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Hello ottoamp --------- 811A - A2

74.109.62.81
Posted on March 23, 2011 at 19:10:24
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011



Hello Everybody and in particular OTTOAMP,
I just got on this list, because this is where I re-found the schematic of ottoamp's 811A amplifier. This schematic was stuck in my memory-banks a long while, but I could not find it on my computer.
Well I happy found it.
I have this question if in the time since 2006 it was necessary to change something on this for its simplicity so beautiful schemo.
I have nice NOS 811A, NOS 6BG7 and wanted to make this my next project.
If possible I like to get close to 20W output. My PS transformer allows for up to 700V B+
It was an 811-10 amp that started my phase of amp building in the later 90ies, but I cannot imagine the 811A will be my last (even so I got more amps than needed - well usually)
Is this A2 811A amp still with you and on duty?
Would love to hear about all your experiences with it.
Cheers
herb

 

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RE: Hello ottoamp --------- 811A - A2, posted on March 23, 2011 at 19:20:53
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011
 POST SCRIPT
I did print out the thread from February 2006 with ottoamp's explanations on choosing tubes and adjusting in bias etc.
Also wondering about using a 26 as input tube with DC on the filament. Amplification and current is pretty similar, bias voltage will be somewhat less. I could get me the 6V7G, but also thought about 7A4.
herb

 

RE: Hello ottoamp --------- 811A - A2, posted on March 24, 2011 at 13:06:21
Neff


 
So, the 811A grid that must be positive at low voltage pulls 20-30 mills of grid current to have the 6BG6 (6L6) function?

 

RE: Hello ottoamp --------- 811A - A2, posted on March 24, 2011 at 14:58:54
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011
 Thank you for writing, you are right. The positive grid gets the electrons going at low plate voltages. What worries me somewhat is the audio-AC voltage swing to drive the 811A. From reading old posts I hear that the CURRENT swing, that is set up between the driver pentode and the grid, is what matters. Still current swing goes along with a voltage swing and I think to cross zero volts on the grid is not desirable.
Because of my Lundale OPTs I am limited to 80mA anode current. That maxes the 811A for its 46Watt plate dissipation at 575V on plate in idle. This brings the grid to about 20V and about 20mA at idle, according to the data sheet curves.
So it may be I need to get OPTs that can do 120mA, so I can bring down B+ and bring up grid voltage and current.
Seems weird, coming from a class A world thinking, but fun to figure out.
So here is where I would like to hear more what experiences diyers made.
herb

 

Are you certain that this has been built sucessfully?, posted on March 24, 2011 at 15:08:47
Chris O
 Running the 6BG6 with only 160 volts across it and 5 watts on the plate and very little voltage swing on the cathode seems dubious to get hardly any power out of the 811A. You need a much more linear voltage swing and current from that driver, IMO. That 811A grid takes close to 6 Watts of driving power in A2.

I will have to admit that using the grid of the 811A as the cathode resistor on the 6BG6 is pretty cool, but you are really locked in to that low draw on the driver.

If this has been built and puts out anywhere close to 5 watts, hats off.

 

RE: Are you certain that this has been built sucessfully?, posted on March 24, 2011 at 15:20:24
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011
 ottoamp did it.
ottoamp posted here and there is an extended thread of pics and discussion in February 2006. He did not say much about output power.
ottoamp is not alone. The pentode-grid CF drive has been done by a handful of fellow builders and I think I read somewhere of close to 20Watt output power as well as great sound.

It is the cool schematic and looks of the tubes chosen, that make me itch to go ahead.
herb

 

This will definitely work - more like 7W at 400V, posted on March 24, 2011 at 16:04:28
Jim Doyle
Audiophile

Posts: 1180
Location: Medford, MA
Joined: July 8, 2001
You might also choke loading the cathode follower driver. You could
also consider a CCS in that role too, but you'd need a stiff negative
voltage supply to make a CCS work properly.

At 700V, you could do an 805 in Class A2 without much problem. You
could also do a 211 in zero-bias mode - which means Class A2 on positive
signal swings, and A1 on negative voltage swings. 211 in zero bias will have
a widely varying drive impedance. Sounds like a design challenge and fertile, unpaved DIY ground to do something not done before.

-- Jim

 

RE: Are you certain that this has been built sucessfully?, posted on March 24, 2011 at 16:07:35
Jim Doyle
Audiophile

Posts: 1180
Location: Medford, MA
Joined: July 8, 2001
 I listened to this on a breadboard for a couple of months while
fabricating a permanent chassis for another amp.. It was very nice ;
and very simple - nothing expensive or complicated...

Does even better in parafeed.

-- Jim

 

"...looks of the tubes" , posted on March 25, 2011 at 05:54:37
Triode_Kingdom
Audiophile

Posts: 10390
Location: Central Texas
Joined: September 24, 2006
 "It is the cool schematic and looks of the tubes chosen, that make me itch to go ahead."

In that case, it might be fun to drive the 811A with a ST-style 6AS7. Regardless of the exact tube, I think linearity would improve if the driver were choke loaded. I also think this amp as shown in your first post has insufficient gain to be compatible with most systems. Two stages of amplification would likely be needed ahead of the driver.

