The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all products

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    Post by Tuninfork on Wed Jan 16, 2019 11:42 am


    I have not posted to the group, but saw some of the new topics and thought I would share some of my experiences with members. I have built, rebuilt or restored hundreds of Dynaco products. I built kits in the 60's and 70's and built many SCA 35's for friends and customers, built so many of them I could do it in a week-end. I have also rebuilt over 100 ST120's(although I knob this a tube forum, but they built some really decent SS equipment also. My suggestions cover all Dynaco equipment, but will limit my discussion to tube equipment.

    We have all see the oxidation and "rusting" of the chassis of Dyna stuff. There are of course people who sell new production, it yours is really bad, but I use a great product to clean and polish decent ones. It is called Micro-Diamon Pro-Polish. It is sold in many car stores and is deigned to remove rust, corrosion, and grime in general, it is safe and effective on most metals and paints, and plastics. I use it extensively on cars and mags. Comes in a 16 oz bottle and is a green liquid. (make sure you shake it well and frequently as you use it. While it of obvious that it is great on chassis, it also works very well on Dyna faceplates to remove small scratches, fading and fog. Use it on a soft rag and GENTLY, as to much enthusiasm will remove lettering. Buff the finished product with a soft terry or microfiber cloth. Great stuff!

    On the subject of restoration there are some hard and fast rules about what to replace first:

    1. ALL electrolytic caps should be replaced before you start anything else.  Original caps(especially the cans) have a rated life of 15 years max. Most today are 40-50 years old. If the piece has been used frequently they still may be working, but I have NEVER had a Dynaco tube amp that didn't have leakage(electrical)  in the can caps and meet specs. I use the German made cans but have used a lot of the new CE ones, with only one failure. That one was right out of package, so I run ALL can caps on my restorer/tester before I put them in the circuit. The German ones are about twice the cost of CE's but you get what you pay for.

    2. I also replace all electrolytic caps and any selenium rectifiers on board.

    3. It is always best to use a metered variac(both current and voltage when powering up anything! If yours doesn't have metering there are some great digital meters that are self powered.(so they don't indicate much until about 50 volts AC) They are well less than $20 and you can build them into a plastic box with AC cord and outlet. With his system you will greatly diminish the likely hood of smoke, fire, exploding stuff and damage to the amps.

    4. Tubes should all be tested and any off value resistors replaced.

    5. I always use grounded line cords, but that is personal choice.

    6. I add a HV fuse in the line to the CT of the output transformer. 1 amp FB works well for most and it will protect valuable transformers should a tube fail.

    Just an aside, I resolder ALL connections and sockets, but sometime you run into some strange things. I recently had a buddy who wanted to rebuild an ST 70. He wanted me to check his work and fire it up for the first time. He did a superb job and replaced ALL the resistors on the board with metal film 5% and replaced all the coupling caps with new dipped caps. After checking all the wiring I brought it up on the variac and set the bias on his new Mullard outputs. Everything was great except the right channel produce no sound. I used a signal tracer to determine that the signal was stopping after the grid of the 6GH8(he used adapters for the original preamp/phase inverter tube- 7199) A quick voltage check showed the voltage to be way off from the working channel. I swapped tubes, then adapter and the problem remained. Flipping the chassis revealed that the right tube filament was not on.(DAH) Started tracing the 6.3 volts and found it right up to the connections from the EL34 to the circuit board. From ground I had 3.6 volts on one leg and 0 on the other. I checked all the wiring and resoldered all the connections, still nothing. Dynaco used small rivets on many connections the accepted hard wiring and once I added a little solder to the connection trace and rivet point it worked perfectly and sounded quite good. I had seen this in some SS Dyna equipment, where connections had been redone several time, but it is the first time I have see it in tube stuff. Sometimes  observatio can save you a bunch of time!!!

    I hope these thoughts are helpful and  will respond to any other or questions.

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    Post by Dave_in_Va on Wed Jan 16, 2019 12:14 pm

    Thanks for posting!

