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6L6 amp with unique phase splitter - check my math

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Showcasing unique new PNP concertina with equal impedance phase split.
Collector voltage feedback via Schade = Emitter voltage feedback via Mu.
Also showcasing tight local feedbacks only, no global loop.

Already I forgot a grid leak resistor on V1a. Just pretend its value 100K?
Is 56H the correct plate to plate primary inductance for 5K 100W CXPP?
See any other problems with these numbers?
 

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I have spotted another snag. The 12AX7 has at most a voltage gain of 100, in reality you will get less than this. The negative feedback to the cathode tries to establish a gain of 100. The net result will be a gain a bit below 50. This is 6dB of feedback, exactly the right amount to create problems. It is enough to create high order terms, but not enough to reduce them. The output stage has no feedback so you get whatever it does - important to get the bias right to reduce distortion.

My guess is that this amp will not sound very nice.
 
The upper 2.2K plate load is impedance multiplied by the transistor Beta and the 220 Ohm pullup (Base to emitter) resistor is bootstrapped. So it's nearly a CCS load on the plate.
If the load were a true CCS, then the cathode feedback could equal the Mu, since no change in plate current would be needed. But clearly some current drive is needed to operate the splitter loads due to finite Beta. Putting a P Mosfet in place of the bipolar could work with constant plate current if the 220 Ohm got CCS'd.

Thinking backwards from the Mosfet case, a tiny amount of required plate current change for the bipolar case would seem to lead to some solution where the cathode feedback would have to be just slightly less than Mu in order to leave some Vgk delta for operation at that new current. Not sure how to calculate that other than using just the tube gm as a first order approx. Or simulate to fully calc.

Lets see. Some delta Vin gets multiplied by approx. Mu, that Mu*deltaVin then activates the Bipolar (/Beta*Rload) for some current change, which leads to subtracting out a deltaVfdbk = Mu*deltaVin*/(Beta*Rload)*(Beta*Rload)*Rattenuator = Mu*deltaVin*Rattenuator from the original Vgk. Whats left of the original deltaVin-deltaVfdbk (*gm1) should then be what is needed to operate the small plate current change for controlling the bipolar Beta. Making sense? I'm not sure myself. And what would the feedback level be then?

Some fixup needed for the tube bias for sure unless tiny input signals (<0.7 V) expected.
 
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OOPs that "deltaVin-deltaVfdbk (*gm1)" above, should read: (deltaVin-deltaVfdbk)*gm1

After consulting some String Theory and Quantum Field Theory Books, looks like it comes out:
Mu*Attenuator = 1 - Mu/(gm1*Beta*Rload)
or Attenuator = 1/Mu -1/(gm1*Beta*Rload)
on a sunny day maybe

So the bottom resistive attenuator needs to attenuate a little beyond 1/Mu to still get approx. Mu gain overall due to finite Beta. I think. So the feedback available must be from using up Beta rather than Mu. Leading me to suspect that a high Beta transistor should give low distortion. Or us a Mosfet like that split-P splitter. So you callin this thang "P soup"?
 
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Cathode swings 50% in phase with the input signal, just as it would in long tail pair.
1.4V peaks would be needed to make the 1st grid forward conduct.
Way more than necessary to drive this gain+splitter to full clip.

Conventional concertinas remain symmetric if both loads are equal impedance on
both rise and fall. Never a problem in AB1, but this amp drives a few milliamps into
Pentode AB2! This was done on purpose to show the improved splitter function.
Misbehavior in A2 is symmetrical about the splitter, not favoring the follower end.
It behaves and misbehaves much more like a long tail pair...
 
Yes Mu/2 per single end. Still Mu Voltage gain if you look at it differentially.
Again I am reminded the behavior of Long Tail Pairs...

Mu/2 is still plenty enough gain to swing the entire concertina headroom
before reasonable danger of V1 grid to cathode forward conduction.

