Hiwatt Lead 50 (SG-50) Noise issues

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JD0x0
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Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

I'm trying to diagnose issues with noise/microphonics with my Hiwatt Lead 50. It happens when I turn the volume and/or gain up weird high pitched oscillating sort of squeal, extremely high pitched. Also hissing with a high pitched squeal, in there, especially if I turn up the presence or treble. On top of all this there seems to be some 'ticks' and thumps going on.

Also, if I tap on the front panel or the top of the amp in a specific area there is often a very loud thump, audible through the amp, even if the preamp gain is set to '0'

Is this filter cap behavior or should I be looking into other things? I'm pretty sure it's not the tubes, no noise with a tap test, and IIRC it happened with the previous set of tubes.

Other than this, the amp sounds phenomenal, as long as the master volume is kept at a low level, there is almost no noise, or static, and there's no squealing or high pitched sounds.
Yeah, it's not a 'real' Hiwatt, but it's still a good amp, IMO. Most of the wiring looks clean, there's no components that appear to be leaking or have popped. In fact, the big Filter caps almost look to be non-Original.
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xtian
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by xtian »

If you can tap on the tubes and hear no trouble, but when you tap the face plate you hear a loud thump, I agree the problem is probably not the tubes. Sounds like a loose connection or bad solder joint. Another possibility is a microphonic coupling cap. I would investigate with a chop stick. Probe, push, tap every component and connection. Can you narrow down the issue?
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JD0x0
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

Thanks for the help!
I poked around, and found 2 microphonic caps

[img:480:360]http://i89.photobucket.com/albums/k201/ ... 50edit.png[/img]
The white .001uF is extremely microphonic, and likely the main culprit. I hope this is the source of my issues, but I guess I'll know more once I get those caps replaced. It's hard to know as, if I turn the amp up enough, I can get a faint tapping almost anywhere in that area on the chassis. I'm assuming it's just because that white cap is so intensely microphonic.
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by Stevem »

I would also remove one output tube at a time and see if the issue changes as having that issue with the gain and master volume down is what that leads me to think.
Your microphonic cap would need to be after the last gain stage circuit wise for that to be the main issue, is that where it is?
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JD0x0
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

Thanks for the input!

The bad caps are coming off V1. upon further inspection, I've discovered that the V2 tube socket seems to be microphonic as well. I tried contact cleaner and re-tensioning the socket, but the tube still feels extremely loose so I am just going to replace the socket, along with V1 which will also rule out a possible bad solder joint at those tube.

With the exception of some hiss, and some sensitivity to outside electronic interference, with the master dimed, the amp doesn't sound too noisy, and doesn't suffer from those other symptoms, as long as the preamp volume is kept off, at this point. Which leads me to think the main issue is at V1. This thing is riddled with old carbon comp resistors, so I dont expect it to be silent.
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

Update: Swapped out the microphonic caps and swapped the V2 tube socket. Improvement. The new socket holds the tube much better, but V2 is still suffering from a small amount of microphonics with the controls dimed.
Let me ask, how likely is it for a carbon resistor from pin 7 to ground to induce microphonics? It's really my biggest lead at this point, though I will triple check my soldering tomorrow, that resistor is essentially the only component on V2 which has not been swapped. I've been trying to pinpoint if it's coming from a specific area or pin, but pretty much anywhere I tap on V2 will induce similar microphonics. I've tried like 5 different tubes which I know are good, just to ensure I wasn't putting a bad tube in that slot.

I'm taking it slow, step by step, to ensure I'm not making anything worse :lol:
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by Stevem »

I think your beating a dead horse at this point, as any component with enough gain so to speak passing thru it will be microphonic !

Hell, hook up a big 1400 volt .01 Ciramic cap on the input grid of any amp and you can talk into the thing as if it where a Mic!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
JD0x0
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

V1 is still ringing like a bell, with the preamp gain set to pretty much anything above '0' and if I turn the preamp gain way up, without an instrument plugged in, I'll get very high pitched feedback with the gain/master/presence set high. Also, if I tap on the front panel or parts of the chassis hard enough a LOUD pop is heard through the amp. The microphonics on V1 are still bad enough that tapping anywhere on the chassis with either the 'bright' and/or 'normal' preamp gain dimed, will easily be audible through the amp. I think once I get V1 sorted, the amp should be playable.
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xtian
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by xtian »

This would be the perfect time to drop in one of those Jet City solid state tube replacements! You could definitively rule out tube microphonics.
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JD0x0
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

xtian wrote:This would be the perfect time to drop in one of those Jet City solid state tube replacements! You could definitively rule out tube microphonics.
Wow! First I've heard I've them, at ~$10 a piece seems like it could be something cool to try. They could come in handy as backup tubes, too.

