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Blowing rectifiers

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Posted on November 3, 2014 at 17:02:36
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 My bud has an ST-70 with the Triode board mod, as well as a SDS Labs power supply board. He had a lightning strike that took out the power transformer, amongst a lot of other gear. We replaced the power transformer with the upgraded power transformer from Triode Electronics. I reset the bias, and all the tubes check out for plate voltage and cathode current. We are running 6P3S-Es at 21 watts dissipation. The power tubes are drawing about 45ma apiece, and with the driver tubes, I must be fairly close to the 250ma max constant duty cycle of the 5AR4.

Ever since we went to the new power transformer we have gone through 2 new 5AR4s one Sovtek and one of those re-issue Genelex 5ar4s, in just about a month and a half. The Sovtek arced over and failed, while the 3 amp fuse blew with the Genelex. I've since tested the Genelex, and it tests very weak, almost nothing, on my heathkit TT-1. I put a known nearly NOS Mullard on the tester to check the tester, and it tested very strong. I've tested all the other tubes for emission and shorts and they test good. I'm pretty sure that these newer production 5AR4 tubes aren't up to the hard use the NOS Mullards are capable of.

Is it possible that the increased power available from the upgraded power transformer combined with the SDS Labs first stage cap value (40uf), is putting too much strain on the rectifier, and blowing them? Would a 5U4GB be a beefier alternative? Any suggestions of what to check that may be causing fuses to blow and rectifiers to fail? I'm better at building stuff, than trouble shooting them when something goes wrong. 8-( Any suggestions would be greatly appreciated.

twystd

 

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RE: Blowing Rectifiers, posted on November 3, 2014 at 18:00:06
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 First thing I would do is check all voltages, including the rectifier filament.


Even though the max capacitor without choke or resistor is 60uf for a 5AR4, 40uf is as hot as I would ever go. Provided the cap is really 40uf, and not way over value like some electrolytics can be. I've seen ones mark 100uf that measured 125. I'm not really knowledgeable about the SDS mods, but I have done the Triode board (though not the transformer). Something is obviously different than before, and it could easily be the voltages being higher (or lower, causing more current pull) but I would start by measuring everything. Stock ST-70 is 30uf on that first cap, IIRC...

Say, one other thing... You didn't accidentally swap the bias tap with the one that goes to the other side of the 5AR4 rectifier did you? They do look similar and all. That would create a truly wacky situation capable of toasting rectifiers I would imagine, a long with a lot more bias voltage probably.

At any rate, it never hurts to go back and double-check your work. Sometimes silly mistakes take fresh eyes to spot. Check the voltages while you're at it.

Good luck


-SF


 

RE: Blowing rectifiers, posted on November 3, 2014 at 18:02:12
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Install the series diode tweak! Then, put a new Sovtek 5AR4 in the socket. The Russian rectifier can deliver the rated 250 mA., but it can get into trouble towards the top of the documented 5AR4 voltage range. The SS diodes in series with the plates provides some extra PIV headroom, which is "exactly what the doctor ordered". :>D UF4007s cost about 30 cents each.

Installing a CL-130 inrush current limiting thermistor between the rectifier and the 1st filter capacitance will give you an extra margin of safety. Even the documented 60 μF. limit in the 1st position should not be problematic, with surge limiting in place.

The OEM C-354 filter choke is a POS. "Shoehorn" a Hammond 159T in.


Eli D.

 

RE: Blowing rectifiers, posted on November 3, 2014 at 18:02:49
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 The 5R4WGB is the high power rectifier that was used in most military grade equipment. I don't know if the 5V filament winding of your transformer is beffy enough to drive it. 40uf of capacitance to me seems alot. Aluminum electrlytics are not the preferred capacitance. Two fast 8 uf paper in oil or met polypropylene one on each side of the 10 Henry choke (M derived Pi filter) or a smoothing and a swinging choke is the preferred. I think you have a bad 40 uf E capacitor, especially if it's an array of them. This is a popular kit so maybe you can direct this problem to the people you obtained the parts from.M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Blowing rectifiers, posted on November 3, 2014 at 18:05:57
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The forward drop of a 5R4 is way too big. At 250 mA. it's a whopping 67 V.

Both the 5AR4 and the 5R4 draw 2 A. of filament current.


Eli D.

