Can cap question; What can it take with it when it goes?

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Can cap question; What can it take with it when it goes?

Postby xlr8 » Mon Dec 28, 2009 7:09 am

Hi folks,

I registered here awhile back after picking up 2 70's off of ebay for a project I'm working on. One has tubes and is currently running in my mix with no problems except for 1 weak 7199. te other arrived with no tubes and the seller did not know if it worked or not.

A few days ago, I decided to see if the other one worked. I plugged it in w/o tubes, turned it on and let it sit for about 15 minutes. No physical hum, no smoke or funny smells. I then installed all tubes minus rectifier to make sure they all lit up, which they did. Again, I let it sit for 10 minutes or so. After this, I installed the rectifier tube. After about 10 more minutes I heard a crack and sizzle and immediately pulled the plug. I could not even touch the can cap and it leaked brown goo.

My question is this: When a can cap craps out like this, can it take other components with it? What is the first thing I need to look at as far as damage is concerned? I'm not terribly concerned about much more than the transformers. I put the tubes back in the other amp and they all seem to work fine.

Please be patient with me folks. I'm not new to listening to tube equipment, but I have never attempted a rebuild like this. I have been woking in the high voltage utility power distribution feild for years now and understand how to handle extremely high voltages, so that's not a problem. I just don't want to burn up any new parts I buy nor cause further damage to what is here now.

Thanks in advance for all advice!

Jon
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Postby WA4SWJ » Mon Dec 28, 2009 7:33 am

Hi Jon,

Sorry to hear about the cap frapping out on you. But it's old and those electrolytics do fail eventually. As long as you didn't leave the amp on too long the rest of it is probably OK. The rectifier (5AR4) would be the most likely other part to fail under those conditions.

Here's one place to get a replacement capacitor. Should fit right into the chassis, but you'll have to unsolder the tabs to pull the old one out if it is built like I think it might be.

http://www.triodeelectronics.com/40ufat52twis.html

Other places have them too. I have ordered from Triode and have been happy with their products and service.

Here's a link to a picture of an ST-70 that I built from all new parts a few years ago. The cap and transformers were from Triode and the driver board is from Shannon on this forum.

Image

There are lots of other amp pictures there if you want to look around. I also refurbished an old rusty ST-70 that works just great. I didn't do anything to the cosmetics on that one.

Image

Good luck with yours and I'm sure you'll find all the help you need here.

Happy New Year to you and all!!!
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Postby xlr8 » Mon Dec 28, 2009 8:08 am

Sorry to hear about the cap frapping out on you. But it's old and those electrolytics do fail eventually. As long as you didn't leave the amp on too long the rest of it is probably OK. The rectifier (5AR4) would be the most likely other part to fail under those conditions.


Thank you for such a quick reply. What you said certainly makes me feel better. The amp in question and the one in your bottom pic look like they came out of the same basement.

About 5 seconds elapsed between pop, sizzle and cord yank. The rec. tube I have is an OS RCA. Only markings are RCA electron tube on the base and Made in Great Britain on the glass, so I don't know what to test. I have borrowed a Mercury 1100 tube tester. How can I find out if it is indeed the 5AR4? Is there a possibility of messing up the tester or the tube if I test the wrong parameters on the wrong tube?
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Postby WA4SWJ » Mon Dec 28, 2009 9:25 am

Jon,

My guess is that you have the original rectifier, which might be a Mullard 5AR4. Hold on to that one if you can! The newer 5AR4's (also known as GZ-34's) are not nearly as robust as the old tubes. I have found the Sovtek 5AR4's work for me whereas the JJ's do not. If it was me, I would try it in the other amp and if it works fine then I wouldn't worry about it.

I'm not familiar with your tube tester at all. Maybe someone else here could comment on that. However, as above, if it works in the other amp it's probably OK.
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Postby EWBrown » Mon Dec 28, 2009 10:07 am

First of all, check your fuse, just to make sure that the original owner didn't install a 15 amp "no-blow" fuse. I've seen such before...

The original rectifier tubes were generally Mullard or Amperex 5AR4/GZ34s, with US or Dynaco markings. Back in that era, when the US Dollar really had some buying power, Mullards, Telefunkens and Amperex were the "cheap imports". That's why all the kit manufacturers used them. One had to know someone in the CIA, in order to get any Russian tubes ;-) Back then, "Made in Japan" still had a certain stigma to it, much as "Made in Chian" can today ;-)

As stated earlier, avoid the J/Js, some may work fine (I have one which has held up admiarbly) but most do not. Definitely avoid the Chinese made 5AR4s, these barely make the grade as a 5Y3GT and will go pyrotechnic just as soon as the EL34/6CA7s' filaments warm up.
Extra demerits if the Chinese rectifier tube is marked with "Made in Chian" and / or "Election Tube"...

