Intermittent buzz in vintage tube amp (Silvertone 1472)

[slidincharlie]

Hi y'all,
I've been away from this forum for maybe two or three years now...
I have a friend's mid-60s Silvertone 1472 amp on my bench. I did some general overhaul, coupling and filter caps replaced, but it has a very annoying problem:
it has an intermittent buzz (like a buzzy low-frequency put-put-put) whose intensity goes along with the volume pots: disappears with either pots at 0 and is higher with the pots at 10. The put-put buzz is there also with the 6AU6 (tremolo) tube off and is not affected by the position of either tremolo pots. Different preamp tubes did not bring any change.
What should I check now?
The schematic is attached.
TIA,
Carlo

[slidincharlie]

Any idea...? 

[Platefire]

Have you tried the chop stick method. With the chassis opened, hooked up to a speaker, power on---move wires around with the chop stick to see if you can effect the sound. It sounds like a components or wires too close that may need some seperation. If when you move something and it makes a difference, you may have found your source. Also, you might try cleaning tube sockets and tube pins with contact cleaner. The way I do that is with amp off, filter caps drained, pull your tubes out and spray contact cleaner on the tube pins and put the tube back in the socket quickly before the cleaner evaporates and work the tube up and down in the socket before letting it rest in its fully installed position. On an old amp the tube sockets and old tube pins tend to get a bit corroded. Platefire

[stratele52]

Probably a bad lead dress, some wire position act like antenna .

[Colas LeGrippa]

As Platefire said, a bad tube socket could be the cause of it. In fact, all the tube sockets should be replaced in an old amplifier, Sooner or later, one or many will become faulty. Filter caps a MUST.

Colas.

[HotBluePlates]

... I did some general overhaul, coupling and filter caps replaced, but it has a very annoying problem:
it has an intermittent buzz (like a buzzy low-frequency put-put-put) ...

Was the "putt-putt" there when you started your overhaul, or did you only notice it after replacing caps?

A discernable putt-putt-putt sound is generally a low-frequency oscillation called motorboating.

While there are other things that could cause motorboating, in an otherwise known-good design, motorboating happens when either filter caps fail or (since yours are apparently brand new) when there's a bad ground for one or more filter caps, rendering those caps effectively "not there".

I'd think that if the noise was only noticed after you began your overhaul, that you should check the grounding of all replaced filter caps for loose connection and/or cold solder joints. Cold solder joints are especially likely if you attempt to solder to the chassis (you need a 60-100w iron in many cases to get a good solder joint on a steel chassis, maybe even a bigger iron or torch if you try to solder to an aluminum chassis).

[slidincharlie]

Thanks a lot guys.
I had already done socket cleaning and retensioning, and the chopstick test - I'll do the latter again with more attention.
I don't think it is motorboating, because it has a buzzy quality to it... Actually, more than put-put-put it's a buzz-buzz-buzz. Anyway I'll check the filter caps grounds.
..Carlo

[Platefire]

I've got two 1482's so the 1472 is kinda like the 1482's old brother 

Have you looked close at the input jacks. If I remember correctly those jacks don't even have a ground
other than their attachment to the chassis. If you wasn't worried about keeping it original I would recomend installing switch jacks with their own seperate ground wire. Anyway rather you install new jacks or not, that's something to check close. Platefire

[Colas LeGrippa]

TIPS ON SOLDERING

even with a torch you won't be able to solder a wire to an aluminum chassis, you would need a TIG welding machine and lots of skills / knowledge

for a steel chasis, if it is thin, you could try a 100W soldering gun, if thicker you'd probably need a heat gun that would melt the parts around , ( or a torch to set parts on fire )

for solid ground, I use small bolts and nylon nuts, and a star washer, after having ripped off any oxyde residue in the local area with a flat screwdriver. Solder a wire to loop terminal before screwing everything to the chassis. measure the resistance to the ground with a voltmeter: one test lead to the ground wire free end, the other lead to the chassis , should read O tenth of an OHM.

good luck  

[stratele52]

I've got two 1482's so the 1472 is kinda like the 1482's old brother 

Have you looked close at the input jacks. If I remember correctly those jacks don't even have a ground
other than their attachment to the chassis. If you wasn't worried about keeping it original I would recomend installing switch jacks with their own seperate ground wire. Anyway rather you install new jacks or not, that's something to check close. Platefire

Right ,   input shorting jack is a must for no noise.

[slidincharlie]

Replacing the original jacks with switching jacks is always my first job on his kind of amps :)
Re: grounding. I have checked all filter caps grounds and they read about 2.3 ohms - is that too much? I admit that almost all grounds I have checked in any amp I have worked on read around 2 ohms, but I've never had grounding issues (buzz, etc.) before.

I will take a differing opinion to this statement, as one of the keys to soldering to aluminum is addressing the removal of the almost always present aluminum oxide coating.  This has to been done under the pool of liquid solder.  Both tin and zinc form alloys with aluminum, thus you have a basis for attachment.  

..."This has to been done under the pool of liquid solder" - I'm not sure I understand this (sorry, it's my poor English). What should I do exactly to prepare the aluminum surface at the grounding point?

[stratele52]

   What should I do exactly to prepare the aluminum surface at the grounding point?

I would only make a hole and using small bolt and nut with teeth washer . No solder on aluminium.

[Ed_Chambley]

Replacing the original jacks with switching jacks is always my first job on his kind of amps :)
Re: grounding. I have checked all filter caps grounds and they read about 2.3 ohms - is that too much? I admit that almost all grounds I have checked in any amp I have worked on read around 2 ohms, but I've never had grounding issues (buzz, etc.) before.

I will take a differing opinion to this statement, as one of the keys to soldering to aluminum is addressing the removal of the almost always present aluminum oxide coating.  This has to been done under the pool of liquid solder.  Both tin and zinc form alloys with aluminum, thus you have a basis for attachment.  

..."This has to been done under the pool of liquid solder" - I'm not sure I understand this (sorry, it's my poor English). What should I do exactly to prepare the aluminum surface at the grounding point?

Think about soldering aluminum wire.  Aluminum quickly becomes oxidized and looks white.  Prep the surface by sanding and applying flux which will keep oxidation from forming while heating, then with with 67Sn-33Pb (soft solder) tin the surface.  Takes lots of heat and you cannot tell visually, without some experience, how hot the aluminum is.  Aluminum will bond.  The best method for someone inexperienced with aluminum is to touch a wooden stick to the area you are heating.  As soon as the wood begins to smoke, you are ready to apply a solder.  When it adheres, you have a pool to attach to.  I have used a pencil butane torch in tight areas, but I have never done this inside an amp chassis.  Really no need and you still run the risk of overheating components.

The best method is TIG with argon gas.  Very little heat.

It can be done, but it is much easier to use a star washer and put thread locker on the screw and nut.

[HotBluePlates]

Re: grounding. I have checked all filter caps grounds and they read about 2.3 ohms - is that too much?

Set your meter to its lowest resistance setting. Touch the meter probes together. What does your meter indicate?

Depending on your meter leads, it may be a little above 0Ω or (more likely) 1-3Ω. If your meter does not have a feature to make a "relative resistance" measurement, you will have to mentally subtract this number from your readings of ground resistance.

NOTE: Sometimes the firmness of your contact with the meter probes will vary the resistance reading (more pressure = lower resistance), and the presence of surface oxidation can cause the reading to be higher than it truly is.

[slidincharlie]

Thank you guys for the precious tips on grounding and on DMM testing.
I haven't had any chance to go back to the amp in these days. Hope I can the next week.
--Carlo