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Driver for 810 SE

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If you have the cathode follower driver grid at a positive voltage versus its cathode, again, you have to deal with that low impedance grid too, not just the low impedance grid of the output tube.
That's not the intention. The intention is to provide a way to adjust the voltage at the cathode of the cathode follower to set the output tube bias. The intention is not to create another stage drawing grid current.
 

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A cathode follower will swing what you feed it provided there's enough compliance. It does not have any voltage gain.
That may be the part of the puzzle I'm missing!

Here's my attempt to design a cathode follower that meets my needs:

Tube: 5687 (I have some lying around)
Ip: 6.5 mA (18V bias for the 810 divided by 2760 Ohm for the choke)
B++: 150V (I already have 150V regulated screen supply for the 12HG7)

For the bias voltage I assume that the effective Vp is 150-18 = 132V. From the 5687 graphs I can see that I need -6.7V bias to get 6.5mA plate current at 132Vp. Given the cathode is at 18V I assume I need 18 - 6.7 = 11.3V on the grid. I used a potential divider to set that voltage (this is what Morgan Jones shows).

I'm attaching a schematic for this stage. Please let me know if I'm on the right track.

Thanks so much for all the help.

Richard

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Yes, you're on the right track. I would encourage you, however, to figure out what peak 810 grid current is. It happens on the left side of your load line. It will be a LOT of current.

Some tubes have a maximum cathode current or peak current rating. The 5687 probably isn't up for the job. You may be looking at something more like an EL34, 6080, etc.
 
I was wondering the exact same thing. The 5687 can handle 28mA at 150V continuously without exceeding its 4.2W plate dissipation limit. At 28mA of grid current the 810 will be seeing around +50V on the grid at about 150Vp. The operating point we picked was 675V so that would give me 525V swing into 5K load for about 27W. I'd be happy with that since it's about the limit of the 1628SEA OPT anyway.

Richard
 
However, if I did drive the 810 all the way it would be +60V at 100V drawing 100mA of grid current. I suppose that would mean turning my volume control up too far could blow the 5687! So does that mean I need a tube capable of 100mA to be safe?

Richard

P.S. It looks like the 5687 peak cathode current is 65mA.
 
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I was wondering the exact same thing. The 5687 can handle 28mA at 150V continuously without exceeding its 4.2W plate dissipation limit. At 28mA of grid current the 810 will be seeing around +50V on the grid at about 150Vp. The operating point we picked was 675V so that would give me 525V swing into 5K load for about 27W. I'd be happy with that since it's about the limit of the 1628SEA OPT anyway.

Your load line shows you will hit +70V on the grid at the line where grid and plate voltage meet (so the plate is also at about 70V). If you go back to your grid current graph, that spot is at about 125mA or so. The 5687 does not show a peak cathode current rating that I can see, nor does it show any operating points that run nearly that much current. Based on that, I would disqualify it.

Note that the plate dissipation does not come into play here, it's more about whether the cathode is beefy enough to tolerate this.
 
They don't perform particularly well in parallel. You'd have to use balancing resistors and that would kind of defeat the point. An 0D3 does have a reasonably high peak current rating though, it's buried in the long GE datasheet and is referenced regarding startup conditions.

The actual performance of the 0D3 under these conditions is still somewhat questionable, but it's a big improvement over a resistor. A stack of zener diodes is another decent option, or a series tube regulator.
 
They don't perform particularly well in parallel. You'd have to use balancing resistors and that would kind of defeat the point. An 0D3 does have a reasonably high peak current rating though, it's buried in the long GE datasheet and is referenced regarding startup conditions.

The actual performance of the 0D3 under these conditions is still somewhat questionable, but it's a big improvement over a resistor. A stack of zener diodes is another decent option, or a series tube regulator.
It will handle 100mA for 10s at startup. That doesn't help though, since it will have to be loaded for the normal 30mA or so and when the 100mA surge comes the voltage drop across the resistor will bring the output voltage close to zero.


However, I have taken a look at the EL34 plate current changes with plate voltage swings and it looks ok to me. If I self bias the EL34 for 6.5mA and the 2750 DCR of the cathode choke I need 164V on the plate. If that were to swing up by 20V the resultant increase on the grid voltage of the 810 would be around 1.5V - which is not that significant. So I think it's fairly stable.
 
