Persistent Ripple in the Filament Supply.

2nd harmonics for the masses

Persistent Ripple in the Filament Supply.

Postby microbus67 » Tue Nov 01, 2016 2:26 pm

Finishing(i hope!) my second GSG amp and am measuring 122mV AC in the 6B4G filament supply, resulting(I'm presuming) in ~2.8mV AC at the output of both channels. The ripple is the same bilaterally, @ the filaments and @ the outputs. All other measurements are correct to specified values. So far I have replaced C11-14, and D1-8(NOT easy in a fully populated board) with no change in the measured values. Maybe I'm missing something simple and obvious, but it escapes me at present. C11-14 were originally 16V 10KuF Nichicons, replaced(nothing fancy this time, but, as I said, no improvement) with 50V 4.7KuF Nichicons. C5's are Panasonic 100V 100uF. Diodes are 50V 3.3A Schottky's.
Any help/enlightenment will be much appreciated!
Thanks...
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Re: Persistent Ripple in the Filament Supply.

Postby jwhitmor » Wed Nov 02, 2016 4:26 pm

I read and article by a competent electrical engineer who was battling a persistent hum, and in the end, the only problem was his build was too compact (too small). As soon as he spread the same parts out on a bigger chassis, the hum went away. I had a little low level hum in my Get*Set*Go too. Most it came from using JAN-6B4G tubes, which were actually 6AV5GT tubes internally wired to work similar to actual 6B4G tubes. I changed tubes and 90% of the hum went away. The other 10% of the hum went away when I tied the PCB ground, the chassis ground, and the AC line ground together at a common point. As a last resort, you could go with real 6AV5GT tubes (not JAN-6B4G). It requires a little rewiring of the tube socket, and a couple 100 ohm resistors. Directions are on the G*S*G board by EWBrown (and others). The 6AV5GT has an indirectly heated cathode so heater supply ripple is not an issue. The 6AV5GT(A) tubes sound great, but if you really want your SET amp directly heated, you will have to hope you find the source of the ripple.
If it is not making X-rays, your B+ is too low.
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Re: Persistent Ripple in the Filament Supply.

Postby microbus67 » Thu Nov 03, 2016 8:19 am

Thank you for your kind response!
My GSG chassis is 12" x 17" and probably sufficiently uncrowded. I will, however revisit wire dress and see if I can improve that.
I've tried some 1964 6C4C's and some NOS Westinghouse 6B4G's without any improvement.(the ripple is also unchanged with the power tubes removed, so I'm not too surprised). I'll try more tube swaps(but would really like the freedom to use any 6B4G quietly).
I have some of the JAN 6B4G(6AV5GT) tubes, but have not tried them in this amp. I listened to a pair in my point-to-point 6B4G monoblock amps, but preferred the sound of directly heated 6B4G's(although they really are much more susceptible to filament noise and have required a thorough capacitor replacement in their heater supplies).
Again, I appreciate your helpfulness and experience.
I'll keep investigating, and hope for a flash of insight.
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Re: Persistent Ripple in the Filament Supply.

Postby TomMcNally » Fri Nov 04, 2016 7:05 pm

Hi Microbus -

I've build at least 25 get*set*go amps, see http://tmamps.com and none of them have ever been probed by a voltmeter. Is the symptom loud hum ? Are you using a Dynaco style PA-774 power transformer ? What kind of tubes ?

The worst that has happened to me when building one of these is forgetting to put fuses in.

Get me some more info so I can help diagnose it.

... tom
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Re: Persistent Ripple in the Filament Supply.

Postby microbus67 » Mon Nov 14, 2016 6:31 pm

Hello Tom, and thanks,

The power tubes are SED 6C4Cs, the preamp tube is an RCA 5691, rectifier is a NOS Tung-Sol.
The power transformer is an Edcor XPWR005, choke is a Hammond 156R, the outputs are EOS LM-2017Us.

I have not listened to the amp, but measure 126mV AC going into the 6B4Gs filaments, and ~8-9mV AC at the outputs. In DC filament supplies I aim for ~.1mV ripple, & certainly less than 1.

Maybe I'm getting a spurious measurement, or maybe the GSG PCB is configured in such a way as to null the AC hum before it becomes problematic at the outputs.
Maybe I damaged the diodes when installing them(all eight of them?, twice? That would be a first, even for me!).
I'm just going on my experience in building Point to Point 6B4G amps and power supplies.

Thanks again,

Mark
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Re: Persistent Ripple in the Filament Supply.

