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Tubelab_com 31st July 2019 03:14 PM

UNSET is coming?
UNSET is coming......

I have mentioned these words a few times in passing, but the truth is that UNSET only existed in LTspice at the time. The simulation showed unreal performance, but that is often the case with vacuum tube simulations, especially those where the tube is not connected up in a conventional manner.

This was the case for about a year. I have too much on my plate to effectively handle right now, so the simulation, along with several others sat in my PC undisturbed for a long time. Late last year I put together a test board for a push pull amp that used UNSET technology in the output stages only. It did work rather well, and I planned to pursue a follow up design, but the need to redesign the TSE board popped up, which is not yet finished. The boards are done, but all the documentation is still a mess.

About 3 weeks ago my wife started exhibiting heart attack / stroke symptoms, but refused to go to a doctor until bad stuff happened. There were good days and bad days for two weeks, which required my full time attention. During the evening of one of the good days we were both at this computer ordering some stuff for a baby shower when she said that she didn't feel well and wanted to go upstairs. Before we got to the stairwell, she passed out and dropped to the floor. I managed to get her upstairs, out to the car, and to the ER. They stabilized her and eventually figured out what the cause was, but this was the beginning of 4 days in the hospital and some full time watchfulness from me once she was home.

During the 4 days in the hospital she was sleeping most of the time so I fetched my laptop and did what I often did with a few days of idle time.....took one of my best simulations and made it into a PC board.

UNSET the simulation became UNSET the test board. it was a hurried up layout, many parts didn't fit right and a few were just left out by mistake. Over the past few days I populated the board with the exact parts from the simulation except where I didn't have the right part, I stuck in whatever I had. What were the chances that this would work? Would it blow up in my face? Am I going to stick expensive tubes into this thing? No way........

So, briefly, what is UNSET?

A SET is a Single Ended Triode amplifier. The SSE, TSE, and new TSE-II amps are examples of this, but the SSE often uses pentode tubes wired as triodes. This works, the pentode takes on triode like qualities with the associated triode disadvantages, most notably the inability to pull its plate down near the cathode voltage thus limiting the available power output. Another issue that needs to be overcome is the screen grid voltage limitations of most TV sweep tubes. Wire them as a triode, and most will eventually blow up when left alone idling which is worse case for a class A amp (maximum dissipation). The tube doesn't know if it's pentode, UL, or triode connected when it's idling, it just gets a constant voltage on it's elements, and if the screen grid is significantly above it's maximum spec, there is a risk that it will blow up (some will, some won't, but most will if left long enough).

What if there was a better way?

I have been searching for this better way for nearly 10 years. I experimented with screen drive, dual drive, and even built my GUT (Grand Unified Theory) board which had the ability to drive every electrode independently in either phase, except the plate and heater. Several toasted (and two exploded) tubes and nearly 10 years later, I arrived at a new topology that I can't find anywhere it recorded vacuum tube history.....yes, there are several close similarities, but this is truly unique.

I called this topology the Composite Electron Device for lack of a better name, since it is a composite of a vacuum tube pentode, a mosfet, and a hand full of discrete parts to create triode like curves. The screen grid specs are not violated, so that nearly any pentode will work with appropriate component values. Note that this statement is still based primarily on simulations, with some limited actual tube testing.

I will not reveal the full schematic just yet, but it is coming.........

The UNSET is an amplifier design that uses CED topology for its driver and output stages. A small CED is built with a high Gm pentode for the input / driver stage. I used a 6EJ7 / EF184 in the simulation, but stuck a $1 tube in the test board.

The output stage uses an identical circuit built with bigger parts. I laid out the board such that several popular octal tubes could be used, but messed this section up pretty bad. I wired it up to use an unloved and unwanted octal sweep tube that I have LOTS of.....A popular tube seller sold me over 100 of them for under 50 cents each!

After finding my mistake with a backwards zener diode, I was really surprised by what happened next.........

zman01 31st July 2019 03:29 PM


Sorry to hear about the health issues, and I hope your wife is stable and recovering well.

Keep unfolding the "UNSET" story to us....

Tubelab_com 31st July 2019 03:50 PM

1 Attachment(s)
I connected the board up to an external bench power supply, my old Fluke 407D. I installed one input / driver tube and attempted to test the input stage by itself, but voltage readings everywhere were all wrong. Nothing came anywhere close to the simulation, and the output was highly distorted. Some poking around with a voltmeter revealed a mistake I often make.....I got a diode in backwards in both channels.

