Hum problem with an old Plush bass amp

[AmberB]

Hello all!
I"m working on an old Plush Royal P1060 bass amp.  This thing has been driving me nuts for a while now.  It has a hum that I have not been able to get rid of.  When I pull out the driver tube and turn it on, it's more or less silent.  I've tried different tubes in different positions, I've done the chopstick probing, so far, nothing has made any difference.  I put a new set of F&T caps in the amp, I thought that was the problem at first.  I've checked my grounds with my Fluke multimeter, they all seem good.  I even rerouted the grounds to try and separate the preamp grounds from the power amp grounds, no change.
I saw the "Uncle Doug" video about the problem with internal capacitance in a preamp tube causing hum, which is why I changed out some of the tubes.
I'm starting to wonder if I have 1 or more bad components in the preamp causing this problem.  I discovered that one of the ceramic caps was microphonic when I was "chopsticking" the amp, so I replaced that, but that didn't solve the hum.
When I was taking AC readings on the hot side of the filter caps, I was getting popping noises when I put the meter on the the test point.  It was like that with all the B+ connections.  It also did that when I was testing DC.
 I don't know if it's because of the meter or if it's another symptom of the problem.  I honestly don't remember if other amps did that, I suppose I'll have to try that on another amp and see what happens.
This one is definitely a head scratcher...

[tubeswell]

Did you replace all the bias supply filter caps as well? Bad cap(s) there can result in hum

[AmberB]

Yes, I did that.  There's only 1 filter cap in the neg bias supply, a 50uf by 100volt.  I replaced it with a 50uf by 150 volt cap.

[tubeswell]

How bad is the hum? If its super bad, it could be a blown/open screen grid resistor. Meter the Rg2s to see if any of them are open


Also, on a related note, measure the tube current on each output tube (and look for noticeable differences).

[Latole]

Picture of the amp may help.
Do you have the schematic ?

How are the input jacks , their shorting contact to ground ?
How is the wiring  ? Lead dress

I f you have removed preamp tubes and noise is there, look a DC filtering issue . Popping noise with meter is not normal.

[tubeswell]

http://www.prowessamplifiers.com/schematics/images/Plush_1060S0001.pdf

[Latole]

http://www.prowessamplifiers.com/schematics/images/Plush_1060S0001.pdf

Thank you.

[AmberB]

I do have the schematic for the amplifier, I found it with an internet search several weeks ago.  I haven't been able to work on it every day, I've just been pecking at it as I have the time.

The amp is nearly silent if I pull the driver tube, so it must be somewhere in the preamp or the driver circuits.
The hum is definitely noticable with all the preamp tubes in.  It doesn't seem to get louder with a guitar plugged in and the volume is turned up a bit.
I checked the ground lugs of the input jacks to the main ground, that's good.  I put shielded wires from the input jacks to the grid of the first input stage tube.  Because of the way the amp was laid out, those wires are pretty long.  The factory didn't use shielded wires there.  There's no noise increase now when I put a finger on the input wires, but the tone control wires pick up noise from the proximity of my fingers.  I don't remember the Fender amps that I've worked on doing that...

[Latole]

Wich driver tube do you talking about ?  V1 V3 ....?
Can you read AC voltage ( ripple ) on each filter caps ?

[tubeswell]

So with the driver tube (V5) in, does pulling V2, or V1, or V3 kill the hum?

[Latole]

So with the driver tube (V5) in, does pulling V2, or V1, or V3 kill the hum?

+

Driver tube is the Phase inverter !     Long time I never hear Driver Tube.

PI tube removed, it always kill all the amp noise and it do not help

[AmberB]

So with the driver tube (V5) in, does pulling V2, or V1, or V3 kill the hum?

With V5 in and V2 out, the amp has a very low buzz that you can only hear if you're close to the speaker, it has a bit of hiss, probably from V5 amplifying basically tube hiss.  If I put either of the channel tubes in, (V1 or V3) it buzzes noticably.

I call the PI tube the driver tube because it drives the power tubes as well as inverting the signal.
The filter caps have the amount of AC ripple on them that you would expect from a functioning amp.  The preamp filter caps have basically no ripple.

[PRR]

You should have a 40uFd 450V cap and clip-leads, so you can shunt any of the B+ filter caps and hear if it makes a difference.

Yes, you can measure, or shot-gun, but 40u on clips is quick and definitive.

[tubeswell]

What PRR said.


As the hum seems to be related to both/either of the input tubes, suspect the preamp filter cap. Check that it’s  ground lead is grounded To the chassis (with your meter)

[Latole]

What PRR said.


