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PostPosted: 21 Mar 2015, 16:35 
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Location: Ennis MT
I am having some trouble trying to solve, what I and friends I have spoken with, seems to think to be blocking distortion on this parallel PP mono block amp.
The symptom is that in pentode, at max volume, I get what sounds like a bad two stroke outboard motor effect sound, which also is causing all the voltages, B+ and filament, to drop and vary up and down whilst this distortion is happening. As soon as a lower the volume a little bit, no more issues.
In triode mode, this distortion starts at about 50% volume, and keeps oscillating, with other words, lowering the volume will not stop it, only switching back to pentode will stop it.
Now, the driver circuit is pretty much a direct copy of the VTA, Tubes4Hifi, octal driver for their M125 mono block. What I have changed is to go from a 'paired' BIAS adjustment to individual BIAS adjustment, by adding two output coupling caps, two extra BIAS trimmers and resistors, and now each output tubes has its own 10R BIAS resistor. Other than that, no other changes have been made to the original driver and output circuits.
Of course, the original driver board with the paired BIAS adjustment has been working fine for many years.
After some discussions with others, I lowered the values of the output coupling caps, C5 to 8, from .33uF to .1uF, decreased the BIAS adjust resistors, R13 to 16, from 150K to 100K, increased the grid tie resistors from 1K to 2.2K and decrease the NFB resistor, R5, from 6K8 to 4K7.
This has almost eliminated the distortion in pentode mode, but in triode, it starts at about 65-70% volume level, still oscillating, only stopping it when switching back to pentode.
Some have suggested to drop the coupling caps from .1uF to .047uF, since I do not have any on hand, I am waiting for some to be shipped.
I would really appreciate thoughts and ideas. Am I heading in the right direction with the above value changes, or is something else the problem.
I have attached diagrams.
Thank you for any thoughts and suggestions.
BTW, I do have permission from the people at VTA & Tubes4Hifi to use their circuit and change it. :thumbsup:


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Last edited by nohum on 21 Mar 2015, 16:39, edited 1 time in total.

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PostPosted: 21 Mar 2015, 16:36 
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here is the driver diagram


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PostPosted: 21 Mar 2015, 17:43 
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I have r.un across this only one time before. It was with a Eico HF50. It was a bad rectifier tube. It was a bad 5ar4 that had low emission and was causing motor boating. One other thing that it does sound like you have your OPT has the phase reversed. Causing positive feed back instead of negative.


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PostPosted: 21 Mar 2015, 18:46 
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wolfdog428 wrote:
I have r.un across this only one time before. It was with a Eico HF50. It was a bad rectifier tube. It was a bad 5ar4 that had low emission and was causing motor boating. One other thing that it does sound like you have your OPT has the phase reversed. Causing positive feed back instead of negative.


thanks. I know its not the rectifier tubes, I am actually using two in parallel, but that was the first thing I swapped over, even put in a Weber Copper Top WZ64.
I also know its not phase reversing, as that would make that sound all the time.....I know from experience here :D
The interesting thing about this problem is that it only occurs at max volume, in pentode, and especially when in a given song vocals and instruments are at high levels, especially bass. Then of course in triode mode, at about, now, 65% volume level with those changed component values, before the changes, in triode, it started at about 50% and in pentode more regular at max volume.
It is certainly a head scratcher right now!....... :confused:
I am hoping that some of the experts here have come across this same problem and found a solution to it.
I have also read this very good article, http://www.aikenamps.com/index.php/what ... distortion, and pretty much have tried all the changes suggested. Maybe I am on the right road, just need to keep playing with the values of the components in questions.


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PostPosted: 21 Mar 2015, 21:07 
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What you are describing is called "motor boating". It is a regenerative feedback that, once started, drives operating poles to the open right half plane. You have insufficient gain margin in whatever feedback loop is in operation. Normally motor-boating is caused by power supply coupling between the driver stage and the power stage. However, you have good isolation there. My guess is that the power stage is way off balance and is self oscillating. This is probably happening when it begins to transition from class A to class AB. Is this a mono-block or do you have two channels hanging off the same power supply?

How are you balancing the output tubes? Remember that total currents in the output transformer must match at zero drive. You need to check your output stage and your LTP for proper balance.

Start by disconnecting the global feed back and checking the drive waveforms with the power tubes removed. If they are clean, then insert the power tubes and recheck the drive waveforms at the input to the power tubes. Look for a power tube going into grid conduction.

Using a single tone, drive the amp and look for low frequency variation in the plate waveform. You may be getting feedback before you actually hear it.

Do these checks and report back.

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PostPosted: 22 Mar 2015, 08:39 
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I have seen this behavior as well when the amp is unstable and oscillating either below audio frequencies or above them. The best way to figure it out is put it on a scope and feed it a sine wave signal (with a load naturally) and see what it is doing. Just swapping parts in and out will prove frustrating and may never solve the problem. The situation is particularly insidious at very low frequencies. I had one of our commercial kits oscillate at 0.4 HZ. It would break into this made at mid to high volume settings. The problem in this case is unlikely to be the same as yours as it was a screen winding problem inside the output transformer that had a short. Without a scope it would have been nearly impossible to find the problem.

