Buzz in my OTHER ST-70....argh!

[joeriz]

It hasn't been my month for tube amps... Yellow_Light_Colorz_PDT_21

Last week I wrote about my stock ST-70. This week it's my other one with the DIYTube board that I wrote about here:

http://www.diytube.com/phpBB2/viewtopic.php?t=2208

Now, every part in this amp is brand new except for the trannies. The amp worked flawlessly and silently for 2 months until this morning. I turned it on and was greeted with a low-level buzz coming from both speakers. Pretty low in level but loud enough that I heard it from my listening seat. So, OK, it's in both channels so I'm thinking power supply. I did quickly try replacing the 12AX7 on the driver board just for the heck of it since it is 'common' to both channels but it's essentially two tubes in one so I didn't really expect anything. Got what I expected..hum still there.

My first guess is the quad cap but it's brand new (I know, not unheard of for it to be bad). It's an 80/40/30/20 from Dynakitparts. I shut off the amp, made sure it was discharged, and did the resistance test. It tested OK. I also tried replacing the rectifier tube just for the heck of it. Hum still present.

I'm still thinking it is the quad cap. I suppose it could also be the power transformer as it is original to the amp.

This drives me nuts as the amp is essentially only 2 months old. Yellow_Light_Colorz_PDT_09

Anyone have any other (less painful) ideas?

Thanks,
Joe

EDIT: Ah yes, almost forgot...I also tried it w/shorting plugs on the inputs so I know the hum is in the amp.

[burnedfingers]

Check your ground connections. Resolder and see if the hum goes away.

[TomMcNally]

Like I always suggest in troubleshooting an amp ... make up a pair of SHORTED RCA plugs, and turn on the amp with them plugged into the input, and no other attachments except your speakers ? Still buzzing ?

Buzz and Hum are two different things ... Buzz is typically caused by bad grounds, as burnedfingers suggested you check, Hum is a power supply issue.

[joeriz]

Per my original post, I've already tried the shorting plugs. It made no difference.

I also tried 'testing' the ground connections while the amp was on, using a screwdriver to touch the main ground point (next to the quad cap) to the chassis and the quad cap lugs to the chassis. This also made no difference.

I'm not 100% sure whether I'd call the noise hum or buzz.

Joe

[EWBrown]

Hum or ripple is a low frequency note (like the low string on a bass guitar)
Buzz will have a bit of a sharp "edge" to it, as there is more HF or transients and spikes content on teh AC power..

I'd suspect the possibility that a near-by switching power supply , compact fluorescent light bulb, or a light dimmer is "crapping up" the power line AC and that is getting in through the power cord. The light dimmers are the worst line polluters, especially if they are set in mid-position. Is there anythig new that was recently plugged in or connected ower circuit or otherwise close by?

I've taken all of the dimmersout of my house (both in NH and NC) except for one ceiling fan speed controller, which will eventually get replaced or removed.


/ed B in NH / NC

[joeriz]

I'd say it's hum then rather than buzz.

I can not think of anything new that has been plugged in over the last few days. And I haven't changed anything in my system since the amp was installed at the end of January. I may take the amp elsewhere in the house just to test and see if that changes things.

My gut is still telling me that it's the quad cap...but I don't want to believe that. Yellow_Light_Colorz_PDT_13

Joe

[kheper]

I'd say it's hum then rather than buzz.

My gut is still telling me that it's the quad cap...but I don't want to believe that.

Solder the twist-lock tabs of the multi-cap to
the chassis. You will need a heavy duty iron
for this task. If there is still hum, it is not a
lack of ground from the multi-cap.

Many of the CE caps seem to fail
prematurely. Is the sound of the amp
affected adversely in any other way? If the
cap is dying there may be severe loading
on the amp, as though the signal running
thru the pre-amp became weak, very weak.

[mesherm]

Pull the three 9 pin tubes out of the driver board and then check the amp for hum. Check the condition of the bias voltage filter caps also.

[joeriz]

Pull the three 9 pin tubes out of the driver board and then check the amp for hum. Check the condition of the bias voltage filter caps also.

The amp was almost dead quiet with the 3 9-pin tubes removed. I had to have my ear almost physically touching the speaker cone to hear anything. The sound was so low in level I couldn't tell for sure if it was the same noise as before only attenuated. But to me it sounded more like what you would hear on a phono input when you turn it way up.

The condition of the bias filter caps looks to be OK and they are brand new along with everything else in the amp.

Thanks,
Joe

[mesherm]

The amp was almost dead quiet with the 3 9-pin tubes removed.

The hum must be coming from somewhere in the driver section. If you can, tack solder in a 10mf or so 400 or 450 volt capacitor at J19 and ground and another at J20 and ground. See if that reduces the hum. If not then its not likely the problem is in the quad cap.

[joeriz]

The hum must be coming from somewhere in the driver section. If you can, tack solder in a 10mf or so 400 or 450 volt capacitor at J19 and ground and another at J20 and ground. See if that reduces the hum. If not then its not likely the problem is in the quad cap.

Thanks.

Let me make sure I understand what you're saying. If I read you correctly, you're saying that if this doesn't reduce the hum, then the problem is most likely not in the quad cap? Am I still supposed to leave J19 and J20 connected to the existing quad cap or am I just "paralleling" these new ones in? I'm assuming I can use something of greater value here? I have some 40 or 50uf caps on hand...

[mesherm]

Let me make sure I understand what you're saying. If I read you correctly, you're saying that if this doesn't reduce the hum, then the problem is most likely not in the quad cap? Am I still supposed to leave J19 and J20 connected to the existing quad cap or am I just "paralleling" these new ones in?

Yup that's pretty much it.
If the cap sections are excessively leaky then you could tell by measuring the voltage across it to see if the voltage is lower than normal. Also a leaky cap usually gets a lot hotter because of the leakage current flow so just feel the cap can to see if it is hot.
If the cap sections were dried out and very low in capacitance then placing another cap in parallel will filter the hum enough to make a noticable difference.
If the caps are not leaky and paralleling caps across the sections doesn't help then chances are the problem is NOT in the power supply.
The 40 or 50 mf caps will work ok. You only have to check the filter caps for the phase inverters power section and the voltage amps power section since there was no hum in the power tube section. A push pull configuration is inherently immune to power supply hum since hum in each tube common mode and is cancelled out in the output trannies.
Any identical common mode signal present coming out from the PI section will also tend to be cancelled out. That leaves you with the hum originating forward of the PI section since it would have to get inverted and fed to the power tubes just like a normal signal to be noticable. This should narrow down the search to the voltage amp stage somewhere.

[joeriz]

Thanks for the explanation. I'll give that a try as soon as I get a chance...maybe not until the weekend.

Joe

[joeriz]

Well, I had a couple of 100uf 450v Panasonic snap-in type caps lying around so I carefully rigged them up to the amp with some alligator clips. Sure enough, the hum went down by at least half...maybe more. When I disconnected the ground connection to the two snap-in caps, the hum came right back.

Guess it's time for a new quad can cap...already! The thing is only two months old. I really don't feel like installing a cap board in this amp. Call me crazy I guess.

Mesherm...thanks for all of your help.

Joe

[SDS-PAGE]

I am not sure if it's necessary to "form" a new cap, but I remember someone here mentioning that it's worth a try. Here is one of the threads:

http://www.diytube.com/phpBB2/viewtopic.php?t=1523