diytube.com

I was flipping my ST-35 around, as I was thinking about putting it into a nicer housing that the plain 'ol Hammond aluminum chassis and noticed a "bloated" capacitor - on the verge of exploding. It's C-3, the cathode bypass on V3, one of the 6BQ5's ... the 470 ohm resistor R-17 looks like it's been hot too.

JJ tubes stike again ? Anyone have any thoughts ?
Too much loud Led Zep ?

(yes the polarity is correct) This is a rev B schematic I'm looking at.

... tom

TomMcNally wrote:I was flipping my ST-35 around, as I was thinking about putting it into a nicer housing that the plain 'ol Hammond aluminum chassis and noticed a "bloated" capacitor - on the verge of exploding. It's C-3, the cathode bypass on V3, one of the 6BQ5's ... the 470 ohm resistor R-17 looks like it's been hot too.

JJ tubes stike again ? Anyone have any thoughts ?
Too much loud Led Zep ?

(yes the polarity is correct) This is a rev B schematic I'm looking at.

... tom

Hi Tom,

Is it the stock Xicon part? Probably a Nichicon sub is in order, eg Mouser pn# 647-UVR1V471MPD. I haven't seen a Xicon blow yet, but for a buck more (for all four cathode bypass caps) the Nichicon are probably worth it.

Shannon

Yep - they are Xicons ... do you think I should go up to the next voltage level ? 35 to 50 or 63 or whatever ?

TomMcNally wrote:Yep - they are Xicons ... do you think I should go up to the next voltage level ? 35 to 50 or 63 or whatever ?

Hi Tom,

I think that was just a bad cap, but using some higher voltage ones certainly won't hurt.

Looking at this now, I need to go back and sweep this area in the low frequency region, as these 470uF caps with 400 ohm resistors creates a low cutoff of just under 1Hz. The original was almost 17Hz (1 / 2pi * 100uF * 95 ohms). For tuning purposes, the "bigger cathode caps are better" may not be optimal. So I'd grab some 33uF ones, too, if you want to experiment (ie, 1 / 2pi * 33uF * 400 ohms = 12Hz). As a radio man, you don't happen to have a function generator that you can sweep down to 5Hz or 1Hz, do you? The poles get pushed out due to the negative feedback applied, so this won't hurt your frequency response.

Shannon

I have a big stock of 100's ... maybe I'll try them and see what happens.
I do have an audio oscillator, distortion analyzer and scope at home, so I'll mess with it a little when I get some time.

Thanks !

If the resistor also looks "cooked" then I'd SWAG that the associated EL84 got a little bit "carried away", and is either drawing some serious current, hence the overvoltage, and bloated cap and crispy resistor, or it's got an internal short.

JJs do tend to bias a bit "hotter", and need to be "tuned down" when replacing old style 6BQ5s or Russian 6P14Ps, and Sovtek / EH EL84s (pretty much the same thing as 6P14Ps)

If you're into Zepp and other heavy duty rockin' I'd suggest a set of the mil-spec 6P14P-EVs, they are like "toughened" versions of 6BQ5s, though some with better tuned ears than I have, say that these -EVs can have a rather "dark" sound. I get mine on e-bay straighe from Russia or the Ukraine, cheaper by the batch of 8 or 16. Last time around it was a tad over $3 each, including shipping.

/ed B in NH

Speaking of bloated capacitors .....

My ST-35 blew a fuse today so I brought it home from the office and while probing around in it noticed C-12 is swollen. Mine are CDE 380 LQ's. I haven't checked to see if it has shorted because I don't want to take the amp apart this evening but it's definitely swollen in comparison to the rest of the power supply filter caps. Looks like I'm gonna get some more caps too. My diodes are OK and there are no burned resistors. Looks like the fuse did its job. My cathode caps are Xicons and they're fine. I'm running JJ EL84's in mine.

I've got a picture but don't know how to post it.

Regards,

[/img]

WA4SWJ wrote:...probing around in it noticed C-12 is swollen. Mine are CDE 380 LQ's..... I've got a picture but don't know how to post it.

Hi Ed,

Is this swelling the top vent "push button", and does it remain flexible? If so, I think it is OK - these Cornell Dubilier's are great caps, unlike some Far East no-names. Also, was your fuse a slow blow? Did it fail on power up, during loud music, etc?

Also, C12 is the last 'cap in the chain' - it has a pair of 3.3K resistors in series with it and the main rectifier cap, so it would be current limited at ~60mA until one of the resistors would blow.

Here's Ed's photo: http://www.diytube.com/forumpix/caps.jpg

Shannon

I built my Stereo 35 Rev D back in 2004 (10 years ago). It has been running great with the original tubes! I opened the unit recently to spray some contact cleaner into the volume control, since the sound was getting "scratchy" as I adjusted the volume. I noticed that 3 out of the 4 Cornell Dubilier 120 uF 450 WV capacitors were swollen (similar to Ed's picture in the previous post in 2006). I don't know when the caps starting swelling. I replaced all 4 caps just in case (Digi-Key 338-3306-ND $3.38 each). Still sounds great!

A bloated cap top is quite unnerving. Fortunately the few CDE's that I've cut the plastic off were 100% flat across the can top. Just the plastic disc at top bowed over time due to the heat and the way it wraps around the edge of the can (thus the plastic naturally wants to bow out). It's just a thin plastic disc to isolate the metal chassis with elevated grounds (e.g. the in-series caps in the Ike). CDE should maybe use a thicker piece of plastic, but then again they probably want something flexible during a true can bloating event (to keep the explosion contained).

Shannon