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Old 25th March 2014, 05:17 AM   #11
sangram is offline sangram  India
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Thanks for that Ian

My problem is that my output is basically a mains toroid with a custom primary. I was looking at a number of strategies to deal with the DC imbalance, and the only thing that seemed to address both DC balance and AC balance was the CCS in the cathode.

Would it help if we could somehow limit the voltage across the cathode cap? Like a 3.3 or 5V zener? Would that help in keeping the 'fart' out of the system? Just a thought I do know it adds one more junction and capacitance, but it might still be worth a shot...

I do experience this sound with the 6F3 amp I was using. That also uses fixed bias a large cathode cap, though like you say, it is actually a hifi amp!

Any thoughts on how I may prevent core saturation if I was to use fixed bias?

--------------------

I am also struggling a bit with conceptual understanding of grid current condition. If I have an input of 250mV and a gain on the first stage, does the grid of the second stage (splitter or crunch channel) need to be at -10V to ensure I do not get grid overdrive? And then since I will be pushing about 30-35V into the output, what would my bias point be? -30V?

And finally, when calculating output stage gain, do we use the exact same method as other stages - ((Rp||Rl)/Rk) is the best I could come up with.
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Old 25th March 2014, 05:40 AM   #12
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Yes get it going and check what teh bias is. Then add on a zener clamp of just higher voltage than that normal running bias.

Grid Current: What you are describing is "grid rectification" current. When you try to take the grid positive with respect to teh cathode then teh grid input strats to look like a forward biased diode. This will clamp teh positive going signal (clip off the top) and this will be reflected at the anode, because of teh phase inversion you will see a signal at teh anode with teh negative side peaks clipped off. Thnat is what appears at teh anode is an exact amplifed and inverted of what is at the grid.


Cheers,
Ian
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Old 25th March 2014, 06:52 AM   #13
sangram is offline sangram  India
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Thanks Ian, once again.

Re grid rectification current, How would I prevent it when the swing of the preceding stage exceeded the grid-cathode voltage of the next stage?

I am confused because the output stage will be running with a bias of a few volts, say 5V. The grid is at ground potential in this scenario (assuming cathode bias). Will the grid of the output tube not overload with any input above 5V? I will obviously not get full power with 5V input, so something is wrong with my understanding here.

Last edited by sangram; 25th March 2014 at 06:54 AM.
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Old 25th March 2014, 01:12 PM   #14
sangram is offline sangram  India
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Riiight.

'ere's version 1, standard PP no global feedback. Provision for an FX in/out loop, which allows me to not use only the power section onwards. CCS uses 7805 regulators, for a 50mA constant current through a 100 Ohm resistor.

The library does not have a UL transformer, so the power sections are currently triode-connected. I am planning on pentode and UL connections too. Also, PS schematic is not shown, as I am not really confident of the voltages used.

Brickbats welcome. My main issue is how close to the edge the triodes are running (max Pa of triodes is 0.5W as per datasheet). This is sort of deliberate, so I don't run out of drive current. Could back it off significantly if required/advised.
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Old 26th March 2014, 01:15 PM   #15
sangram is offline sangram  India
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Well, I finished drawing up to the best and worst of my ability. Will start sourcing parts next week.

How much insurance will I need? And what should the ER be prepared for?
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Old 27th March 2014, 02:39 AM   #16
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Hi again Sangram. How did you choose the size of your coupling capacitors? 0.22 is pretty big for a guitar amp - encouraging blocking distortion and lower note flatulance. I rarely use more than .010u. Just asking
Cheers
JimG
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Old 27th March 2014, 03:02 AM   #17
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Yes 0.22uF is a bit too big for guitar amp use. Try 22nF or as Jim suggests 10nF.

I only had a quick look at your proposed schematics but noted the following:

MOSFET M1 needs a 220 Ohm or biggger (no need to go over 1K) gate stop resistor. The propensity for parasitic oscillation is roughly inline with a devices gm. Mosfets have X100 to X1000 the gm of a typical audio output pentode and so you can assume that without a gate stop they are X100 to X1000 times as likely to go into parasitic oscillation.

The concertina splitter V3a needs some bias components. Probably best to cathode bias it and return R7, C16 etc to the bottom of the cathode bias resistor (bootstrapped input).

Check out some early Fender Schematics - e.g.
http://www.recproaudio.com/diy_pro_a..._5e3_schem.gif
See the way they biased it?

Don't worry about the 56K vs the 33K anode and cathode loads. The triode section of your 6GV8 is "grunty'er" than a 12AX7 triode section so you can use the 33K you had shown.
If you want to get fancy you can use LED(s) in place of the bias resistor. IR and Red LEDS about 1.4 Volts, Green LEDs about 1.8 to 2.0 Volts.

When you go to pentode mode the 2K grid stop resistors on the output pentodes will want to be MUCH larger (suggest 47K) to help prevent blocking distortion and improve the overdrive sound.

Cheers,
Ian

Last edited by gingertube; 27th March 2014 at 03:12 AM.
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Old 27th March 2014, 03:44 AM   #18
sangram is offline sangram  India
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Thanks for the help :-)

I chose 0.22 randomly, and I will back those down to 22 and 10nF. I may have so high voltage 22nF types already, saves me a few more pennies.

I will perform the rest of the changes on the schematic and part values, and hope to fire up the iron soon. I am still a bit confused about bias I guess, but I hope to have that figured out after a few builds.
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Old 27th March 2014, 04:15 AM   #19
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Bias (the basics):
Devices come in 2 types.

Enhancement Mode Devices, like Transistor and Mosfets. With no bias they are OFF, you have to apply some "bias" voltage to get them to conduct current.

Depletion Mode Devices, like TUBES and JFets. With no bias they are ON "flat out" - conducting maximum current. you need to apply some "bias" voltage to limit the current or turn them off.

For a tube you want the grid to be at a negative voltage with respect to the cathode to control the tube current. This is the same as saying you want the cathode to be at a positive voltage with respect to the grid, and that is the way we normally arrange it in practice.
You put a bias resistor in the cathode circuit and tie the grid reference back to the bottom of that resistor. The bottom of that bias resistor might be, but does not have to be, at 0V (In this case, for teh concertina splitter, it will not be at 0V, but the grid to cathode voltage is still what controls the current). The voltage drop across that bias resistor when current flows makes the cathode more positive than the grid. The more the current the higher the voltage but the higher the voltage the more the current is "turned off". The circuit will find its own equalibrium.
Want a higher idle current - use a smaller bias resistor and vv.

Cheers,
Ian

Last edited by gingertube; 27th March 2014 at 04:22 AM.
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Old 27th March 2014, 06:20 AM   #20
sangram is offline sangram  India
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Thanks again Ian.

My question was actually the calculations I was using anode load to set the max current through the tube, looks like my current and power calculations are off so I will need to go back and see what those are working out to. For academic interest at least!

I could not figure out how to change the gate stoppers in pentode mode only, not enough poles on a switch (I assume to help with the increased miller capacitance in pentode mode?), so I'm just using a single value now and hopefully it will work in all the proposed modes.

Eventually I hope to evolve a preference for a specific type of configuration. In all the hifi amps I've heard, I seem to prefer UL modes or amps configured permanently in UL. I suppose it might not be the same for guitar.

I'm guessing I might get about 10-12W out of this thing in Triode mode, which is enough for me and hopefully has a sound decent enough to record some basic stuff with. Not really looking for much overdrive, basically some soft clipping. Like an old Fender, rather than Marshall. That is why I haven't yet put in a bridge rectifier.

Really appreciate all the help I'm getting.



Cheers!
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