New tube amp noise...

[Cornholio Farquarth III]

There are also other things to consider with this noise problem. How are the power and output transformers oriented to one another?

[MacFangus]

Thanks MacFangus. The hum increases as the volume is turned up. It also increases when the reverb is turned up.

So I don't know if that means two separate problems?

 

Make sure you try rolling an alternate set of known viable tubes before going too far. Spray all of your inputs, outputs, tubes sockets using Deoxit D5 as well. As to the reverb, you're going to need to focus on the reverb cables, but BE CAREFUL... Current is lethal! It could be the cable shielding and/or the reverb tank. You can bypass the tank using an RCA jumper cable to check the amp without the tank installed. If you need a new tank, I have an USA made Accutronics tank that will work. You need to have the impedance adjusted, but that's cheap. If the amp is new, it more than likely has a China made tank in it.

 

Also... Make sure there is a shield between the chassis and the cabinet.

[Jazzer2020]

 

A few random thoughts:

1) "Lead dress" is supposedly an important factor in minimizing amplifier noise....

 

2) Is the top of your amp's chassis shielded? ...

 

3) Yes it IS possible to add additional components to reduce an amp's hiss, but of course there is a potential trade off...

 

4) Hum from the reverb? ...

 

In any case, if you payed "mega bucks" for a custom, boutique build, the BUILDER should be the one you are having this conversation with.

 

We would LOVE to see a picture of the amp's circuitry. We'd also like to know the name of the builder and how much you paid.

 

 

Thanks Grumpy et al for your recent comments.

 

I just opened her up now to peek inside and see what's going on.

I have very limited technical amp knowledge so I will upload some photos and let you guys see if you can spot anything.

 

The first thing I noticed is that the top of the cab does not have any shielding.

 

As for the name of the builder... At this point I prefer not to disclose it. We will see how things go with him.

He has not been very responsive overall thus far.

 

Delays with the build and now after I told him about the noises end of April, he said he'd be getting back to me on May 1

after the weekend. Still haven't heard back from him!

 

As you will see it was a very tight build. Not sure why. I did mention having a light build, but not at the expense of introducing noise.

 

 

[Jazzer2020]

I don't know if I'll have figured this out by the time I get a reply but...

 

When I was removing the chassis from the cabinet I had a very tricky situation to navigate.

The builder put washers and nuts to hold the long chassis screws. I don't know if this is standard or not.

Anyway, one of the the nuts/washers was almost impossible to get at.

Transformers were blocking it.

 

Somehow I managed to stick needle nose pliers in just to touch the nut (couldn't even get the pliers

to grip the nut because of poor positioning).

Applying pressure to the nut from the side, I was able to unscrew the screw from the chassis.

 

But I knew I'd be in big trouble when I had to re-assemble.

 

So I'm going to have to either leave that nut/washer off when I re-assemble now or figure something out.

Any suggestions? There must be a secret to this.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Mr.Grumpy]

Thanks for the pictures, it's lovely. I don't see any issues with workmanship. But...6L6 power tubes? I thought this was a Princeton clone.

 

The first thing I noticed is that the top of the cab does not have any shielding.

 

That could be the problem. As I mentioned in my previous post, my Music Man uses what appears to be plain old window screen, which I presume is galvanized steel wire, and it's just stapled to the inside top of the cabinet.

 

 

 

...There must be a secret to this.

 

No secret, just the proper tool. You need a nut driver, it's basically a small socket wrench with a screwdriver handle. Usually they are sold in sets of 5 or six with various sizes. Should be well under $10 US for a set like these. They are excellent for tightening and loosening the nuts on pots and jacks too.

 

[ATTACH=CONFIG]n31979182[/ATTACH]

 

 

[onelife]

Thanks for the pictures' date=' it's lovely. I don't see any issues with workmanship. But...6L6 power tubes? I thought this was a Princeton clone.[/quote']

 

Those were my thoughts as well.

 

It looks like a well built Princeton Reverb clone but 6L6 power tubes would pull a lot more filament current than a stock (Fender) Princeton power transformer could handle.

 

 

 

 

[Jazzer2020]

Thanks again guys for your help, I really appreciate it!

 

I'll try to fill in more of the picture now.

First the noise, the most important part.

 

Yesterday I thought of an additional item that could be very helpful in diagnosing the noise.

