Cooked my Dynaco ST-70?

[lokerola]

Well folks, I bought a really nice Dynaco ST-70 a few months ago from here on BT, and it's been great. Lovely sound, performs flawlessly, etc.

However, I think I cooked it. I usually listen to it for 30 mins to a few hours a day. Earlier this week I had it on and was listening to tunes, but started to feel sort crummy and went to take a nap. Came down with a nasty cold that wiped me out for a few days. I had left the amp on continuously for 3 days.

I went over to the Dynaco and nothing. The preamp, DAC, and fan that blows on the Dynaco power transformer are all on and working, but not the amp. I switched it on and off a few times, just to make sure I wasn't losing my mind - but I seem to have cooked the poor thing.

Is it possible that one of the tubes is fried, preventing it from powering up? If so, I can dig up my spares and put them in.

If not, I guess I need to find a amp tech to help me out.

 

[Oerets]

Didn't smell anything see smoke evidently. Both good signs!

Anything leaking like oil, out the bottom? Another good sign if not.

Does your unit have a fuse?

 

[lokerola]

Didn't smell anything see smoke evidently. Both good signs!

Anything leaking like oil, out the bottom? Another good sign if not.

Does your unit have a fuse?

No smoke or smell. I haven't checked for leakage or a fuse. Going to go check now...

 

[lokerola]

No smoke or smell. I haven't checked for leakage or a fuse. Going to go check now...

That was it. Blown fuse. Thank you @Oerets !!!

3amp slow blow is the replacement, if I'm not mistaken? There's print on the tiny fuse, but I'd need a freaking magnifying glass to read it.

 

[Oerets]

Don't have an ST-70 so of little help.
Glad it was the fuse.

 

[KB007]

The real question isn't what size fuse is it, but more why did the fuse blow in the first place?? Did you have any bad electrical storms while it was on? Any power brown outs and return spikes? Just leaving a tube amp on for days shouldn't pop a slow blow fuse. Is there something else happening in the amp that you can see or measure? You might want to grab your meter and the voltage data for the ST-70 and do a little snooping at the various known voltage points to check everything is in spec. You may have something drifting that needs replacement.
Is the amp completely original or have the been any updates or mods done? If it's completely original, at least replace the selenium rectifier as they are basically a time-bomb waiting to go off. Are all the caps original or replaced?

 

[lokerola]

The real question isn't what size fuse is it, but more why did the fuse blow in the first place?? Did you have any bad electrical storms while it was on? Any power brown outs and return spikes? Just leaving a tube amp on for days shouldn't pop a slow blow fuse. Is there something else happening in the amp that you can see or measure? You might want to grab your meter and the voltage data for the ST-70 and do a little snooping at the various known voltage points to check everything is in spec. You may have something drifting that needs replacement.
Is the amp completely original or have the been any updates or mods done? If it's completely original, at least replace the selenium rectifier as they are basically a time-bomb waiting to go off. Are all the caps original or replaced?

Yes, I thought if that. It's been reworked by some previous members here at AK, and it's in near mint condition. But something was amiss, for sure. We did have some storms roll through, so that may be it. But none of the clocks in the house are reset, so I don't think we lost power.

I admin to being a tube n00b, so I don't know the inner workings of these very well. Here's a clip from the selling post (from @CT Jim , great guy to work with):

"...upgraded with the SDS ST-70 Capboard, selenium rectifier change, new RCA gold-plated inputs, and more"

I've never done anything more than bias the tubes, so I'd have to get a crash course on what else to check.

 

[airtime]

That little fuse saves a lot of heart ache. And I've seen it bypassed on some stereos I've worked on.

Don't worry about leaving the stereo on for a few days. If you remember there are usually threads that pop up from time to time about the virtues of leaving tube gear on 24/7. Me personally, I don't think it's a good idea but some people do. So don't worry too much about it.

