Amp Whistles

[musicfixerupper]

Hi, i have a super reverb build that makes a whistling noise when the standby switch is shut off and then fades out. What could cause this?

[Matty_V]

Hopefully someone can answer this. I believe it was my JMP that would groan when I flipped the standby. I rewired much of the circuit and cured whatever it was. I'd like to know what it was I fixed in case I run into this again!

[RicharD]

It's nothing to be concerned about.  When you disconnect B+, the caps remain charged until the loads drain them.  Probably what is happening is as one or more of the preamp tubes falls out of bias and starts oscillating.  The output tubes, being of fixed bias in in a Stupor Reverb, will simply fade out as B+ fades out. 

[Matty_V]

Thats a pretty good explanation. Now that I think about it...I had my situation backwards. It would make a funky sound when I would flip from standby to play. What might cause that?

[RicharD]

Same kinda deal except perhaps there is a large charge time somewhere in the power supply, so a tube might oscillate before it reaches quiescent state.  These anomalies are much more prevalent in fixed bias amps because if the filaments are at temp, and there is already a bias voltage present, the output tubes are gonna do their thang right away as soon as B+ is applied and will continue to do their thang after B+ is disconnected.  Meanwhile at the low end of the power supply it might take a few extra seconds for the circuit to reach a quiescent state. Again, it's nothing to worry about.  When experimenting and cranking the variac slowly, I've heard a plethora of whistles and buzzes.  My Kelsey Bass circuit with the filaments left energized will amplify to some degree for almost a minute after disconnecting B+.  A whole lot of filtering and very little current draw.

[DummyLoad]

hmmm... i've overhauled 2 supers and did not have that issue. you said it was a silverface? i converted a 71 silverface to blackface and the 2nd was a 67 blackface. neither whistled before or after their respective overhaul. perhaps it's a quirk with the revision you have, or maybe you have PS cap(s) that are tired and taking some time to come up to idle. does it buzz? the SR is a quiet amp. what model SR is it? if it settles down, i wouldn't worry. the 71 silverface i overhauled had a popping in the reverb circuit, because the fish-paper board became conductive. the popping would stop after about 10-15 warm-up. sold it as-is where-is and new owner was not discouraged.

--DL 

[overtone]

could it be induced by the choke?
I cannot remember, but I am sure Merlin Blencowe mentioned this possibility under certain circumstances, in combination with the standby switch, in his latest power supply book. Oh man, my memory... I am travelling right now so I cannot look it up for a while.

[overtone]

I am now back at Amp HQ and have refreshed my brain with Merlin's power supply book (Page 78).
Butterylicious nailed it and the book explains in some more detail how a choke can be part of the cause.

A quick summary of the points made in the book for your information:

At switch-on there is an inrush of current.
The HT will tend to oscillate at the resonant frequency of the choke.
DC current & ripple voltage across the choke will both be abnormally large for a short time.
This can push the choke into saturation, causing its inductance to fall and the resonant frequency will rise into the audio range.
For this reason you may hear throbbing or squealing sounds during switch on.
(This is also possible in switch off.)
It will be made worse if a stand-by switch is used because the inrush will be heavier & the amp is warmed up and ready to amplify any oscillations.

By the way you may <edit fix it> preclude the issue at the design stage by damping, or critically damping, the choke. This involves adding some resistance after the choke but you will drop more DC voltage. I found the Duncan Powers Supply Designer 2 was very useful at dialing that value in.
Best, Tony

[Matty_V]

I guess one way you could verify that would be to replace the choke with a resistor and see if it still oscillates like that?

[jojokeo]

I guess one way you could verify that would be to replace the choke with a resistor and see if it still oscillates like that?

Perhaps, but on pg 80 in "tuned LC filters" he says you can simply add a smoothing cap in parallel across the choke and the filter can be "tuned" to reject a particular frequency more effectively than all others. If one particular frequency gives trouble it may be possible to suppress it via this method & gives formulas for figuring what it is and what cap value to use.

[RicharD]

What's to fix?  When you power stuff up -n- down, it's gonna sound funny for a few seconds.  It ain't hurtin nothing.

[jojokeo]

What's to fix? It ain't hurtin nothing.

I agree but if someone gets bugged about something and wants to pay me to fix it, then I won't argue.

[Matty_V]

That oscillation noise could arguably be the best sound that comes out of my speakers, depending on who you ask! I personally like my amps to be as poorly grounded and hum inducing as possible. 

[RicharD]

My understanding was the oscillation only happens when switching to and from standby and not when in normal playing mode.  I'm a big believer in, "if it ain't broke, don't fix it."  I also won't sell somebody a repair they don't need.  Maybe I missed something somewhere?   

[DummyLoad]

That oscillation noise could arguably be the best sound that comes out of my speakers, depending on who you ask! I personally like my amps to be as poorly grounded and hum inducing as possible. 

you want a late model hi-watt then... ask frank_57...   

[overtone]

My understanding was the oscillation only happens when switching to and from standby and not when in normal playing mode.  I'm a big believer in, "if it ain't broke, don't fix it."  I also won't sell somebody a repair they don't need.  Maybe I missed something somewhere?   

No you did not miss anything, Matty_V asked what the causes were and in my reply I mentioned, as a by the way, that there are ways to avoid the issue at the design stage.
I subscribe to the "if it ain't broke, don't fix it" philosophy too and my use of the words "fix it" were careless in this case. I will edit that reply to "preclude the issue at the design stage",
best to you all, tony

[jojokeo]

I also won't sell somebody a repair they don't need.  Maybe I missed something somewhere?   

That wasn't my point nor does it describe my moral values. I get some guys that even after explaining things like this, will say to do it anyways AND aren't worried about a few more dollars for something that they want. What am I going to do or say then? "No! No amp work for you!", the Amp Nazi. (reference taken from the Seinfeld soup nazi episode in case anyone doesn't get the humor?)

[RicharD]

oops!  I certainly didn't mean any disrespect.  Jojokeo is a long time, highly respected member of this forum.  Please accept my apology.  I gotta quit posting when I'm drinking.

[jojokeo]

Hey, "no worries mate" as my Aussie drummer friend likes to say. I just don't want anyone thinking I'd do whatever (repair that is) for money in that way. I just enjoy helping people and making them happy even if they want something special or whatever but may not always be of special value or worth it or a difference maker. Fellow musicians can be quite quirky w/ their "special needs".

If they feel Sprague Atoms, mustard caps, and carbon comps give them that special tone, mojo, and confidence edge to their playing then that's exactly what they've got to have ya know? It almost doesn't matter if it really does make a difference or not as long as they believe it does and if it emanates by way of better performance then it's all good in my book.

Cheers mate! 
Joe

[TIMBO]

Hey jojokeo, It's about time you guys learn't how to speak australian, it would make it far easier to understand when you explain the technical JARGON.      And the Quote is more like "no Wucken Forrires mate" or in short "no Wuckers" Hope this helps.

[jojokeo]

Hey jojokeo, It's about time you guys learn't how to speak australian

Isn't all I have to say is "Foster's" (according to our TV adds anyway)? Ironically I hear the aussie's don't even like Fosters?

My buddy says our way of talking is considered boring and when he tries to immitate it, it sounds like a really bad attempt of John Wayne meets Ozzie Osbourne. Very funny in itself to hear that.

A couple more close friends are from Belgium (French speaking side) & Holland w/ heavy accents and have a way of word useage that combined w/ a few other local cultural ethnicities (LA/OC area) all cracking back on each other can get gut-bustingly funny in a hurry. Those moments are magic.