‘69 era 50 Watt Plexi build squealing problem

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Webby
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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Sat Sep 12, 2020 5:23 am

Thanks Neil!
It was a lot neater when I first finished the build actually, and then I decided to change the circuit to Bass Amp specs (didn't care for that), then later ‘70’s Lead Specs (didn't care for that either!), and eventually back to ‘69 (which sounds best to my ears!).
I haven't got a shield in the bottom of the cabinet like some Marshall heads. I did have the chassis out on the bench though, and slid a thin sheet of aluminium underneath while testing. Didnt make any difference!
Thanks for your observations. Could you pinpoint the couple of places where the grid and plate wires are a bit close?
Thank you
Steve

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by danman » Sat Sep 12, 2020 6:30 pm

I also noticed that your blue (plate) wires and green (grid) wires are running right beside each other on all three preamp sockets. Ideally you would want a slight bit more lead length on the grid wires so that you can get a little separation between them and the plate wires. The plate wires should also be laying flat against the chassis while the grid wires are slightly elevated off the chassis to give them more distance from the plate wire. Also make sure that the plate leads cross the filament wires at 90 degree angles as much as possible. Some builds can be very sensitive to little lead dress errors while other builds work perfectly. Try these little fixes and see if the issue persist.

You can also try removing each preamp tube one at a time and see if the noise is still present. If the noise stops when you remove a certain tube, you have found the gain stage where the problem is occurring.

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Sun Sep 13, 2020 2:59 am

Hi Danman, and Neil.
OK, thanks for your advice. I’m away for a couple of day’s vacation, but I will try all of that when I get back.
Neil, that ‘twisted loop’ is already gone as I have re-installed the LarMar MV with shorter lead length.
Danman, which are the ‘plate leads’ and the ‘filament wires’ that I have to cross at 90 degrees if possible? Excuse my ignorance (I am an intelligent person, but I have very little knowledge of electronics, I just followed the build instructions and can solder to a reasonable standard!). 🙂

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by danman » Sun Sep 13, 2020 5:56 pm

In most Marshall style builds, the filament wires will be the twisted red and black pair if wires that connect to pins 4/5 and 9 of the preamp tubes. Plate wires are usually blue and connect to pins 1 and 6. Grid wires are green and connect at 2 and 7 and cathodes are yellow and connect at 3 and 8. Ideally you would want a bit of space between all of these wires as they make their way to the socket and always try to cross at 90 degree angle if a wire must cross over another. The pics in the build instructions should give you an idea of the perfect lead dress.

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Mon Sep 14, 2020 2:50 am

Hi Danman
Thanks for your reply. I will definitely try and get everything to follow the original build pics as far as I possibly can (the wires coming from under the board are not stretched tightly, so I should be able to improve on lead separation).
I never realised that the wire dress was that critical! I guess I was so concerned with making sure every wire was connected to the right place, and that the solder connections were cleanly executed, that I didn't pay enough attention to the lead dress.
Steve 😬👍

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by danman » Mon Sep 14, 2020 6:47 pm

Some amps seem to get by with less than optimal lead dress while others will exhibit issues like yours. Since you don't have a scope to trace down the issue, this will be the easiest place to start. I'm guessing that you have already tried different preamp and power tubes to make sure that one isn't acting up and causing the issue.

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Tue Sep 15, 2020 3:01 am

Hi Danman

I don’t have a scope, but I know a man who has! He’s a long retired ex-Royal Air Force Radar Engineer who never fails me. What he doesn't know about tube technology, you could write on the head of a pin! I always try and solve any problems initially by myself though, before I have to throw in the towel, and knock on his door!
Actually, I haven't tried different tubes, but I have done the ‘pencil tap test’ on all of the pre-amp tubes, and there is absolutely no hint of microphony. All of the tube filaments (Pre-amp and Output), are lit up normally, no blackening of the glass, and no obvious ‘firework displays’.
Back from vacation today, so I’ll be getting to work on the amp in the next couple of days.
I’ll start by switching out the tubes one by one, and if that doesn't throw anything obvious up, I’ll be trying all of yours and Neil’s suggestions.

I will report back when I’ve hopefully sorted the problem.

Thanks again

Steve

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by danman » Wed Sep 16, 2020 6:59 pm

In also had one other thought. Where on the chassis are you grounding the sleeve side of the output jacks? I see one black wire from the OT and I can also see a second black wire which I believe is your ground wire. Where on the chassis did you connect this? You may want to try a different grounding point and see if it helps. Just unsolder the lead from the ground lug and use a couple alligator clips and a short piece of wire as a test lead to try other grounding points nearby.

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Thu Sep 17, 2020 4:41 am

Hi Danman

Difficult to see in the picture, but there is a short black wire going from the right hand jack sleeve terminal, to a ground lug that is attached to the left hand socket mounting bolt of V4.

I have done absolutely all of the things suggested by everyone (except an Aluminium Shield in the bottom of the amp casing, which I will install shortly), but unfortunately nothing has made any difference whatsoever ☹️

I will try a different grounding point as you suggest, but if that doesn't work, it’s off to my friend with his scope!

I have been in touch with Tone King as well, because after all of this, there could be a problem with the Ironman Attenuator of course! They have asked if I could send them a short video clip highlighting the problems I am experiencing, so I will post a link to Google Photos on here for you to view/listen to.

Thanks again Danman and Neil

Steve 😎👍

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Thu Sep 17, 2020 11:16 am

Hi Danman and Neil.

OK ..... Here (hopefully 🤞) are 2 links to 2 short videos of the amp and attenuator combination screeching away!

https://photos.app.goo.gl/BA6vZ2vGrw4UpmKbA

https://photos.app.goo.gl/HA6udTgyPHGr6CN19

So there you have it! Go figure!

Steve 😬

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by danman » Thu Sep 17, 2020 6:51 pm

Definitely strange. Maybe the attenuator company will have some ideas about what's causing it.

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Fri Sep 18, 2020 3:36 am

Hi Neil.

Yes, let’s hope so! An interesting aside ....
A friend of mine has a Marshall 6100 30th Anniversary Head, and he says that at extreme gain levels, using a Passive, Resistive Load Attenuator (even though it’s a Master Volume equipped head, he still likes to get the power amp stage cooking a little), the amp will exhibit similar squealing sounds to the ones I am getting, but nowhere near as loud, and you have to be as close as a foot away from the amp Head/Attenuator. Also, touching the Attenuator casing has no extra effect on the level of the squeal, unlike the Ironman II, which is a very sophisticated (and expensive!), active, reactive load design.
I’m visiting my friend later today to see if the same thing happens with the Ironman II and his 6100 head.

Thanks again

Steve 😎👍

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Sun Sep 20, 2020 1:20 pm

Hi all.

Well .... after trying all of your suggestions (the last one being the fitting of an Aluminium Shield underneath the amp chassis), there is absolutely zero improvement! ☹️

So I have concluded that after exhaustive testing, there is absolutely no chance that the Ironman Attenuator is at fault, and that the problem has to be 100% within the amplifier itself! ☹️

So ..... it’s off to my my amp tech friend, his test bench and his scope! I will let you know the outcome.

Thanks again everyone.

Steve 👍

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by danman » Sun Sep 20, 2020 5:32 pm

Good luck! Please keep us posted on the results.

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Re: ‘69 era 50 Watt Plexi build squealing problem

Post by Webby » Sun Sep 20, 2020 6:48 pm

Will do! 😎👍

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