After fixing the backwards diodes the input / driver stage fired right up and revealed performance close to the simulated circuit. The DC voltages were a bit off, but I didn't even have the right tube in the board, so how could I complain.

I cranked up the input until the driver hit clipping and backed up a wee bit. I was getting 56 volts RMS! This is more than enough to drive the grids right out of just about anything. The THD at a sane 20 V RMS was 0.2%, no complaints here. Frequency response? What's that? The driver is flat from 5 Hz (generator limit) to somewhere beyond 100 KHz where the 8903A stops reading.

I stuck in a pair of 25DN6 tubes and wired their heaters to a 24 volt SMPS, turned up to 25 volts. The 25DN6 is an old sweep tube dating back to the mid 50's. They can still be found in large quantities for about $3. They are NOT all created equal. The old ones from the 50's are built to their 15 watt plate dissipation ratings. Some of the newer vintage tubes have BIG plates inside! The GE's have a plate that is identical to what's inside their 24 and 28 watt tubes, so that's what I stuck in the amp.

I set the Fluke on 450 volts, and turned the bias up to 100 mA per tube.....yeah that's about 40 watts (OPT and other losses). I used a pair of 3000 ohm Transcendar OPT's that were still on the bench from my TSE-II board testing. The amp came to life immediately and surprised me with performance.

The THD at 5 watts was under 1% except at the frequency extremes where the OPT limited the performance.

The -3dB point at the low end was 8 Hz where the THD was 26% due to OPT saturation. At 20 Hz it was 0.54 dB down at 4.03% THD.

The upper -3dB point was arounf 50 KHz where the THD was 1.58%. At 20 KHz the amp was 0.62 dB down at 0.56% THD. NOT crank it up!

Thd was 0.197% at 100 mW, rising to 0.235% at 1 watt. 5 watts brings 0.662%, 10 watts 1.61%, with 2.48% at 15 watts and 4.04% at 20 watts clipping sets in at 20.8 watts where the THD hits 5%.

I then hooked up some speakers and cranked on it for several hours. It's one of the best sounding amps I have heard.....I really want to do a side by side with a TSE-II.....

I did discover that the amp will exhibit a foldover situation when both channels are driven well into clipping. This was traced to my conservative settings on the current limiter in the screen regulator which feeds all 4 tubes. It would be easy to just turn up the current, but maybe two separate regulators is a better solution......time will tell.

w5jag 31st July 2019 05:43 PM

I hope Sherry is feeling better now, scary stuff ...

Really looking forward to trying this - sounds like maximum output is roughly about 85-90% the plate dissipation rating of the tube. Quite an improvement on the usual 20 - 30% ...

Will we need a screen voltage? Any advance information on power supply voltages, or other special stuff we will need to be prepared for this board, or is that too much of a reveal at this point in the development?

Mr_Zenith 31st July 2019 06:24 PM

I'm sorry to hear of Sherri's ordeal also, and I truly hope she's recovering well and that there won't be any lasting effects. You've both been through a lot.

I'm as excited as anyone else about the UNSET, but I can wait. Your family comes first.

Tubelab_com 31st July 2019 08:04 PM


Will we need a screen voltage?
The current test board has a screen regulator (zener stabilizer for those who like to argue the details) to derive the screen supply from the main B+. All other voltages are derived on board.

The typical board as I see it will require a transformer like the SSE, a HV winding with a CT, a 5V winding if a tube rectifier is used, and a heater winding. Some big fat sweep tubes need as much as 2.65 amps each (6LW6) and the two driver tubes need 300 mA each. Since there are LOTS of odd voltage TV sweep tubes out there the heater connection for the output tubes are brought out to a separate connector so that a separate transformer or SMPS can be used for them. Most of the usual driver tubes are available in 6.3 volt and lower heater voltages. The two heater connectors can be wired in parallel if 6.3 volt output tubes are used.


Any advance information on power supply voltages, or other special stuff we will need
After testing some amps with the UD board which require several power supplies, I gave some serious thought into simplification. The "dream" goal was to get TSE sound quality from a simplified power source and cheap tubes........300B = $100+, 25DN6 = $3, 36LW6 about $15. I really need to stuff one in there and see what I can get out of it on say 600 volts, if that much doesn't set an OPT on fire!