The hum/hiss starts as soon as the amp is switched on, before the tubes have any chance to warm up (or before I can play any guitar).

Would this point in the direction of the power section?

 

The amp builder is not local and this is one reason I haven't been in a big rush to send him back the amp for repairs right away. In addition to the time for repairs it will cost at least $100+ each way for shipping.

 

Re: the 6L6 tubes.

Yes this is a Princeton clone, but 'souped up'.

A stock Princeton had 6V6's and was 12-15W with a 10" speaker.

This one is 25W with a 12" speaker.

It is configured to also accept 6V6's.

 

Re: the nut driver.

Had I been able to gain access to place such a tool over the nut I surely would have used one!

But the fourth nut was completely inaccessible for such a tool.

The chassis was inside the cab. The front speaker cover could not be removed until the chassis was taken out.

Anyway, in the end I figured out a way to put the nut onto the screw when reassembling.

[Jazzer2020]

Originally posted by onelife View Post

 

It could be a filter capacitor in the power supply.

 

 

That's where I'd place my vote.

 

I don't know if Youtube Uncle Doug is an amp expert but he sure sounds like one.

This is what he says about filter capacitor noise:

 

60 Hz hum ----> tube

120 Hz hum ----> filter capacitors

 

In my case I have a 60 Hz hum, but as I mentioned above, the hum starts as soon as the amp is powered on,

before the tubes have had a chance to warm up. Whether they can add noise before they can conduct sound, I don't know.

 

 

[WRGKMC]

After looking at the chassis pics I can tell you exactly where the hum is coming from.

 

That amp is set up like my bassman is. It uses a Bias/Hum adjustment for balancing tube bias.

 

What you do is tweak the bias pot till you hear minimal hum. That's it. Turn the standby on and off a few times to make sure the hum remains very low. If you cant get rid of the hum completely try a different set of power tubes. This is really common with these style amps. How low you can get the hum is purely based on having a good matching set of tubes and tweaking the balance.

 

Allot of people hate the design because it assumes tubes are all the same and the fixed bias of the one tube is in an ideal range. Many in fact modify the amps to put in separate the bias of the two tubes and set each tube separately. I never bothered myself. I've used the amp since 67 and it only takes seconds to balance the two tubes using the bias pot and the most that might happen is the pot may move a little due to vibrations in transport or the two may vary as the tubes get some hours on them. you simply give it a little tweak and you're done.

 

I do like the fact they mounted that adjustment in the back. On the Bassman they mount it between the transformers which makes it nearly impossible to tweak with the head mounted in the cab. You can tweak that one from the back with a small - screwdriver Once the tubes are broken in you shouldn't have to mess with it any more. You may hear a little hum when voltage surges occur but that's normal.

 

What I do, instead of just using my ears is connect an AC meter with a low voltage setting of 2Vac to the speaker jack, no input, volume turned completely down and then adjust the pot till I get the lowest AC voltage. After that when I turn the standby switch on and off, I may get a slight surge where the hum might be a little louder, but after the surge it should be unnoticeable.

 

Hum on this kind of bias balance amp is fairly normal. The type of tubes makes a big difference on how low you can get the hum. I've run Groove tubes and had good results. I used JJ's recently and the hum level would come and go so I went to Electro Harmonix and there's no hum at all (plus low hiss levels. If the tubes aren't well matched you may have to live with a minimal amont of hum till you change the tubes.

 

The other possibility are the 470 ohm screen resistors aren't well matched. For awhile I was running a high impedance on my Bassman and I kept having the tubes blow and take those screen resistors out. I eventually put in a set of military grade screen resistors with 1% tolerance and got a perfect balance between the tubes. The ones in the picture look like they have a silver band which indicates a 20% tolerance. That means you can have as much as 80 ohms variance per resistor, especially when they get hot. I'd lift one end and measure each to see if they are a close match. If they're only off a few ohms I wouldn't worry and simply focus on the tubes and the bias balance.

 

You did mention about shielding inside the amp case. Whet the old fenders like mine used was steel screen stapled to the cab casing. When you installed the head the chassis would press against it grounding the screen making a faraday cage. It works very well grounding the open side of the chassis. The only thing that might be better is copper foil. It has to be stapled in place and flat as possible. You don't want any rough edges extending inside the chassis causing a possible short.

