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Dynaco St70 hum. Not so subtle on my Klipsch kg4.2s!

poorbastards

poorbastards

Active Member
I bought a st70 a few days ago. There wasn't any hum when I first used it but wanted to wait until I had installed a potentiometer to fully test/use it.
The prior owner installed a different switch for power and that sparked on me last night,I went in and cleaned up the connections and there is more a slight hum. Very subtle on my bench speakers but noticable on my Klipsch and it's driving me insane.

I feel like I would have noticed the hum the first time I powered it on and I don't think it's from the potentiometer.

Really not sure what could use started causing this. Is the switch damaged from the zap it took?
 
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Is the hum in one channel or equal in both.

If it's both then the problem lies in the power supply.

So let's start - post your results

Also I know this sounds stupid but is it a real hum or a hiss.
 
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airtime said:
Is the hum in one channel or equal in both.

If it's both then the problem lies in the power supply.

So let's start - post your results

Also I know this sounds stupid but is it a real hum or a hiss.
Click to expand...
It's a hum for sure. Out of both speakers equal in sound and does not strengthen nor weaken with volume adjustment
 
Ok then the problem is most likely in your power supply circuit.

First remove the 7199 tubes or what ever driver tubes you are using.
If the hum disappears then replace those tubes. Or lets hope it's not something on that board.

If the hum remains after you pull the driver tubes then it would seem your quad cap has aged out.

I'm sorry but I didn't notice - has this amp been worked on. restored or modified???
 
airtime said:
Ok then the problem is most likely in your power supply circuit.

First remove the 7199 tubes or what ever driver tubes you are using.
If the hum disappears then replace those tubes. Or lets hope it's not something on that board.

If the hum remains after you pull the driver tubes then it would seem your quad cap has aged out.

I'm sorry but I didn't notice - has this amp been worked on. restored or modified???
Click to expand...
I should have added that I'm sorry. He claimed the quad cap was replaced, it does have the new vta board and he modified the toggle switch, I added a potentiometer.
I'll give that a try
 
poorbastards said:
Yeah I knew that, if they weren't so efficient it wouldn't bother me. The question is how it what went wrong...
Click to expand...

Ahh, you didn't say whether you used it with the KIipsch before instaIIing the voIume pot- my bad I aIways run stuff on the bench through junk/cheap speakers before hooking it up to anything good
 
I should add, you did make sure the hum was coming from the amp and not a preamp????

Disconnect the preamp and listen for the hum, or lack of.
 
airtime said:
I should add, you did make sure the hum was coming from the amp and not a preamp????

Disconnect the preamp and listen for the hum, or lack of.
Click to expand...
This is all without a preamp, that's why I'm installing the potentiometer. So with nothing hooked up to the RCA and the 3 12au7's removed I turn it on and it slowly starts to hum. It may be slightly worse in the left channel but not much.
No difference when turning the onboard potentiometer I installed.
When I turn if off it hum stops instantly, no fade.

Like I had mentioned in my original post the power switch sparked on me last night. It still shows an open and closed circuit on my voltmeter. He did wire if odd, it's a mini toggle with 6 solder prongs. 2 for ground, 2 different amp levels and 2 different volt levels. He had it soldered each line across 2 prongs.

It worked before just fine, this hum seemed to have started once it sparked on me. I wish I knew more but that's it
 
Measure the quad cap voltages

put the tubes back in, hook up the speakers and measure the quad cap voltages. Be careful they can hurt you.

Section “A” (Designated by a Square): 305 VDC
Section “B” (Designated by a Triangle): 375 VDC
Section “C” (There is No Designator): 415 VDC
Section “D” (Designated by a Semi Circle): 435 VDC

Do you know what driver board those 3 12au7 tubes are in?

I would undo that power switch.

Are you sure you wired the volume control with a proper ground?
 
also, short the inputs to the amp, floating inputs have been known to cause hum. See if the hum persists when the inputs to the amp are shorted
Also, what is the bias set to. The VTA allows for individual biasing of each EL34, so check each bias setting, 400mV is what I use for EL34's.
 
airtime said:
Measure the quad cap voltages

put the tubes back in, hook up the speakers and measure the quad cap voltages. Be careful they can hurt you.

Section “A” (Designated by a Square): 305 VDC
Section “B” (Designated by a Triangle): 375 VDC
Section “C” (There is No Designator): 415 VDC
Section “D” (Designated by a Semi Circle): 435 VDC

Do you know what driver board those 3 12au7 tubes are in?

I would undo that power switch.

Are you sure you wired the volume control with a proper ground?
Click to expand...
[ ] - 454
∆ - 375
D - 440
- 430

While it was on is side that thing zapped again. I couldn't tell where it came from but again it seemed to come from where the toggle is.
That sound always gets you're hear racing!!
 
Erhard-Audio said:
also, short the inputs to the amp, floating inputs have been known to cause hum. See if the hum persists when the inputs to the amp are shorted
Also, what is the bias set to. The VTA allows for individual biasing of each EL34, so check each bias setting, 400mV is what I use for EL34's.
Click to expand...
I have several amps that will hum without something plugged into them. Do try shorting plugs if you find your voltages right. Or plug a source into it.
 
I would remove that toggle switch and redo it right. That switch could be your problem. The voltages are right so the cap should be OK.

What is your bias voltage at? check it

The first section is a little high
 
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Lavane said:
I have several amps that will hum without something plugged into them. Do try shorting plugs if you find your voltages right. Or plug a source into it.
Click to expand...
I feel like a dunce but I can't find an explanation online. Does shorting the inputs just mean grounding them to the actual chassis and not to where they normally ground to?
 
Doesn't matter usually. My shorting plugs are some cheap RCA plugs that have a wire soldered from the center pin to the outer ring. Doesn't need to be anything too special.
 
poorbastards said:
I feel like a dunce but I can't find an explanation online. Does shorting the inputs just mean grounding them to the actual chassis and not to where they normally ground to?
Click to expand...

signal ground and chassis ground/B+ ground are the same in the ST70. So basically yes, short the center of the RCA input sockets, which is signal, to the chassis or grab a couple of RCA plugs and short the center pin to the outer/RCA plug case pin.
 
Just for a baseline, my rebuilt ST70 is dead quiet into KEF 104/2 speakers, just using a pot between CDP or phono preamp.
 
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