound Bob Latino mentions in his thread may make it worth using again.

                            ​​​I assembled this ST 70 as a kit 50 years ago, used it for about 20 years, then lent it to friend for about 20. When he returned it, I upgraded it, but can't say I was all that impressed with the result, although I did enjoy doing the upgrade work. Perhaps triode mode will prove worthwhile.


                            • #15
                              Hi TN: Call me Roger, I lack the age, wisdom and maturity to require "Mr." In your position, I would finish the Mk III's with the triode/ultralinear switch. You can't have too many bells and whistles. I added that as an option when I built my Cary/AES Superamp 20 years ago. It is a PP that can be operated with EL34, kt88, 6550 etc. tubes by merely changing the tubes and resetting the idle current. As before, I can't hear the difference between triode an ultralinear but that may have something to do with the 77 year aging of my ears. The rest of me is much younger......;-)