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SE ATS25/807 two stage amp
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Author:  mwhouston [ 18 Jul 2010, 08:04 ]
Post subject:  SE ATS25/807 two stage amp

I'm after a schematic for a two stage SE ATS25/807 amp. I just recently bought some ATS25 tubes for AU$7 each.

Author:  Suncalc [ 18 Jul 2010, 18:50 ]
Post subject:  Re: SE ATS25/807 two stage amp

So what kind of an amp are you thinking about? Single ended of course, but triode strapped, ultra linear, or pure tetrode. From the data sheets it looks like the 807 really shines in the triode strapped or UL mode of operation. The compression of the plate curves toward cutoff is going to produce some pretty high distortions in the pure tetrode mode.

If you pick one, I'd be happy to take a look at the data sheets and see if I can come up with something. What kind of driving voltage were you thinking about for full output? iPods and CD players are generally about 1.5v to 2v peak so that's what I generally shoot for. An 6SL7 set up for a gain in the 14 v/v region ought to give about the right swing to drive an 807 in SE operation.

What do you think?

Author:  mwhouston [ 18 Jul 2010, 19:24 ]
Post subject:  Re: SE ATS25/807 two stage amp


I wouild like to go SE UL - it is sort of what I like in sound and design. Seeing you are offering I would like to drive the ATS25 with a 6SN7. I have just recently used a 6SN7 to drive KT120s and are very pleased with the result. I'm happy to develop the amp with you.

Author:  mwhouston [ 19 Jul 2010, 07:17 ]
Post subject:  Re: SE ATS25/807 two stage amp

Requires a serious look: ... c00764.jpg

Author:  Suncalc [ 19 Jul 2010, 20:37 ]
Post subject:  Re: SE ATS25/807 two stage amp

Well, here is the first cut at a two stage design.

I went and read the entire STC Application Report on the 807 (June 1954) and looked at the recommendations provided in that report. The report contains SE curves for triode and tetrode operation, but not UL. Normally under these circumstances I would pick something about 30% of the way from triode toward tetrode and see what I get. However, as luck would have it, the Automatica web site ( has some published curves for 807 UL operation. So I decided that I'd do a derived design from scratch.

The resultant design is trade off between supply current, power, and distortion. I wanted to preserve as much output power as possible without getting the AC swing into the compression region before tube cutoff. As such I decided to keep the operating point at a lower voltage and let the bias current run higher. As a result, I preserved almost all the output power of the tetrode mode, kept the 2nd harmonic distortion at about the same level but drove the 3rd harmonic distortion way down. The trade off is that the bias current is about 25% higher then in tetrode mode. This should produce a very smooth sounding amplifier with plenty of detail. See the attached operating curves along with the associated design data.

The bias point requires only a 20 volt drive so a 6SN7 makes a good choice for a driver stage. The circuit topology I chose is one I prefer for two stage SE amps. It is entirely auto biased with no feedback of any kind. It places a master gain control in between the two stages to keep the control noise out of the driver stage and also controls the frequency roll off points for the amplifier. All the important design information is included on the schematic. This design requires about 1.58v peak input to produce the full 6.3W output per channel. This level is well within the capabilities of most CD player and iPods so the overall dynamic range should be ok.
UL Plate Curves.png

Now for some specifics. I recommend the Edcor GXSE25-8-2.5k output transformers. These are a relatively expensive transformer but I am concerned about the bias current level (~103mA). I know that the GSXE10-8-2.5k can't quite handle that kind of bias current. The GSXE15-8-2.5k might be able to handle it but a chat with a design engineer at Edcor would probably be required to make sure. The low end roll offs due to cathode bypass are pretty benign. The low end roll off is dominated by the coupling capacitor and the volume control/grid leak resistor. I would have put in a bigger bypass cap to get better low end performance but I am concerned about bias excursions if the 807 is temporally overdriven. The RC time constant is 10.7ms giving about a 54ms bias excursion recovery time. This is about a third of a beat at 400 beats per minute (similar to a relatively fast guitar lick or fast classical flute solos). I am afraid that any larger a cap and occasional excursions would become noticeable at high drive levels. The high end roll off due to the 807 Miller capacitance shouldn't be a problem even at low volume levels. If it is a concern, the volume control can be replaced with a fixed 100k grid leak resistor, and then have the volume control moved to the input. The 5k grid stop resistors are recommended in the application report to prevent RF oscillation. This value should prevent any oscillation problems and also reduce problems with bias excursion.

The power supply requirements are a 300v B+ supply capable of at least 220ma. I didn't design a power supply yet because I didn't know whether this would be done in mono-blocks or in a single chassis. I also don't know what is the rectifier preference. Personally I like a big 5U4 in an amp like this. The 807 is a big tube and I think the other tubes should reflect that large size if possible.
NOTE: Please see this SE ATS25/807 two stage amp - SCHEMATIC WARNING!

So this is my first cut. Take a look at the overall design, poke holes in it, recommend any changes you think would improve the design, etc. This is just the first cut at a workable design so comments, suggestions, changes, etc. are more then welcome.

Let me know what you think.

Author:  mwhouston [ 19 Jul 2010, 21:27 ]
Post subject:  Re: SE ATS25/807 two stage amp

Suncalc - Haven't had time for a read - I must dash out for lunch appointment.

BUT a quick look at the schematic has put a smile on my face. I'll get backto you later today.

Author:  mwhouston [ 20 Jul 2010, 00:29 ]
Post subject:  Re: SE ATS25/807 two stage amp


You have my mouth watering. If you make any changes get back to me fast, the soldering iron maybe doing what it is good at. I like big time.

Pick an Edcor power tranni. Lets go tube rec. I have the OPTs already. Lets build this baby.

I've started on the 6AS7 amp so it may be a week or two. By then if you have found an improvement it will easy to impliment.

Keep me informed. At this point it is the schematic above.

Just a remninder the tubes I ahve are ATS25s. IS THIS OK. ANY CHANGES? Ceramic sockets and anode caps on the way.

Author:  Suncalc [ 20 Jul 2010, 09:16 ]
Post subject:  Re: SE ATS25/807 two stage amp

Everything I have found says that the ATS25 and the 807 are functional equivalents so I think that the design should be OK.

I'll pick out a power transformer and whip out an appropriate power supply design. I'll assume a single supply for both channels and size everything accordingly. The total supply load is about 1.36K so I think I'll look into some appropriate 2 to 4 Henry chokes for the supply. I should have something in a couple of days.

Just a caution, you might want to prototype one output stage before diving into the final amp. I think I've done the design right but I'd feel really bad if you got the whole amp built and them found that I made some major mistake. :( This just goes to my personal propensity for prototyping. (

Let me know when you start the build.

Author:  mwhouston [ 20 Jul 2010, 09:27 ]
Post subject:  Re: SE ATS25/807 two stage amp

Long story short. If I mock-up one channel the PS sees something diffrent to real ilfe.

Author:  Gio [ 20 Jul 2010, 17:35 ]
Post subject:  Re: SE ATS25/807 two stage amp

Hi Matt, nice modeling. OT, but what did you use to create the plots and schematic? They look great!

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