A Marshall AFD Sqealing problem (video included)

[yalesmith]

yeah the timers are ok...... I have tried a bunch of different tubes but i may try a 12au7 or a 12at7 in position 1 or 2 and see if that does anything.

thanks

[ampdoc1]

Yale,

The only times I have heard that level of squealing was when the OT leads were backwards. I don't know your level of expertise, but the chance of having them backwards is 50% in any given build, unless the polarity of the wires are labeled.

I'd give it a try if you haven't done it yet.

a'doc1

[yalesmith]

I have tried switching them...the squealing only occurs when the gain is past half way. The Plexi channel is fine, only the hot rodded channel squeals

[yalesmith]

any chance my transformer is underpowered? I ask because i have no clue as to how mA relates or affects amp design and performance. The amp is running 6550's - Thanks

[yalesmith]

I have figured it out and i am embarrassed to say it was lead dress on all the shielded wires. I always thought that the more shield, the better job it would do of shielding. I learned (the hard way) that you need to give a little space between the shield and the wire.

First problem was an issue where the wire was touching the shield, there had to been an area where the plastic wire covering melted or split. The second thing that helped with noise was when i lenghened the distance between the shield and the wire the amp got quieter.....i assuming there was some leakage.

Again thanks for all the tips and hopefully someone can learn from such silly mistakes.

Dave

[mhartman]

Congrats on your new amp! I built one of these a while back (https://tubeamparchive.com/viewtopic.php?t=17499) and I'm still enjoying it. At first I played a lot of GNR with it, but I've discovered that it can do much more, especially with the nice sounding plexi channel. Nice job!

[yalesmith]

where you strucked about how scooped the sound was and how on the edge (about to blow up) the amp was? There is an insane amount of gain, it threw me off, almost soldano SLO like (next build).

[mhartman]

where you strucked about how scooped the sound was and how on the edge (about to blow up) the amp was? There is an insane amount of gain, it threw me off, almost soldano SLO like (next build).

Yeah, the gain on the high gain channel is off the charts. I find myself using it at 9:00 or below on the dial. I considered tweaking it for less gain, but got sucked into my next project and decided to leave it. Play with the multiple master volumes to tweak the gain a bit too (I like allowing for some power tube gain instead of all preamp gain).

[Structo]

I see the input cable has the shield connected to pin 1.

While that can lower noise somewhat, it is a dangerous practice to put high voltage anywhere that can have a fault and pose a hazard to the operator.

[Lindz]

Structo, your comment is timely for me, I was going to post about this very topic. I have a Ceriatone built Yeti here that I just popped out of the cab to have a look at and it has the input jack cable shielding soldered to pin 1 but with a 1 meg resistor in series on the pin end. There is a 56k grid stopper on the lead and a 1 meg on the shield both attached to pin 2

I had never seen this before and was curious about it. I assumed it was a method of reducing noise but since I had never seen this before I thought I would post a dumb question, but your comment and pics of the AFD amp beat me to it. I am thinking that Nik is doing this on a few of his high gain amps

The input jack has aluminum shielding around it inside the chassis and there is no 1 meg to ground on the jack either - Nik has essentially moved the 1 meg from the jack that would normally go to ground and instead moved it to pin 2 via the shield

If attaching the shield like is indeed not best practice (safety wise) would you think the resistor would add enough additional safety margin? I assume OP's amp has the resistor on the jack side

To me any trick in the book to reduce noise is good as long as its safe

[Koop]

I think Mark is right (good eye!). I see the middle leg of all three trim pots hanging near the edge of the board unconnected. The layout appears to show the middle leg attached to the 1k resistor on the two trim pots on the right and to the 0.47uf cap on the left one.

Oops...I posted this after reading page one, didn't realize the answer was already found.

[M Fowler]

I think Mark is right (good eye!). I see the middle leg of all three trim pots hanging near the edge of the board unconnected. The layout appears to show the middle leg attached to the 1k resistor on the two trim pots on the right and to the 0.47uf cap on the left one.

Apparently the wiper is connected but we can't see that from the photo.

Mark

[Structo]

Structo, your comment is timely for me, I was going to post about this very topic. I have a Ceriatone built Yeti here that I just popped out of the cab to have a look at and it has the input jack cable shielding soldered to pin 1 but with a 1 meg resistor in series on the pin end. There is a 56k grid stopper on the lead and a 1 meg on the shield both attached to pin 1

I had never seen this before and was curious about it. I assumed it was a method of reducing noise but since I had never seen this before I thought I would post a dumb question, but your comment and pics of the AFD amp beat me to it. I am thinking that Nik is doing this on a few of his high gain amps

The input jack has aluminum shielding around it inside the chassis and there is no 1 meg to ground on the jack either - Nik has essentially moved the 1 meg from the jack that would normally go to ground and instead moved it to pin 1 via the shield

If attaching the shield like is indeed not best practice (safety wise) would you think the resistor would add enough additional safety margin? I assume OP's amp has the resistor on the jack side

To me any trick in the book to reduce noise is good as long as its safe

Its a method sometimes used in higher gain amps to lift the ground of the input above ground potential.

I remember reading about it in one of Gerald Weber's books.

I have also read that it is not a safe practice and probably shouldn't be used.
I'm sure it would not pass UL or CE guidelines.

Good lead dress and appropriate grounding methods should be enough to keep it quiet.

The only thing I have seen that is done sometimes is to lift the heater voltage by connecting the center tap or balance resistors to a higher voltage, like 12 volts or above to get rid of 60Hz noise.

[ChrisM]

Structo, your comment is timely for me, I was going to post about this very topic. I have a Ceriatone built Yeti here that I just popped out of the cab to have a look at and it has the input jack cable shielding soldered to pin 1 but with a 1 meg resistor in series on the pin end. There is a 56k grid stopper on the lead and a 1 meg on the shield both attached to pin 1

I had never seen this before and was curious about it. I assumed it was a method of reducing noise but since I had never seen this before I thought I would post a dumb question, but your comment and pics of the AFD amp beat me to it. I am thinking that Nik is doing this on a few of his high gain amps

The input jack has aluminum shielding around it inside the chassis and there is no 1 meg to ground on the jack either - Nik has essentially moved the 1 meg from the jack that would normally go to ground and instead moved it to pin 1 via the shield

If attaching the shield like is indeed not best practice (safety wise) would you think the resistor would add enough additional safety margin? I assume OP's amp has the resistor on the jack side

To me any trick in the book to reduce noise is good as long as its safe

Its a method sometimes used in higher gain amps to lift the ground of the input above ground potential.

I remember reading about it in one of Gerald Weber's books.

I have also read that it is not a safe practice and probably shouldn't be used.
I'm sure it would not pass UL or CE guidelines.

Good lead dress and appropriate grounding methods should be enough to keep it quiet.

The only thing I have seen that is done sometimes is to lift the heater voltage by connecting the center tap or balance resistors to a higher voltage, like 12 volts or above to get rid of 60Hz noise.

Gerald suggests connecting the shield to the plate. Now if you have a co-ax cable connecting the plate and grid what do you get? A small snubber cap! Yes co-ax has capacitance associated with it.

Yes kind of risky though. Say you did this trick on V1 leading from the input jack. If that co-ax run failed and shorted you'd have HV going back into your guitar!

[Lindz]

I noticed in my 1st post that I said the shielding went to pin 1 - I wasn't thinking, its pin 2. I edited it but people's quotes still show it as pin 1