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Old 16th February 2014, 12:23 AM   #1
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Default Direct Coupled Output Stage, DC Bias Balance

Hi All,


I have a small 2XEL84 amp that I decided to experiment with a DC coupled output stage on… So far I like the results but I'm having trouble balancing the bias voltage at the cathode followers and I'm not sure why. I don't have schematic drawn up yet so I'll describe the sections below, maybe someone will spot an issue.

The phase splitter is a standard 12AX7 with 100K resistors and .022uF coupling caps, there is no feedback fro the output. These caps feed the grids of a set of cathode followers (12AU7) via 150k grid stoppers. A negative bias voltage of about -25vDC is delivered to the 150k resistors via 475k resistors. The cathode followers' cathodes are tied to a -40vDC rail via 47k resistors. The cathodes are tied directly to the power tube grids.

The strange thing is that the bias outage that shows up at the cathodes is not equal on both tubes… one tube is always about 1.2 voltages lower that the other. This results in a big disparity in plate current on the el84s. I've verified all the resistors and they are all matched within 1%, there is no evidence of leakage in any caps, and I've tried a few different AU7 tubes… the bias creeps up and down but it's always imbalanced by about the same amount. The power tubes are a fresh, matched set.

I've noticed that other cathode follower drivers tend to have separate bias pots - maybe this is why - but I figured that internal feedback in the follower would effectively force the cathode to follow the grid. I could try to hack a second bias pot but I've built myself into a corner and I'm not looking forward to the surgery.

Does it sound like I messed up by not using separate bias pots or is it possible I have some other issue here? Would it make sense to seek out a matched pair 12AU7?

Sorry for no diagram… I'll try to sketch something and post it.



Brian
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Old 14th April 2014, 01:07 AM   #2
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Hey,


I'm not surprised I didn't get any replies…. wordy post, no pictures. Well I went back to this build with a match pair 12AU7 and things worked a it better. I'm getting a little disparity in plate current on my el84s. There's exactly -35 volts on both grids of the 12au7 followers, -17.4 and -17.0vDC on the el84 grids, but I'm getting a huge disparity in grid current: 12 and 20ma. I'm pretty sure these are a matched pair but I'm wondering if anyone with more experience thinks that the .4vDC different could account for the nearly 50% difference in plate current?


Any thoughts?



Brian
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Old 14th April 2014, 04:33 AM   #3
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Brian,
You want individual bias adjustments for each side of the push pull.

I can see a few other things which are not optimal.

First the cathode followers with a 47K load to a -40V rail: The EL84 will require approx -10 to -12V on its grid (assuming no more than say 10 Ohms between EL84 cathode and 0V), check this out first, the -17V you read seems wrong.
That means a say 30V drop across the 47K resistor => 640uA. 640 microamps in a 12AU7 is just not enough, it is also not enough to get decent high frequncy performance (slewrate) from the amp particularly if you have higher Miller Capacitance at the EL84 grids from Ultralinear or Triode Mode operation.

Your negative rail neds to be enough voltage for the output tubes to be driven into cutoff. The "rule of thumb" is 3 times the normal bias voltage, so -40V should be OK (just).

Make sure you have grid stops on the EL84 (4K7 minimum). If they are oscillating at VHF frequencies then the voltage readings will be strange.

For your possible interest:
See post #603 here:
EL84 Amp - Baby Huey

Mosfet source follower in lieu of your 12AU7 cathode follower. Current source load in lieu of your 47K else much the same idea.

Cheers,
Ian
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Old 14th April 2014, 04:48 AM   #4
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150K grid stop resistor? Really?
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Old 14th April 2014, 01:43 PM   #5
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You can use CCS in the cathodes of your output valves (suitably bypassed) to ensure that DC balance is maintained at all times. It works very well.

Shoog
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Old 14th April 2014, 02:41 PM   #6
Merlinb is offline Merlinb  United Kingdom
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Quote:
one tube is always about 1.2 voltages lower that the other.
So you're saying one of the cathode followers is resting with 1.2V more bias than the other? That does sound like quite a large discrepency for a 12AU7, since both presumably have identical anode and grid voltages...

Quote:
Originally Posted by Hearinspace View Post
150K grid stop resistor? Really?
I think he's talking about a 150k + 475k potential divider between stages.

Last edited by Merlinb; 14th April 2014 at 02:47 PM.
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Old 14th April 2014, 06:36 PM   #7
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Thanks Merlin, but I still don't understand what he's doing. Why run your audio signal through a 150K grid stopper? . . . . . and as part of a divider, why throw away your signal voltage?

150K x 1uA is .15V, why add that complication to the circuit?

The GE data sheet calls for a max recommended grid resistance value of 250K for fixed bias . . . . .
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Old 14th April 2014, 09:05 PM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Hearinspace View Post
Thanks Merlin, but I still don't understand what he's doing. Why run your audio signal through a 150K grid stopper? . . . . . and as part of a divider, why throw away your signal voltage?
Dunno!

Quote:
The GE data sheet calls for a max recommended grid resistance value of 250K for fixed bias . . . . .
A cathode follower has plenty of degeneration, so at least this isn't a problem.
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Old 15th April 2014, 02:37 AM   #9
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Folks,


Sorry, let me play catch up a bit. Below is a quick (incomplete) schematic and a picture of the build. It's basically a matchless spitfire type amp with a direct coupled cathode follower for each el84 inserted after the LTP phase splitter. I pulled the cathode follower design from the only instrument amp I know of with a negative rail and followers… the SVT. That's where I got the 150k grid stoppers.

There are errors on this. For example, the signal enters and leaves the phase splitter triodes via the grids… rather than leave via the plate (whoops). Aside from that you can get the general idea. I really need to draw something else up that is more representative but the signal leaves the LTP at the plates, passes through the 150k grid stopper to the grid of the followers; and there is a 475k bias feed (shown as 150k per the SVT schematic but increased to not load down the LTP). There should be a ground symbol at the power tube cathodes (far right), and the unmarked resistors are there to set the bias range… I'll have to double check them.

Click the image to open in full size.

Click the image to open in full size.

I'm not doing a great job of helping the forum help me I think… that's why I wanted to refocus on smaller pieces of the problem. Does the mis-match in cathode bias voltage in the followers make sense? Should I need matched a matched triode 12au7? Could the large cathode resistors be working against me?


-Brian
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Old 15th April 2014, 09:00 AM   #10
Merlinb is offline Merlinb  United Kingdom
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I can see no reason for such a large mismatch between the 12AU7s, assuming you really have built what you think you have built. Are you sure you have enough cathode resistance for the power tubes? Maybe one of them is biased too hot and is drawing grid current? Stick a couple of small grid stoppers on them (100R say). Not only is this good practice, it allows you to check for grid current too.

Are both the V- points actually the same point?

Check your resistances. Maybe one of them isn't what you think it is.

Nice looking build, BTW.

What is that black twisted pair disappearing off the bottom of the photo? It looks like it should go to the grid stoppers of the cathode followers??

Last edited by Merlinb; 15th April 2014 at 09:10 AM.
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