As the hum seems to be related to both/either of the input tubes, suspect the preamp filter cap. Check that it’s  ground lead is grounded To the chassis (with your meter)

That is what I suspect and wrote yesterday
___________

" The filter caps have the amount of AC ripple on them that you would expect from a functioning amp"

-AmberB

You may be right, sorry I don't know who are you, a newbie in tube amp repair or a pro ?

[AmberB]

What PRR said.






You may be right, sorry I don't know who are you, a newbie in tube amp repair or a pro ?

I'm not a newbie, but I don't do it for a living either.

[AmberB]

What PRR said.


As the hum seems to be related to both/either of the input tubes, suspect the preamp filter cap. Check that it’s  ground lead is grounded To the chassis (with your meter)

Well, these filter caps are new F&T caps that I just installed, but I suppose I could have a defective one.  I'll have to try the 40uf cap and clip leads and see what happens.

[Latole]

Well, these filter caps are new F&T caps that I just installed, but I suppose I could have a defective one.   

Not IMO.

[tubeswell]

... I suppose I could have a defective one.  I'll have to try the 40uf cap and clip leads and see what happens.

Meter out the preamp filter cap's ground lead to chassis first - to check for DC continuity

[Latole]

... I suppose I could have a defective one.  I'll have to try the 40uf cap and clip leads and see what happens.

Meter out the preamp filter cap's ground lead to chassis first - to check for DC continuity

Second time you ask, with no report....

[AmberB]

I haven't quite figured out the quote thing in a reply yet, so I'll have to answer the questions without references.
My preamp filter cap grounds are all tied to the main chassis ground point by a heavy gauge wire.  The center tap of the B+ winding is also tied to that chassis ground point, after going straight to the first section filter cap ground point.  The second filter cap, the screen grid filter cap, is also grounded directly to the B+ winding center tap.
When I do continuity testing, I get no resistance from any of the filter cap ground leads to the chassis ground point or to the B+ winding center tap mounting point.  This includes the cathode caps for the preamp tubes, and the ground lugs of the input jacks.
Also, the wire from the input jacks to the first triode grid is shielded with the shield wire grounded at only 1 end, in this case the input jack ground lug.

[AmberB]

Another thing that I've found is that our AV wall voltage is "dirty".  One of the local radio stations seems to be riding on the AC.  You can see it in the B+ with the oscilloscope when you have the volts per division way down.  It only comes through the speaker when you turn the amp way up.
It's weird that when T-2 is pulled, there's almost no hum at all, but with it in circuit, the hum is rather noticeable.
I'm going to replace the ceramic caps in the driver tube circuit and see what happens.  Why Plush used ceramic disk caps between the PI tube plates and the power tube grids is beyond me...
Another thing, all of the tone capacitors and stage isolation caps are 400 volt.  Seems a bit low to me, but that's what Plush used. 

[tubeswell]

If you clip a 600v 0.1uF film cap on each plate for the V2 tube, which one kills the hum?

Could be several things:

• improperly closing input jack tip switch contacts for the V2 tube channel
• loose tube socket pin clamp(s)
• bad plate resistor(s)
• cracked solder joint(s)

(I take it you tried a tube swap?)

[AmberB]

If you clip a 600v 0.1uF film cap on each plate for the V2 tube, which one kills the hum?


[/list](I take it you tried a tube swap?)

On the schematic for this amp, V1 is the preamp tube for channel 1 and V3 is the preamp tube for channel 2.  V2 is the combiner tube (for lack of a better description), and it only uses 1 of the triodes, which sends the signal to the PI tube.
I have tried tube swaps, and I tested all the preamp tubes in a different amp to see if they hummed in that amp.  All the tubes tested good, and swapping them around made no difference.
I'll try the .1 film cap on the plates and see what happens.

[AmberB]

Well, I definitely have to remember the 0.1uf from plate to ground test for hum problems in the future.
The amp stops buzzing when you put the 0.1 cap between the plate of the second triode of V1 and ground.  That seems to be the only place that makes a difference.  So, now I have to figure out what's wrong with that circuit.  I don't know if it's picking up the hum from the tone section or from the V1-b circuit it's self.

[tubeswell]

Well, I definitely have to remember the 0.1uf from plate to ground test for hum problems in the future.
The amp stops buzzing when you put the 0.1 cap between the plate of the second triode of V1 and ground.  That seems to be the only place that makes a difference.  So, now I have to figure out what's wrong with that circuit.  I don't know if it's picking up the hum from the tone section or from the V1-b circuit it's self.

Inspect the socket pin clamps for that tube - may need to be re-tensioned


Another possibility is a bad plate resistor or cold/cracked solder joint - carefully check the solder joints for V1.

