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While everyone has the troubleshooting hat on...

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Posted on August 30, 2020 at 16:18:44
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001

Ive got a loud Hummmmm on one of my DIY HIFI Supply 'Billies' 300B Monoblocks.

They've been in storage (with tubes removed) for the last couple years.

Worked fine when I put them into storage prior to our move to Santa Fe.

Cranked them up and one has a LOUD Hum. Swapped tubes and no, not the tubes.

With a 10 Ohm load resistor across the output terminals and no input on the RCA in I get .003 VAC for the 'quiet' one and .016 VAC on the one with LOUD Humm.

106 dB Edgar Horns, so any AC hum on the output is going to be loud.

Has hum pots with all humm reduced as much as the hum pot will allow.

AC on both 300B filaments is the same at .24 VAC.

Another difference from the schematic is that the lower section of the 6SL7 is biased by a 1.2V NiCad. In static condition .004 VAC across that NiCad on both the HUMM unit and the 'quiet' unit so likely not the NiCad.

Only other difference I can find, and I suspect it's the clue if there is one, is that the HUMing unit reads .076 VAC on the input of the grid resistor (not the grid itself) of the 300B while the 'quiet' one reads a mere .004 VAC. Again, in a static state with no input on the RCA input of either amp and a 10 OHM load on the output.

My kingdom for a scope!

Sadly, my old Heathkit scope died long ago and I have not replaced it.

My cheap Chinese DVM will not read AC at B+ DC voltages so I have no idea what the PS may be adding to the problem but no puffy caps or smells and the DC levels look to be the same and near normal.

Where should I start?






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

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RE: While everyone has the troubleshooting hat on..., posted on August 30, 2020 at 16:29:10
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 I had a hum similar to yours, and determined that it was coming from my Welborne Tube Pre-Amp. I eventually found a bad solder joint on a PCB Board.. It had gotten 'Old' and the joint had finally cracked.... Sucked up all the old Solder.. RE-fluxed the area and flowed some fresh solder into place..... All is Quiet now....

So, I would start with a Magnifying Glass at each solder joint and/or re-solder every joint until it's nice an shiny.....

BTW -- I built a pair of Billies once about 25 years ago..... It was my first foray into SET amps. They were pretty inexpensive at the time, but remembering them sounding decent enough that I was hooked on the SET sound.




 

Pretty much point to point but would make sense to..., posted on August 30, 2020 at 16:43:34
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 do a bit of re-soldering (which I have done around the tubes) but not the PS, which does have the caps on a PCB.




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: Pretty much point to point but would make sense to..., posted on August 30, 2020 at 16:59:28
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 Yes, I remember building the Billies..... All Point to Point.... You could pretty much much solder every joint in short order.....



 

OK folks, 16 mv across 10 ohms (dummy load on the bench) is only 1.6 ma..., posted on August 31, 2020 at 07:21:25
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 If my spread sheet calculations are not amiss.

1.6 ma times 16 mv is only 32 uw so how is 32 MICROWATTS causing a LOUD hum even with 106dB horns?

What am I doing wrong here?

Maybe something else has gone amiss?


First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: OK folks, 16 mv across 10 ohms (dummy load on the bench) is only 1.6 ma..., posted on August 31, 2020 at 09:31:45
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 Your Calculation seems correct to me. -- I'm still betting on a faulty connection somewhere.



 

RE: While everyone has the troubleshooting hat on..., posted on August 31, 2020 at 10:10:34
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 Jump a known good cap across the electrolytics, one at a time. We used to do this at the TV shop I worked at, with the power on. But you get some arcing that way.

 

RE: While everyone has the troubleshooting hat on..., posted on August 31, 2020 at 11:21:38
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 you might want to check the .47u coupling cap.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: OK folks, 16 mv across 10 ohms (dummy load on the bench) is only 1.6 ma..., posted on August 31, 2020 at 13:27:15
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 I get 25 microwatts, which is 45dB below 1 watt, so 106 - 45 = 61dB - about the level of normal conversation.

 

Or, said another way..., posted on August 31, 2020 at 13:48:55
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 too much him! :-)

Thanks, I feel a bit better now.

Will try replacing the coupling cap.

I think it a Jensen foil and oil?

Noy original but I have some .22 dark brown orange drops out of an old Jolida I upgraded years ago in my parts bin I can pair up for .45 ufd and try.

What's everyone using for coupling caps in SET 300B amps these days?




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

I got a similar result when Manually Calculating ..........., posted on August 31, 2020 at 14:01:03
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 But, when I used this on-line Calculator I got 30 Micro Watts

https://ohmslawcalculator.com/ohms-law-calculator




 

Indeed I did..., posted on August 31, 2020 at 16:36:07
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 Replaced it with a .33 Solen. Same .014mv AC on the output across 10 ohms but not as nice a sounding noise. :-)




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: While everyone has the troubleshooting hat on..., posted on August 31, 2020 at 17:46:08
trobbins
Audiophile

Posts: 189
Location: Melbourne
Joined: March 26, 2012
 Buy some electronics contact cleaner spray with micro-nozal and remove/insert and spray all valve pins and any other plugs/sockets, battery connections, and pots if you can access them.

If you have a soundcard in your PC then suggest trying to use a spectrum analyser software to look at the speaker output when idle.

If you remove the 6SL7 then you will only have the output stage connected, so that is one way to locate the section of circuitry likely at fault.

 

+1 on the Contact Cleaner .............., posted on September 1, 2020 at 05:10:58
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 When I refurbished a Fisher Tube Receiver, the biggest single improvement that I made was Cleaning the Tube Sockets and Controls and other connection points. All of these had decades of dust, dirt and oil film.



