My very first tube amps!

Well, I bought my very first tube amp and I'm excited to get them going. I was initially buying one amplifier but after negotiating a price on one, the seller told me he had a second one for sale also. So we made a deal on 2 of them. I know they power up from a video the seller made.

The seller has no information on these amps cause he saved them from going into the trash heap. He works for a junk removal company and he saved a bunch of stuff and these.

I know that these are identical and were made on the 27th week in 1974. I have no idea if these have been updated since 1974? I know several people on other forums have said to change the selenium rectifier before you start messing with these amps. I have no idea if the rectifier has been messed with!?

I will be taking these to my technician soon but thought I would try and get more info about what I actually have. I'm hoping that these won't need a ton of work to get them running before I even think about doing mods to them. I know it has the 7199 tubes that are hard to find and expensive. I think I might upgrade in the boarts to the VTA in the future but these will stay like this for now.

I went ahead and cleaned them up before I drop them off. I believe the can cap is still orginal and will need to be replaced?

If you see anything that you would do please don't hesitate to mention.

Thanks!

I think you made a very good deal.
Apart from se rectifier with associated components and the can cap i don't
think you have to do anything before listening test.
There might be thinks to replace , power tubes is one, but test the amps first.
Whatever you do try to keep them original, thats where they have the highest value, it
might also be the best sounding combination.
Your 7199 might be good, if not you can use 6u8/ecf82 with a simple adapter.

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PeterCapo wrote:You have a couple of original Dynaco Stereo 70s that The Absolute Sound magazine tagged as being among the most significant amplifiers in the history of hi-fi and that still sound good by "today's standards."  There's a lot to know about them and about what to do or not do with them.  If you want to realize their potential, you'll need to sort-out fact from fancy.

I'd suggest taking your time and not being in any rush to hand them off to someone to work on (if you ever do, then I'd suggest having a detailed discussion with your tech so that you understand exactly what he would do to them).

The best first things would be to study two "must read" documents about the Stereo 70.  You don't want to miss them.  The first one has a lot of technical talk, but it also includes sensible philosophy that anyone can understand in approaching this classic: http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

The second document is essential to finding your way around the Stereo 70: https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf

Finally, I wouldn't assume that any of the vaunted aftermarket "mods" represent a definite upgrade or improvement over the original Dynaco circuit design.  The biggest handicap that decades-old amplifiers like yours might have is exactly that they are decades old.  But old age in this kind of product does not imply inferior or obsolete circuit design.  If need be, they can be either partially or extensively restored with fresh parts 1:1 without making substantial changes to the original Dynaco circuit.  Many who have done simple 1:1 parts refreshing while retaining the original Dynaco circuit design have found that they sound great and really don't need a change to a different circuit.

I plan on keeping them orginal as possible for the time being.

I'm taking them to my technician to make sure all the tubes are working and that all the bias are set correctly. If he suggests replacing anything because they are out of spec then I will replace them with equivalent items. I love that they have not been modified.

I believe I got these for a fair price $550 (set). I just want to get them up and running and enjoy them with my Klipsch Forte ii.

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PeterCapo wrote:$550 for both is amazing.

Setting the bias is easy.  Instructions are in the manual, if you might be so inclined.  Setting the bias is not a once-and-done thing.  Variations in AC mains (from the wall socket) changes the bias.  It should be checked at some regular interval at home.  An inexpensive handheld multimeter would do the job for you.

I'd avoid Chinese-made octal base tubes for the simple reason that their pins tend to be splayed, and tend to have lumpy or bulbous solder, all of which deform the socket contacts and can cause significant problems.  

An advisable preventive maintenance item would be to clean and re-tension the octal socket contacts.  If you'd like a procedure that you could then pass on to your tech, or just do yourself, let me know and I'll post it.  There is more than one way to go about it, but one way may be safer than another.

Wise advice.
If i may add one advice : don't remove tubes for other purposes then to replace them
with new. Both sockets and tubes may be damaged each time.
As for bias adjustments, yes they will shift with mains voltage, but that is ok.

PeterCapo wrote:$550 for both is amazing.

Setting the bias is easy.  Instructions are in the manual, if you might be so inclined.  Setting the bias is not a once-and-done thing.  Variations in AC mains (from the wall socket) changes the bias.  It should be checked at some regular interval at home.  An inexpensive handheld multimeter would do the job for you.