I'm not sure DF96's bit about -6dB being enough to cause high order terms
but not enough to fix anything??? Here its all about faking the cathode
feedback seen by a LTP. I never seen LTP having issue on account of -6dB.

Output stage has local cathode feedback per ARC ST-70-C3 style.
 
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Never a problem in AB1, but this amp drives a few milliamps into
Pentode AB2!

I played with the simulation a bit at work today. You can see the output grid bias shift if you push the amp into clipping. The sim shows 4 ma grid current peaks dwindling over time. All you need now is to figure out how to put some mosfets in there to drive the grid harder. Even at the edge of clipping the plate is only swinging down to 200 volts. That seems awful high to me. Maybe a bit more screen voltage is needed.

The maximum plate voltage for a 6L6GC is 500 volts DC. I have ventured beyond this and found that 600 volts is on the edge of flashover. Not the tube but the socket! The 6L6GC and all of the popular audio tubes have the plate on pin 3 and the heater on pin 2. The black bakelite sockets commonly found in audio gear will arc from pin 3 to pin 2 at the point where the pins come out of the plastic on the bottom. If you plan to venture into this territory, use a seperate heater supply for the output tubes and GROUND pin 2. This way an arc won't blow your power transformer or driver tubes. Use only ceramic sockets. The glue used to hold the base on to the tube may absorb moisture and arc over inside the base.
 
My power supply makes 650 volts without a load, and my octal sockets are plastic
relay sockets for purpose of quickie breadboard testing. This might be a problem....
Do I need something with anode cap and perhaps ceramic socket savers?
I'm figuring 650 will come down a bit once the tubes conduct...

I have RCA 814's with top cap but 10VAC to heat the filaments is a whole other
lump of iron I'd need to lug around. Also one GU50 with tank socket that probably
wouldn't mind 650 plate volts, but no matched pair...

Also 6HV5 triodes with ceramic compactron sockets, but already promised away...

So them indestructible Chinese 6L6 you were braggin' on isn't gonna cut 600V?
Or its just the socket???
 
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By the A2 way, MJE350 supposedly should tolerate 133mA (20W) with heatsink???
That would be 560ohm where you see 2K2's now, and 5.6ohm cathode feedback.
I'm not entirely comfortable the idea sand at full rated dissipation, I'm just sayin'.
But then the grid leaks could be reduced to 10K or 4K7 for mo' current in A2...

You can't stay in A2 forever this way, its still gonna pump down the grid caps.
Just showing off this concertina's potential for the occasional big transient.
A few mA grid current now and again is a small drop in a large sandy bucket.

MJE350 is toasting comfortably at 5W in the original drawing.
 
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So them indestructible Chinese 6L6 you were braggin' on isn't gonna cut 600V? Or its just the socket???

All of my testing on the Chinese 6L6GC's were done before I got the new flamethrowing power supply. The old Fluke 407D goes to 550 volts. It set one of the plastic sockets on fire, but the tube survived and is still alive. I got a few hundred of the black bakelite chassis mount sockets at a hamfest years ago. The plastic will carbonize if the arc persists. The B+ will then set the socket on fire even if the arc stops. My new power supply goes from 0 to 650 volts at 1.7 amps. It just blows things in half, including electrolytic caps. They don't have time to vent they just explode.

I made a lot of guitar amps several years ago and I saw the pin 3 to pin 2 arc happen several times. It is easy to provoke this type of arc by improperly loading the amp. Some metal heads used to run an 8 ohm speaker on the 4 ohm tap for a screaming disortion. This has set more than one OPT on fire and fried a few tubes and sockets. If the metal head cranks things high enough to blow a speaker open, the amp will fry. The arc usually starts at the tube socket, but I have seen OPT's, tubes, power transformers, impedance selector switches, PC boards and other parts fried.

Do I need something with anode cap and perhaps ceramic socket savers?