/ed B
Last edited by EWBrown on Mon Dec 28, 2009 8:22 pm, edited 1 time in total.
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Postby xlr8 » Mon Dec 28, 2009 4:53 pm

My guess is that you have the original rectifier, which might be a Mullard 5AR4. Hold on to that one if you can!


Yeah, I was thinking of going with something along the lines of the Weber Copper Cap anyway and will use this tube to make some comparisons when I'm done. I wasn't sure if Mullard and RCA were the same, but I guess they are?

Extra demerits if the Chinese rectifier tube is marked with "Made in Chian" or "Election Tube"...



Dear lord, please tell me you're kidding! We have the same problem in the knife collecting world. Seems the Chinese will TRY to clone anything... As for the fuse, it has been replaced with a 5A fast blow. I think it was just a convenient replacement, but then again, that's why I'm here.
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Postby mesherm » Mon Dec 28, 2009 6:05 pm

An original multi can cap in a ST-70 should be replaced anyway.
Also if the original selenium rectifier in the bias circuit is still there it should be replaced with a modern diode like a 1N4003 or higher.
As a matter of course I would replace the two bias fiter capacitors also.
Remember that the bias voltage has to be negative with respect to ground.
When powering up an ST-70 with new tubes or if its in unknown condition you should first make sure that the maximum amount of negative bias voltage is applied to each pair of tubes.
With no tubes installed, power up the amp and check the voltage at pin 5 of each of the EL34 sockets. Connect the black DMM lead to the chassis and place the red probe in the pin 5 tube socket of each tube. The voltage should read as a NEGATIVE. Adjust the bias pot for the side you are checking until that voltage is at its MAXIMUM NEGATIVE value...around -55 volts or so. The right side pot adjusts the right side power tubes and the left side pot the left power tubes. That way the tubes will conduct the least amount of current when all the tubes are installed for the first time. Then comes the process of adjusting each pot ping-ponging from side to side until the correct bias current is reached. Most of us run only 40 to 45 ma per tube (80-90 ma per pair) rather than the 50 ma per tube (100 ma per pair) as recommended by Dynaco.
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Postby EWBrown » Mon Dec 28, 2009 8:18 pm

And if you have the original phenolic Dynaco 7199 driver board, those big black cylindrical paper caps are also another serious liability, the "black cats" and "bumble bees" weren't meant to last for 40-50 years. The outer cases can crack, and the caps will go leaky if not just plain short out, which can kill the EL34s, at worst, or just really mess up the bias settings at the minimum.

generally these phenolic boards turn into "crispy critters' after a few decades...

Speaking of Chinese Clones, a few years ago, I picked up a nice looking, but cheap, genuine fake Rolex "Oyster" watch clone, for $25 at the local Sunday flea market. It looks real, and feels real, but it has a quartz movement, instead of the true Rolex mechanical multi-jewel movement.
it is also not waterproof, as is the true Rolex. So it is more of a sand crab rather than an Oyster...

The second hand "stepping" is a dead give-away. But then it looks good enough to fool a few Rolex owners I've shown it to. Before then, at least they had the decency to give the fake Rolex watches names like "Bolex" or "Rolux", or some other intentional mis-spelling.

Hmmm... Flowery Branch, GA is just about 60-70 miles south of here.
I hear that name on WDUN 550AM, which comes in OK around here.

/ed B
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Postby xlr8 » Tue Dec 29, 2009 10:09 am

As a matter of course I would replace the two bias fiter capacitors also.
Remember that the bias voltage has to be negative with respect to ground.
When powering up an ST-70 with new tubes or if its in unknown condition you should first make sure that the maximum amount of negative bias voltage is applied to each pair of tubes.
With no tubes installed, power up the amp and check the voltage at pin 5 of each of the EL34 sockets. Connect the black DMM lead to the chassis and place the red probe in the pin 5 tube socket of each tube. The voltage should read as a NEGATIVE. Adjust the bias pot for the side you are checking until that voltage is at its MAXIMUM NEGATIVE value...around -55 volts or so. The right side pot adjusts the right side power tubes and the left side pot the left power tubes. That way the tubes will conduct the least amount of current when all the tubes are installed for the first time. Then comes the process of adjusting each pot ping-ponging from side to side until the correct bias current is reached. Most of us run only 40 to 45 ma per tube (80-90 ma per pair) rather than the 50 ma per tube (100 ma per pair) as recommended by Dynaco.
_________________

I'm printing this and putting it in my notes as a reference. Thank you very much!

