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Old 29th March 2012, 12:19 AM   #1
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Default Differential Amp CCS Tail SVCS Plate - Noise Issues

I've been playing with this circuit now for a few days. It can sound very good but there is a noise issue I'm not sure how to approach. When the "right side" SVCS board is set to anything above 108 VDC there is a white noise (pfffffff) audible on the speaker. (The noise off-on knee is sharp, over something like a volt or two) If the other SVCS is taken up above about 120 that noise increases by something like a third. If I take the right hand plate on its own up to about 180VDC the noise then stops.

Thinking there might be some kind of current source interaction between the SVCS and the LM317 I have tried bypassing the SVCS loads with 180K and also 100K resistors. I've also tried bypassing the driver tubes with a 180K resistor from plate to ground. I think the noise is lowered a little in each case but by hardly enough to justify leaving the resistors in.

The amp sounds great with the driver plates below 108V but the operating point can hardly be dictated by the limits of a problem. I believe something is oscillating but where and why I don't know.

I also tried resistors for plate loads. No hiss but the sound isn't nearly as good as with the SVCS. Can anyone offer suggestions?

The SVCS board circuits are as in the Schematic posted here but with Q1 = ST P3NK80Z , Q2 = FQU11P06TU , Q3 = MJE5731A

Also not shown on my drawing both 8233's have 100R stoppers on G1
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Old 29th March 2012, 12:52 AM   #2
avp1 is offline avp1  United States
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Originally Posted by Hearinspace View Post
I've been playing with this circuit now for a few days. It can sound very good but there is a noise issue I'm not sure how to approach. When the "right side" SVCS board is set to anything above 108 VDC there is a white noise (pfffffff) audible on the speaker. (The noise off-on knee is sharp, over something like a volt or two) If the other SVCS is taken up above about 120 that noise increases by something like a third. If I take the right hand plate on its own up to about 180VDC the noise then stops.

Thinking there might be some kind of current source interaction between the SVCS and the LM317 I have tried bypassing the SVCS loads with 180K and also 100K resistors. I've also tried bypassing the driver tubes with a 180K resistor from plate to ground. I think the noise is lowered a little in each case but by hardly enough to justify leaving the resistors in.

The amp sounds great with the driver plates below 108V but the operating point can hardly be dictated by the limits of a problem. I believe something is oscillating but where and why I don't know.

I also tried resistors for plate loads. No hiss but the sound isn't nearly as good as with the SVCS. Can anyone offer suggestions?

The SVCS board circuits are as in the Schematic posted here but with Q1 = ST P3NK80Z , Q2 = FQU11P06TU , Q3 = MJE5731A

Also not shown on my drawing both 8233's have 100R stoppers on G1
Are you sure that this is nose and not oscillation? Did you measure noise spectrum (FFT)? When you use complex circuits like SVCS, they can oscillate at frequencies up to tens of MHz.

Last edited by avp1; 29th March 2012 at 12:54 AM.
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Old 29th March 2012, 01:20 AM   #3
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With no grid stops and no power supply decoupling, you've got several possibilities for parasitics. Do the obvious stuff first.

All good fortune,
Chris
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Old 29th March 2012, 06:14 AM   #4
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Quote:
Originally Posted by Hearinspace View Post
... I believe something is oscillating but where and why I don't know.

Also not shown on my drawing both 8233's have 100R stoppers on G1
So you do think it's oscillating, which seems likely to me also.

The power supply is totally decoupled by R7, C3, and Q1 which set a steady DC voltage to the gyrator.

You do have grid stoppers on the 8233s

So the obvious is covered...

Your triode connection already provides plate resistance, which is why your addition of load resistors only changed the onset of the oscillation.

I'd get a wideband scope on it and see if the oscillation is common mode or differential.

Last edited by Michael Koster; 29th March 2012 at 06:16 AM.
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Old 29th March 2012, 07:22 AM   #5
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Hi Ian,

First question - If you have CCS-type anode load, why CCS cathode bias? The dc path is confined to each 8233, so if the two CCS unit get different ideas about what the current level should be then there could be a fight. Does the trouble stay away if you switch to resistors in the cathode (or LED(s) if you want all the gain)? I would start with resistors, as lowered gain may be helpful.

If the 8233 is the type I'm thinking of [aka E55L] then you make 50mA/V in triode! That will need some serious precautions against oscillation in any connexion. I doubt that 100R grid stopper is high enough - I would experiment with much higher values. Please check that the anode wiring is as short as can be, and that the SVCS units are reasonably spaced away from the grid-associated parts: coupling of only a few pF between these sections can provide the undesired feedback at VHF.