 

Don't use aluminum foil or aluminum flashing tape. Aluminum sucks for blocking hum AC hum passes right through it. It will work if its 1/4" or more but paper thin copper foil will work twice as well.

[Jazzer2020]

Thanks again WRG for the tips!

 

 

After looking at the chassis pics I can tell you exactly where the hum is coming from.

 

That amp is set up like my bassman is. It uses a Bias/Hum adjustment for balancing tube bias.

 

What you do is tweak the bias pot till you hear minimal hum. That's it. Turn the standby on and off a few times to make sure the hum remains very low. If you cant get rid of the hum completely try a different set of power tubes. This is really common with these style amps. How low you can get the hum is purely based on having a good matching set of tubes and tweaking the balance.

 

I got up the courage to adjust the bias/hum pot a couple of weeks ago. I didn't know if I could make adjustments with the amp powered on, so not taking a chance I would turn the pot slightly and then power up and test, power down, and repeat...

It was a bit tedious, but in the end, going through the complete turn, the hum noise went down just marginally, if at all.

 

What I do, instead of just using my ears is connect an AC meter with a low voltage setting of 2Vac to the speaker jack, no input, volume turned completely down and then adjust the pot till I get the lowest AC voltage. After that when I turn the standby switch on and off, I may get a slight surge where the hum might be a little louder, but after the surge it should be unnoticeable.

 

How do you get at the speaker jack leads to do the test? Can you just use a speaker cable plugged into the jack and gain access to the leads on the other end of the cable? Can you use a metal screw driver to adjust the bias pot while the amp is on?

 

Hum on this kind of bias balance amp is fairly normal. The type of tubes makes a big difference on how low you can get the hum. I've run Groove tubes and had good results. I used JJ's recently and the hum level would come and go so I went to Electro Harmonix and there's no hum at all (plus low hiss levels. If the tubes aren't well matched you may have to live with a minimal amont of hum till you change the tubes...

 

...You did mention about shielding inside the amp case. Whet the old fenders like mine used was steel screen stapled to the cab casing.

 

I don't really know what to do next to tell you the truth.

On the one hand, this is a brand new amp with a lifetime warranty.

I could certainly swap some tubes to see what happens but... that would be an extra investment on my part that would be covered by the builder.

And I wouldn't want to mess with shielding the cab either. This would be his job to do.

 

But as I mentioned just before, just getting the amp to the builder and back is going to cost someone at least $200.

So I think you can see the conundrum I am in.

 

Don't use aluminum foil or aluminum flashing tape. Aluminum sucks for blocking hum AC hum passes right through it. It will work if its 1/4" or more but paper thin copper foil will work twice as well.

 

Speaking of aluminum flashing tape...

When I was separating my Fender Princeton II combo into head and speaker cab a couple of years ago I knew I was supposed to shield the top of the cab.

I had already bought some window screening to use as the shielding but decided to use aluminum flashing tape because I looked at my Fender Vibroverb cab and saw a sheet of metal tape stuck to the top of the cabs. Granted, it was a different tape than what I was using, but it seemed like it would be easier to install.

 

Well fast forward... My Princeton II also has a hum/hiss now on par with the new amp.

Before I split the amp into head and speaker cab it was dead quiet. It had a metal sheet at the top of the cab.

So maybe that's where I should be concentrating on?

 

 

 

 

 

[onelife]

...

How do you get at the speaker jack leads to do the test? Can you just use a speaker cable plugged into the jack and gain access to the leads on the other end of the cable? Can you use a metal screw driver to adjust the bias pot while the amp is on? ...

 

leave the speaker plugged into the amplifier - connect the leads of your voltmeter to the terminals of the speaker

 

yes you can use a metal screwdriver while the amplifier is switched on - if you have a non-metallic tool then use that

[WRGKMC]

The minimum hum setting should be close to the center of the bias pot rotation. You wont hurt anything going to either extreme but I wouldn't leave it there If the hum is worse. There's a null point between two slightly different kinds of hum which you need to find. If you're having to turn it to one extreme or the other it may indicate one tube is weaker then the other. I'd swap the two around and try again. If you still cant get a minimum hum buy a new matched set.