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    Post by WLT on Wed Jan 16, 2019 6:00 pm

    I agree with all of it but have not used the fused CT idea. I have had three failures of the CE caps and none with the German F&T.

    Interesting problem and solution. I just had a very noisy Dyna SCA 35. It was a factory wired version and was fixed by re soldering all connections on the right audio board. It may have been bad from day one but got worse with age. Weird.
    Peter W.
    Peter W.

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    Post by Peter W. on Thu Jan 17, 2019 10:50 am

    WLT wrote:

    Interesting problem and solution. I just had a very noisy Dyna SCA 35. It was a factory wired version and was fixed by re soldering all connections on the right audio board. It may have been bad from day one but got worse with age. Weird.

    Not so much. Until late in the Jefferson Street phase, most Dynaco "Factory Assembled" units were made by Drexel students on piecework. And there were any number of stylistic variations. Further to this, each part-and-piece but for the transformers and tubes came from the lowest-bid supplier with emphasis on locals. Items were boxed for shipment at sheltered workshops, you get the idea. Other peculiarities:
    a) Solid wire was used almost exclusively for assembly. The cheap wire-cutters of the day (  ) would almost invariably nick the copper with each use, leaving a weak point right at the soldering location.
    b) Good solder, back in the day as now, was not cheap. A Drexel student could like-as-not cheap out on the solder and go with 60:40 or even 50:50, rarely. Almost inevitably leading to dry and cold solder joints.
    c) And, even then, time was money - so technique applied only as it expedited results. And the results need only last about a year from first-use.

    Things we forget:  Back in the day, pretty much right into the late 1970s, some areas until the 1980s. Air was DIRTY. acidic, and people smoked. A lot! So corrosion was a common problem at every level. A chassis did not rust spontaneously. Audio systems were commodities very nearly as common as televisions, more common in some demographics. They were treated as such, without much care or reverence. Tubes were much more rugged then as well, to the point that they would survive all sorts of horrors - poor bias, dirty conditions, short-cycling and much worse. Not a cult. No altars. Just another appliance.

    On electrolytic caps - their survivability is based on a combination of quality, storage and operating conditions and time between uses. A Dynaco OEM ST70 tube amplifier built in 1963, used in reasonable ambient conditions on a regular and consistent basis until today has every chance of retaining its original electrolytic caps in fully operational condition. I have such a unit, and it is by no means the only one I am familiar with. That same unit put away in 1980, and fired up today has about a 30% chance, at best.  If stored in poor conditions, a near-0% chance. However the OEM caps may be reformed, or not.

    The life of a modern well-made electrolytic cap properly matched to operating conditions, from about the year 2000 is indefinite, NOS or in-use. Film caps, polyester caps and mica-dipped caps, the same. My feelings on boutique caps, PIO caps and similar choices are well known. Vishay-Sprague orange-drops are about as "boutique" as I can support rationally.

    In any case, I suspect that most of us here are far more 'in touch' with our systems than our parents or grandparents, those few of us that actually go to the trouble, that is.

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    Post by Tuninfork on Thu Jan 17, 2019 11:22 am


    Thanks for your insight. I didn't know about the college students building "factory kits". I have had factory units that were rather poorly built and some kits that look like a Nasa engineer built them. I have had units brought to me that were kits, that never functioned from the start(my guess), usually the cause is one or 2 wires in the wrong place. I also have had units brought to me that actually came close to Dyna Specs after 50 plus years.  Can caps have a finite life, as you say, units that used frequently will still reform and function, however you would be surprised how drifted and leaky some of them are. Dyna was pretty amazing in the designs that gave really good performance at a very reasonable price point. I still marvel at some of the stuff that is still functioning after 50-60 years. Of course that was the way things ere built back then.
    I didn't spend much time on coupling caps, but the technology there has greatly improved. I recently had an amp that after I rebuilt the power supply and bias supply, I went to the coupling caps. My normal procedure is to measure each one for value before the amp is fired up, so if they are really bad I am not putting a bunch of DC on the grids of the next stage. Once the amp is operating I measure the DC on each side of the cap, so one side is high voltage DC and the other side should be almost none(obvious you have to be aware that the bias system on some amp is feeding -DC to the grids as bias) This particular amp had fairly close values 0.50 and 0.43, but when measuring DC it was in the area of 10 volts and 5 volts, which is way too much for a cathode bias system. I never leave a paper and wax cap in place, but some of the sprague (black beauties) and "bumble bee"  are still pretty good, but check their value.