Thanks for the heads up!
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

Update: I've managed to subdue the issue quite a bit, to the point I can now crank the preamp and master volumes to '10' without squealing, EXCEPT when the presence is cranked. At about 2:00 the control introduces a large amount of audible hiss, and if the volume is set high, there's a very nasty squeal/screech. V1 still has noticeable microphonics, but it's much more manageable, now. Also, the microphonics seem to be notably worse with the 'bright' volume turned up compared to the 'normal' volume, at this point.



FWIW, this is happening without an instrument or cable plugged in.

Almost there :D
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JD0x0
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

I seem to have diagnosed the issue. I tried clipping a 330pF cap across pins 1 and 3 of V1 and viola, seems I can crank every dial without the amp screaming out of control. So I guess it was HF oscillation at V1. While I wait for some parts, I guess my dilemma is whether to actually add a 47pF-100pF there, or just leave the amp. As is, the amp is sounding great and I have the issue under control enough that the amp is pretty much gig worthy without any worries, as pretty much no one would crank the Preamp, Master and Presence all to '10' in a Live situation, which is pretty much the only way I can get the issue to show up, now. My question is, would the 47pF change or degrade the tone noticeably? The 330pF seemed to subdue the highs, but I didn't have a smaller value cap on hand to try.

Also, I got the noise floor WAY lower, with some metal films I had on hand. I gave the amp a pretty decent overhaul, though. It was more for curiosity, than anything, but now, the amp sounds way better. And I thought it sounded really good, before. It's now, tighter, smoother, no nasty grit or 'hash' and blooms more than my ODS clone. :shock: Sounds more like a Hiwatt than a Marshall now, but it's got some brighter Dumble flavor in the OD. I really love this thing, now, which is why I'm skeptical to make any changes to V1.
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by Stevem »

Oscillations take place between a minimum of 3 gain stages due to coupling when two are in phase and reinforcing each other.

The first gain stage flips the the incoming signals phase as does the next stage, and after the phase flip at the 3rd stage then stage 1 and 3 are in phase and any of there wires or components that are within 3 inches of each other can make for the oscillation issue!
Something's to check.

1) Many times with a issue like yours if you are already using sheilded grid wire and your power supply filter caps are big enough and also good ( good as in low ESR ) to uncouple the gain stages then simply swapping the plate coupling caps end for end with kill a oscillation issue. Coupling caps have a self sheilding effect when installed in the right signal flow direction.

2) the grid leads are one of the most likely to a sourse of this type of issue and as such grid leads need to very short.
On the output side of any gain stage once you get down stream of the blocking/ coupling cap then at that point the next gain stages grid lead starts!
What this means build wise is that it's better to have the plate leads be long if need be then the grid leads. Grid leads act like antenna's!
On some builds depending on the layout anything longer than a 4 inch grid lead can get you in trouble!

3) many times anything more than two gain stages on one power supply node is asking for it also. Adding another node fed by even only a 2k dropping resistor may be enough to lick the problem and make for just a minor voltage drop that does not effect the gain your looking for!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
JD0x0
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Re: Hiwatt Lead 50 (SG-50) Noise issues

Post by JD0x0 »

Got some low quality clips recorded on my 5 year old android phone, now that I've got the amp in good running condition. :P
Really doesn't do the amp justice at all but gives an idea what the amp can do. The phone mic obviously adds a lot of compression and isn't able to pickup up the higher highs, and lower lows, that you can hear in the room.

Custom tele with a strat pickup in the neck, overwound A2 single coil in the bridge. Kept all controls on the guitar at '10.' preamp gains both maxed out. Tried just using picking dynamics, but of course the mic is compressing them quite a bit.
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