Buzzing Choke

knowledge base for the classic Dynaco ST70

Buzzing Choke

Postby mgalusha » Sat Aug 08, 2009 9:57 pm

Hello all, my first post here so be gentle. ;)

I've got a friend with a Bob Latino built ST70 and he's having trouble with the choke buzzing. I've been tinkering with audio for about 35 years but somehow never manged to own one of these amps so I figured I'd ask those who do.

Anyway, he brought it by last week to have me install a new choke to replace the buzzing one. This is the second choke it's had, the first replacement was installed by the builder when he sent it back for repair. I installed the new choke and fired it up on the bench and it was dead silent mechanically, which of course was great. Hooked it up to my system and quiet electrically as well, no hum or buzz and it was sounding quite nice. After a bit he asked me to check the bias and to set it at 50mA per tube. He's running GEC KT-77 reissues. The bias was at about 44mA and as soon as I turned it up to 50mA it started buzzing again and of course wouldn't stop when I dropped the bias.

Looking up the specs of the choke from dynakit parts it seems to be rated at 200mA and at 50mA per tube plus the input/driver tubes this is of course somewhat over the 200mA rating of the choke.

My question is this, has anyone had trouble with the chokes in their ST70's or is he just wanting to bias too hard? 50mA seems pretty high to me but I tend to run tubes pretty conservatively. Not really any room in the chassis for a higher current choke. Since I don't have any real experience with this amp or this tube I'd love some input on what the best course of action is. I'm guessing 40mA would be a much better bias current for this tube but I'd prefer some more data points.

Thanks in advance for any thoughts, always appreciated.

Mike
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Postby TerrySmith » Sat Aug 08, 2009 11:36 pm

The cap right off the rectifier may be going bad causing the choke to see 120hz unfiltered DC.
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Postby Geek » Sat Aug 08, 2009 11:37 pm

Hi,

Welcome!

I've seen many rotten chokes hitting the market nowadays :(

I suggest a choke from a reputable dealer.

(IMO, ST 70's are biased way to hot @ 50 mA)


The cap right off the rectifier may be going bad causing the choke to see 120hz unfiltered DC.


True too! Check that cap out.


Cheers!
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Postby mesherm » Sun Aug 09, 2009 12:34 am

I have run KT88s in an ST70 equipped with an upgraded PT, a Weber Coppercap WZ68, and an 80/40/30/20 quad can cap with the 30mf after the rectifier. The KT88s were all biased at 62ma and there was no sound from the 1.5H choke so just the extra current alone I doubt would make it buzz. Take Terry's suggestion and check the cap section right after the rectifier. Also make sure both plates of the rectifier are working and equal.

As Geek says, there is no real advantage to running the EL34s/KT77s at 50ma idle. In Dynaco's day foreign made EL34s were plentiful and cheap. Drop the idle down to 40ma and your rectifier and output tubes will thank you. A higher idle current does not mean more output power
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Postby mgalusha » Sun Aug 09, 2009 6:31 am

Thanks gents, most appreciated. I haven't done any troubleshooting on it yet other than changing the choke for him. I had thought about the cap but haven't really poked it yet. It's a new build so one would hope the quad cap was good but I've seen brand new caps that were bad.

If I have to replace it, is there any difference between the quad cap from Dynakit Parts or the CE quad cap from Antique Electronic Supply? Same price and values so perhaps they are the same part?

I am going to tell him to bias them around 40mA, that was already in my plan. I saw the posts about adding a pair of diodes in front of the rectifier, what an excellent idea and I'll be doing that on my preamp as well. :)

Any other tips to ensure it's reliable for him? He's not a DIY type and is pretty much limited to setting the bias, so I would like to help him make it as reliable as possible. The two prong cord kinda bugs me, I like safety grounds on tube gear but I don't want problems with ground loops.

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Postby davygrvy » Sun Aug 09, 2009 9:50 am

Ground loops? Don't get me started ;) I used to build recording studios.

You really can't avoid them with unbalanced interconnects... For example, your FM antenna might be mounted on the roof and come in on 75 Ohm coaxial which legally has to be grounded where it enters the building. Same for cable TV. And your amp might be grounded through the power cord, too, so when you connect the two in your system, you have this huge loop that causes common impedance coupling across your preamp.. no fun.

IMO, balanced (differential) inputs for an amp is a really good idea. I dropped the $60 each on a pair of Jensen JT-11P4-1s for my ST-70 as well as using a three-prong power cord. This gives you the option to lift the screen connection on the inputs.

Yes, please add a three-prong for safety reasons. Converting the audio inputs to balanced by using a transformer can solve any ground loop issue.
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Postby Bob01605 » Mon Aug 10, 2009 7:12 am

mgalusha,

I built that amp and it had no hum/buzz when it left here. The power transformer on that amp was bad (It did not give the proper voltages) so I replaced it at no charge for the customer under warranty. This amp was tested when it left here the second time and played perfectly with EL34 tubes with no hum, buzz or any noise whatsoever.

A couple of things ...

1. IMHO you shouldn't bias KT77's at 50 milliamps per tube. That is a little high for that tube and will shorten tube life with no real increase in sound quality. 40 to 45 milliamps per tube is about right. As mersherm mentioned > "A higher idle current does not mean more power."

2. The fact the choke buzzes even after you changed it out tells us that the choke is probably not the problem. Although the quad cap checked out fine here it is possible that when the B+ DC current draw gets high the quad cap/choke combination starts leaking some ripple into the B+ DC. It is possible that the quad cap is bad (probably the first section) but only when stressed at higher current draws. Checking this quad cap with a cap meter may not allow you to see the problem. Your meter is only hitting the cap with 9 volts and this seems to be an issue that crops up only with higher voltages and higher current draws.

Remove that quad cap and send it back to me and I will send you a new quad cap. Your customer should have my ship to address. If he doesn't then Email me at > Bob01605@aol.com and I will provide you with my address.

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Postby mgalusha » Tue Aug 11, 2009 1:50 pm

Bob,

Thanks for the info. I have not had a chance to look at since I posted but was planning on wiring in a 20uf/600V cap in place of the first section of the quad cap to see if that helps matters. If that takes care of it I'll send you the quad cap, appreciate the assistance. I know it's smaller than the current value but it's the closest thing I have on hand with a high enough voltage rating.

I fully agree on the bias and I've informed my friend that he really needs to run them around 40mA.

I have no doubt that you sent him a quiet and fully functional amp. He did mention you had replaced the power transformer so that was one thing not to worry about. I am in no way trying to imply there was anything wrong with the build of the amp, I'm just trying to help him out so he doesn't have to ship it back east and risk the potential damage from the shipping companies. It's a win - win for me, I get to tinker with something new and he brings me some nice single malt Scotch for my time. :)

I'll email you with the results of trying a new cap in place of the first section of the quad cap.

Mike
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Postby Bob01605 » Wed Sep 09, 2009 1:04 pm

Mike,

I was glad to hear that you got the new quad cap from me, installed it and that the installation of the new quad cap corrected the problem. That bad quad cap that you sent to me measured exactly as you stated. It is strange that TWO of the 4 sections (20 uF and the 40uF) BOTH went bad and the other two sections are still OK? I might note that these CE quad caps have been dead reliable for me in over 200 amps and they always measure "over spec". The 80 section usually measures about 90-95 uF and the other 3 sections are also usually 10-15% over spec also. Glad that we got the issue taken care of without too much aggravation ..

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http://www.tubes4hifi.com/bob.htm
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