I'd avoid Chinese-made octal base tubes for the simple reason that their pins tend to be splayed, and tend to have lumpy or bulbous solder, all of which deform the socket contacts and can cause significant problems sooner or later.  

An advisable preventive maintenance item would be to clean and re-tension the octal socket contacts.  If you'd like a procedure that you could then pass on to your tech, or just do yourself, let me know and I'll post it.  There is more than one way to go about it, but one way may be safer than another.

This is all really good information!

I do have a multimeter that I can use to check the bias as a regular routine maintenance check that I can always add to my goggle calendar

If there is no reason to pull tubes out tgen I will definitely follow that guideline. Thank you

I don't plan on buying any Chinese made tubes thats for sure. The guy I bought the st70's from has a quad set of
EL34 tubes that he would sell to me along with some GZ34.

Any documentation is great in my book!

Don't pay overprice for tubes, but do buy matched tubes from a reputable vendor.
Eurotubes.com is one, the best would be to buy 8 matched ( or 2 matched quads) this will
make sure you have a lot of matched pairs for many years. This since any two of these
will be a perfect match.
And don't try to get NOS, they arn't better you may be fooled. New tubes will
have warranty with them ( from reputable dealers).

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peterh wrote:Don't pay overprice for tubes, but do buy matched tubes from a reputable vendor.
Eurotubes.com is one, the best would be to buy 8 matched ( or 2 matched quads) this will
make sure you have a lot of matched pairs for many years. This since any two of these
will be a perfect match.
And don't try to get NOS, they arn't better you may be fooled. New tubes will
have warranty with them ( from reputable dealers).

I hear you on not paying for overpriced tubes. I'm always on a budget so it has to be a deal for me to pay for stuff that's used. The set I posted he want's $150 for the quad which is a pretty fair price. If I were to buy new it would more than likely be Electro-Harmonix 6CA7 EH Power Tubes.


The 7199's are the ones I'd like to keep orginal as possible.

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Heck of a score! I just got ONE ST70 for a bit more than your two, and NO tubes came with it! Congratulations!

erniegiro wrote:Heck of a score! I just got ONE ST70 for a bit more than your two, and NO tubes came with it! Congratulations!

Thanks man... I'm super stoked and I can't wait to get them up and running. It's probably going to change me from SS amps to Tubes.

I just a phone call from my tech. He said that these amps are in really great shape and that they are pretty much good to go. He did recommend that I should probably get rid of the selenium rectifier and add some silicone diodes. He also mentioned that one of the amps could use some new el34's since 3 of them did test a little weak. Good thing I bought a box of tubes from the guy I bought these amps from. It had 7 EL 34's in it and a matching quad set of xf2 dual halo tubes it. I might just have to install those and have the selenium rectifier changed out. I'm so stoked and I can't wait to get them home soon. I think I might have him install new capacitor cap on them while he's in there working on them.

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PeterCapo wrote:1. Replacing selenium rectifier in the bias supply with a single UF4007 silicon diode = good idea.

2. Adding two silicon diodes to the tube rectifier socket per the "yellow sheet diode mod" has been a go-to technique for some time now.  However, it was recently the subject of some interesting discussion on a different forum that IMO now places in question its value in protecting the rectifier tube.  

3. On the matter of tubes, I'd have to reiterate the importance of using only a burned-in and closely matched quad from a tube specialist like McShane Design or Eurotubes.  I know you're excited, but I would caution against being too eager to use your quad set of xf2 dual halo tubes.  Reason for this is that your quad set of xf2 dual halo tubes is rare and worth $$.  And in spite of what your tech may be telling you, the cleanliness and tension of your octal socket contacts could still be a vulnerability if they have not been properly cleaned and retensioned.  A dirty or loose socket contact can result in irreparable internal damage to whatever EL34 tube you install.

I'll be sure to mention this info to him on installing a single silicone diode UF4007

Thanks for the info on the sockets... He did mention that the sockets were replaced and that they were in great tight condition. This was a relief to hear especially after you mentioning this in an earlier post. I'm in no rush to install them since they are worth so much money. I have been looking at buying a new matched burned in set and keeping the ones I have. Sometimes it bothers me when I see people with so much old collector items and no one uses them. I used to collect old school bmx parts and I would see people show off their collection of stems and I'm like dude 20 people could actually use those to build their bikes. Sometimes it's just the way it is.