 

[primosounds]

Yes, I thought if that. It's been reworked by some previous members here at AK, and it's in near mint condition. But something was amiss, for sure. We did have some storms roll through, so that may be it. But none of the clocks in the house are reset, so I don't think we lost power.

I admin to being a tube n00b, so I don't know the inner workings of these very well. Here's a clip from the selling post (from @CT Jim , great guy to work with):

"...upgraded with the SDS ST-70 Capboard, selenium rectifier change, new RCA gold-plated inputs, and more"

I've never done anything more than bias the tubes, so I'd have to get a crash course on what else to check.

Besides replacing the fuse you should definitely re set the bias on the power tubes. If you have a tube tester it would also be prudent to check your tubes.

 

[lokerola]

Besides replacing the fuse you should definitely re set the bias on the power tubes. If you have a tube tester it would also be prudent to check your tubes.

Good idea, will do. Thanks.

 

[Oerets]

If the fuse is in a place where heat could have effect then this may be the cause. After time may be the cause of failure. Heat will cause more current to flow.

I would run it for a few hours and see. If it blows again then a trip to service land may be needed.

 

[lokerola]

If the fuse is in a place where heat could have effect. This after time may be the cause of failure. Heat will cause more current to flow.

I would run it for a few hours and see. If it blows again then a trip to service land may be needed.

I did have a small fan blowing on the back of it, but the fan may have been too small. When I replace the fuse I'm going to run it outside the cabinet, with a larger fan blowing on it and see what happens. After checking bias on the tubes.

 

[Oerets]

I would leave it alone right now. Proper size fuse installed and run attended for a few hours checking bias periodically.

If it blows service. The old fuse may have just gone bad. I've seen it happen on one with some age and multiple power on cycles.

 

[Lavane]

I would leave it alone right now. Proper size fuse installed and run attended for a few hours checking bias periodically.

If it blows service. The old fuse may have just gone bad. I've seen it happen on one with some age and multiple power on cycles.

On off cycling could be it. There is a sudden in rush at startup. It could wear on the fuse over time. I've had fuses pop and not found any equipment damage before.

 

[peterh]

Fuses will, just like the filament on a lamp, have a finite although not very exact lifetime. Each time
amp is turned on a short surge will heat the fuse, some time it will be enough to blow it.

Just replace the fise with a correct one and continue listening!

 

[triode17]

Recently experienced a power outage here where the power rapidly cycled off and on a few times before it went out completely, which is one reason I use this: http://www.trci.net/products/shock-shield/power-strip It will likely open before any damage to the system happens.

Lokorola - This is good advice. i do the same thing. You may have had a momentary power loss. With the tube rectifier in the ST70, depending on when the power came back on, if the main power supply caps (the big can) discharges enough, and then the power comes back on, while the rect. is still hot, it will charge the caps. back up instantly, which can cause the rectifier tube to short out. This in turn blows the fuse (hopefully). Replace the fuse with the proper 3A Slo-Blo and try it again. If it blows twice, something is wrong. Don't attempt it again until its looked at for damage. Install a PORTABLE GFCI , now.

 

[KB007]

OK, if you're a total tube noob and you don't know a heater supply from a getter, then run with a new fuse for a while and if it does blow again, get it to someone who knows about tubes (and how to not get killed)...

I always liked my ST-70, but eventually sold it in the pursuit of "more".

 

[s-petersen]

when you replace the fuse, pull the rectifier for startup, and see if the fuse holds for a minute, then power down for a couple of minutes before starting it with the rectifier, to make sure the transformer is ok

 

[Bob]

probably a brown-out if your clocks didn't reset, sometimes the spike or brown-out occurs within
the margins of an electronic outage. My UPS rings with a long outage (say 40ms) but not with a
flicker (which should be much shorter).

I've had several ST70s and left them on for days. you're looking for something that happened.

 

[lokerola]

Thanks everyone. I don't think this thing has the rectifier tube any more. I believe it was bypassed with something solid state/diode/etc. I'll definitely look into the GFCI - makes sense.