What do you think of this? (211 A2 SE)

[Giaime]

Hello everybody!

I just finished designing a 211 A2 SE amp, just for the curiosity. I'm not planning to build it: the objective was taking more experience and familiarity with mosfet drive and A2.

What do you think of the design, is there anything obviously wrong, or something that could be done better? The circuit is explained in the article linked.

Also I'm not too sure about the setting of the quiescent current in the mosfet, as you can read in the article I made a supposition about how it works.

I've uploaded it in my site:
http://www.giaime.altervista.org/hivolt.html

Feel free to make any comments!

Please note that this is not a project suited to beginners due to high voltage

[Eli Duttman]

Giaime,

I LIKE FETs serving as "spear carriers" for tubes; so, you'll get no argument from me. Perhaps MOSFET Follies contains information of use to you.

A "stiff" B+ rail seems indicated. Think about a hybrid bridge rectifier using stacked sets of UF4007s and 2X 6AU4GTA damper diodes feeding a choke I/P filter. Given the SLOW B+ rise, you could forego the use of a bleeder resistor across the 1st filter cap. 211s heat up and conduct FAST.



Edit: fixed typos

[kevinkr]

Just wondering what dictated the choice of an ECC83 mu follower? I haven't found this topology to sound particularly good with this tube. Looks great on paper, in practice lousy bandwidth and very slow sounding.

Also the gain will very closely approach mu, making this amplifier excessively sensitive unless some global feedback is employed. (No, I'm not advocating this..) Off the cuff about 350mVrms would drive this thing to full output.

If it were me I might go for the 6SL7, probably in SRPP, full output power would be achieved at a little over 1Vrms.

[Giaime]

Hello Kevin!

In fact, gain is pretty high, around 85, and it needs 1.2Vpk to drive the 211 to full output. Frequency response is artificially limited due to 6.8k grid resistors, to have a -3dB point of around 150kHz.

And obviously yes, I would use global NFB, as explained in the article, for a factor of 6dB.

That's not hard to implement a 6SL7 in that stage, neither any other high gain tube, maybe we can use the full 6dB feedback only using ECC83.

Edit: further investigations show that only with ECC83 you could use feedback: otherwise, you need a very thought preamp that puts out hefty swings

I'll redraw the amp schematics adding nfb.

[Tubes4e4]

Hi Giaime,

IRF840 has a horrendous input capacitance of 1300pF, so you might check if the peak current swing of your tiny current trickler ECC83 mu follower really is able to drive it to the required bandwidth. I doubt it.

You might consider IRF820 instead, which has only 1/3 of input capacitance (about 360pF).

Tom

[Giaime]

Hi Tom,

good point, and thank you very much, I have to dig into datasheets. But since it is a source follower, the input capacitance that datasheet says is still applicable?

I ask because in the article "Mosfet Follies":

"But wait!" I hear you yell. "The MOSFET has a big gate-source capacitance. Won't that suck all the high frequencies out of the signal?".
No, it won't. The big gate-source capacitor is there, OK, but when the device is hooked up as a follower, the apparent capacitance seen by a circuit driving the MOSFET gate is reduced to a very small level by the fact that the source is following the gate almost perfectly, so the apparent capacitance is reduced by the local feedback to an inconsequential level. Even a high impedance 12AX7 plate is not affected in the audio range by this capacitance. Even the big power, big capacitance MOSFETs do this one OK. I have done this, and what's more, done the necessary measurements to find the effect on the response of the composite stage - and there is no detectable change in frequency response below 30kHz. None.

So?

[Giaime]

Updated schematic...

[Eli Duttman]

In the case of a source follower the gate capacitance is not much of an issue. HOWEVER, the reverse transfer capacitance does matter. The reverse transfer capacitance of the IRFBC20 is a low 8.6 pF. The IRFBC20 data sheet here.

BTW, if you CCS load a 12AT7 section, you will get lots of linear gain and better drive capability than the high Rp/high mu triodes offer.

[Giaime]

Yes, I see. Thank you very much: however IRF820 has a reverse capacitance of 40pF, I don't think this will be an issue. Given the similar rating, surely one could use the IRFBC20 instead.

About the CCS-loaded 12AT7: surely, but there the gain will be no more than the mu of the valve, instead here I need at least 38-39dB, not 35, to employ negative feedback.

Three ways:

1) ECC83 mu follower with nfb,

2) 12AT7 CCS loaded (or even totem pole)

3) two simple grounded cathode stages, in cascade. There should be substantial harmonical cancellation, however to tweak this and to evaluate the designs one should prototype the whole amp (or at least the input + mosfet stage).

I don't see other options, because I don't generally like cascode circuits nor interstage transformers.

[tubelab.com]

Been here built that! Look at the 845SE on my web page.

http://www.tubelab.com/845SE.htm

Look at the second schematic on that page.


I started out building an Ongaku clone, and after several itterations wound up with the amp that you see on that page. The power supply makes a BIG difference in the sound. I have done two power supply designs, and I am still not happy with it. I have a power supply from an old Motorola police radio base station that puts out 1500 volts at 1/2 AMP! It gives the best sound, but weighs 120 pounds and the transformer buzzes loudly.

I use mofet drive. I wanted to be able to drive 211's and 845's interchangeably which requires at least 200 volts peak to peak! I have since used this circuit to drive an 833A, which can draw grid current of over 100mA. You can't get this from a single stage. Even if you could, I don't think that an ECC83 has the slew rate capabilities to swing near this much voltage at say 50 KHz. For that you need current. I think you might get there with the 12AT7 at about 10 mA (or both sides in parallel at 20mA). I have been using them (CCS loaded) in my latest amps because 5842's have become scarce and expensive.

I have called the CCS, mosfet drive combination PowerDrive. I spent a couple of months experimenting with this when I first figured it out. I thought about the large capacitances associated with power mosfets (gate - source and reverse transfer) and didn't think that they apply in a source follower. There is however a significant gate to drain capacitance. This is not always specified. I work in a building full of CMOS IC designers, so I picked a few brains. I was told that this capacitance is not constant, and depends on the "channel depth" and "channel width". These are big words that are related to the voltage and current across the device. OK, you just can't look this up on a data sheet, it depends on the application. So, I ordered a bunch of high voltage mosfets and set up a test fixture. I drove them with a CCS loaded 5842 and measured the frequency response. I wasn't smart enough to document everything, but I did find that the Toshiba 2SK2700 has no problem hitting 500KHz at 100 volts P-P. I use this one in all PowerDrive designs. It can take 900 volts.

I don't see other options, because I don't generally like cascode circuits nor interstage transformers.

Just a suggestion, but use 1/2 of a 12AT7 CCS loaded for the first stage. A low Mu triode or triode connected pentode that can handle a plate voltage of 500 volts. I would try a 6BL7, 6BX7, or a triode wired EL-34. Use a CCS load and mosfet followers between each stage. In other words copy my design without the expensive tubes. It works with 211's AND 845's only by adjusting the bias.