    Thanks again

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    Post by Wharfcreek on Fri Feb 08, 2019 1:24 pm

    I figured I'd toss my $0.02 in since I haven't posted here in a while.   I've spent a lot of time with my old Sprague Cap tester, my 'newer' digital tester, and now some time with an ESR tester.   What I've found is that 'most' old electrolytic caps do leak.   Some more than others, but some quite badly.  What's disappointing is that some 'new' electrolytic caps also show signs of similar leakage.   Going back to the comment about old can caps; I'd agree that properly used, stored, formed, and otherwise maintained units are perhaps as usable as anything 'new' is today.....but I don't know that I'd add 'reliable' to that list.   That said.....'track record' may be worth considering such that if it's still working properly after 50 some years.....maybe it's worth keeping?   I've had 'new' caps fail, and that ranges from the multi type can units as well as the axial/radial types. While the 'new' technology has definitely allowed for the manufacture of these types of caps in smaller and smaller enclosures..... I can't help but wonder if the continued effort to 'shrink' them is perhaps detrimental in some way?  

    As to 'signal' caps......again, the leakage issue is about the same.   To be honest, I'm almost more concerned about leakage in the signal path than I am in the power supply.   At least in the PS if things are starting to fail you can hear it pretty readily.  But, in the signal path, you can have a good amp going bad over time......and as they say about 'training your ears'..... you simply may not realize that your beloved amplifier is actually 'hurting'!!  

    Probably the best approach is the replacement of everything; caps, resistors, tubes.....maybe even the iron!   But, since we all know that's not going to happen, let's be realistic.  Yes, replacing all the power supply caps is 'good form'!!  Likewise, replacing the signal caps is also good idea.  And, since all those old resistors tend to drift over time.....they can go too.  But, in all likelihood, the purchase of a new-to-you amp is for the purpose of being listened too.....not to make a 2, 4, or 6 month (or longer) project out of.    So, How about some basics here:  1) What's it's history?  Has it been in use lately, or on a shelf for 20 years?    If it was in use last week, it's probably a pretty safe bet that you can use it now.  What 'tools' have you got?  If limited to a soldering iron and a DMM, then you kind of have one hand tied behind your back.   If you plan to do restoration / repair type work, then you might want to add a variac, a signal generator, a capacitor testor, or maybe even a scope to your 'inventory' of test equipment.  And.....DUMMY LOADS!!   If you're going to work on tube amps, these are about as essential as any other piece of equipement.

    Anyway..... my process for an old amp is really pretty simple:  Put all the tubes in it, install dummy loads, and use a variac.  I run it up to about 40 VAC for a few hours and check the temps on the filter caps.   If nothing is cooking, I run it up another 10 V every couple of hours until I get to about 80 or 90 VAC.   At that point, again having no significant heat within the filter caps, I move from dummy loads to 'test' speakers.   If all is quiet, I put a test signal to it.   If it's humming badly, the PS gets evaluated and repaired as necessary.  From this point it's a matter of 'fixing what's broke'.  Once that's done, it becomes a matter of whether or not the amp will be a museum item, on someone's 'collection' shelf, a daily-driver, or a 'complete' restoration.   The process does vary somewhat based on 'intended purpose after the fact'.   But.... again, I try to keep it simple...but not 'risk' damage.  In the end, it's keeping this stuff safe, reliable, and working well that makes the hobby fun to me!!
    Peter W.
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    Post by Peter W. on Fri Feb 08, 2019 3:10 pm

    Don't sell yourself short. Your $0.02 is worth at least $0.50 in my book!

