Hum in SVT power amp?

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Mark
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Hum in SVT power amp?

Post by Mark »

I have been trying to get the 50hz hum out off my brothers SVT power amp. The hum appears to be in the PI and driver stages as far as I can tell. The amp is quiet with only the output valves in place.

I reasoned that there can be three possible causes for the hum. One being filter caps, earthing, and 6.3 vac filaments.

I did check the hum balance pot and it is working fine. I have put the filament to the board on a switch so I can turn them off while the amp is running to if the hum decreases and the hum didn't decrease at all. I also checked bias of the amp so that wasn't a factor.

I haven't done much with earthing other move the location of the 10 ohm about chassis earth resistor. That made no difference to the hum.

As far as the power supply goes I measured the B+ and I have a rail voltage of 675VDC with a ripple voltage of 5.6vac, the second node has 560VDC with a ripple voltage of 0.12vac. The bias cap has -160VDC with 0.2vac.
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Stevem
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Re: Hum in SVT power amp?

Post by Stevem »

That's a strange problem because once you kill the filiments you have taken 50 cycle hum out of the picture for those 9 pin tubes and at that pint with no gain that can take place from those tubes then even 100 cycle AC ripple hum on the D.C side of the power supply would not be a issue!
My first best guess here is that one of those 9 pin tubes has a short.
Have you subbed in known good driver tubes?
Has the amp been recapped and do the filter ground points look like they where moved, also is this newer SVT ?
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Mark
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Re: Hum in SVT power amp?

Post by Mark »

Thanks for your reply Steven, I should elaborate I switched the heaters out for the nine pine valves out for a couple of seconds so the cathodes were still warm. I figured turning the heaters off momentarily would result in the hum stopping,but there was no change in the hum.

I have tried changing the PI valve but not the drivers. I'll give that a go tomorrow. They are NOS AWA 12BH7's and have been in the amp for a while.

The filter caps are original, but they appear to be fine as the the hum levels are what I would expect. I don't like the 100uF used for the bias as there is a saw tooth waveform on the bias. I did sub a new 100uF cap in place of the original cap and there was no difference. I put both caps in parallel and the hum was reduced. I'm thinking the blackface Fender value of 470uF is required. However this isn't the source of the hum. I know this as the amp is quiet without the PI and driver valves in place.

The SVT is a Japanese black face model built in the early 80's.
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nworbetan
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Re: Hum in SVT power amp?

Post by nworbetan »

Mark wrote: Sun Aug 26, 2018 1:35 pm I don't like the 100uF used for the bias as there is a saw tooth waveform on the bias. I did sub a new 100uF cap in place of the original cap and there was no difference. I put both caps in parallel and the hum was reduced. I'm thinking the blackface Fender value of 470uF is required. However this isn't the source of the hum. I know this as the amp is quiet without the PI and driver valves in place.
It looks like you're on the right path if pulling the PI and driver tubes removes the hum. Try this to narrow it down even more: pull just the PI first, then put it back and pull just the drivers.

I don't know what else you've measured and not posted about, so I'll just ask a couple questions based on the numbers you've posted.

What's the amplitude of the sawtooth wave both with and without the driver tubes? Where are you measuring the sawtooth at? Does it show up at points E and H also, or just D?
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Re: Hum in SVT power amp?

Post by tubeswell »

Ohm out the Rg2s anyway - just to make sure
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Mark
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Re: Hum in SVT power amp?

Post by Mark »

]This is the ripple on the centre tap of the output transformer.
IMG_0693.JPG
This is the output from the output transformer secondary.
IMG_0697.JPG
Increasing the filtering of the main node.
IMG_0695.JPG
Ripple at the main node to the output transformer.
IMG_0694.JPG
Hum at the speaker with increased filtering.
IMG_0696.JPG
As shown the filtering hasn't had much impact upon the hum. When the amp is connected to a load it is dead quiet.
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Last edited by Mark on Tue Aug 28, 2018 10:04 am, edited 7 times in total.
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tubeswell
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Re: Hum in SVT power amp?

Post by tubeswell »

Re: the bias sawtooth - I see you've tried subbing the bias supply filter cap. I still think you should check for an open Rg2.
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Mark
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Re: Hum in SVT power amp?

Post by Mark »

I'll check the Rg2 resistor, by this do you mean the 47K grid stoppers or the screen grid resistors?

This is the sawtooth waveform on the bias supply, it also has an underlying 50hz hum too. I would like to vastly increase it's value to 470uF but I'm not sure how the time constant of charging the supply up would affect the whole bias supply.
IMG_0698.JPG
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Mark
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Re: Hum in SVT power amp?

Post by Mark »

I decided to measured all resistors around the output valves, they were all fine.

I looked at waveforms on the plate, screen and input of the output valves.

