6L6GC AB2 Amp

[kenpeter]

Originally Posted by chrish

Well, as for the oscillation, I tried just about everything.

Well hard to say what stopper best stops a lumped parasitic oscillation?
Those that interact locally, only within the circuit itself... A resistor is
not going to "stop" a phase margin problem or design error of that sort...

But if its interacting with free space (radio), thats much easier to say!
Mono and dipoles will have an impedance between 25 to 75 ohms cross
the feedpoint, and maybe 450 to 800 ohms (to GND) at far open ends.
Impedance of free space is 377ohms. I don't know if that last figure is
actually relevant in this argument?

So the point I make: is that a 1 ohm stopper is not going to significantly
spoil the efficiency of a parasitic antenna stub, nor is 10K doing anything
but breaking the problem into two separate resonant stubs... Capacitively
coupled right across the stopper from one High Z stub end to another...
If the stop resistance is not between 25 < R < 600, I don't think it stands
much chance for spoiling the Q of the stub or killing RF.

If higher resistance has to be used for some other reason, then stick a
smaller value in series on either side to spoil the two new child stubs.
A string of small value resistors is probably more effective than any one
large lump resistor of equal sum.

[chrish]

Thanks Kenpeter. I think my problem was the dodgy leads that I made up for my test setup. Everything appears OK now. I will try monitoring the output when hooked up to the main system again just to be sure that I don't have a systemic problem with the amp layout.

Tubelab, thanks again for your support and assistance. I will hook up balance adjustment as you describe. The trim pots in my parts bin are 10 turn types, so no hair trigger on adjustment. I think I will also try reducing the plate load resistors in the diff amps to up the current in them to about 7mA each. Thanks for the tip regarding the software, will investigate!

Also, when initially looking at design setup, I thought 400 volt B+ would be suitable as I was looking at 6L6GC, 6BG6GA and 7591. My test tubes are 6P3S. They don't like 400 volts (I had a Tubelab moment!)

Cheers,

Chris

[chrish]

Also a note about the IRF820 MOSFETs. I have cranked the drive section of the amp to heavy clipping to the point of having the 75V regulator tube extinguish and the MOSFETs have not blown with the protection zeners.

[chrish]

For the balance control, does this look like what is described?

Thanks,

Chris



Uploaded with ImageShack.us

[tubelab.com]

For the balance control, does this look like what is described?

That looks like it. Set the voltage at the grid pin to zero with no tubes installed to avoid any surprises on start up.

[chrish]

Originally Posted by tubelab.com

That looks like it. Set the voltage at the grid pin to zero with no tubes installed to avoid any surprises on start up.

Good tip! Thanks

[chrish]

Whilst waiting for parts I have been doing some thinking/tinkering. My heater voltage lift is only set for +55 volts. I will re-set for +75 volts for better heater to cathode limit compliance. On power up now, watching the voltages, the -ive supply does not pull it out of limits, so don't think I need any clamping device as suggested in past.

Looking at the 7-8mA 6SN7 likes for linearity, I was quoting Morgan Jones. With some re-reading, I have seen that he has about 5mA per triode in his diff amp stage of his Crystal Palace amp, as the diff amp cancels second order harmonic distortion. I decided to do what I could to see any distortion without any analysis equipment. I hooked up my cheap and simple battery operated signal generator, removed the output tubes and placed the scope probes on the input and output. Inverse on channel 2 of the scope and placed one waveform over the other to see any differences.

First scope picture is at 1kH square wave, output superimposed over input. Not too bad!



Next sine wave at 1kHz



Here is sine wave at 20kHz. Some phase shift.



Did not have square wave at 20kHz due to limitations of cheap signal generator. Overall, pretty happy with results and don't feel I need to tweek too much, maybe just change the 47K first stage load resistors for 33k and get the current up just a wee bit.

Still waiting on parts to implement the balance circuit above. Maybe this weekend.

[chrish]

Had the opportunity to work on the amp for a few hours today. After some research reading MJ, I saw that he had similarly low current flow through his diff amps in the Crystal Palace amp, noting that the diff amp naturally cancels the second order harmonic distortion. Therefore, I will keep values as suggested by Tubelab.

I was in the process of replacing one resistor in the heater lift divider (old schematic had about 55Volts, new value is about 80volts) when I blew my power supply. The resistor is on the power supply board. After replacing it and re-mounting the board, I powered up and noticed I was getting no voltage. Some initial trouble shooting revealed that the 100R 10W resistor in the input C R C was open circuit. A little odd, but I had mounted it high on its leads to give it some cooling room, and the constant jiggling around as I was building may have damaged something. I replaced it and powered up. Got a good 396 volt reading (what I was getting previously), for about 3 seconds... Then BANG! A bright flash and I killed the power. there was some evidence of arcing from the tab of the power MOSFET to the chassis. Figuring it was probably blown, I replaced it. Powered up, and getting about 260 volts output. Hmm, measured adjust to out on the LM317 and it was about 0.8 volts, so thought this may have gone out in sympathy, so replaced it too. Re-powered and same result.

With all tubes removed, I get about 430 volts out of the supply. With the 2 6SN7s in place, I get about 260 volts. Raw DC at the input filter caps is around 540 volts. I measure 260 at the source and drain of the 2SK1119 MOSFET and at the output of the LM317, and about 0.8v between OUT and ADJ on the LM317.

Any suggestions about how to trouble shoot this? What is likely to have gone out in sympathy when the 2SK1119 arced to ground?

Unfortunately I am about to step out the door and will be away for a couple of days with work, so no response to any suggestions does not indicate a lack of appreciation.

Regards,

Chris

Schematic of power supply:

http://img171.imageshack.us/img171/855/powersupply.pdf

[rknize]

Whenever I have "blown" a Miada-style regualtor, the LM317, the pass transistor, the small series resistor, and the "protection" diode all blow.

[chrish]

Thanks Russ, home for a few days now, so will investigate following your advice.

Cheers,

Chris