 

I would suggest a set of EH over some others. I tried Ruby and JJ's and they didn't come close to sounding good. The EH sound like the old RCA tubes I used to use. Groove tubes can be good too. They are a bit tricky buying however. They come in low, (clean) Medium (Normal) and High (Hot) bias. I bought a set before Fender bought the company but I cant remember what range they were in and the site doesn't tell you what they recommend for different amp types. Not that it matter that much on this type of amp because the bias is fixed on one tube and the pot simply balanced the second tube to the first.

 

You also mentioned using screen over the amp head. Normal door screen is nylon, not metal so it would have no shielding capabilities. Some other types may contain aluminum or other metals that simply don't shield well. If you go to a good hardware store they may have a section with screen on reels. The one store by me has this. They sell Nylon, Steel and Brass screen. The brass would work best.

 

Like I said, aluminum sucks for shielding AC hum. If you're using it, get something better. You can buy thin galvanized steel at hardware stores, cut it to size, drill holes and screw it to the inside of the cab. That will knock out the hum quite well. Copper foil tape can be purchased too. They make shielding paint but I'd be a bit leery using that inside a cab. It works well enough inside guitars but the heat from tubes and vibration might cause it to flake off and short circuits.

 

For using the meter, I simple unscrew one of the speaker jack covers. As Onelife says, the speaker must be connected and the amp running. No input and volume turned off. You connect one lead to the chassis. Then you carefully touch the center lead from the jack with the meter set for low AC volts. You can start higher and work your way down on settings till you get a reading. Its normally a volt or less when you can hear a hum.

 

From there you can turn the pot to find the lowest reading. It will be like a valley as you turn it, the voltage will reach bottom then start rising up. Tweak it back and forth till you find the lowest point. At the same time you'll hear it change with your ears too. You may hit a point where you see no change on the meter but you hear a small change. Just use your ears at that point to get the lowest level between the left and right.

 

As I said, with good tubes, the hum should disappear below the amps normal hiss level. The hiss is normal and will vary depending on what tubes you use and the type of speakers you use. When the amps were designed the speakers weren't the best and they didn't produce allot of high frequencies so the hiss was masked. If you bough cabs with upgraded Altecs like mine had the hiss was more noticeable but added loudness made it well worth it.

[Jazzer2020]

Thanks onelife and WRG. I have my work cut out for me.

[Jazzer2020]

I had a hunch. It occurred to me last night.

 

This new amp is basically a Princeton Reverb clone.

Built as a head and speaker cabinet, just like my Princeton Reverb II

after I separated it into the same form (from original combo form).

 

My Princeton Reverb II was always dead quiet.

Up until the time I separated it into head/cab form.

 

Since then there has been a bit if a hum and hiss.

And the hum/hiss is on par with the new amp build.

 

So I thought, what is going on here?

 

I was initially thinking about the grounding plate that I removed from

my Princeton (it was rattling) and had replaced with aluminum tape

in its new head housing. Maybe that was not sufficient grounding?

 

And recently I discovered the new amp doesn't even have any grounding

in its head housing.

 

I thought beyond this. What else had changed?

Well now there was separate head and speaker cab, instead of a combo.

 

Then I thought, "What about the speaker cable that I built?"

The 7.5' cable that I was using to connect the head and speaker?

Maybe, just maybe it was acting like an antenna?

 

The new amp was shipped with a 2' cable which I had straight away set aside

and never used.

 

I plugged her in today and the hum was reduced by more than 75%.

Plugged her into my Princeton Reverb II and the same thing happened.

 

Is it dead silent now? No.

But I think I can live with the hum now.

[WRGKMC]

The speaker cable length shouldn't not make a difference. Longer cables or cables with thinner wire have more resistance and may attenuate the signal, but that's it.

 

It may be acting like an antenna because you still have aluminum tape which is acting as an RF collector instead of a shield. Aluminum reflects 60% of hum compared to copper of a similar thickness and even worse at low frequencies. It works well shielding high frequencies in the mega Hz range but doesn't block low frequencies very well at all. Its also got problems with oxidation and it cant be soldered, only welded. Used as a chassis cover it can loose contact with the amp frame and actually collect more noise then it blocks.

 

Your best reflective metals are Silver Copper and Gold. Silver and Gold are two expensive which leaves copper as a top choice for reflectors.

 

Your best absorptive metals are Nickle, Iron/ Tin plated steel Stainless Steel and various Mu Metal and super alloys containing Chromium, Molybdenum Iron, and other exotic alloys which are manufacturers to block/collect specific frequencies.

