6L6GC AB2 Amp

[chrish]

Thanks SpreadSpectrum!

Any chance someone can comment on my calculation regarding the direct coupling of the two stages? Do I take the -75V tail supply from the (nominally at this stage) 150V anode voltage to get +75V on the grid of the second stage?

Thanks,

Chris

[tubelab.com]

The LTP first input stage has a CCS on the tail. I understand that this makes the load line horizontal at the current set by the CCS over all plate voltages.

A CCS instead of a plate load resistor (like the second stage) does indeed make a horizontal load line. This means that the second stage will have a gain very near 20. The CCS in the tail works more like an unbypassed cathode resistor. The first stage will have a gain of about 10, maybe less. The total gain of the driver should be 140 to 200. This should crank the 7591's and I think that the 6L6's will be happy too, because I find that you will need more drive to wake them up in triode mode, but you will find that you need less feedback with triodes too.

The cathode will need to be positive with respect to the grid, but how do I work out what this is? The only component is the 10M45 and it's adjustment resistor and stopper resistor. How many volts are dropped across this?

A CCS will drop whatever voltage it needs to force the desired current. This makes things much easier than it looks. As you pointed out the cathode will be a few volts positive with respect to the grid. The grid is at zero volts since it draws (theoretically) no current and there is a resistor to ground. So the cathode will be at a small positive voltage (3 to 8 volts). I have verified that you can indeed turn the power supply from -25 to - 400 volts, the cathode voltage doesn't change.

If you have access to a scope, the best way to set this up is to set the CCS to the desired current, drive the stage with 2 to 3 times the "normal" drive voltage, and tweak the load resistors for the best looking sine wave. Pick a value that results in the lowest plate voltage that gives good results. You can also tweak the CCS current a bit, and try all of your 6SN7's. They are not all the same! Verify that the distortion decreases as you lower the drive voltage.

So for our example, if we have -75V supply to the first stage, and if the plate voltage was 150V, then the grid of the second stage would be +75V?

If the plate voltage of the first stage is 150 volts, the grid of the second stage is 150 volts, they are connected together. We already know that the cathode will be slightly more positive than this, so we can guess that it will be about 155 volts. The negative supply is -75 volts, so the cathode resistor must drop 230 volts. It passes twice the set current of the plate CCS's (two tubes). In this case 14 mA, so Mr. OHM tells us that we need roughly a 16K resistor. Start here and tweak for best distortion.

Now tweaking the CCS current in the first stage (this is why there is a pot here) will set the operating points for the entire driver, and can be used to optimize the output voltage swing potential. As you guessed, it is a trade off, drive voltage VS distortion. You will tweak this again when the whole amp is done.

I take Sherri to the airport tomorrow. Then I will have 3 weeks to make some glow. I have found a box full of E130L's and a whole bunch of sweep tubes. I think they would look right with some 6SN7's driving them. Soon.

I'd get the soldermask, it stops the copper from corroding and the solder from sticking where you don't want it too.

The "barebones" boards from Advanced are solder plated, so there is no copper showing. Solder shorts are a possibility, but I have done some high density QFP IC's on these boards without issue. They are not pretty though, see picture.

[chrish]

Thanks for that George. Was doing some thinking on the flight today, and realised, of course, that the plate of the first stage is tied to the grid of the second, so it will be the same voltage... Also, as I will not have a negative supply on the second stage, I will have to drop around 158V, so figured using 11K here (14mA @ 158V = 11.3K). I am guessing that I can tweak the current in the loads of the second stage also to get exactly the bias I need here.

Yes i have a signal generator and a scope, so will follow your advice and tweak!

Getting very close to finalising the design here and going live! Looking forward to it!

Thanks again for the help!

Chris

[oldmanStrat]

The "barebones" boards from Advanced are solder plated, so there is no copper showing. Solder shorts are a possibility, but I have done some high density QFP IC's on these boards without issue. They are not pretty though, see picture.

yes of course... HASL over copper - plates everything. I never run my boards that way.... almost all of our stuff is BGA's these days - and you need soldermask.

I take Sherri to the airport tomorrow. Then I will have 3 weeks to make some glow. I have found a box full of E130L's and a whole bunch of sweep tubes. I think they would look right with some 6SN7's driving them. Soon.

I have a request. I have a need for a small SPUD type amp for the office, and I know that you have one "almost" done. Care to share schematics/layout? I wouldn't mind playing around with it...

end of thread jacking - I'll start a new one on the SPUD topic ...

[Mickeystan]

Hello Chris,

Thought I would write and mention that there is a few of obvious schematic errors you will want to fix. The errors I observed in the CAD schematic are as follows;

1. The lower mosfet source follower's output is connected to screen grid of the lower output tube rather than the control grid.

2. The suppressor grid of output tube(s) should be tied to the cathode on each tube.

3. The minus gas regulator tube used for the bias circuit needs to have its plate and cathode connection reversed. The plate should go to ground and the cathode to the negative supply.

I am sure you would have found these errors eventually, but I figured sooner is better than later. Have fun with the amplifier.

Doug S.

[chrish]

Thanks for taking the time to have a look at the schematic.

The program I am using is DesignWorks Lite.

1. Connecting to suppressor from Mosfet was a mistake, have now corrected, thanks.

2. I had to make my own tube components, should have remembered to connect cathode and suppressor. A bit hard to modify now, but thanks for pointing it out.

3. Gas regulator polarity, thanks for correcting me on that, have modified.

Attached is latest, with corrections...

Thinking of lifting the voltages of the heaters for the 6SN7s. Is this required? I was thinking of 50-75volts...

Thanks,

Chris

[tubelab.com]

Thinking of lifting the voltages of the heaters for the 6SN7s. Is this required? I was thinking of 50-75volts...

Yes! The cathode voltage of the input tube is near zero. The cathode voltage of the second tube will be in the 150 volt range. In order to keep both 6SN7's within their H-K voltage ratings the filament windings should be at about 75 volts.

I laid out a simple PC board with a pair of 6SN7's using a schematic very similar to yours. I haven't had time to make one yet. Maybe this weekend.

[Mickeystan]

Chris,

I agree with Tubelab on his recommendation to elevate the heaters on the second 6SN7. The design thought I wanted to offer you was to simply tie your regulated +105v supply to an isolated filament winding that serves that tube, rather than to add more parts in the way of a resistive divider.

Doug S.
(aka Mickeystan)

[chrish]

Thanks, glad I am on track here. Unfortunately I only have one 6.3 volt winding that has to supply all tubes, so it will have to be 75volts from a resistive divider from B+.

Now, for dumb question time again...

In another thread going at the moment talking about MOSFET source followers, members are talking about the amount of current they are setting for the MOSFET using either just a resistor or CCS. In our example here (same as MOSFET Follies and tubelab driver board) we have a 10K resistor from the source to the negative supply. Is the current calculated by looking at the voltage drop across this resistor, ie difference between bias voltage (about -20v in this instance) and negative supply (-75v here) for 55v dropped across a 10K resistor to give 5.5mA?

Also, what would be a reasonable current to aim for?

Thanks again for the help!

Chris

[tubelab.com]

Is the current calculated by looking at the voltage drop across this resistor, ie difference between bias voltage (about -20v in this instance) and negative supply (-75v here) for 55v dropped across a 10K resistor to give 5.5mA?

Yes.

Also, what would be a reasonable current to aim for?

It is a good place to start. I have been running my fets in the 5 to 10 mA range. Too much current and they get too hot.