DIY Audio Projects Forum

SE ATS25/807 two stage amp
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Author:  Fernan [ 21 Aug 2010, 14:24 ]
Post subject:  Re: SE ATS25/807 two stage amp

mwhouston wrote:
Got some early snaps

With frustration, shock, anger, pain,......... more the 39°C here in Spain, I have not touched anything this afternoon.
I can take some snapshot

I looked on ebay, had 2 for sale but used, with description of 67% and 70% measure what I will not buy. I recalled also have a couple 5933/807W (possibly with much use), on the contrary, is a small and without beauty bulb.


Author:  mwhouston [ 22 Aug 2010, 13:41 ]
Post subject:  Re: SE ATS25/807 two stage amp

Those following this thread maybe interested in some of the latest posts on the SET thread:

Author:  mwhouston [ 22 Aug 2010, 13:44 ]
Post subject:  Re: SE ATS25/807 two stage amp

Suncalc: Are you aware of any other 6SN7/807 builds in the past? If so do you have a link?

Author:  Suncalc [ 22 Aug 2010, 16:21 ]
Post subject:  Re: SE ATS25/807 two stage amp

Unfortunately I don't know of any 6SN7/807 (or 6SN7/1625) SE designs floating around on the net at all. All of the designs of which I know are for 807 PP modulators and audio stages.

I think the oldest (and best known) is this one. Unfortunately this design has several features on my personal list of things to avoid. (PP output stage, global NFB, nonstandard LTP phase inverter, and way too many trimmers). This one appears to be better, but it still is not something that I think I would ever build. Here is a single ended 1625 design, but it uses a rather strange pentode drive stage instead of a nice simple triode. I just don't think there are a lot of people out there that really go for the SE design sound. Either that or everyone thinks anything under 10W per channel is not worth building. I don't know which.

Not that I'm biased or anything ;) , but I think our design is definitely the best single ended design I've seen yet.

Author:  mwhouston [ 22 Aug 2010, 19:30 ]
Post subject:  Re: SE ATS25/807 two stage amp

Suncalc: Thanks but no thanks. I'm sticking with your design. Still waiting on the power tranni. But as mentioned in an early post I can start some parts of the amp. I have a timber base put aside which needs a little work, I may do that also while wating.

You sound like you enjoy your SE equipment as do I. My SET (6AS7) is excellent-just love it.

Author:  mwhouston [ 23 Aug 2010, 03:41 ]
Post subject:  Re: SE ATS25/807 two stage amp

I went back and had another look at the schematics. I could warm to this:


Author:  Suncalc [ 07 Sep 2010, 18:25 ]
Post subject:  Re: SE ATS25/807 two stage amp - SCHEMATIC WARNING!


I have been looking at the design for this amp and I have a concern. I believe that the ATS25/807 may be at risk of thermal runaway using this design. I believe that the appropriate fix is the addition of a screen resistor of approximately 1.7kΩ in series with the transformer UL tap.

Please allow me to explain. The nature of UL operation is such that, at the zero signal condition, the screen voltage is slightly above the plate voltage. This happens because the bias current flowing through the output transformer primary resistance causes a voltage drop between the B+ tap and the plate tap. Because the screen tap is typically 40% of the way "down" the potential well from the B+ supply, the resultant voltage at that tap is higher then the voltage at the plate. For some power tubes this is acceptable so long as the transformer resistance is relatively "small" (with the definition of "small" depending on the bias current, and tube construction). Tubes such as the KT88, 6L6, 6V6, EL34, EL84 etc. can deal with the few volts differential without undue heating.

However, the ATS25/807 is an RF transmitter tube. In this tube, the location of the screen is relatively close to the control grid. This is why in tetrode operation, the max plate rating is 600v and the max screen rating is only 300v. If the screen potential is too high relative to the cathode/plate E-field potential at it's location, the screen dissipation will go up, its ability to modulate the plate current flow will be lost, and the tube will go into thermal runaway. Therefore it is imperative that the screen potential be kept low enough relative to the cathode/plate E-field potential at it's location that the overall tube function is not impaired.

Now, the calculations for the schematic have a 97ma plate current and a 6.8mA screen current as calculated from the published conditions in the STC807 report. I have estimated the output transformer primary resistance at 200Ω (This is the actual resistance of the GXSE10-8-5K, but since I don't have the data for the CXSE25-8-2.5k, which should be lower, I specified what I had). This means that the plate-to-screen differential voltage is 0.6*200Ω*0.097A=11.6v. Assuming the 6.8mA screen current at zero signal, this implies that a resistance of 11.6v/0.0068A=1.7kΩ will drop the necessary voltage to bring the screen back down to the plate voltage level. I don't believe that this level of screen voltage suppression will have a perceptible effect on the UL operation of the tube.

I have attached the modified schematic below. Note that if the actual primary resistance of the transformer is different, the formulas above can be used to calculate the appropriate size of the screen grid resistor.
Modified Schematic.png

This whole concern may be the product of my overly developed sense of paranoia but I believe that a little caution up front can prevent much cursing later on. Please let me know when you've seen this post.

Author:  mwhouston [ 07 Sep 2010, 20:27 ]
Post subject:  Re: SE ATS25/807 two stage amp

Suncalc: Noted. I have printed the updated schematic. STILL waiting trannies from Edor but have had shipping notice of late so hopefully soon. I'm going to use one of my full size wooden bases and I may do a polished copper top plate on this-just for something different. Err, screening of course.

Author:  Gio [ 07 Sep 2010, 20:45 ]
Post subject:  Re: SE ATS25/807 two stage amp

Hi guys, I put a note under the first schematic that points to the revised one so there are no surprises.

Author:  mwhouston [ 14 Sep 2010, 06:39 ]
Post subject:  Re: SE ATS25/807 two stage amp

Suncalc: The news we have all been waiting for (well OK maybe just me) is the Edor power tranni you suggested has landed. In fact I have it unpacked and sitting beside me. I have always had the OPTS and from a local Hammond supplier I purchased the choke some months back.

At the last minute I have decided I want the top plate of the amp to be polished copper. I also want to have an Al plate under that. My farher-in-law makes my Mirbeu bases. To accommidate both the copper and Al top plate the routing at the top of the base has to be deepened. Therefore it may be another week or so before I can start the layout.

I name all my amps and preamps etc. One of the brand names I work under is "retro-thermionic". Each series of amps is generally based on a tube or a standardised form of construction with maybe different power tube selection. Each series bears a "wine style" name. All amps are SE.

The first amp built was the "retro-thermionic Merlot". Which supports a number of power tubes and different top plate colours. It is my intention to build basic and in-expensive amps in the Merlot range and move through other "wine style" named amps to the Shiraz types. The Shiraz will be the biggest and most powerful SE amps I will build in this series. The "retro-thermionic Shiraz "Black"" is based on the KT120 tubes which I used in my BeBamp (bread board amp). I also just recieved the power tranni to build it.

The ATS-25/807 amps fall into what I have decided is the Cabernet series. There maybe only one. In this case the model is the "Luciano". It will have a remote control attenuator. So the full name of the amp is the "retro-thermionic Cabenet "Luciano"". It is obvious where I got the name from. And yes I dedicate it to Luciano Pavarotti. If I like the amp and I purchase more 807s (Sinos for $25) I may make some variates.

Finally I can move ahead on this project.

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