Dynaco ST70 big Snorting noise. - diyAudio
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Old 23rd December 2012, 07:10 PM   #1
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Default Dynaco ST70 big Snorting noise.

Not a hardcore diy question but it's about as diy as I've gotten so far... A few years back I did a socket-up rebuild of an ST70 using some upgrades from Joe Curcio. It works and sounds good but I noticed that when there's no input, the right channel quickly crescendos from quiet to a very loud (i.e. turn it off QUICK!) howling snorting noise. It seems like a feedback sort of thing. With a shorting plug in place it's completely quiet. With the right speaker accidentally disconnected, you can hear the noise mechanically in the output transformer, yikes. The left channel is fine either way.

I tried supplementing the ground to the right input rca but that made no difference.

I put a page together with some pictures of the wiring and an audio clip in which I momentarily take out the shorting rca a couple of times and replace it. I am wondering if the routing of one of my wires for the right channel might be the problem. Any ideas?

Pics & audio here: Untitled Document
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Old 23rd December 2012, 08:54 PM   #2
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I would start by cleaning all the flux off the driver board. After you have finished that I would get the soldering iron out and warmed up and touch up all the solder joints and then try it again. Make sure not to forget to load the output of the amp.
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Old 23rd December 2012, 09:43 PM   #3
chip647 is offline chip647  United States
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Your amp is oscillating with the open input. Double check the input resistance to ground. If this is 100k or something, simply do not run the amp with no input. Simple.
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Old 23rd December 2012, 09:47 PM   #4
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O'Burned - This problem existed immediately after my rebuild of the amp at which point all the solder connections were fresh. New driver board, new sockets, new rca's etc.

Re. flux, ok I'll take a close look at that.

Whatever it is completely stabilizes and disappears when there's an input or a short across the input. I wonder if I should check for a voltage across the input rca - should there be 0 volts across the input?

Thanks for your reply, hopefully I'll find a big bunch of flux somewhere and that will be the end of it
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Old 23rd December 2012, 09:52 PM   #5
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Chip647, oscillating with the open input is a perfect description.

Obviously the L & R should be the same so if they're different then this would be a good hint; I've been using it but it's always kind of seemed less than ideal to have an unknown issue like that and just ignore it.

Would this oscillation be inherent to the design or more likely something not quite right in wiring, components etc.? I thought maybe since I hadn't built any amps before maybe it was wire routing or something I got wrong... Thanks.
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Old 23rd December 2012, 11:53 PM   #6
Tesla88 is offline Tesla88  Italy
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Is the driver based on the orginal Dynaco or on the newer driver using triodes only?
Anyway when you say "when there's no input" you mean input connectors removed or signal from source set to 0 ?
Are you sure the NFB phase is correct?
It can also be a problem related to wiring...try lowering input impedance
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Old 24th December 2012, 12:21 AM   #7
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Quote:
Originally Posted by Tesla88 View Post
Is the driver based on the orginal Dynaco or on the newer driver using triodes only?
All triode I believe.

Quote:
Originally Posted by Tesla88 View Post
Anyway when you say "when there's no input" you mean input connectors removed or signal from source set to 0 ?
When there is nothing plugged into the right-side RCA input jack. With a preamp plugged into it it's fine, and fine with a shorting plug inserted.

Quote:
Originally Posted by Tesla88 View Post
Are you sure the NFB phase is correct?
Nope in fact I don't know what that is Guessing "no feedback", ? How would I check that phasing?

Quote:
Originally Posted by Tesla88 View Post
It can also be a problem related to wiring...try lowering input impedance
How would I go about lowering input impedance, is there a resistor across the input?

Thanks...
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Old 24th December 2012, 03:20 AM   #8
6L6 is online now 6L6  United States
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Have you swapped the tubes left to right and it still does the same thing?
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Old 24th December 2012, 03:52 AM   #9
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Got it. Thank you all! Basic diagnostics I guess, I should have thought of it, if you have one good channel, start comparing and see what's different. Per Chip647, I checked the resistance on each of the inputs. ~150K on the good side and open on the other. That made it easy for me to go over the board with a loupe and I found a sneaky little missed connection. Soldered that up and bingo, silence. Yay! Thanks again.
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Old 24th December 2012, 08:52 AM   #10
Tesla88 is offline Tesla88  Italy
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Great ! So the problem was due to the high input impedance and also the fact the grid of input triode was not closed to ground, maybe.

Quote:
How would I go about lowering input impedance, is there a resistor across the input?
Yes , but yuo spotted yet! a 150k is a resonable value , a 100 - 47 K is a more classic value , but untill the amp is ok with a 150K there's no reason to lower it.

Quote:
Nope in fact I don't know what that is Guessing "no feedback", ? How would I check that phasing?
If you swapped the Output Trasformer primary connections for mistake , the NFB work like a positive feedback , in most case you get a self oscillating amplifier that produces a squealing sound so is easy to spot because amp wont work at all
Sometimes when feedback is low (not the dynaco , with like 20dB of NFB) or when the amp oscillates over 20KHz , you just wont notice the fact of the wrong NFB phase ...
This appened to a friend of mine's amp i had to repair ,a guitar amp , the shop changed the OPT , amp was working but bad sounding.
When i tested it with a sinewve at input , the output displayed a triangular wave
Just swap the primary of OPT and the amp is ready to rock again

Cheers
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