 

No switches in the signal path and Humm is being measured..., posted on September 1, 2020 at 10:12:22
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 as a pretty steady .014 mVAC with no input.

Did clean the pins on the tubes and pin sockets and wiggling the tubes about make NO difference in the .014 mVAC on the output or even the higher mvAC reading at the input of the 300B.

Suspect PS and for that I will need a scope.

Should have one, used one every day of my work life for a good 15 years so why not now when I spend so much time diddling with electronic stuff?

Used scopes go for a couple hundred and I don't need a calibrated one so lots to choose from on eBay.

Have a decent Tektronix x10 probe but that's about it. :-)




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: No switches in the signal path and Humm is being measured..., posted on September 1, 2020 at 17:31:21
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 You shouldn't need a scope to troubleshoot the PS. The primary cause of hum are electrolytics that dry out. The easiest way to find the problem is to just jump each electrolytic with a known good one. If it is the problem, the hum will vanish when you place the good electrolytic across the failed one.

 

Still need a scope..., posted on September 2, 2020 at 07:51:15
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 But as long as I have to tear into it to 'bridge' the caps I'm thinking of just rebuilding the PS and be done with it.

All the caps likely could stand an upgrade anyway as the amp is at least 15 years old.

Might upgrade the tube sockets while I'm at it.

Eventually it will be used to bi-amp the bass bins of my EdgarHorns. 80-500Hz, so maybe Nichicon electrolytics will be an upgrade and 'good enough for 80-500 Hz. That said, the original caps say 'Rubycon USA' so maybe a small step down?




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: Still need a scope..., posted on September 2, 2020 at 08:51:37
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 Scopes are handy to have around. What I use the most is a Fluke ScopeMeter. The portability is great. I have recently seen scopes of all kinds on CL at very good prices. I have also used PC scope plug-ins, also a good option if you have a PC as part of your bench.

Regarding the caps, 15 years, depends on the caps and how they were stored. A company that I worked for shipped a product to the military that would sometimes sit in a warehouse for years. We included a procedure to reform the caps that seemed to have worked out well.

 

DIY-HIFI Supply still sells Rubycon caps that go into this amp..., posted on September 2, 2020 at 09:42:56
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 Likely been in stock for years as well.

I have no 450V electrolytics laying about and my intention was to rebuild both mono-blocks anyway so that's what I will do.

If one still hums and the other does not after ALL caps in both units have been changed, tubes have been swapped and all connections re-soldered, then I'm stumped.

Scope on the bench, probe in hand is still second nature even after 40 years away from the bench.






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: DIY-HIFI Supply still sells Rubycon caps that go into this amp..., posted on September 2, 2020 at 11:58:12
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 You'll fix it. I usually don't try to track down hum with a scope, the reason is that I usually see it everywhere. There are some exceptions.

I mentioned before about jumping in electrolytics at the TV shop I worked, even with the power on. Volt meter and that cap, with jumper leads, was the most used diagnostic tool at the shop, followed by a scope. The value isn't that important, it was about 100uf, 450 volts that we used for just about anything. It would tell us in a second if electrolytic was the problem. All it needed to do was reduce the problem, to tell us what to replace.

 

"then I'm stumped." ..............................., posted on September 2, 2020 at 12:51:43
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 Are you positive it is the Amplifier? -- I just had an issue and it turned out to be the Pre-amp Tubes... I had stuck a more powerful amp in place of my 45 amps which made the Tube Noise more pronounced.

Turned out my Pre-amp Tubes had 'Gone Noisy'... Put a Quad of Old Production 6SN7 Tubes in the Pre-Amp and the problem was solved.... I was first convinced it was the Amp, as that is what I had changed. But the more powerful amps had highlighted the problem that was always there. Of course it took me about 4 hours of 'trouble shooting' to find the problem.




 

Both monoblocks on bench w/ no input..., posted on September 2, 2020 at 14:20:46
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 Inputs held at Gnd by a 100K resistor to Gnd on the grid of the input tube.

Noisy amp has .014 mVAC on the output across 10 ohm load resistor, good (quiet on speakers) has .003 mVAC across the same 10 ohm load resistor.

And swapped tubes with no change.

And noisy amp has .076 mVAC on the 300B side of the .47ufd coupling cap while the 'quiet' amp has .004 mVAC at the same point.

Doesn't leave much more than caps and they're all about to get changed.

15 years, it's time.






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: Both monoblocks on bench w/ no input..., posted on September 2, 2020 at 14:26:59
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 You should change 1 at a time, so you can identify the 'offender'.



 

I have no such curiosity..., posted on September 2, 2020 at 15:52:19
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 just want quiet amps so I can enjoy my horns.




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

Well, at least let us know if you get it solved .............., posted on September 3, 2020 at 11:41:10
Cut-Throat
Audiophile

Posts: 18285
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 Our curious minds would like to know.



 

Filter caps, as fretr has mentioned, is the obvious goto., posted on September 3, 2020 at 11:45:25
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 Filter caps don't like to be left uncharged for 10 years. They have a way of going bad, not unlike a battery. With this sort of complaint, not only would I be looking at the caps in the amp that hums but also in the one that does not, as its likely they they would be slipping downhill too.

 

Only been in storage a bit over two years..., posted on September 3, 2020 at 17:39:00
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 since we packed up and moved to Santa Fe in April, 2018.

But yes, all the caps in both monoblocks are going.

Just waiting for Parts Connection to deliver the goods.

Maybe even a tube socket or two if I hear ANY pops or clicks.




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

If the part is substandard, it may have failed regardless of storage in that time. nt, posted on September 4, 2020 at 09:30:38
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 -

 

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