Two tube class a2 se trioded el84 with 12dw7 cathode follower driver? - diyAudio
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Old 31st January 2012, 03:52 AM   #1
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Default Two tube class a2 se trioded el84 with 12dw7 cathode follower driver?

Would it be ok to run a trioded 6bq5/el84 in class a2? If so would a 12au7 cathode follower source enough current to do it? The idea is to try a very simple two tube amp with the 12au7 side of a 12dw7 configured as a cathode follower to drive the grid of the el84. ~Austin

Last edited by Austin Translation; 31st January 2012 at 07:45 AM.
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Old 31st January 2012, 10:17 AM   #2
DF96 is offline DF96  England
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Three questions:
1. Why? (A2 is usually used for high power PA conditions)
2. Does the EL84 data sheet give A2 conditions (I haven't looked, but I suspect not)?
3. Is the EL84 grid robust enough to cope (probably)?
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Old 1st February 2012, 10:16 PM   #3
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Thanks for the reply, 1. I have a couple 12dw7s and I was just trying to figure out something interesting to do with them and coax a little more power out.
2. I haven't seen any data sheet charts that list positive grid voltage example but
3. I suspect the grids would be fine as long as I didn't push them too hard but I don't really know for sure.
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Old 2nd February 2012, 01:29 AM   #4
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The tubes have no problem handling control grid current, and do continuously when used in Class C r.f. power amplifiers in transmitting equipment. With the small current flowing only towards the tip of positive peaks in audio service, the average grid power is negligible. As in pentode operation, it is the screen grid dissipation that needs to be watched. Many beam pentodes specify doubled screen ratings on music/speech peaks in class AB or B (1 or 2) as seen with ultra-linear operation, but with the high bias currents many people run tube life is severely impaired. There is also a compounding heating effect where the screen rating should not be pushed when the plate is run near rated value. Bias current down a bit, triode operation is actually not as tough on the screen as the worst case pentode operation which is seen when the plate voltage falls below the screen voltage. That's particularly a problem when their is heavy bass content and the speaker system has a impedance peak due to resonance at those frequencies because the higher load resistance makes it more likely that the available plate current results in a bigger plate voltage swing for a given current. If your driver is capable of producing much larger plate currents than needed and you try pentode mode, a series grid resistor can soft-limit the drive. Some slight compression of drive voltage as grid current rises can actually reduce distortion since it would tend to cancel the effects of transconductance rising with current. Also, being progressive compression it makes for a more pleasant sounding overload characteristic. The Ampeg SVT series AB2 amps have particularly large drive-limiting resistors. What to limit drive to depends on a number of circuit conditions including your choice for the plate load. Screen overload is much less of a problem with lower plate circuit impedance values.