My latest experiments have been using sweep tubes because they were designed for big plate voltage, they have high peak current capability, and they can pull the plate down to 30 volts or so. Compare that to the 200 volts you are seeing now. I have one of Petes red boards running 6HJ5's ($5 each and no plate cap) on a 600 volt supply. It makes 75WPC with a 6.6K load and 125 WPC with a 3.3K load. It is hooked up to my big speakers now for a rock concert in the living room. It has seen full throttle operation for multiple hours at a time with no issues. The long thread is here. An 18 WPC amp makes up to 250 WPC. Most fun I have had with an amp in a long time and it isn't even one of mine!

http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html

I have RCA 814's with top cap

I have a bunch of 828's which are similar. You need big voltage and a very high load impedance to make them work. A 15000 ohm 200 watt OPT isn't on the shelf at Walmart.

Also 6HV5 triodes with ceramic compactron sockets, but already promised away

The very similar 6JD5 is on the dollar menu at ESRC so I bought some. I haven't found a way to tame them yet.

650V problem solved. I've been offered a quad of 6BG6.
Most 6BG6's and 807's are cut from the 6L6GA mold which carries a 19 to 21 watt plate rating. You can still get 60+ watts from a pair, just dont plan on long term sine wave testing.


I had a bunch of 26EW6WG's. I gave half of them away, which started this thread. http://www.diyaudio.com/forums/tubes-valves/178561-george-re-26e6wg.html

In order to answer the usual how hard can I lean on these tubes questions, I wired them into one of my Simple P-P boards. I squeezed 35 watts out of a pair which was more than expected. I then unplugged the 26EW6WG's and popped in some 6BQ6GA's. After turning a few knobs I was getting 75WPC.

http://www.diyaudio.com/forums/tubes-valves/178561-george-re-26e6wg.html
 
If an amplifier has a gain of A, and your feedback network has an attenuation of 1/A, the the total gain will be A/2. This comes from the standard feedback equation. It also means that the feedback signal will be about half of the input signal, so the remaining half then drives the amp.

This in turn means that whatever distortion your amp makes (open-loop), will be mixed into itself again via the feedback loop. Let's assume the amp has pure second-order at 5%. The feedback signal means that it will now in addition have 2.5% third-order. Both will then be reduced by the feedback to half their value: 2.5% 2nd, 1.25% 3rd. But the 3rd will also mix to create some 4th etc etc. The moral of the story is that unless your amp is so low in distortion that it needs no feedback, you either need no feedback or enough to suppress distortion. Maybe 14-20dB feedback minimum?

You have misunderstood my concerns about the -0.7V bias. I am not saying that grid current will flow for signals over 0.7V peak. I am saying that grid current is likely to flow at -0.7V. Yes, current can flow against the voltage! This is because the grid-cathode system can act as a thermolectric generator. You need to get bias to around -1V before you can probably ignore grid current. I am not talking about the mA's of grid current in an AB2 output, but a few uA of grid current are enough to create distortion unless the input comes from a low impedance. Simulators vary in their ability to model this; it depends on how good your valve model is.
 
I can throw a silicon diode into the constant current stream under V1 cathode
and bump up the bias there another 0.66V. Just have to lower the plate current
accordingly, so that that concertina's operating points don't shift. Meaning the
12AX7 will be at the same plate and grid voltages, but higher cathode and only
2mA instead of 3mA... This doesn't seem beneficial in the sim, but if V1 grid
currents flow in reality, it could be a backup plan.

I got a quad of the 6BG6, my iron can take it, so I'll probably run all four off
the one concertina. I got pics in my cell phone, I'll try and find the data cable.
No reason I should have to overcook a single pair.

Duncan 6L6 sim models don't want to conduct more than 330mA, even if you
pull screens up to 600V (which I would never do in real life!). I'm not sure the
lower limit of 180V plate swing seen in sim is realistic? But with two 6BG6GA
in parallel per end, I should have no worries to move enough current without
having to run the screens crazy high...
 
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