[AmberB]

I resoldered all the connections for the preamp tubes, and checked the tube socket pin clamps.  All seem OK.
Channel 1 has really long wires from the controls to the grommet board.  I replaced the wire from the volume control to the second triode grid with a shielded wire and it cut the buzz by half.  I already had replaced the wire from the input jacks to the first triode with a shielded wire.  The amp had no shielded wires from the factory.
The way this amp is built is that 1 set of channel controls are are mounted inside the chassis, and the other set is mounted above the chassis on the faceplate.  The wires for the upper controls, channel 1, go through a hole in the chassis to reach the grommet board.

[tubeswell]

Do you have a solid plastic chopstick you can use to safely give the resistors and caps for the tube stage a good prod with (to see if anything in particular -like a plate resistor- makes a popping noise)?

[PRR]

...Channel 1 has really long wires from the controls to the grommet board.  I replaced the wire from the volume control to the second triode grid with a shielded wire and it cut the buzz by half.  I already had replaced the wire from the input jacks to the first triode with a shielded wire.  The amp had no shielded wires from the factory.....The wires for the upper controls, channel 1, go through a hole in the chassis to reach the grommet board.

http://www.plush-amps.com/PhotoPage2.htm
http://www.plush-amps.com/P1060-blue-chassis.JPG

[AmberB]

That blue Plush sure is cleaner than the one I'm working on!

Tubeswell, I have a nylon rod that I use for poking and prodding stuff in the amp when it's on.  I found a bad ceramic cap in this amp that way.  I've replaced all the ceramic disk caps in the amp now.
I'm wondering if the film tone caps can get microphonic, or if I'm just making a microphonic tube act up.  I probably should replace them anyway.
(microphonic 7025s and 12AX7s are one of the things that plague me...)
I've been resisting replacing the plate resistors with carbon film 1 amp resistors, I've been using 1/2 watt carbon comp resistors to replace the original ones in the amp.

[tubeswell]

...
I've been resisting replacing the plate resistors with carbon film 1 amp resistors, I've been using 1/2 watt carbon comp resistors to replace the original ones in the amp.

Did you have the hum problem before you replaced the plate resistor(s)? (You still might have a defective Rp)

[AmberB]

Yes, the hum problem is what started this mess.  It's why I started working on the amp.

[PRR]

I don't see why a bad plate resistor would hum. It doesn't know the pitch. It would need some other problem involving wall-power filtering to make noise at 50/60/100/120Hz.

It is an odd construction, perhaps cheap, but I don't see a fundamental problem.

I don't see how tone-caps matter much.

The 0.1u off the plate is diagnostic, not a fix.

The ground layout is better than some. The rectifier return (PT CT) is big crap, and they wired it correctly and directly to the first cap. The preamp caps are downstream. The G2 cap could return either place. The main thing is that PT/rectifier must NOT dump into preamp ground reference.

Have you made a concise list of what knob does or does not change the hum?

Have you got to jumping a 40uFd cap on the power rails?

Replaced the bias supply cap 50u 100V?

Move it around on the bench to be sure it is not an external source? (I have war stories.)

Take it out in the sunlight and critically inspect ground joints and solders? If you are over 30, you should have a magnifier. (Over 60, you know where to nab a 50mm SLR camera lens, a great optic for getting the eye 2 inches from the object.)

[AmberB]

Have you made a concise list of what knob does or does not change the hum?

Have you got to jumping a 40uFd cap on the power rails?

Replaced the bias supply cap 50u 100V?

Move it around on the bench to be sure it is not an external source? (I have war stories.)

Take it out in the sunlight and critically inspect ground joints and solders? If you are over 30, you should have a magnifier. (Over 60, you know where to nab a 50mm SLR camera lens, a great optic for getting the eye 2 inches from the object.)

Actually, none of the knobs change the hum.

I finally scrounged up a 47uf-500 volt cap to try jumping on the power rail.  I discovered that I'm currently woefully short on power supply caps.

I did replace the neg bias cap with a new Sprague 50uf-150 volt cap, properly wired to ground.

I'm not beyond believing that it may be an external source, but another amp that I've been working on doesn't have any buzz on the same bench, so I don't know about that.  I have wondered if my work area is extra electronically noisy.  I don't have any flourescent lights in the room, but there's one on the aquarium in the next room.  I turned it off and that made no difference.  All my lights are LED bulbs, I don't know if those are electrically noisy or not.

I am 60 plus years old...just turned 65...the old eyes are not what they used to be...
I use a 100 watt solder gun on these old amps with grommet boards and point to point wiring.  I've redone the entire grounding system on this amp because I replaced the cap can with individual filter caps.  The owner of the amp didn't want to spend the 40 bucks on a new cap can for whatever reason.  I'm sure the solder connections are good.
I should take a picture so that you can see what the amp chassis looks like...

[AmberB]

Well, the 47uf-500volt cap makes no difference...time to try something else...

[tubeswell]

How are the amp's ground return wires arranged?


