What can I expect from a Dynaco ST-70?

[Doc B.]

Bear in mind guys, that the legend is that those 500V rated quad cans were really just 450V cans that were custom stamped 500V for Dynaco.

[JC]

Well, I imagine legends get started for a lot of reasons.  In the end, though, I either have to trust AeroVox, or a legend about AeroVox.
Since the cap didn't show any signs of failure after 40+ years, I'm inclined to trust AeroVox.

[Paul Joppa]

While there is not much gear that will measure accurately the capacitance of an electrolytic cap, it's also my understanding that the best measurement is the equivalent series resistance (ESR). This will increase by something like a factor of four over the life of a cap, and is a good measure of remaining lifetime - at least, if you know the ESR of a fresh new one. A decade or two ago there was a DIY circuit to make this measurement, possibly in Wireless World(?) - if I recall correctly there was even a kit, and there may be commercial meters that exist.

Here's an article from the CDE web site, with more than anyone really wants to know about electrolytic caps:

http://www.cde.com/catalogs/AEappGUIDE.pdf

[JC]

Thanks!  I have that filed away for future reference, for sure.

[mediumjim]

The fact still remains that in just about every instance there will be tell tale signs of problems in the Can Cap before it fails.  The primary sign will be audible hum at idle. The second will be white powder residue on or about the bottom of the Can itself. 

I make a living in the vintage guitar world and that involves vintage tube amplifiers that have filter/electrolytic capacitors.  Yes, they do go bad for whatever reason, but I have never in my years seen one that has suddenly failed that didn't hum like a banshee or have visual signs of leakage. 

Look, if changing the Quad Cap Can gives you peace of mind, then by all means go for it and I respect you for doing so.  But at the same time respect the choice of those who chose to use their senses to determine when to change it. 

Jim

[Wanderer]

The fact still remains that in just about every instance there will be tell tale signs of problems in the Can Cap before it fails.  The primary sign will be audible hum at idle. The second will be white powder residue on or about the bottom of the Can itself. 

I am not going out of my way to be unduly contradictory. My experiance does not match the above. I have had old multi-section caps fail as shorts in a number pieces of gear. I recall off the top of my head a Dyna Mk3, ST-70 and a Scott tube integrated (a 222 or a 229, can't recall which). No white powder, no out of ordinary hum, no phone calls at 1 am and not even a tersely worded note, just POP!, SNAP and a blown fuse. One section of the cap now tested as a dead short. 

Perhaps unique to me but that has been my experiance.   

         

[Doc B.]

I had a Heathkit integrated suddenly start hissing and piss its innards all over the chassis after bringing it up slowly on a variac and then having it run for quite a few hours of quiet operation.

[mediumjim]

Maybe I'm the luckiest guy in the world!  Doc B., so how many times have you restored an old tube amp?  I'm going to go out on a limb and guess well into the hundreds over your years and to have only one fart and sputter.  Wanderer, I have no doubt that what your telling me is true.   Maybe I will live and die by the sword that I wield, maybe not. 

I find it ironic that is all but one instance mentioned by both Doc B. and Wanderer, they were on DIY's where the integrity of the original build possibly may be called into question!  If we were talking McIntosh, Marantz, Harmon Kardon or other high-end amps, then maybe an argument could be made.  In the mean time, I'm going to enjoy my humble 1963 Dynaco ST-70 with an original Quad Can. 

Jim

[JC]

And, sometimes they don't.

I wonder how many people here change their light bulbs before they burn out?

[Wanderer]

they were on DIY's where the integrity of the original build possibly may be called into question! 

The Scott was factory wired as I believe the Mk III was also  (so long ago....such fading memory).

My thinking is more along the lines of HI-FI amps running wth voltages nearer the limit in an attempt to get lower distortion. Aren't music amps run cooler and distortion is viewed as a plus? I keep hearing about guitar amps having "nice tone" and "good crunch".      

....oh, and when my light bulbs burn out they don't take $100 of matched output tubes with them. 

[JC]

Guitar amps are generally run at higher Voltages and less accommodating environments.  Many models met or even exceeded maximum recommended Voltages on plates and screens, even at the legendary lower line Voltages back in the day.

If you have had an amp take out a set of output tubes due to a power supply filter failure, that would indicate that the tubes must have sacrificed themselves to protect a 50 cent fuse.  Not to mention several other parts in the power supply.  I would have several questions about such a design.

I have replaced many a power supply filter cap in many vintage guitar amps because they showed visible signs of leaking electrolyte, bulging outer containers, etc.  Many caps have not, and have continued in service; I suspect they may fail someday, just like a brand new part may fail someday. 

In all those amps, though, the only one that burned new-manufacture output tubes was one that was "legendary" for having a narrow range of negative bias adjustment.  Which it had 37 years ago when it left the factory with Mullard EL-34s that evidently tolerated it better, or required a smidgeon less negative bias to set up properly. 

[Wanderer]

I certainly bow before those with greater experiance with musicial instrument amps - not something I have much backgound with and my comments about them were uninformed.   

[Grainger49]

Jim has chosen not to head the warnings that are all through this thread. 

We shouldn't beat this dead horse further.

[mediumjim]

Jim has chosen not to head the warnings that are all through this thread. 

We shouldn't beat this dead horse further.

Grainger:

Not about listening to or taking heed of the said warnings, just that no tangible evidence has been brought to the table to convince me to change what is not broken.  JC is dead on right about Guitar Amps and their operating ranges.  As JC aptly noted, new parts fail, old parts fail.   I find comfort in the fact that in my ST-70, the Quad Cap is 47+ years old and still works as designed.   Trust me, if it starts to hum, or show signs of leakage, it will be replaced, but not until then.

As far as tubes go, I have a NOS quad of Mullards in her that cost $100.00 per tube...Let's take the other road and replace the Quad Can...what's the going price for a good one, $70.00, and Wanderer indicated that a matched quad of new EL34's is around $100.00.   My point is that the changes of a catastrophic Can Failure is rather minimal and even if you change the Can, it still can suddenly fail if you buy-into the rhetoric.  Let's extrapolate the numbers...if the assumption is one in a hundred will fail and have collateral damage....

100x $70.00 (cost of a new can)= $7000.00

Under this assumption, those who replace the can will spend $7000.00 to avoid a potential loss of say $100 to $400.00. 


Look, I understand and get it that the advice to change the Can was made in good faith, but I feel based on false premises. 

Jim

[Doc B.]

I don't really like the look and feel of where this thread has headed, so now that everyone has expressed their opinion I am going to lock it.