And if you have the original phenolic Dynaco 7199 driver board, those big black cylindrical paper caps are also another serious liability, the "black cats" and "bumble bees" weren't meant to last for 40-50 years. The outer cases can crack, and the caps will go leaky if not just plain short out, shich can kill the EL34s, at worst, or just really mess up the bias settings at the minimum.

generally these phenolic boards turn into "crispy critters' after a few decades...


This is something I've been studying on ever since I purchased these amps last year. These are the 3rd and 4th st 70's i've owned. The first one was all stock. Something went wrong on the board about 6 months after I first powered it up and ruined more than I could afford to fix at the time. I was very happy with the way it made my old Klipch LaScalas sound...VERY happy! So happy that I went on to purchase the second one which had been modified. I don't know what board was in it, but I know it had 3 input tubes and giant yellow Wonder Caps on it. I remember it sounded phenomenal on those speakers. That amp was built by a fella named Robert Cunningham in Chamblee GA (I was sort of hoping he might be around here somewhere). After a marriage and then subsequent divorce, I had to sell the entire setup and start over. That system is what has spurred me on into the world of DIYTUBE and ultimately led me to purchase the 3rd and 4th st 70.

I have done quite a bit of reading about some of the different options that are available today. I've read about Curcio, Van Alstine, Triode Electronics, the DIY Tube board, VTA and Mapletree. I'm leaning toward VTA and have recently contacted them. Believe me, the stock stuff in these amps won't be around for long and this is where I'll be needing help with things I know I'll run into in the process. I should have all of the parts together in a few weeks...

Hmmm... Flowery Branch, GA is just about 60-70 miles south of here.
I hear that name on WDUN 550AM, which comes in OK around here.


Well, we were really more of a twigg than a branch when I moved here 6 years ago, but urban sprawl has arrived. And yes, I get WDUN pretty strong up here. I switch to it when I lose 640 WGST on my way home from ATL. You must be pretty close.

WA4SWJ, your handle would lead me to believe I should listen out for you sometime. What bands do you frequent?
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Postby WA4SWJ » Tue Dec 29, 2009 12:08 pm

Well, right now I'm not real active on the bands because I just moved and don't have an antenna up. I could get on the satellites I guess (I'm the editor of The AMSAT Journal), but to be honest I've been busy unpacking stuff. I like to work on tube amplifiers even more than ham radio so I've been mostly doing that.

Maybe during the long weekend coming up! When I'm on I like the higher bands like 20 m or higher.
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Postby EWBrown » Tue Dec 29, 2009 12:30 pm

I can say just about the same , except that I sold most of my satellite stuff (145 / 435 MHz) a couple years ago, and I also don't yet have any antennae up here, so HF is currently a no-go.

Only usable gear I have is just a collection of somewhat old 2meter, 220 and 440 HTs, and 1200, which is running somewhat flakey lately, I suspect one of the internal flat cables between the tiny PCBs has worked loose again,,, I also have a literal "basket case" Azden 6 M HT, which is opened up and stored away in a large plastic "zip-lock" storage box until I get around to replacing its CPU back-up battery, and re-stuffing its battery pack with new NiMH cells.

I also have to send the FCC my new address, as they still have my old NH information.

ed B KB1MZ Clay County, NC EM85
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Postby Geek » Tue Dec 29, 2009 5:05 pm

WA4SWJ wrote:When I'm on I like the higher bands like 20 m or higher.


You have propagation there?

Wow, I haven't even been able to hear 5MHz WWV for over a week now... even the morning west coast crew on 3720 is barely audible. And flat out nothing above 4MHz is going anywhere by my ears in the CN99 grid :-/

Cheers!
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Postby WA4SWJ » Wed Dec 30, 2009 6:08 am

Guys,

Well, I said "when I'm on", which ain't often. I usually like to work Field Day. During the last event I made a contact or two on 6 m with a wire tossed up in a tree. One year when I lived in Georgia I actually made a 432 SSB contact from the Atlanta area over into Alabama during Field Day. Amazing!

Among other radios I have (I have a bunch of them), I have a Flex 5000A and you can zoom in on active areas of the band pretty easily and can tell when a band is dead or not most of the time - love those SDR's! Obviously the solar cycle is low right now, but lets hope those sunspots fire up again soon!!

Gridsquare: EL89xb

Happy New Year!!!
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Postby WA4SWJ » Wed Dec 30, 2009 6:12 am

Jon,

BTW - sorry to get off topic. Ham radio is fun though.
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Postby EWBrown » Wed Dec 30, 2009 9:49 am

As an alternative to the can cap, the various cap boards are excellent, these mount under the OPT, and use the same mounting screw hole pattern. I've seen these from Triode (SDS Cap Boards) and Cyrus Chong (cchong on e-bay), and others. These boards generally also contain the bias rectifier, caps and support components..

/ed B
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