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Old 30th December 2017, 04:26 PM   #21
GeorgK is offline GeorgK  Austria
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If turnung the master down does not change anything you can rule out the preamp and focus on the power stage.
Does it depend on the speaker load?
I do not see any NFB loop in the power amp, is there any?
Try to remove the tube before the PI, does anything change?
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Old 30th December 2017, 04:49 PM   #22
blackwhaleamp is offline blackwhaleamp  Belgium
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Actually, when I move the Master, the oscillation goes away. So, it is at the pre-amp. I'll start removing some tubes to see if I can spot the issue. There is no NFB loop in the amp
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Old 30th December 2017, 05:38 PM   #23
GeorgK is offline GeorgK  Austria
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Didn't you say the controls had no effect?

Does the oscillation continue on the pre with the master down?
If yes, can you find them on some preamp supply rail?

If so there may be in fact a problem with the supply.

If some stage is oscillating by itself, i.e. without feedback via the supply rails, the reason way even be some bad wiring layout.
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Old 31st December 2017, 10:39 AM   #24
blackwhaleamp is offline blackwhaleamp  Belgium
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Quote:
Originally Posted by GeorgK View Post
Didn't you say the controls had no effect?
Sorry, I was not specific enough

Quote:
Originally Posted by GeorgK View Post
Does the oscillation continue on the pre with the master down?
If yes, can you find them on some preamp supply rail?

If so there may be in fact a problem with the supply.

If some stage is oscillating by itself, i.e. without feedback via the supply rails, the reason way even be some bad wiring layout.
I removed V1 and V2, and the oscillation still there. If I remove V3 or V4 it dissapears.

Then, I decided to leave all the tubes and start to remove the plate voltage. So, if I also removed the power for V3 at R31, the oscillation still there. Same case if I remove the power for V4 at R33.

Another thing about V4, only half tube is used. The other half is no connected, but the fillament at PIN 5 is connected.
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Old 31st December 2017, 12:00 PM   #25
GeorgK is offline GeorgK  Austria
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The unused electrodes of V4 are tied to GND?

Did I get that right? When pulling either V3 or V4 oscillation stops, but as long as they are installed, you would have to remove plate voltage on both V3 and V4 to stop it ?
It still is not clear to me whether the oscillation depends on the master volume setting. Is it gone when turned down or just not audible any more?
Have you measured at the output of V4?
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Old 1st January 2018, 05:30 PM   #26
blackwhaleamp is offline blackwhaleamp  Belgium
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Quote:
Originally Posted by GeorgK View Post
The unused electrodes of V4 are tied to GND?
Yes, the electrodes are grounded

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Originally Posted by GeorgK View Post
Did I get that right? When pulling either V3 or V4 oscillation stops, but as long as they are installed, you would have to remove plate voltage on both V3 and V4 to stop it ?
So, if I remove the tubes V3/V4 and if I remove the plate voltage, the oscillation still present

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It still is not clear to me whether the oscillation depends on the master volume setting. Is it gone when turned down or just not audible any more?
Have you measured at the output of V4?
Yes, the oscillation depends on the master volume. So, as soon as I put the master on zero, the oscillation is gone.

I measure with the scope before V3, and I couldn't see any oscillation. However, when I measure on PIN1 of the Master Pot I see some noise in the scope. I am not sure if that is the PLUK noise, because I hear it every 3/4 seconds
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Old 2nd January 2018, 05:47 PM   #27
GeorgK is offline GeorgK  Austria
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The timebase of the scope is set to 5ms? Then this can't be spikes occurring every 3 or 4 seconds.
You should be able to see the "your" oscillation on the speaker output. You will probably find it on the grids of the power tubes as well. Do you see it on any point of the HV supply?
Going backwards from the speaker, look for any signs of your noise on the signal path. You should to find out if it caused by interference over the supply.

I am still not sure if you get it from the pre or power stage...

You say it is still present with V3&4 removed?? This would point to the power stage, but it does not make sense that something should change with the master vol position then.

