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Old 24th March 2010, 04:40 AM   #411
AJT is offline AJT  Philippines
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the zeners are to be closest to the mosfet gates after the gate stopper resistor...this is standard practice..
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Old 24th March 2010, 03:30 PM   #412
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I've read that they start regulating about there, but don't freak out if it doesn't. It should be regulating by 10mA I think.

I'm sure Tony is right and they should be tied directly to gate, but I have found it difficult to solder two things directly to the pin of a mosfet that is not on a circuit board. Gets messy. If you can make it work, do it.
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Old 24th March 2010, 03:50 PM   #413
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I had the recent experience of blowing these FETS in a similar set-up. The zeners should tie directly to the gate after the gate resistor. Or use a FET with the diodes built in. i.e. 2SK3563 or a 2SK3564 for higher voltages. IMHO I would think the later two solutions would provide the least capacitance and is certainly easier to implement in a P2P.

Based on a solution that I used in my design, I reduced the resistance of R54, and R55(per your schema). If you took these to 250k, you would reduce (not eliminate) the chance that the gate will exceed its limits wrt the source duing the power up and power down cycle. In any case you would be less reliant on the zeners. I don't know what it would do to your gain structure however.
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Last edited by SGregory; 24th March 2010 at 04:02 PM.
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Old 24th March 2010, 10:32 PM   #414
chrish is offline chrish  Australia
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Thanks SpreadSpectrum and others. I have redrawn the schematic with back to back 15V zeners to gate. I will be constructing a mini-board with perf board for these, so I should be able to go straight to the pin.

Thanks SGregory. I might leave the values as designed for now, but if I start blowing MOSFETS, I will take your advice. Had a look at your design. Looks nice, some similarities here... I like your build too, looks well laid out and tidy. I note you have CCS for the MOSFET followers. I did not see the discussion regarding the benefit of their inclusion. Any chance you could spare a few words regarding the expected benefits?

Thanks again everyone for your continued support!

Chris
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Last edited by chrish; 24th March 2010 at 10:35 PM.
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Old 1st April 2010, 05:59 AM   #415
chrish is offline chrish  Australia
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Work has begun...

Spent a few hours going over posts today, as I could not remember why some values of components were chosen. I have reviewed the VR tube supplies, I think I mis-calculated the current limiting resistors there. Also looking at the anode load resistors of the LTPs, they require some high wattage types. I am not sure if I have the right types for these. Murphy says you discover these things at 5pm on the day before Easter, won't be able to get parts till tuesday next week.

Chassis construction has begun. The top plate is 7mm thick, so I am attempting to mount the tube sockets from underneath and tapping the thread for the bolts directly in to the top plate. Hopefully this will have the top plate free of nut-heads showing.

Had an issue drilling the holes for the tubes, I had to get a bi-metal 27mm hole saw. Tried it on a scrap of perf-board and it made the right size hole. After drilling the top plate, I find that it is not perfectly round and it is drilling a 28mm hole. The valve bases will hide the sloppy fit, but I am not happy. Ended up buying a 27mm Starrett 27mm bimetal hole saw for the second chassis. Hopefully it will work out better.

Here are some shots of how the chassis layout looks. Heatsink for the Maida regs is on the back plate.

Note, photo is of top plate only, it is sitting on an MDF jig I made to hold in place when inverted for easier wiring etc.

Cheers,

Chris

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Old 2nd April 2010, 05:37 PM   #416
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Looks nice. Heatsinks look good. Is that 7mm thick aluminum, then? That thing is going to be a tank!
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Old 17th April 2010, 12:45 PM   #417
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Between domestic issues and university study, I have managed to spend a little time on this. I am only building one at the moment, after the inevitable trouble-shooting will build its twin.

I have completed the heater wiring, mounted the transformers and built three small perf board sub-assemblies. The one at the rear is, not surprisingly, the power supply. 400V regulated, +150V and -150V. The mid board has the MOSFET followers and bias adjust, as well as a couple of the load resistors for the second diff amp. Front board has the CCS and load resistors and power supply decoupling cap for the first diff amp stage. CCS and MOSFETs are attached to the (7mm thick) top plate for heatsinking. I was going to attach to the sides of the chassis initially, but wanted to go for a no screw head showing look and side walls were not thick enough for a blind tap.

Have not tested the power supply yet. As it is significantly more complex than previous power supplies I have built, I wanted to have it attached to the chassis and heat sink and have the mains supplied from a proper IEC connector with fuse. This is for safety and to try to avoid screw ups and subsequent smokey moments with dodgy alligator clip setups.

Hope to have the mains supply and switch connected up tomorrow. Fingers crossed I did not screw up the point to point wiring of the regulated supply. It was a bit of a pain in the neck to make on perf board. I should learn how to design and fabricate printed circuit boards!

Will keep posted on further updates.

Cheers,

Chris

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Old 18th April 2010, 11:55 AM   #418
chrish is offline chrish  Australia
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Did the mains wiring today. The +/- 155 volt supplies are working, yet to be connected to their respective voltage regulator tubes. The regulated 400 volt supply is putting out around 445 volts. There is no load at the moment, so their is just the 8-10mA through the set resistors, plus a mA or two through the heater reference lift voltage divider. I see Spreadspectrum's advice above to not worry if it is not regulating with no load. At least there was no smoke! Voltage at the second power supply capacitor after the diode was 575 volts, so it is dropping about 130 volts with about 0.02 VAC showing on my Fluke DMM. Will see how this looks with some load.

About to go away with work for a few days, then have a couple of days off before a licence renewal exam. Hopefully will get some of the signal wiring completed and post some results.

After a very long time I am making some progress! Thanks for all the help along the way guys!

Chris
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Old 18th April 2010, 04:03 PM   #419
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Lookin' good! Nice metalwork too.
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Old 20th April 2010, 07:06 PM   #420
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Quote:
Originally Posted by N1ESE View Post
The '2' means that the grids ARE driven positive and grid current will flow. An output stage in which there is grid current such as AB2 or B must be transformer coupled instead of R-C coupled. If R-C coupling IS used the grid behaves like the plate of a diode and rectifies some of the audio signal which makes the grids more negative. Instead of class AB2 you would have a kind of moving class AB1 and a lot more distortion than you had counted on. If you were to attempt class B with R-C coupling, the extra grid bias could actually bias the tubes beyond cutoff and lead to heavy distortion. The rise in average, DC, plate current may be anywhere from 50 % to 200 %.
A good point, but I was wondering if one could have their cake and eat it too to some extent here by keeping the grid bias resistances on the low side (say 22K) and using large grid coupling capacitors, say 10uF. I'm thinking that this approach should allow a few volts swing into positive grid territory while the bias shift resulting from prolonged high level outputs would reduce output stage heat dissipation at the cost of a temporary mild rise in distortion. If properly matched to the output transformer/load, this bias shift might even allow somewhat more continuous maximum power output from the amplifier than a conventional fixed bias approach (either AB1 or AB2 for a given quiescent bias) without overdissipating the output tubes.
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