Yes, the 810 grid current DC path goes right through the cathode follower.
Yes - but do I have to take that into account when biasing the tube? If I bias the tube for Ia=6.5mA and then connect the cathode to the grid of the 810, which pulls another 12mA, will anything else change? Or will there still be 6.5mA being pulled through the choke?


Thanks for all your patience and answers. I've ordered the 155C and EL34.


Richard
 
I would use the 156C, the extra DCR won't hurt.

The grid current from the 810 is a given quantity, as is the P-K voltage of your cathode follower. The variable will be how much current flows through that choke. Of course if insufficient current flows through that choke, then the 810 won't be biased up properly. This is why I use the biasing arrangement that I posted to provide some adjustment. I have never been able to build an amp like this without having adjustable bias to keep things dialed in.
 
I had ordered the 156C as well since they weren't expensive. So I can try both.

I think I skipped over your bias diagram yesterdaay. I just went back and took a look - it makes sense now. I'll do the same thing so I can adjust the bias on the cathode follower to indirectly adjust the bias on the 810.

Richard
 
If I connect a 5K resistor across the IST in place of the 810 I see a nice sine wave. With the 810 in place I get significant distortion. I will post the waveform later when I'm home. As I recall it looks like one half of the wave is distorted. I'm guessing it's the half when grid current is being drawn and my driver is not capable of delivering it. Could it be that the output impedance of the driver is too high?
The Zo of the driver is way too high. The A Number One difference between driving a 600R resistor and a 810 is that the 600R resistance doesn't change. When the grid of the 810 isn't pulling current, there is no drop across the driver's Zo. When it does pull current, there's a serious voltage drop that leads to the asymetrical results you saw. ISTs and pentodes are a terrible combination for another reason: the extremely high rp of the pentode: you won't have any low frequencies. You need triodes with sufficiently low rp.

I'd appreciate any guidance. At some point I will want to look at other drivers including FETs, but for now would like to learn how to use the IST.

I wouldn't. A power MOSFET source follower can provide hella current and do so with a very low Zo. If you're still insisting on IST drive, your best bets for drivers would be something like a 6BX7 (rp= 1K3). Even better would be a 6AS7 (rp= 280R). Being dual triodes, I'd get an IST with a center tapped primary for PP, and a voltage step down turns ratio to drop the Zo seen at the 810 grid even more.


You could also parallel up the sections of either the 6BX7 or 6AS7 and use it as a cathode follower driving the 810. That would be almost as good as a MOSFET and better than an IST. You get that extra source of phase misbehaviors out of the signal path so you can add gNFB.

"I already had an 811A and 812A from when I was trying some RF stuff".

The main problem with these RF power triodes is that a lot of them were designed for high-u, high rp, and power sensitivity. Using them for SE audio usually requires +VGK to set a reasonable Q-point. (Same problem for RF pentodes: 814, 1624 -- both need bias points approaching zero bias and positive grids for decent output) A few (806, 845) were designed with low-u and rp for Class C plate modulation for AM. Since this is an audio application, the same criteria apply for good quality AM as for audio reproduction.
 
Thanks Miles - this is very useful. As you will see I have moved away from the IST to a cathode follower approach. I realize that I'm using these power tubes in a way they weren't intended for. I thought it would be an interesting challenge that would force me to learn more about drivers and tube amp design in general. I will certainly want to try an FET source follower once I get some results from a tube based approach.
 
I've made some progress. I replaced the IST with an EL34 choke-loaded cathode follower. See attached schematic. It's definitely not optimal - I cut corners to get started. The operating points have been chosen more for convenience than anything else.

For my initial results I'm getting 1% THD at 0.1W, 2.6% at 1W, 5.3% at 10W and 9.0% at 20W. By adding -13dB GNFB these improve to 0.5% THD at 0.1W, 1.6% at 1W, 2.3% at 10W, 3.5% at 20W and 8.5% at 30W.

I need to figure out the source of the distortion - I think that at lower power much of it may be coming from the driver. At higher power I may well be suffering power supply sag for my 150V supply from the high grid current.

I still have work to do but have something to work from. Thanks for all the help so far. More suggestions are very welcome.

Richard


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