 

RE: Hello ottoamp --------- 811A - A2, posted on March 25, 2011 at 07:38:49
Davin Carter


 
I have built an 811A amplifier like this. I would definately do it again. You should be able to get at least 15W with a 700V supply. I moved on to a 211 amp just because I had to build something new.
One difference, is I used a 5763 tube for the driver. An el84 would be about the same. I used a 6sl7 for the driver.
I do not know what voltage the 6bg6 has on the plate. You need to see what the maximum grid current required by the 811 and make sure the 6bg6 has enough voltage to pass that current. Plus add the eak grid voltage on the 811.

The 811 is fairly cheap and should be able to do 15 W class A2 for alot of hours.

 

RE: "...looks of the tubes" , posted on March 25, 2011 at 07:48:30
Chris O
 You can put all of the gain stages you want ahead of the driver, but the design is still limited by the 25 v bias over ground for that cathode follower. The AC voltage swing on the cathode on the 6BG6 can't go below 10 volts or so on that cathode without running into big non-linearities. It is stuck with PP 15 volts for the AC drive.

The 6AS7 would be a better choice, but you run into that 35mA problem and you still don't have any relief from the AC cathode voltage swing.

 

RE: Hello ottoamp --------- 811A - A2, posted on March 25, 2011 at 08:57:28
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011
 Thank you for your encouraging comments. I believe I will need a heavy duty pot and breadboard the circuit to find the operating point for the 6BG6. Will also try an SE-OPT primary as cathode load choke.
Will report back here what I found working well.
herb

 

RE: Are you certain that this has been built sucessfully?, posted on March 25, 2011 at 09:10:00
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011
 Thank you Jim for sharing your experience and the great pics of your work plus schematic. I have a collection of parts on hand to start bread-boarding and will also try another SE-OPT as a cathode load choke. Right now I don't want to strain the budget with a pair of 805 or 211 and the 811A setup will provide hours of good times figuring it out and then tube rolling for first stage and CF driver.
Will let you know here if I get somewhere with it.
herb

 

RE: "...looks of the tubes" , posted on March 25, 2011 at 09:18:58
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011
 Yep 6AS7 is one I thought of to try once I get into driver tube rolling.
The small space define by 25V and 25 mA worries me too. Just all depends how much impact the current swing has on top of the voltage swing to modulate the electron stream. It worked for some people like Jim Doyle and I'm not afraid to run my trials.
herb

 

lessons learned, posted on March 25, 2011 at 11:33:29
Jim Doyle
Audiophile

Posts: 1180
Location: Medford, MA
Joined: July 8, 2001
 At 400ish plate volts, an a 24V grid bias - the grid supplies 24mA of
current. So, the driver needs to load down a 1000 ohm impedance ; which
means you want a driver with at least 300 ohms source impedance (!!!).

With a cathode follower, a high-gm tube will present a driving impedance
of 1/gm. So, if assuming a minimum desired 300ohm source ; shoot for S (gm) larger than 3. Looking at the tube database, 6V6 S=3.6, 6W6 S=3.8, EL84 S=11, EL34 S=9, 6AS7 S=7, 6BX7 S-7, So, no surprise the 6V6/6W6 are regularly used because they do the job. However, you have to keep in mind the Va max of these tubes because you are using them as a cathode follower! For that reason, the EL84 and the EL34 are actually the better drivers tubes because not only do that the highest gm out of the limited set of choices, but they take take the full anode B+. Of course, there are other choices out there - and like you mentioned - hunting down big beefy sweep tubes will probably win the game for all three parameters of max gm, max peak current, max anode voltage rating.

If you run the 811A at higher plate, the job of the driver becomes easier because the grid impedance rises. You need more voltage swing on the grid, but on average less grid current to manhandle.

I remember seeing BIZARRE things on my scope and FFT when playing with the 811A. For instance, at 8W output, the FFT showed only D2 components and higher harmonics were down in the grass (close to the detection limits of my shitty gear). HOWEVER - when I put the FFT analyzer onto the grid pin of the 811A, I saw a whole series of 60Hz harmonics (60, 120, 240, 480) that were way out of the grass. These were intermodulation productions against the 60Hz AC filament supply of the 811A - and they weren't showing up on the plate, but were very prominent on the grid. Crazy.... Physically this makes sense - because on the 811A the grid cage is so close to the filament that when it's pulled positive, the electrons are stripped from the region around the filament and you can actually measure the change of cathode current when the AC line voltage crosses 0V at 60Hz.

FYI - I used AC filaments... I had noticeable hum on 98dB eff speakers, and was able to trim the hum to "tolerable" background only after amending the 100 ohm 2W filament pot with two 150 ohm resistors on either side.

-- Jim

 

RE: lessons learned, posted on March 25, 2011 at 20:41:41
7Herb
Audiophile

Posts: 15
Location: Philadelphia
Joined: March 22, 2011
 Hi Jim,
Love your report on your findings and bringing me back up to speed on THE parameter, buy which to choose the driver tube.
I also emailed with Douglas Piccard and he brought up the schematic of the Altec 1570. It is PP but fully balanced in the driver and output circuit.
However B+ is 930Volt, where the grid is likely at zero and the audio swings around zero - wonder what distortions that brings. Also thinking of using 3D21A as driver, (because I have two), a smaller transmitting tube that has no problems with high potentials. A SE-OPT will be doing choke service on the breadboard. Have do do some mechanic prep work over the weekend to get some tubes fired up later.
Thanks a million for your suggestions and advice! I appreciate it.
herb

 

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