One of the next steps will be to hook up an Antek toroid.


sounds like maximum output is roughly about 85-90% the plate dissipation rating
Not really. Yes, the 25DN6 has a 15 watt plate rating. The data sheet was written in 1955. Tubes produced in the late 60's and 70's all used a standardized plate structure. The GE 25DN6's that are in there now have the same plate found in their 24 and 28 watt sweep tubes, which is larger than the plate found in a 6550. They just show a hint of red in a dark room at about 45 watts. Most of the RCA 25DN6's that I have have the same plate structure as their 6DQ5. I haven't tried them yet to see their true capabilities.

I would guess that this amp gets close to the theoretical maximum 50% efficiency, so I'm putting 40 watts into the tube to get 20 watts out.....that's when it is cranking out a pure sine wave at just below clipping. Turn the volume down to zero and the tube eats ALL of those 40 watts.....that's the true limiting factor in any SE amp that runs true class A.

I didn't mention this before, but as currently built there is no global feedback applied anywhere in the amp. Each stage has the local feedback loops needed to make triode curves from pentodes. Each stages operates completely independently from the other. I plan to pull the driver tubes, connect a push pull OPT to the output stages, and drive them with a UD board, but that's another project.

A global feedback resistor can be applied from either side of the OPT if desired. This shows a serious further reduction in THD in the simulation, but has not yet been tested in the amp with music running through it. It might help squeeze more sound through some budget OPT's due to its lowering of the output impedance. I still have a pair of Hammond 1628SEA's that might eat 600 volts and pass 30 to 40 watts............

Tubelab_com 1st August 2019 02:25 PM


I hope Sherry is feeling better now, scary stuff ...
She is doing much better now, thanks. This mess started with an unusual spike in her resting blood pressure, from normal to quite high, which drew the usual response from her cardiologist......up her meds. A couple weeks in Florida coupled with double doses of her blood pressure meds did not reduce the BP, it kept climbing. So upon return they added a diuretic, which along with some dehydration from the Florida heat caused her electrolytes to drop to "near coma" levels. 4 days of IV drips brought them and her back to a functioning, but not completely normal condition. The BP problem remains however.

On top of that our youngest grandkid, age 5, shattered his wrist yesterday. The fun continues.

Back to the UNSET.....I listened to it for several hours last night, still quite amazed by it's stunning realism, but in true Tubelab style, I had to mess with it's dead.

It started when I swapped the Transcendar OPT's for the big Hammond SEA1628's which have never sounded quite right when fed by the usual suspects, a 300B, or triode wired KT88, so would a triode emulating sweep tube do any better? The transcendars are 3000 ohms, the Hammonds are 5000 ohms, but could be wired for 2500. I chose 5000 for the first test.

I had seen the screen supply collapsing due to a built in current limit when driving it hard with music, but it seemed worse when I drove both channels into clipping with a 1KHz sine wave, so I did the usual Tubelab quick fix. I put a clip lead jumper across the sense resistor.

Power output went way up, but before I could record any numbers there was a bang, and some mosfet parts scattered on the workbench. The amp however kept on making power, so I believe that the mosfet is probably the only casualty, but my sense resistor had unsoldered itself, so the fet obviously needs a bigger heat sink. I'll get back to if after it cools off.

zman01 1st August 2019 04:50 PM


Originally Posted by Tubelab_com (
On top of that our youngest grandkid, age 5, shattered his wrist yesterday. The fun continues.

Mischief grand pa style? :)

Tubelab_com 1st August 2019 07:52 PM


Mischief grand pa style
Not intentionally. Apparently there were random clothes scattered on the floor in the room he shares with the 7 year old. He tripped on something and fell, whacking his wrist on part of the bed frame or a night stand on the way down. The same kid broke his elbow last year by crashing down the stairs. The other three kids have had no issues, so he must be clumsy, fragile or both.

Power has been restored to the UNSET board. The mosfet was scattered, but it was the only casualty. I had soldered a resistor in parallel with the sense resistor in my first attempt to boost power, which I subsequently jumpered. The resistor became unsoldered and the jumper which was connected to B+ left its mark on some small parts, but none of them were blown. A bigger mosfet and a 5 watt sense resistor have allowed for continuous operation well into clipping. Some quick knob turning seems to indicate that the big Hammond likes this circuit and it's HF response extends to beyond 30KHz.

I have also discovered that the distortion figured on my 8903A are meaningless random numbers when used below 20 Hz. The amp was measuring 0.5something percent THD at 20.5 Hz but transitioned to 40 something percent when the knob was bumped to 19.5 Hz. The scope display continued to show a perfect sine wave. More testing tonight maybe.

Zen Mod 1st August 2019 08:09 PM

fingers crossed for your wife health situation

regarding UNSET - subscribed , big green all around my head

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