Strange Noise From Amp

sportoghese · 1st July 2010, 10:21 PM

Hi guys,

I have basic understanding of amplifier operation, and I'm trying to gain some further knowlege. I have a problem with an amp I acquired second hand about six months ago:

Starting a few days ago when I first turn it on it makes a motorboating type noise until it warms up, then it's fine. The amp is a Wavac MD-300B I can not seem to obtain a schematic of its circuit. Right now I only have extra input tubes to roll, and that doesn't seem to be the problem. Anyone have any ideas? How about schematics for this amp? They seem to be sequestered. Thanks for the help.

Best,
Steve

mjf · 2nd July 2010, 06:29 AM

hello.
have you an idea how old this amp is?
.......if it is older than 20 years or so you can begin with changing old bypass electroytics.there are often some small elcos inside like 4,7uf.........10uf....... parallel to resistors.perhaps any bad elco is the reason for motorboating.
greetings

sportoghese · 2nd July 2010, 02:23 PM

Thanks for the response. I don't know how old it is. I can't even seem to find out how long they've been making it for.

Replace bypass caps: check.
Could you explain why the motorboating subsides after warm up? Thanks again.

Steve

DF96 · 2nd July 2010, 03:07 PM

Two reasons why LF oscillation can sometimes reduce when warmed up:
1. The whole feedback loop has such a peculiar phase response that reducing gain causes oscillation. This sounds counter-intuitive but the maths of loop stability says it is possible. It is not often seen though.
2. The unwanted feedback is via the HT supply but under normal conditions there is some degree of cancellation. During warm-up some stages come up faster than others so the balance is temporarily disturbed.

The second one is more likely, and could be caused by a capacitor changing value due to age. Without a circuit diagram it is difficult to say more.

sportoghese · 2nd July 2010, 08:29 PM

Follow up: This problem turns out to be intermittent. Today when I fired the amp up, it didn't let out any unpleasant burps at all. Would this point to caps that are teetering on the edge of giving out? Could an iffy tube make this type of noise? I've heard crackly tubes,and wavery sounding tubes, etc., but not burping.

Matt.B.H. · 2nd July 2010, 09:26 PM

Stick some photos on of the guts. It shouldnt be too much to work out

Cheers Matt.

1st July 2010, 10:21 PM	#1
sportoghese diyAudio Member Join Date: May 2010 Location: st.paul	Strange Noise From Amp Hi guys, I have basic understanding of amplifier operation, and I'm trying to gain some further knowlege. I have a problem with an amp I acquired second hand about six months ago: Starting a few days ago when I first turn it on it makes a motorboating type noise until it warms up, then it's fine. The amp is a Wavac MD-300B I can not seem to obtain a schematic of its circuit. Right now I only have extra input tubes to roll, and that doesn't seem to be the problem. Anyone have any ideas? How about schematics for this amp? They seem to be sequestered. Thanks for the help. Best, Steve

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2nd July 2010, 06:29 AM	#2
mjf diyAudio Member Join Date: Jan 2009 Location: upper austria/near linz	hello. have you an idea how old this amp is? .......if it is older than 20 years or so you can begin with changing old bypass electroytics.there are often some small elcos inside like 4,7uf.........10uf....... parallel to resistors.perhaps any bad elco is the reason for motorboating. greetings

2nd July 2010, 02:23 PM	#3
sportoghese diyAudio Member Join Date: May 2010 Location: st.paul	Thanks for the response. I don't know how old it is. I can't even seem to find out how long they've been making it for. Replace bypass caps: check. Could you explain why the motorboating subsides after warm up? Thanks again. Steve

2nd July 2010, 03:07 PM	#4
DF96 diyAudio Member Join Date: May 2007	Two reasons why LF oscillation can sometimes reduce when warmed up: 1. The whole feedback loop has such a peculiar phase response that reducing gain causes oscillation. This sounds counter-intuitive but the maths of loop stability says it is possible. It is not often seen though. 2. The unwanted feedback is via the HT supply but under normal conditions there is some degree of cancellation. During warm-up some stages come up faster than others so the balance is temporarily disturbed. The second one is more likely, and could be caused by a capacitor changing value due to age. Without a circuit diagram it is difficult to say more.

2nd July 2010, 08:29 PM	#5
sportoghese diyAudio Member Join Date: May 2010 Location: st.paul	Follow up: This problem turns out to be intermittent. Today when I fired the amp up, it didn't let out any unpleasant burps at all. Would this point to caps that are teetering on the edge of giving out? Could an iffy tube make this type of noise? I've heard crackly tubes,and wavery sounding tubes, etc., but not burping.

2nd July 2010, 09:26 PM	#6
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	Stick some photos on of the guts. It shouldnt be too much to work out Cheers Matt.

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