blown choke. what else went bad?

Hi all,
My ST70 popped and started to smoke. the right front power tube started to glow and arc.

I pulled it apart and found the choke had melted.

Its due for a retube and I'm sure the choke needs to be replaced. should I be concerned with anything else being damaged?

all capacitors were replaced a year ago. none had any visual signs of damage.
thanks

First off, check your fuse and make sure no one installed a 10 amp fuse in place of the 3 amp slow blow fuse. Second make sure your quad capacitor is good. And third, if you have a AC ammeter, with the bias set at 1.56 volts, you should measure about 1.6-1.7 amps A.C., if it pulls more than that, then there is another problem with your amp.

Sal

I've seen working chokes that "leaked" bees wax or whatever brown sticky ooze you are seeing. Are you certain that the choke has failed? As a test you could go "Mark II style" and replace the choke with a 50 Ohm, 10W resistor, retube and see what happens. The choke is not expensive--it's just usually not as available as a 50Ohm power resistor. The "pop" that you mentioned sure sounds like a cap letting go. Did you happen to feel the temperature of your quad cap after you event? Your local guitar amp repair place should be able to sell you a 20-30uF, 450-500V cap to bypass a bad segment in your quad cap to confirn the problem. Do you have a multimeter with a capacitance function? That makes checkingthe wuad cap segments easy.

it was most definitely the choke. it was raging hot. 10 min later it was still popping and sizzling. It smelled terrible. I dont think it has any wax left inside it. its all over the inside of the amp. I'm running a cap board and not the quad cap. visually they all look fine. I'm not sure if my multimeter measures capacitance.

I ordered another choke from triode and an upgrade power transformer. the old one was constantly running very hot.
I've also got a new set of tubes coming. but ill fire it up with the old ones from the left channel in it just in case.

The only reason why I mentioned to check the fuse is to protect the transformers in case of a catastrophic failure of a tube. If someone replaced the 3 amp fuse with a larger one, the fuse might blow if a tube shorts.

Sal

fuse is the appropriate 3 amp. I was surprised it was still intact. but my sunn 200s (mk3) has blown tubes without popping the fuse. however with a lot less pomp and circumstance. (smoke and fireworks)

Take an ohms meter and check your caps. Each section of your quad cap and both bias caps. I think the choke heated up because one of the caps shorted and cause to much current to flow threw it. My can cap shorted and showed no outward signs of being bad but when tested with an ohms meter it showed a direct short zero ohms.

Tested every cap and couldn't find a short. would a runaway tube cause the choke to burn up?

How did you check the quad cap? If you used a meter, without the B+ on the cap, you might not see any leakage.

When it comes to debugging Dynaco equipment that blows fuses... This was the best $40 I spent. It has a 200uF capacitance range and is available at Fry's or Harbor Freight. Some segments of my quad caps pass the resistance checks but fail for capacitance. For a few dollars more they may have one with inductance ranges too.

Bob speaks the truth... one of my Mark III amps had a quad cap that looked okay at low voltage but made sounds like a coffee percolator for few seconds before the fuse blew!

Bob that cap meter on ebay looks good but it seems to have the same range as my Harbor Freight DVM--200uF max. As you said--that's great for tube amp hobbyists. As of this morning I have a near new solid state integrated amp (from the media system) that refuses to come out of standby . I sure could use a cap meter with a (much) higher range today! A schematic would be nice too Debugging an old Dynaco amp is fun and almost certain to please, especially with help from TDTAF. Debugging a "modern" integrated amp that won't turn on is another matter altogether. This must be how the old school mechanics with their timing lights and feeler gauges feel when they open the hood of a Prius.
...j

Where DIY types get schematics for new(er) equipment? For that matter where do pro techs get this detail? Anyone know of a source for the hobbyist?

Thanks for the advice on the capacitance meter. I went ahead and ordered one. my ST70 is using a cap board and not a quad cap. I got my replacement choke and power tranny yesterday. so hopefully today Ill have it back together.

To J Beede. As an old school mechanic I would have to say that worse then a prius (even though I strongly dislike those cars) are the new ford diesel trucks. good luck getting a wrench on anything. again thanks for the reply's and advice.

SUNNFRK wrote:Thanks for the advice on the capacitance meter. I went ahead and ordered one. my ST70 is using a cap board and not a quad cap. I got my replacement choke and power tranny yesterday. so hopefully today Ill have it back together.

In that case the cap meter will add even more value as you should be able to sort through the caps on the board and replace just the bad ones--if there are any. I am no fan of old style quad caps and have had great results using a simple 25-50-25-25uF board using series sets of 450WVDC radial caps. The 4" x 2.5" boards fit nicely inside the Mark III chassis.

Well I got everything back together. It seems to be working fantastic. the new power Xformer doesn't run anywhere near as hot as the last one. my old chassis and output trannys where rusty and pitted so I repainted everything black to match everything else. it now looks as good as it sounds.

I guess it had to be either the tube or the choke.