And can you post some pictures?

[AmberB]

I'm in the process of revising the ground setup.  When I get that done, I'll take a picture.  This amp has the typical input jacks grounded on the faceplate, so I've been thinking about how that ground relates to the preamp tube circuit grounds and the power supply cap ground for the preamp B+.  I can't help but think there's a ground loop somewhere that's causing this hum.

[tubeswell]

Have you read Merlin Blencowe’s (‘Valve Wizard’) Grounding article? Very handy tips

[AmberB]

I have not read that article.  I'll have to see if I can find it on the internet.

[AmberB]

Here's a picture of the chassis that I just took.

[tubeswell]

I have not read that article.  I'll have to see if I can find it on the internet.

On this page


http://www.valvewizard.co.uk/Grounding.html

[Williamblake]

I guess you already gave chopstick hell to where the bias resistor connects to the old filter can?

[AmberB]

I guess you already gave chopstick hell to where the bias resistor connects to the old filter can?

Yes, I did that.  I'm currently in the process of rewiring the grounds to the grounding scheme that Merlin Blencowe wrote about in the article that's linked in the comment before yours.  I'm going to have to move thew grounds of bias circuit as part of that.

[AmberB]

So, I changed the ground system on this amp to a more or less buss bar setup using heavier gauge wire.  I started at the first B+ filter ground, which is where the PT center tap is also grounded, as well as the power tube cathodes.  The fground wire runs from there to the negative bias ground mount, then to the screen grid cap ground, then to the preamp caps ground point, and from there to the ground of the input jack.  The input jacks are mounted direct to the chassis, as are most older amps.  The AC cable ground is mounted to one of the PT mounting bolts.  The heater ground is mounted to the chassis on one of the old cap can grounds.  Apparently it's not supposed to matter exactly where you mount that as it's just a reference point.
This setup kills about 90-95 percent of the background hum that I was getting.
Now I just have to figure out what I did to make the bass channel hum when you turn it up.  I'm pretty sure it's a problem with where I grounded the shield on one of the shielded wires, probably the one going from the volume control back to the second stage grid.

[AmberB]

I've identified the buzz in the bass channel when you turn it up as 120 hz. The buzz is not audible when the channel is turned all the way down, it gets louder as you turn it up.  Different problem from the original problem that is now fixed.  The buzz is there with or without an instrument plugged in.

 The normal channel just makes the normal noise when you turn it up with no input.

I guess it's time to do some more reading...

Did I mention that microphonic tubes are a pain in the ass?  I have 5 or 6 of them in my tube stash apparently.  A couple of them are bad enough that they would ring when I bumped the circuit board with my plastic probe...

[tubeswell]

I've identified the buzz in the bass channel when you turn it up as 120 hz. The buzz is not audible when the channel is turned all the way down, it gets louder as you turn it up.  Different problem from the original problem that is now fixed.  The buzz is there with or without an instrument plugged in.

See comments attached

[AmberB]

See comments attached
[/quote]

The first set of two filter caps are in series, like the way Fender does it's first B+ capacitor, so that first place with a circle on it is not a ground point.  It's the connection between the neg of the first cap and the pos of the second cap in series.
I have the entire ground line connected to the chassis at only that one point you circled, the ground lug of the input jack.  I've tried to run all of the individual cathode resistor/capacitor grounds from the preamp tubes to the ground side of the power supply cap that supplies that triode.  The grounds of the volume and tone controls are run back to the ground of the triode that drives the tone controls.

The problem with the amp is now just with the bass channel buzzing when you turn it up, so I seem to have solved the basic background hum problem at least.

[AmberB]

I finally got the hum problem fixed on this amp.  I figured that it was a ground loop somewhere that was causing the hum in the bass channel.  The inputs for the bass channel are mounted above the chassis on the faceplate.  The faceplate is aluminum and the chassis is steel.  I figured that there was a ground problem with the input jacks because of the faceplate not getting a good ground to the chassis.  I had tried several things to solve the problem, to no avail.  I put the amp aside out of frustration for a while, but finally got back to it this week.  The first thing I did was to create a firm chassis ground on the chassis by tinning the chassis next to a small hole in the chassis that is near the input jacks of the normal channel.  Then I used a machine screw to fasten a solder lug to the chassis at the hole, and soldered it to the chassis where it was tinned to make sure I had a good ground.
In the previous work that I had done to the amp, I had made a ground "buss" for all the power supply capacitors and had run that to chassis ground at the input jacks on the chassis.  I took the chassis ground off the input jack ground and soldered it to the new chassis ground.  Then I ran a wire from the ground lug of the bass channel input jacks to the normal channel input jacks so that all the jacks are grounded to the chassis.  That seems to have solved the ground loop problem.