As I noticed the amp was quieter without the driver valves installed I decided to measure around the output valves under four conditions mentioned below.

I took four sets of measurements, first with the AWA 12BH7 driver valves in the amp (the noisiest).

The plate has a 24Vpp sawtooth waveform on both push-pull sides of the output valves.
The screen grid has a 1.7Vpp sawtooth waveform on it.
The control grid has a distorted 220mV sawtooth waveform.

Second without any 12BH7's in the amp (the quietest.)

The plate has a 2.5Vpp sawtooth waveform
The screen grid has a 10mVpp sawtooth waveform on it.
The control grid has a distorted 300mVpp sawtooth waveform.

Thirdly a set of GE 12BH7 driver valves in the amp (quieter).

The plate has a 14Vpp sawtooth waveform
The screen grid has a 1.4Vpp sawtooth waveform on it.
The control grid has a distorted 110mVpp sawtooth waveform.

AWA 12BH7's back in the SVT with the input pin 2 grounded (acceptably quiet.)

The plate has a 19Vpp sawtooth waveform
The screen grid has a 1.5Vpp sawtooth waveform on it.
The control grid has a distorted 90mVpp sawtooth waveform.

My thoughts on this are the amp is quietest with the 12BH7's removed as the output valves are biased hard off and there is little demand on the power supply, thus very little ripple.

The GE 12BH7's have less ripple but about the same hum as the bias has to be reset when the 12BH7's are replaced. The question is how is the hum getting onto pin 2 of the 12BH7?
Last edited by Mark on Fri Aug 31, 2018 12:27 pm, edited 7 times in total.
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tubeswell
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Re: Hum in SVT power amp?

Post by tubeswell »

Mark wrote: Tue Aug 28, 2018 10:08 am I'll check the Rg2 resistor, by this do you mean the 47K grid stoppers or the screen grid resistors?

This is the sawtooth waveform on the bias supply, it also has an underlying 50hz hum too. I would like to vastly increase it's value to 470uF but I'm not sure how the time constant of charging the supply up would affect the whole bias supply.

IMG_0698.JPG
A sawtooth wave like that on a power supply can indicate a bad smoothing cap.
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Mark
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Re: Hum in SVT power amp?

Post by Mark »

A sawtooth wave like that on a power supply can indicate a bad smoothing cap.
Yeah, that's a possibility, I have tried putting caps in parallel with the main filter caps, this does improve the hum but it doesn't establish if there is a faulty filter cap.
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Mark
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Re: Hum in SVT power amp?

Post by Mark »

I have traced the signal back to it's origin though that doesn't really help either.

I find there is roughly 20mV of hum on the plate of the first triode stage, this is then amplified through the subsequential stages.

I have put a wire on pin 7 and earth and there was no change to the signal. I removed the 12DW7 and the hum is gone. This dispels any thoughts that the hum is coming from the AC filaments of perhaps the socket is conductive.

Any thoughts?

https://drtube.com/schematics/ampeg/v9pwr.gif

https://www.thetubestore.com/lib/thetub ... ematic.pdf

IMG_0701.JPG
IMG_0702.JPG
IMG_0703.JPG
IMG_0704.JPG
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Re: Hum in SVT power amp?

Post by martin manning »

What is the frequency of this waveform? 50 Hz? What happens if you ground the grid of that first stage to the bottom of its 470k grid leak resistor (i.e., short R2)? Does the hum go away?
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Re: Hum in SVT power amp?

Post by Mark »

Thanks for your reply Martin. The input of the power amp is shorted to ground. In addition I shorted the input to ground with a wire going from the ground on the PCB to the grid (pin 7) on the socket of the valve. No change.

I did check the output and it was 50hz. I will recheck and confirm the frequency this afternoon.

EDIT: The hum is 20mS wide, so yes it is 50hz hum.

I have had one guy say the amp has grounding issues in it's design. I have tried a few things and I can't prove or disprove a grounding issue. I'll try and post images that show what is actually happening.

Check out the SVT earthing diagram in my first post it shows how the earthing is really layed out. For example there are two tracks that are used for earthing on the PCB. One track handles the bridge rectifier and 6550 cathodes. The other track is used for the PI and the driver follower valves earth connections. The PI cap and the lower half of the bridge rectifier filter cap share the same earth connection as they are in the same can cap.
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Re: Hum in SVT power amp?

Post by martin manning »

Thanks for the update. What I was hoping to do by shorting the grid leak resistor (eliminating any input signal) was to determine whether the hum originated in the input network or the first triode itself. Since the hum is 50 Hz, it’s source is not any of the rectified voltages. More likely is due to a poor ground connection. Perhaps cleaning, tightening, and or resoldering all of the grounds around the input and first triode stage would help. I also recall reading about grounding issues with this amp.
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