    Some points - more stylistic than criticism, and a bit of a mini-rant, again, not as criticism:

    A Variable Autotransformer as a test instrument, without a fine-pitch current meter attached is dangerous, and worse than useless. Unless the intention is to dim lights. Full Stop.

    Devices with solid-state rectifiers will respond to slow uptake in voltage as a means to test (form) electrolytic filter caps. Devices with tube rectifiers will not see B+ until the rectifier passes DC, and that will not be before between 70% and 85% of operating voltage is achieved - not hardly a soft start, or a slow uptake.

    Using cap temperature as a means of testing their condition is OK, even useful. Where I worry is on the "few hours" part. If one-or-more of those caps is very nearly dead-shorted, it may sacrifice the B+ transformer winding before it actually heats to an alarming degree. Hence the need for a proper current meter - whereby such a condition becomes instantly visible.

    And the rant ensues:

    Testing new-to-you equipment is always an adventure. So the goal is to minimize the potential for surprises, and should such surprises occur, mitigate and minimize any damage. When a piece of equipment hits my bench - it is treated as-follows:

    a) Basic cleaning - all controls & switches. Dust removal. Tube removal (testing as well) and tightening of sockets.
    b) Tugging of connections and checking for burns, breaks and visible damage. I use a dental pick for the tugging.
    c) When the above is complete, turn on power *with no tubes* and check for any current draw beyond normal power transformer losses - a few watts, anyway. Give this 10 - 15 minutes.

    Shorted inputs from here.

    d) Re-install (tested) tubes but for the rectifier if so equipped. As above. Current draw should be a reasonable reflection of power-transformer losses and tube filament current. If a solid-state rectifier, note the onset of B+ on the current meter - and where it winds up. This should be predictable and reasonable, and, especially, below nameplate current.
    e) As above, with rectifier. Minimum 30 minutes. With someone THERE.
    f) Look for problems. Smell for odors. Feel for heat. If so-inclined, and if properly skilled, and if the unit is isolated - test voltages at appropriate points.
    g) Now with speakers and a signal.

    This is a 'get-acquainted' process, and in my case, will take place before the first part is replaced. I may not actually complete a) -  g),  as should a bad cap, tube or resistor (or three) be revealed, I may have to abort the process until all such issues are addressed. But, this a) - g) will be completed first-and-last before the item is allowed back into polite society.

    On Dummy Loads (yes, I have several): I am generally more inclined to short inputs and otherwise let the amp rip - if I have any parasitic oscillations, they will reveal themselves as OPT heat in short order  - not so much if the OPTs are loaded. That is Look/Smell/Feel above. I also keep a heat-gun that will show me point-sources, differences between, and similar. The little ST-35 playing behind me as I sit has all four output tubes within 2 F, of each other, as are the driver tubes, as are the OPTs, and the power transformer is about 130F. After 7 straight hours today, so far.

    It is when one is nominally "done" with an amp that I would trot out the dummy loads, load the amp to near-clipping, and observe how things go. Again, look, smell, feel. And observe current draw. It MUST always be at/below nameplate at the specified voltage. Always. Keeping in mind that dummy loads are constant resistance, not variable impedance as are real (conventional) speakers.

    All this is my particular 'style' when approaching these things. Not hardly the only way, certainly not the best way, but just my way. I love doing this 'work' (therapy). I hate doing it twice.

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    Post by sKiZo on Fri Feb 08, 2019 3:29 pm

    One thing I'd add to the list. A fairly common cause of hummmmmm is inadequate ground on the metal case of the multi-cap or ground side of the RCA jacks. Either or both - that was one issue I had with my ST120 on first fire, but an easy enough fix.

    Well, two things. Better make that several?

    I'd also stress following the directions provided with the Latino kits, or the short version ... RTFM! Check off each step as you go along, check them again, and pay particular attention to the "solder later" joints, as "later" expires on first fire ... hopefully without damage.  ;-/

    Oh. And close clip any component leads that could possibly short on the chassis, also not a good thing.

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