Intro and Question about Quad Cap

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Intro and Question about Quad Cap

Postby deathman68 » Wed Dec 30, 2015 7:23 pm

Hi all,

I just joined the forum after lurking for a few days and reading/digesting several posts. I came by the (mis)fortune of the world's best Xmas gift from my father...his old Dynaco ST-70 combo with a PAS-2 pre-amp. Before powering up either unit, I took a couple of days to research them and to see what potential upgrades are available or recommended given that they are completely untouched since their original build. In my foolishness, I decided not to do the main board CAP upgrade on either until I had a chance to plug the units in and to take some measurements with my trusty multimeter so I could order all replacement parts in one go. Here was the result:

1. PAS-2: Powered up and left on for 30 minutes with no noticeable issue other than the power switch and unit received power regardless of switch position. I traced it back to a bad 0.02 mfd cap and soldered in a new one that I had and voila...problem solved! Though I noticed some seepage from the 2000 mfd cylinder caps so added those to the list. Now for the important part...

2. ST-70. Powered up and watched the tubes slowly come to life while monitoring the tube bias. As expected, it fluctuated slightly through warm-up until the 10 min mark when the GZ34 shot a blue arc and blew the slow-blow 3A fuse. Per factory rec, I yanked the GZ34, replaced the fuse, and fired it up without the GZ34...all worked as expected until I replaced the GZ34 and the fuse blew again...bad GZ34. Fast forward three days...new JJ GZ34 showed up and in the meantime, I switched the red power leads (verified position three times before soldering) and added the diodes to the tube socket per internet recs. I plugged in the new GZ34 and powered up...all good for the first 10 min. I adjusted/monitored the bias the whole way until I heard a pop and gray smoke POURED out of the right front jack and my house filled with noxious fumes/smoke. My first reaction, was "oh sh*$, I just fried the selenium rectifier and doomed myself to a slow painful death." Post-mortem teardown revealed a brown goo coating over the internals as shown in the pics below:

https://drive.google.com/folderview?id=0B_zD_oadVZipcEdBdDMyaTh3TUU&usp=sharing
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Image
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Image

I suspect that I fried the quad cap, but the question is why? I've searched forum, but didn't see any ideas. That said, I turn to the tube gods of the forum to help point me in the right direction. Aside from comments about my foolishness in frying a perfectly good amp and potentially killing myself (and my family), I 'd be interested to hear speculation about potential causes and next steps. I'm a noob in terms of tube amps, but have the usual amateur tools and experience with crude circuit design (555 timers, leds, etc).

I'm putting together a full upgrade list at this point, and have seen several options, but would love to hear from those with first-hand experience. Thanks all!

All the best,
Jeff
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Re: Intro and Question about Quad Cap

Postby TomMcNally » Wed Dec 30, 2015 10:14 pm

Hi Jeff and welcome to diytube.

*You* didn't blow the quad cap, it was a casualty of old age. That amp might be 50+ years old. They don't last forever. Easy fix is grab a new quad cap from one of the many suppliers, or install a cap board instead. Do the selenium mod, either rebuild or replace the driver board. Check the values of all the resistors and replace the whacky ones.

The good thing is there is more info online about that Dynaco stuff than any other amps.

good luck !

... tom
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Re: Intro and Question about Quad Cap

Postby burnedfingers » Thu Dec 31, 2015 5:21 am

The better fix is to leave the original cap on the board and unhook the wires to it. Once that is done purchase a ST 70 capacitor board and install that. What that gives you is more filtering and the ability to move the bias circuit to this board. One source is Triode Electronics.
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Re: Intro and Question about Quad Cap

Postby burnedfingers » Thu Dec 31, 2015 5:38 am

Some like the original driver board. It all depends on how much time and effort you want to put forth to bring it back to life and to put in to make it sound better. Some like myself like to modify the heck out of the amp and actually make it sound like an amp instead of a kit amp. One of the best mods is to replace the power supply board like I mentioned because it will allow for better filtering and a stiffer power supply. About the same amount of money will be spent on either a GOOD quality quad cap or the add on power supply board. In addition to the info on the power supply board there are a number of driver boards out there or you can build your own like I do.
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Re: Intro and Question about Quad Cap

Postby deathman68 » Thu Dec 31, 2015 7:30 am

Hi everyone,

Firstly thanks for the quick replies. I'm hoping that there's no collateral damage to surrounding parts and that the brown goo has lost some of its corrosive properties over time. I will be cleaning everything up with a hi grade electrical parts cleaner.

In the meantime, I reviewed the multiple options out there and decided to go with the Triode upgraded driver and cap boards. At this point, I want to get everything up and running before doing further tweaks. I'll also plan to selenium rectifier delete and swap out the speaker connectors and front rcas with some gold plated ones.

I also ordered the cap board for the PAS-2 and will recap and do the selenium delete as well.

I'll report back whenever I have some progress. Thanks again!

Jeff
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Re: Intro and Question about Quad Cap

Postby burnedfingers » Sun Jan 03, 2016 5:24 am

Gold plated RCA connectors do nothing for the sound. They only help with corrosion.
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Re: Intro and Question about Quad Cap

Postby Quad » Wed Jan 06, 2016 1:53 am

burnedfingers wrote:In addition to the info on the power supply board there are a number of driver boards out there or you can build your own like I do.


Joe, what is this board? Is it similar to the Mapletree board?
Any pics of the underside, or schematic, if it is public ?
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Re: Intro and Question about Quad Cap

Postby Geek » Wed Jan 06, 2016 4:08 am

-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: Intro and Question about Quad Cap

Postby burnedfingers » Wed Jan 06, 2016 6:31 pm

Quad wrote:
burnedfingers wrote:In addition to the info on the power supply board there are a number of driver boards out there or you can build your own like I do.


Joe, what is this board? Is it similar to the Mapletree board?
Any pics of the underside, or schematic, if it is public ?

Actually the board is a copy of the Mapletree board and yes I like the board a lot. But, to be perfectly honest Geek has a better board in his dynomutt (sp?) I bought one and it was great. Its priced right and it downright performs. You should also consider his power supply boards. He offers driver boards and other boards at very good prices that frankly are way ahead of others. I have tried about everything on the market including my own. The next 70 I do will have Gregs board.

I could give you a copy of the Mapletree board but my honest opinion would be to buy Gregs. I've just had the two board laying around and I took a picture to show you what you could do if you wished. Anything can be accomplished with enough drive and ambition.
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Re: Intro and Question about Quad Cap

Postby Quad » Mon Jan 11, 2016 2:54 am

Thanks! My question was more out of curiosity about the board in your pic.
For now, my amps live in storage, as I had to relocate for my job.
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