Last edited by GeorgK; 2nd January 2018 at 05:49 PM.
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Old 5th January 2018, 06:43 PM   #28
blackwhaleamp is offline blackwhaleamp  Belgium
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Quote:
Originally Posted by GeorgK View Post
The timebase of the scope is set to 5ms? Then this can't be spikes occurring every 3 or 4 seconds.
You should be able to see the "your" oscillation on the speaker output. You will probably find it on the grids of the power tubes as well. Do you see it on any point of the HV supply?
Going backwards from the speaker, look for any signs of your noise on the signal path. You should to find out if it caused by interference over the supply.

I am still not sure if you get it from the pre or power stage...

You say it is still present with V3&4 removed?? This would point to the power stage, but it does not make sense that something should change with the master vol position then.
I made a small video (Shared album - Gustavo Rangel - Google Photos), so you can hear better how it sounds. The volume of the pluk is a bit low, so I hope you can hear it.

I let the amplifier on for a while, and the pluk-pluk disappear by itself. Then it comes again. Then it goes again. It's like it comes always around 10min after I switch ON the amp, stays for 10min and then it goes

I will measure the signal from the grid at the KT88, and let you know how it looks like

Thank a lot for your help, it is very valuable for me
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Old 5th January 2018, 09:21 PM   #29
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by blackwhaleamp View Post
However, when I measure on PIN1 of the Master Pot I see some noise in the scope. I am not sure if that is the PLUK noise, because I hear it every 3/4 seconds
Sorry, that was some misunderstanding of mine, I thought of one in 3 seconds, you meant 3 times a second. Not that important.
Was the control up or down?

I would proceed like that:

As the spikes on your scope cannot be your noise (what you measured is probably line related, 200 Hz) see what the speaker output looks like. Set your scope to about 250ms/div, if possible. Or choose another convenient setting where it can be clearly displayed. Then trace the signal back.
In similar cases, I switch the input of my scope to AC-coupling and look for every sign of this signal, also on the supply rails. But I don't know if this is possible with your PC device? And if so, make sure the coupling withstands the HV and use a 10:1 probe for the supply.
Shorten the input (or the grid R, respectively) of V4a, V5a. Does it go away?
Pull one pair of power tubes, then the other. Changes?

Do you feel the master volume just controls the level of this signal as if it would be coming from the pre?
Or is there a point of potentiometer setting where the oscillation rather stops?
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Old 7th January 2018, 10:52 AM   #30
Parafeed813 is offline Parafeed813  Netherlands
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You leave the amp on for a while and it comes and goes?
Low freq instability could come from the power supply as well.
Can you measure the voltages on the anode and g2 supplies for the KT88s?
Both dc and the ac ripple. Are these readings different when the amp is oscillating and when it is silent?

Low volume, low freq can still be high voltage swings and ditto currents.
I once had an amp that had a low freq oscillation when the volume was higher than zero. It turned out that there was not enough decoupling between the anode and g2 supply of the power valves:a 150 ohm choke and similar capacity before and after it. The result was that any sag on the anode supply caused a reverse current: the electrolytic for g2 was charging the electrolytic for the anodes. A simple diode blocking this reverse current fixed it.
Now in your amp, there is only 470 ohms between anode and g2 supply and similar capacitance around it. Could be a similar problem. Put a diode in series with this 470 ohm resistor to try this out.

Besides all this, there are a few suggestions I'd like to make.
A 3A slow blow fuse for the HT is dangerous. If something goes catastrophically wrong on the HT side, you want it to blow fast and with less margin.
I know the GEC datasheet lists the max g1 grid leak for fixed bias at 100k, but I'd prefer lower values, specially for new production. The similar 6550(a) has a 50k max. In your amp you have 100k+22k.
You have 100uF as the first reservoir cap, which is directly feeding 4 KT88s (200 in series with 200). This is not much considering KT88s can draw a lot of current.
Lasty, 600+ Volts and 200W from a quad of KT88s is a heavy task for any KT88. Both for NOS and new production. I wouldn't expect long life.
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