EDIT: BTW you are running Ultra Linear mode and triode modes not true pentode mode.

Good listening
Bruce

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PostPosted: 22 Mar 2015, 10:00 
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Location: Ennis MT
Suncalc wrote:
What you are describing is called "motor boating". It is a regenerative feedback that, once started, drives operating poles to the open right half plane. You have insufficient gain margin in whatever feedback loop is in operation. Normally motor-boating is caused by power supply coupling between the driver stage and the power stage. However, you have good isolation there. My guess is that the power stage is way off balance and is self oscillating. This is probably happening when it begins to transition from class A to class AB. Is this a mono-block or do you have two channels hanging off the same power supply?

How are you balancing the output tubes? Remember that total currents in the output transformer must match at zero drive. You need to check your output stage and your LTP for proper balance.

Start by disconnecting the global feed back and checking the drive waveforms with the power tubes removed. If they are clean, then insert the power tubes and recheck the drive waveforms at the input to the power tubes. Look for a power tube going into grid conduction.

Using a single tone, drive the amp and look for low frequency variation in the plate waveform. You may be getting feedback before you actually hear it.

Do these checks and report back.


yes, it is a mono block. I have also reverted back to the original power supply circuit, that is, using a multi cap. I designed my own filter cap board, in order to eliminate having to use the old style can multi cap. It made no difference reverting back to the multi cap filter stage, as well, the filter cap board I designed is used in other builds with no issues.
I did disconnect the NFB, which actually made it worse, the onset of the oscillation was much sooner. But I have not yet connected the scope, which I guess I should have done from the get go! I will do so, pump in a 1V sine wave at say 50Hz and see what it all looks like at various stages in the circuit.
What I still don't quite understand is that from going from a dual BIAS circuit, that is, each pair of paralleled output tubes originally had one BIAS adjustment, to individual power tube BIASing, by adding the two extra coupling caps, extra BIAS trimmers, has made such an impact.
I will take some reading and get back to you guys.
Hey, I really appreciate your help here, I was starting to go bald tearing my hair out! :eek:

forgot to add: I am balancing the output tubes with individual BIAS adjustments, circuit details are on the driver circuit schematic. And as I've mentioned above, this is the only thing I changed from the original VTA circuit, going from BIASing two tubes together to individual BIASing, and thats when this problem started.


Last edited by nohum on 22 Mar 2015, 10:09, edited 1 time in total.

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PostPosted: 22 Mar 2015, 10:02 
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Joined: 12 Mar 2015, 17:24
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Location: Ennis MT
gofar99 wrote:
I have seen this behavior as well when the amp is unstable and oscillating either below audio frequencies or above them. The best way to figure it out is put it on a scope and feed it a sine wave signal (with a load naturally) and see what it is doing. Just swapping parts in and out will prove frustrating and may never solve the problem. The situation is particularly insidious at very low frequencies. I had one of our commercial kits oscillate at 0.4 HZ. It would break into this made at mid to high volume settings. The problem in this case is unlikely to be the same as yours as it was a screen winding problem inside the output transformer that had a short. Without a scope it would have been nearly impossible to find the problem.

EDIT: BTW you are running Ultra Linear mode and triode modes not true pentode mode.

Good listening
Bruce


yes, I stand corrected on the ultra linear mode.
Thank you for your suggestion, I will scope it out and see what I get and report back.


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PostPosted: 23 Mar 2015, 21:06 
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Joined: 12 Mar 2015, 17:24
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Location: Ennis MT
OK, today I had a short window when I was able to hook up my scope and signal generator. The attached photo's show the result of the distortion coming in. Top is the input signal @ 1.5V, bottom trace is looking at the output, speaker post, with an 8OHM dummy load.
The first photo shows all is well in 'pentode' mode, bottom trace, output, is at max volume.
The remaining photos show the result of the distortion in 'triode mode', volume at about 65%
In all photo's, input remained the same, @ 1.5V.
Notice how the input too seems to be affected, like some feed back through the preamp circuit, and of course the amplitude of the output drops quite dramatically.
Scale for the top trace is 1V per division, bottom trace is 10V per division.
That is all I could do today. Next will be with the output tubes removed and checking the signal through the 'preamp' stage.


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PostPosted: 23 Mar 2015, 21:35 
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If your input is being affected, there are one of two most probable answers. Either, you set the scope to trigger on the output waveform and your view of the driving waveform was thus compromised or (and more likely) the level of feedback is so massive that it's driving the input triode into grid conduction do to an inexact phase lag.

Disconnect the feedback input to the driver triode, make sure that you're triggering on the input waveform directly out of the signal generator, and run the exact same tests again.

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