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Newly built amp was performing well...now oscillating

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Posted on August 22, 2015 at 02:11:59
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I am having some stability issues with this circuit;
http://www.southampton.ac.uk/~apm3/diyaudio/Croft5_circuit.jpg
It was performing well for about a week and now there is an audible oscillation which sounds like chirping. Any idea what may have caused the amp to go hawwire? regards, Dak

 

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RE: Newly built amp was performing well...now oscillating, posted on August 22, 2015 at 04:26:34
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 First thing I would check are the tubes.

 

RE: yeah, that is the first thing i will check, posted on August 22, 2015 at 09:13:04
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 The tubes are used ones so they might be bad. Although, I can't think of any instances where that kind of oscillation was "bad" tube related. I don't have the amp yet I am just describing the symptoms as told to me. Thank you, Dak

 

RE: Newly built amp was performing well...now oscillating, posted on August 22, 2015 at 19:02:05
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Did you turn off your cell phone?

Just asking because of the symptoms - it happens often.

 

RE: Newly built amp was performing well...now oscillating, posted on August 22, 2015 at 20:07:33
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Would that be the dreaded RFI? Actually, the amp is not mine but I had just built it and sold it. It was well behaved with me but I don't know what the situation is at its new home. I will ask the new owner about cell phone signals and such. thanx for the tip, Dak

 

RE: yeah, that is the first thing i will check, posted on August 23, 2015 at 14:58:32
FenderLover
 Did the new owner alter the global feedback network?

 

RE: yeah, that is the first thing i will check, posted on August 23, 2015 at 21:02:31
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 No, everything still looks to be the same. Check the update

 

RE:Update, posted on August 23, 2015 at 21:21:01
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I got the amp back from the owner to check on its condition and everything looks ok. I retested all the tubes and they are all good. So, I removed the 5v4 rectifiers and installed some SS rectifiers and did some voltage checks at 80 vac. Voltages look good EXCEPT; the bias circuit.
I have installed an "EFB" circuit which is the brainchild of Dave Gillespie. This uses an LM337 transistor along with an adjusting pot and some resistors and caps. With the EFB I was having a stable output of .56 vdc which translates into about 24 ma static dissipation per el84 tube. The problem is now I am reading 1.5vdc on the test point. Is it possible that the LM337 has failed? Anyway, I put back the 100 ohm 5 watt bias resistors for now and disconnected the EFB. I will run some sound checks and if the amp is stable I will probably just leave it at good ole cathode resistor bias. If the EFB is unreliable I would not consider it an improvement over a cathode resistor. regards, Dak

 

RE:Update, posted on August 24, 2015 at 11:08:55
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 When I first saw the EFB circuit, it occurred to me that even a momentary short in an output tube could destroy the LM337. I don't know if your LM337 is defective, but it might be advisable to install a TVS (transient voltage suppressor) to ground at the output of the chip.

 

RE:thankS TK, posted on August 24, 2015 at 11:33:48
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 If that is the case I would rather do with out one. It has been my experience with SS devices that they are too sensitive to over voltage situations which in tubed gear is a very likely occurrence. Adding another component just makes it that much more of humbug versus a single resistor. I am not that savvy with using SS but was willing to give it a try. So, with this unit I gave it a chance, but it only lasted a week before failing. I don't think I can recommend the EFB circuit inspite of its advantages. Reliability is a high priority with me. Best regards, Dak

 

RE:thankS TK, posted on August 24, 2015 at 16:03:13
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



This seems to be another case of tubes being mechanically fragile and electrically tough, while SS is mechanically tough and electrically fragile.

Yes sir, be very careful about exposing SS parts to over voltage, lest they fail. The IRFBC20 and ZVN0545A MOSFETs are favorites of mine in the DC coupled voltage follower role. At this time, I would not call for either FET, unless a protective Zener diode, like that shown in the uploaded preamp schematic, is employed. FETs and other SS parts turn on instantly and voltages are "squirrely" before tube conduction starts.


Eli D.

 

RE: Newly built amp was performing well...now oscillating, posted on August 24, 2015 at 16:32:31
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 It could just be a noisy tube.
Hard to say without hearing it.

I have had some in the past make random subtle low level chirping sounds that go up and down in frequency, that I don't think was typical oscillation. It followed the tube from left to right channels.
Replacing the tube fixed the problem.

I just chalked it up to buying tubes on eBay.
You can get a barely used set of tubes that looks measures well, yet is a "noisy" tube.
I guess eBay is a good spot to unload these little sins as most might not notice right away.

△This message will self destruct in 10 seconds△

 

RE: Newly built amp was performing well...now oscillating, posted on August 24, 2015 at 16:49:44
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 You might be right about a bad tube but the SS device looks like it is fried. I switched the amp back to simply cathode resistor bias and will try out the amp to see if there are any more issues. cheers, Dak

 

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