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Old 12th June 2013, 05:52 PM   #271
rdf is offline rdf  Canada
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Love the 6E5P but man was it microphonic. The application was a preamp with a step-down output transformer so it might not be an issue in your application.
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Old 12th June 2013, 07:28 PM   #272
Magz is offline Magz  United States
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Originally Posted by hpeter View Post
how much Vp/Vrms this driver gives ? @1V input/max
6E5P mu is about 30, so 1Vp in gives 30Vp out. That's plenty considering the 833C has a mu of 35 itself, it's not hard to drive. For a 2V input, 2*30*35 = 2100V.
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Old 12th June 2013, 07:33 PM   #273
Magz is offline Magz  United States
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Love the 6E5P but man was it microphonic. The application was a preamp with a step-down output transformer so it might not be an issue in your application.
I've dealt with the 26 in my preamp, which can be very microphonic. If I run into issues with that I'll look into mounting the tube socket in a vibration-isolating manner, that seemed to work for the 26.
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Old 21st June 2013, 04:04 AM   #274
Magz is offline Magz  United States
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While still waiting on the OPTs, I made up two Vref boards for the driver circuit using the cascaded 10M90S chips feeding a 47K resistor, and upgraded the adjust pot from a Bourns to a Vishay metal foil with a 10ppm/degC tempco. I also put the 2N60 mosfet follower on this board, along with its associated gate stopper and zener protection diodes (picture 1). In this way I can mount the board with the 2N60 using the chassis as a heat sink and save some space while shortening the wiring considerably over the previous version. A small hole in the side of the chassis will allow for adjustment as needed without cracking open the amp. I then tested it in the circuit and all went well; voltage at the 2N60 source (and thus at the 833C grid) rose 2V or so as it warmed up for the first 10 minutes, then became rock solid. Sines and squares look excellent.

So, this simple Vref works much better than the shunt regulator for setting the 2N60 gate voltage, but I still need a stable shunt reg under the 833C...what to do? I contacted Kevin at K&K Audio and after a few emails back and forth he kindly sent me a tiny upgrade board for his shunt regulator that contains a temperature stabilized Vref - I'll be wiring that in and testing it this weekend. Stay tuned.
Attached Images
File Type: jpg 833 Amp Voltage Source - Mosfet Board.jpg (278.9 KB, 936 views)
File Type: jpg 833 Amp Voltage Source with input board.jpg (333.2 KB, 924 views)
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Old 23rd June 2013, 02:19 AM   #275
Magz is offline Magz  United States
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The improved, temp compensated Vref from K&K is definitely a better fit for this application when installed in the shunt regulator. After drifting downward steadily (~3V total) during warmup, the voltage was now steady within +/- 0.1V as opposed to fluctuations of up to 1.6V using the non-compensated, simpler stock Vref. Much better for holding the 833C bias steady. Of course the true test will be in the functioning amp itself.

Picture 1 shows the small Vref board installed on the main board, and picture 2 shows the traces I had to cut and the resistor and cap I needed to add in to get the rest of the circuit compatible with the upgrade. I still need to add in a high quality resistor and fine adjustment pot to R9 on the board (on order), for the test I just used an old 1M pot I had to dial in the voltage. The big TO-247 shunt mosfet is mounted off the board so it's not in the picture.
Attached Images
File Type: jpg 833 Amp Shunt regulator new Vref.jpg (241.4 KB, 833 views)
File Type: jpg 833 Amp Underside of shunt reg board.jpg (257.5 KB, 824 views)

Last edited by Magz; 23rd June 2013 at 02:21 AM.
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Old 23rd June 2013, 06:51 AM   #276
hpeter is offline hpeter  Europe
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Old 25th June 2013, 07:43 PM   #277
Magz is offline Magz  United States
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Received a snapshot today from Monolith, this time the powder-coated OPT cans. Next they will pot the OPTs in the cans, let them cure for a few days, and ship them to me, along with the first power supply chokes for the 833 B+.

Anticipation...
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Old 26th June 2013, 01:56 AM   #278
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Originally Posted by Magz View Post
Received a snapshot today from Monolith, this time the powder-coated OPT cans. Next they will pot the OPTs in the cans, let them cure for a few days, and ship them to me, along with the first power supply chokes for the 833 B+.

Anticipation...
Look really small though,

...I meant the tube next to those transformers
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Old 26th June 2013, 08:05 PM   #279
hpeter is offline hpeter  Europe
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Old 14th July 2013, 10:40 PM   #280
Magz is offline Magz  United States
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Originally Posted by Magz View Post
The improved, temp compensated Vref from K&K is definitely a better fit for this application when installed in the shunt regulator. After drifting downward steadily (~3V total) during warmup, the voltage was now steady within +/- 0.1V as opposed to fluctuations of up to 1.6V using the non-compensated, simpler stock Vref. Much better for holding the 833C bias steady. Of course the true test will be in the functioning amp itself.

Picture 1 shows the small Vref board installed on the main board, and picture 2 shows the traces I had to cut and the resistor and cap I needed to add in to get the rest of the circuit compatible with the upgrade. I still need to add in a high quality resistor and fine adjustment pot to R9 on the board (on order), for the test I just used an old 1M pot I had to dial in the voltage. The big TO-247 shunt mosfet is mounted off the board so it's not in the picture.
So I replaced the 1M carbon pot in the modified shunt regulator with a 220k Vishay bulk foil resistor plus a 50k Bourne Cermet pot, since I suspected that the lousy temp coefficient of the carbon was not optimal - wow, was I right! Now the shunt reg stabilizes to the set voltage within 15 seconds, and stays there without moving at all. That problem is solved, now I just need to see how it holds up while sinking the 833C cathode current.

Monolith has informed me that my OPTs and HV PS chokes are ready, but unfortunately I'll be traveling next week so I won't be able to pick them up at the airport until the week after. Then it's time to put this bad boy together!
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