 

High MU metals with high permeability like Iron, Nickle, Copper, Steel, Chromium, Molybdenum. Copper, Solver, Gold, and Tin plated Steel collect magnetic waves and carry them to ground before they get inside the chassis. Copper, Gold, Silver reflect the waves before they get inside the chassis. The result is the same in audio circuits but different materials will be used for other types of circuitry. One example is radio which as emitters in the circuits.

 

 

This chart shows how effective aluminum is at various frequencies. Compare the numbers. Hi MU metals are hundreds of times more effective, especially at low frequencies where aluminum barely registers in the 60hz range. Aluminum sucks for 60Hz and is only effective at extremely high frequencies well up in the mega/giga Hz ranges.

 

 

[Jazzer2020]

Good grief. That didn't last too long!

Thanks WRG for your reply.

 

I must have had a 'bad hear' day last week when I made my 'discovery'.

Just did an A/B test with the 7.5' and 2' speaker cables and the hum is basically the same.

 

So back to the drawing board...

 

I listened carefully to the hum last night. It is comprised of two components. 60Hz and 120Hz tones.

the 120Hz tone is actually noticeably louder than the 60Hz one.

 

 

[Jazzer2020]

Time for an update…

 

I was finally given the go ahead to take the amp in to a local repair shop

to have the hum checked out after waiting for more than three months.

 

The night before I brought the amp in I did some more A/B tests and documented

the differences to show the repair guy.

 

As I was doing the A/B-ing I made a huge discovery.

 

I started off A/B-ing the New amp/New cab and my Princeton Reverb II amp/cab.

 

The New amp had its hum/hiss. Hum increased as the volume pot and also the reverb pots were turned up.

The Princeton was almost dead quiet.

 

Next I plugged in the Princeton to the New cab.

Still almost dead quiet.

However with Master at 4, reverb hum increased steadily and considerably as the pot was turned up.

 

 

Up until this time I had never plugged the New amp into the Princeton cab!

When I did, the amp was much much quieter!

 

What is your take on this and what should the next step be?

I really like the sound I am getting from the New amp/New cab (minus the hum of course).

 

Is there something that can be done with the New amp head to correct the hum, or is another speaker cab the only way?

 

[Grant Harding]

I didn't notice if you mentioned if your head is sitting on the speaker cab during all of this. If not see if physical separation changes it.

 

I know that Bogner use aircraft grade circuitry to really make sure that they're in control of the signal path and their amps are really quiet. If your amp is point-to-point wired maybe there's a wire that's too close to something it doesn't like?

[Jazzer2020]

I didn't notice if you mentioned if your head is sitting on the speaker cab during all of this. If not see if physical separation changes it.

 

I know that Bogner use aircraft grade circuitry to really make sure that they're in control of the signal path and their amps are really quiet. If your amp is point-to-point wired maybe there's a wire that's too close to something it doesn't like?

 

Thanks Grant for your suggestions.

I have tried all kinds of configurations:

head sitting on speaker cab, and away from it, at distance etc.

 

The only change in the hum occurred when I swapped the speaker cab a couple of days ago, from New cab to Princeton cab.

 

Yes the amp is point-to-point wired.

 

The tech guy on it now has sent me two very lengthy e-mails describing avenues to approach this.

They start from simple things and quickly get to expensive intense work.

 

He stated the reverb pot noise is likely due to the proximity of the tank in the small head, as opposed to its normal location in a combo.

 

Then he's looking to the EH tubes, possible fender copper ground plate under the controls, and even a total rewiring of the amp and isolating all the grounds and star grounding everything.

 

Does not sound good...

 

 

 

[Grant Harding]

I'd personally send it back to make them get it right and if not provide a credit. I realise that might not be an option, but it's what I'd do if I were them.

[Jazzer2020]

I'd personally send it back to make them get it right and if not provide a credit. I realise that might not be an option' date=' but it's what I'd do if I were them.[/quote']

 

Thanks Grant. Yes that is something I will consider discussing with the maker if nothing else can be done that's practical.

 

The thing that is still leaving me scratching my head though, is the result I got when I swapped the New speaker cab with my Princeton cab.

 

Since almost all the hum went away when I did this, why would grounding modifications need to be done?

 

Thoughts?