 

RE: Blowing rectifiers, posted on November 3, 2014 at 18:33:31
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 Actually, my book says 1.9A for the 5AR4 and 2A for the 5R4 but that's a lot of forward drop. These are very good fast caps much better than Aluminum Electrolytics.
http://www.mouser.com/ProductDetail/Panasonic/EZP-E50206LTA/?qs=sGAEpiMZZMv1cc3ydrPrF7yRxWMKYHonLiaHGpKe970%3d
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Blowing Rectifiers, posted on November 3, 2014 at 18:41:30
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 Oh one other thing - Electrolytic caps CAN be lightning damaged! It can perforate the dielectric and make it so that the cap leaks a lot of current when it gets warmed up. I used to work as a TV engineer, and I've seen lightning chew up caps before. I remember one device that had fried diodes after a lightning strike, we replaced the diodes twice and they kept blowing up. It turned out to be the entire bank of electrolytics was shot. If this ST-70 took a big enough wallop to knock out the power transformer, you can just about bet the caps are toast. I dunno why I didn't think about that before.

I'd replace them anyway to be on the safe side, and I bet your problem goes away.

 

RE: Blowing Rectifiers, posted on November 3, 2014 at 19:16:57
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 Yeah, great suggestion, now that I think of it, those caps are about 10 years old, time to replace them anyway.

twystd

 

RE: Blowing rectifiers, posted on November 3, 2014 at 19:29:56
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 Thanks Eli, I'll take your suggestions on both the diodes and the thermistor. I have a bunch of the UF4007s in stock, but not familiar with that particular tweak. Are you talking about installing them on the plates of the rectifier? I would assume that if that is it, I would orient them with the cathodes on the transformer side, correct?

BTW, IIRC you like the Panasonic electrolytics? If so, could you tell me which series you prefer? I don't know much about electrolytics anymore. Since I DIY, I always make space to accommodate ridiculously large Russian caps over electrolytics. However this is a different situation with the size limits of the Dynaco chassis.

twystd

 

RE: Blowing rectifiers, posted on November 3, 2014 at 19:35:21
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 Thanks for the suggestion, but space limitations of the ST-70 chassis, make it hard to use anything other than electrolytics. I'm not a big fan of electrolytics either, I only use them if I positively have to.

twystd

 

RE: Blowing rectifiers, posted on November 3, 2014 at 20:19:09
Tre'
Industry Professional

Posts: 17293
Location: So. Cal.
Joined: February 9, 2002
 It could be that the new transformer has less DCR in the secondary than the stock PT.

The data sheet shows a minimum of 75-200 ohms per leg "limiting resistor" (depending on the AC applied) for a 60uf input cap.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Blowing rectifiers, posted on November 3, 2014 at 20:23:45
Eli Duttman
Audiophile

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



I've uploaded a graphic describing the series diode tweak. That neat drawing was made by somebody else. I produce "hen scratches". ;>) While the drawing shows 1N4007s, definitely use UF4007s. Even with the vacuum rectifier blocking SS diode switching noise, less noise to begin with has to be better.

There's BAD news on the Panasonic cap. front. :>(( For whatever reason, Matsushita Electric is cutting way back on the parts we like to use. Contact Jim McShane next week to get more "skinny". Hit Mouser's web site and look at 105o C. 'lytics by Nichicon. Those parts may be the best we can source, given the Panasonic situation.


Eli D.

 

RE: Blowing rectifiers, posted on November 3, 2014 at 20:24:51
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 If he had a lightning strike that took out the power trafo,you no doubt did damage to the filter caps on the SDS board.Take it out of there and put in a 35uf poly off the GZ34...If you don't want to do that,change those caps on the SDS board because I am sure they are not functioning correctly after the power surge or,there is an issue with the choke.After you do that,put the diodes and in=rush limiters in like Eli suggested.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Blowing rectifiers, posted on November 3, 2014 at 20:51:22
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 Hey Mike,

I've heard you mention these 35uf polys that you're fond of putting on the 5AR4 of our favorite S-5000. I meant to ask you more about what brand/flavor you use. Can you post more info or a pic?

Cheers,

-SF

 

RE: Blowing rectifiers, posted on November 3, 2014 at 20:56:22
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 That drawing reminds me of the great Forrest Mims! Kudos!

-SF

 

Thanks Eli! (nt), posted on November 4, 2014 at 00:10:46
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 .

 

Right here you go., posted on November 4, 2014 at 11:29:03
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 These are pulls but they are brand new pulls out of new equipment but you can save a little money this way..
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Right here you go., posted on November 5, 2014 at 07:24:10
Thom
Audiophile

Posts: 232
Joined: March 6, 2000
 Mike,

These are about twice the price of the 30uF/600V version as priced both new on Mouser. Do you find there is a sonic difference or just go for the add'l 5uF when buying pulls?

Thx,
Thom

 

RE: Right here you go., posted on November 16, 2014 at 21:59:56
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 It depends where they are used..Off the rectifier tubes I would say they are better.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

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