I also did get some cool rectifier tubes as well. So this is a good thing!

First of all, nice buy. You got these at a really good price.

Regarding the recommended reading from Curcio, Audio Regenesis, etc, please do that. It will help you understand the circuit and various options.

I agree on replacing the bias selenium rectifier with a diode, preferably a UF4007 or other "fast recovery" type diode. Also replace the bias caps with 100 vdc units.

The ST-70 was designed to run on 115-117 vac, unfortunately, today the AC voltage is usually 121-122 vac which leads us to a problem... I would measure the AC voltage on the output tubes, pins 2 and 7, these really should be 6.3 volts. Unfortunately they will probably be 6.8 vac or so which will shorten tube life and affect the sound. One solution would be to add a CL-80 to the hot side of the power cord. This is an NTC, a Negative Temperature Coefficient device that starts at a high resistance and as it warms up, drops to a low resistance. It serves several purposes: first, it will lower your incoming line AC voltage which in turn lowers the voltage on the tube heaters. It also reduced the AC in-rush current to the tube heaters, which increases the tube heater life by a factor of 4 according to Robert Tomer, a tube expert from the 1950s. It also allows you to use a regular fast blow fuse instead of a slow blo fuse which means if something does happen in your amp, the fuse will blow much more quickly. BTW, if the voltage on the tube pins 2/7 isn't lowered enough, add a 2nd CL-80 on the other side of the AC line.

One recommendation would be to remove the current bias measuring resistors and replace them with a 1 ohm, 1/2 or 1 percent resistor from pins 1/8 to ground. With that you can use the 200 ma scale on your meter to measure the milliamps going thru your tube while reducing the internal impedance of your amp. I believe the amp's internal impedance is covered in the Audio Regenisis article. I would run 40 ma per channel or so if I were running today's 6CA7s / EL34s instead of the 50 ma in the original Dynaco.

I would also recommend putting (at least) a 100 ohm 3-5 watt wire wound or metal film resistor between pin 4 of the output tubes and the green or green/white leads. This will somewhat lower the voltage on the screen grids but more importantly prevent an arcing of the screens. It will also tighten up the sound a bit. I say at least a 100 ohm, some people suggest up to 1200 ohm although I think that is high, I use a 560 ohm personally.

A third recommendation: replace the 6.8k and 22k resistors on the main power supply cap. The originals are carbon comp and they've probably drifted over time due to the constant load on them. I would replace them with Vishay CPF2 or CPF3 (2-3 watt) metal film resistors (was recently working on some other old gear, a 25k carbon comp resistor measured 18k).

The ST-70 is a very nice amp. I run mine with the Triode Electronics EF86 driver board and use an ECC-99 for the cathodyne phase splitter. I have the Triode power transformer and run KT-90s as the output tubes in triode mode. Of course, YMMV and you'll run yours as you wish. I'm sure you'll enjoy it.

Best of luck,

-Ed

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PeterCapo wrote:These are two later production original Stereo 70s and might have 120VAC power transformers instead of the 117VAC PT.

Yes Peter, I see the plastic leads on the transformers indicating a later unit. The output tranny's are probably not A-470s. The unit "might" have a 120 volt power transformer, not the original PA-060. Measuring the AC from pins 2 to 7 will verify if the voltage is too high. I would put a CL-80 in anyway to preserve tube life, a tube with heaters running at 6.15 or 6.2 vac is going to last a lot longer than one running a 6.8 volts. The one watt of output power you might lose due to lower B+ voltage is much more minor in my opinion.

Plus a regular fuse instead of a Slow Blo will blow faster if there's an issue AND the vast reduction in heater shock when you "flip the switch" is another issue.

The only real downside is that if you are measuring voltages, you'll have to wait for a few minutes for the CL-80 to fully reach its lower resistance, but that's not really an issue in tube equipment, you'll have to wait for the tubes to warm up and the bias to stabilize before making any precision voltage measurements.

Just my opinion, for what it's worth (maybe 2 cents...)

-Ed