Last edited by riccoryder; 2nd February 2012 at 01:35 AM. Reason: clarity
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Old 2nd February 2012, 07:42 PM   #5
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So correct me if I am wrong but when a when a tube works into a resistor for the load the energy is converted to heat in the resistor, and the idea is that same energy could be conserved and passed on by driving and interacting with the grid of the next tube as part of it's load instead. Is this right? What kind of a load does swinging the next tubes screen positive present to the previous stage?
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Old 3rd February 2012, 09:10 PM   #6
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Well if you're thinking of where the power that gets to the grid came from and changes in the power dissipated in the resistor between the driver plate and the power supply when there is a load, the power lost in the resistor actually increases when load current is drawn, because the current through the resistor rises. When the output goes positive, the load current goes through the resistor. When the output goes towards ground, the tube current is higher too, since it sees both current from the supply resistor and the load current coming the other way (using charge from the capacitor). It is easiest to visualize that if you think of the capacitor as acting like a low-capacity battery that's charged to match the resting voltages in the surrounding circuit and can keep the voltage across itself constant in the short term. If the tube switched off, the rising voltage on the plate would cause charging current into through the capacitor that also flows through the load. With the tube switching to ground, the discharge current flows through the load in the other direction. The problem with that circuit for driving grid current is that current the other direction can't flow through the grid, it acting much like an ideal diode and resistor in series only passes current in one way. With more current flowing through the capacitor when the output goes positive, and only a very tiny current back through the grid bias supply resistor when it goes negative, over time the capacitor would charge up to a higher voltage. At that point the average voltage at the grid would be lower than the average voltage at the driver plate by a greater amount, the voltage across the cap. The effect is that the bias voltage becomes increasingly negative, and the grid current obtained falls to match the small amount of that can flow back through the high value bias resistor, typically hundreds of K Ohms. If the amp is over driven, that increased negative bias can go beyond cutoff, with only a portion of the positive audio peaks breaking through to where the output tube conducts. Those spikes of audio represent very severe distortion, more than what symmetrical clipping would give. Also, dips in the audio level would have the lowest level sounds muted, and the bias wouldn't go back to normal right away as the excess voltage across the cap could only be discharged by the small current that could flow through the large grid bias resistor. Ironically, when some people modify an amp working to get better low frequency response with a larger coupling cap, they make it the recovery time when overdriven even longer. That overload distortion with a bias shift and breakup/muting is called blocking distortion, and is most often seen by musicians. If the distortion event wasn't intentional, it can particularly disruptive when the amp needs a long time to recover. Usually the caps are smaller in that application. Then the distortion still occurs, available as an effect when levels are cranked, but the recovery to undistorted operation on backing off the levels is faster. Looking at things another way, grid current has an average dc value at a point in time, and you can't maintain DC through a capacitor. The current can only match what goes through the bias resistor. So for class 2 operation, either another path must be added for current flow on negative excursions, or a different circuit with no capacitor is needed where current can flow without disrupting the bias voltage. Some like the vintage method with a driver transformer (see 4-65A datasheet), but good transformers are very hard to find and expensive, and complicate life for any considering negative feedback. Sometimes driving with an op-amp can work very well and provide some excellent current sampling feedback opportunities, but especially with the lower gain triodes, getting enough voltage swing requires higher voltage chips or added buffers. With op-amps it may beneficial to run mismatched positive and negative supply voltages to get the most optimal output swing while staying within the maximum total supply voltage limit. Direct coupled drive from a cathode or transistor follower is common. A classic example of class AB2 drive and biasing with a follower is the Heathkit W6A or W6M amp. Inserting a MOSFET follower into an existing tube amp between the grid and existing circuit is very simple, but the the negative supply for the source resistor returns to needs to be beefed up compared to the usual bias supply. Many simple negative bias supplies were made for extremely light loads. Another trick is to use a low impedance adjustable negative bias supply in the normal capacitor coupled circuit, but add a fast diode or even tube diode (6AL5, 6H6, 6JU8 etc) sometimes with series resistance, across the bias resistor, cathode of diode to the grid drive. The clamping action of that diode, can either force the bias to go more positive, or at least keep it from going more negative when grid current flows. That method usually introduces some distortion since the higher output impedance of an ordinary driver sags some when added current starts to flow, but it is a way to avoid blocking distortion and see more power than class 1 under heavy drive conditions. Fixed bias generally works better than cathode bias for class 2 stages. With time for cooling at low bias currents, and the low average screen power being sufficient when the control grid has more drive, the ratio of possible music/speech output power to plate dissipation is increased. For instance in class 2 a pair of 4-65As can deliver 270 Watts. That's 95 Watts more than in AB1. The Heath amp in AB2 delivers 70 Watts with ultra-linear connected 6550's even though ultra-linear usually gives only half the power of straight pentode operation. Of course it takes some added work to get very low distortion in high power amps with very low idle currents, but when pushing power an SET does too. Class 2 will get you the peak currents to deliver more power from triodes too. To get maximum power you'll also want drive capable of swinging all the way to cutoff. Some semi-local negative feedback, like tying the secondary output tap to the output cathode, can help linearize the amp against transconductance changing with current. That's increasingly needed when trying to operate over a very wide current range for more power.
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Old 3rd February 2012, 10:08 PM   #7
DF96 is offline DF96  England
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Quote:
What kind of a load does swinging the next tubes screen positive present to the previous stage?
A very non-linear load, that is why the driver needs low impedance and good current drive capability.
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Old 4th February 2012, 02:45 AM   #8
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The 6BQ5/EL84 is a pretty high perveance valve, so won't benefit much in class A2, maybe a dB or two at most. Still won't hurt to try it - you might gain another dB or two lowering the load impedance if it really matters.

And be sure to think through the turn on and turn off sequences; they're critical in a direct-coupled amp.

All good fortune,
Chris
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Old 4th February 2012, 12:03 PM   #9
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The 6BQ5 delivers higher currents than the 6CM6, also a 9-pin 12 Watt pentode, but the data sheet for that may still be worth a look to provide some insight into the sort of behavior seen. Curves are provided for drive levels up to 20 Volts for both triode and pentode modes. There are also curves showing grid current which is low, rising at the lower plate voltages. It is worth asking what is planned for the output transformer. If it is a leftover from pentode operation is may be worth moving an 8 Ohm load to the 16 Ohm output tap if there is one. Less compression at peak current may result in slightly less third harmonic and more second harmonic distortion since there would be less symmetry with the waveform compression seen at low currents from falling transconductance.
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Old 4th February 2012, 02:38 PM   #10
DF96 is offline DF96  England
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It can be a bad idea to try to balance off distortion from grid current and distortion from changing transconductance, even though this technique was encouraged 60 years ago. The reason is that the two curves don't match, so you get a reduction in lower orders and an increase in high orders. It may measure better but sound worse.
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