Sometimes, audible oscillation is actually HF or VHF oscillation, with an audio envelope. Means to check for high frequency behaviour is worth looking into: scope for preference, but a portable radio is useful as a guide.
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Old 29th March 2012, 10:45 AM   #6
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Just a simple side observation. I am an advocate of the cathode arrangement you have on the front end, but LM317 are not linear enough to allow the circuit to give of its best. I would be looking to replace them with something better.

Shoog
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Old 29th March 2012, 03:23 PM   #7
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The circuit has a constant voltage gyrator on top, a resistance in the middle, and a CCS on the bottom. Sound familiar Rod ;-)

But actually the CCS is a tail so only maintains constant total current in the 2 cathodes.

That's why I would suspect a common mode oscillation. Both tubes get involved due to the high impedance tail. More stopper-age may help but maybe you can puzzle out what's really happening.

It most likely is RF oscillation sounding like a sharp hiss.

Last edited by Michael Koster; 29th March 2012 at 03:27 PM.
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Old 29th March 2012, 03:45 PM   #8
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Well, I'd put grid stops on the 2A3's first thing. But that's just me.

All good fortune,
Chris
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Old 29th March 2012, 06:41 PM   #9
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Wow you guys. I'm overwhelmed. How to put together a cohesive reply ? . . . . .

FIrst two replies are now moot. But to Chris I'll say that on your suggestion I did put 100R grid stoppers on the 2A3 but no effect on the oscillation. On your suggestion of decoupling, I took it to refer to channel to channel PS filter split on the last stage, one for each. As this is only a mono amp that wouldn't apply but your suggestion prompted me to try a separate RC (220R/200uF) stage ahead of each of the SVCS boards. It didn't kill the oscillation but I got the impression it might have raised the Q a little. This was from watching the voltages as I adjusted the driver loads. They seemed to be a little more tightly averse to settling whenever I got close to matching them exactly. A little like magnets with both north poles facing getting the most skittish the closer to perfect alignment you get.


Michael, Thanks for jumping in. I've tried shorting the output stage grids to each other with a cap to see of noise source was input or output stage and the noise stopped. If the oscillation was common mode in the output stage would the cap stop it?
I also appreciate your intent to locate the source. I don't think stoppers improve sound and would rather eliminate the need for them.

Rod, What can I say dude. I still don't know how to use LT Spice and have to build everything that looks interesting enough. It's tedious sometimes but there's nothing for it I'm afraid. I got interested in the driver cathode arrangement after seeing Shoog's post on his own amp with that config on the output stage. It's just struck me as being worth trying. Then, being a big fan of Wavebourn's SVCS circuit it occurred to me that using it would allow me to direct couple the two stages, using the ccs's in the drivers to get them operating in a more or less balanced way and using the SVCS output voltages to balance the output tubes. That's more or less the idea. Sheldon explained the SVCS circuit to me in a way that made me see how it is a morph of a typical CCS so I can understand your thinking. But I also ran across references to the possibility of using it this way ( - though I confess I found no examples of its actually being tried.) I will try upping the stoppers and will tidy up the plate wiring, though it IS a breadboard and spaghetti is the side dish.


Shoog, It's clearly a neat sounding circuit. Thanks very much for posting on your amp. What do you recommend?

ANyway, I'll try putting a scope on it tonight. I don't have a differential probe that I trust but maybe with two probes and channels I can see what's what.
If anyone has setup suggestions I'm all ears.
Thanks
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Old 29th March 2012, 06:57 PM   #10
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I used the simple cascode MJE340 double transistor CCS which was popularized on this board in a similar situation. I use LM317's on output stages, but at that point the requirements aren't quite so exacting as in the front end - where imbalance will propagate all the way to the output.

I had real issues with the output stage on my amps using CCs and direct coupling. Gary Pimm had the same issues with his Tabor amp (which my amp designs are based upon). Basically the full voltage momentarily developed across the CCS at power up - which in your case would be 420V, or possibly more due to inductive spiking across the OT. A simple LM317 will toast instantly in this situation. I eventually overcame the problem on my latest amp by using a LM317HV equivalent which is good to 250V and also protected it with a 100V zener. The B+ on this amp is only 120V so all has been fine so far.
I also built a Tabor clone using 807's and went through a year of hell trying to find CCS which would withstand power up transients, eventually I ended up using some of the IXY 900V chips which have been fine ever since.

The great and amazing thing is that this arrangement of input-output constant-current operated direct coupled stages has been in operation for over a year at 15hrs a day with no issues and no need for any form of adjustment. Eventually I expect the voltage across the CCS to drop below a functional level as the valves age and then the amp will start to oscillate - but so far so good.

My gut feeling as to your problem is that above about 10Khz the LM317's are dropping in impedance to far below the gyrators impedance and there is a conflict at the point of transition. If that is the case then making the cathode CCS much stiffer will probably sort the problem out.

Shoog

Last edited by Shoog; 29th March 2012 at 07:27 PM.
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