Postby microbus67 » Fri Nov 18, 2016 9:50 am

Tom and all,
I have listened to the GSG this am, connected to a pair of RS 40-1354A's($3.97 a pair on close out!), 88.7dB, and I can now say beyond doubt that 9mV AC on the outputs does well and truly hum(Knew it would).
Sounds like 60Hz to my aged ears.
Not TOO loud, but still unusable for music reproduction.
After TWO sets of Schottky diodes, and TWO sets of filter capacitors, coming up with perfect voltages everywhere else, and exactly the same AC ripple bilaterally is seems very odd, unlikely, and suggestive of some simple, central error.
B+ ripple is low.
What else could inject/transmit the noise?
I'm at a loss, and, unless the error declares itself pretty soon, am thinking of removing the PCB and doing a point to point from the major components already in place.
I know the degree of apostasy this move is likely to represent to devotees(and I respectfully remain one), but I've got to get this amp finished and (quietly) playing.
Thanks!
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Re: Persistent Ripple in the Filament Supply.

Postby TomMcNally » Sat Nov 19, 2016 2:40 pm

Mark -

There must be a short someplace, or a component of the wrong value. These amps don't hum. Are your connections to the tube sockets verified ? No solder blob under the barrier strip ? Tube socket not keyed correctly ? Maybe posting or emailing some hi-res pictures would help. We've found problems that way in the past.

The interesting thing is that I assume it's affecting both channels.

... tom
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Re: Persistent Ripple in the Filament Supply.

Postby microbus67 » Sun Nov 20, 2016 8:27 am

Tom and all,
Thanks for your suggestions. I'll carefully examine all these areas again.

Yesterday I changed out the spiffy(or so I thought- with the serious looking red base and all those extra reinforcing rods!) 5691 for a Plain Jane, ancient, "Bell and Howell" 6SL7.
The 13mV hum at the outputs dropped to 3mV. Not stone silent by any means, but one does have to get right up to the drivers in order to hear any noise at all.
THAT I can live with.
Maybe I can find some other noise-inducing faux pas and get the noise even lower- I'll certainly look carefully at everything you suggested,
starting with re-flowing the capacitor connections to assure appropriate filtration.
All resistors were measured and matched to within a gnat's eyelash just before installation, but I'll look at them again.
I've already moved all movable wires around while on the meter to see if lead dress could be the problem. It wasn't.
I'll look for any possible shorts again.
The symmetry of the values really is interesting, and possibly telling...I keep getting the feeling that something simple and obvious is about to jump out and dramatically reveal itself.
Anyway....3mV is a pretty quiet start, and will keep me from having to pull out the board and create a whole new point to point design!

Thanks again,

Mark
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Re: Persistent Ripple in the Filament Supply.

Postby jwhitmor » Wed Feb 01, 2017 12:13 pm

I experimented with the 6SL7 tube also. The best I have is a Slyvania brown base 6SL7WGT. The anodes are round, black, and very short. The worst I tried was a H-P 6SL7GT made for computer circuits (I am told). I also tried a brand new Sovtek 6SL7GT, which was "just OK" . The VT-229 tube is nice too. It is better than the Sovtek, and sounds similar to the Sylvania WGT.

You put in so very much effort, so I decided I would get out my oscilloscope, and check my G*S*G for comparison. I have no measurable dcV offset. I have 1.1 mVac 60Hz on both L and R speaker terminals open circuit, but it drops to 0.8 mVac with the speakers connected. You do need some kind of loading on the inputs. When I leave the inputs open circuit, I can hear a little hum. With the preamp connected, no hum. For testing, I suppose 250 ohms across the inputs would work.
Last edited by jwhitmor on Wed Feb 01, 2017 12:35 pm, edited 1 time in total.
If it is not making X-rays, your B+ is too low.
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Re: Persistent Ripple in the Filament Supply.

Postby microbus67 » Wed Feb 01, 2017 12:31 pm

Thanks!
The amp is now in daily service with a pair of Klipsch 98dB loudspeakers, a Boozhound phono stage, and a Broskie Aikido Cathode Follower line stage using a pair of NOS Sylvania 6CG7s.
The owner is absolutely delighted with the sound, and says that only a slight amount of hum is detectable very close to the drivers.
Good enough for me!
Two days ago I had the opportunity to do some listening to a prior build(by me : ) of a GSG, with no line stage, a Boozeound phono, and a pair of Hsu speakers augmented with a Velodyne sub.
Stealthy system, with a chillingly black background and excellent dynamics. Sounds even better than I remember!
My favorite 6SL7s are the old Sylvanias, and some JAN VT-229s. I'm inordinately fond of 6SL7s, and have collected more of them than any reasonable person has any cause to...
Thanks again for your help!
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Re: Persistent Ripple in the Filament Supply.

Postby jwhitmor » Wed Feb 01, 2017 12:52 pm

I am always experimenting (of course). I am currently using with my G*S*G, a Nelson Pass design "B1" FET line stage with a passive attenuator (Broske) and a "Budgie"phono stage (or a DurioSound DAC with HD audio files). Single driver (Fostex) back loaded horn speakers (no crossover or filters). A lot of music, for not a lot of money. I have a Budgie preamp also, it is a lot of fun to build and use.
If it is not making X-rays, your B+ is too low.
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