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    newbie with some questions about ST70

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    colin86325

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    Join date : 2014-04-01

    newbie with some questions about ST70

    Post by colin86325 on Tue Apr 01, 2014 7:55 am

    Hi guys, I've been lurking for a while as I did not have an amp, but I finally bought one from Ebay and decided to join the forum as it has been very helpful.

    So, I bought a Dynakit ST70 and have a couple questions.  I hope they aren't too obvious, as i am just a beginner and will probably ask  some dumb questions.  Smile

    Here's the front of the amp. I have a couple questions here:

    • In the far left corner there is a toggle switch.  What function does this perform?  I couldn't find any similar pictures.  Is this for triode/pentode mode?
    • It looks like I have 2 quad caps.  Is this common? The tube between them seems to be like a relay.




    Here's the label on one of the cans.  it's made by Sprague.


    Here's a peek underneath.  Looks like two chokes. Is this unusual?  

    Here's a couple closer details.  I am wondering whether the dreaded selenium rectifier has been replaced.



    Here is the power transformer.  I know that the original PT could get rather hot.  Is this transformer up to the task of driving KT88, or should it be upgraded?


    Lastly, the OPT.


    Thanks for any tips you guys can give me.  I'm really excited to start using this amp!

    peterh

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    Re: newbie with some questions about ST70

    Post by peterh on Tue Apr 01, 2014 8:49 am

    Well, this in not a dynakit-70, it's a modified dynakit-70.

    Without decoding what's done it's difficult to answer your questions. Was there no schematics or other documentation whith the amp ??

    The simplest way is to rebuild it according to a dynaco st-70 description ( can be picked up here ) ,
    even simpler is to have someone do it :-)

    MontanaWay

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    Re: newbie with some questions about ST70

    Post by MontanaWay on Tue Apr 01, 2014 10:36 am

    well congrats of joining the abyss of the tube world!!  Laughing ....like some famous lyrics go....you can check out any time you like...but you can never leave!

    Anyway, if this was mine, I would probably strip it right down to the bare chassis. Clean and polish the chassis and proceed to re-assemble it. As peterh has said, getting hold of wiring diagrams is easy, so that should not be a problem for you.
    I would probably also look at getting a new driver board, VTA or Poseidon. The existing driver board looks like a later edition though, but still, I would go with one of the new designs.
    That transformer looks to me like the original unit, it has been found that it is kind of border line, but should work OK.
    Pretty much do a full frame up restoration.
    Unless you can figure out the mods that have been done to it, bringing it back to original, with the above mentioned updates, should then give you a great sounding amp.
    Of course we are assuming that all tubes and multi caps are good. Speaking of tubes, that small tube in the middle I am assuming is the rectifier tube, can you read any model number off it?

    colin86325

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    Re: newbie with some questions about ST70

    Post by colin86325 on Tue Apr 01, 2014 11:00 am

    Thanks guys, i'd like to get my hands dirty with this and learn a bit in the process.

    I will try to contact the seller, but the amp did not come with any schematics. I've been searching, but Google hasn't returned any amps that look similar to mine. I guess that somebody did a dual power supply modification.
    I don't know what that toggle switch does on the front, and whether it's related to the power supply.

    Montana--I took a look at the smaller tube in the center and it is a AMPERITE 6N030T MINI DELAY Relay. I guess that this is to prevent cathode stripping? I don't think this amp has a tube rectifier.

    MontanaWay

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    Re: newbie with some questions about ST70

    Post by MontanaWay on Tue Apr 01, 2014 11:04 am

    colin86325 wrote:Thanks guys, i'd like to get my hands dirty with this and learn a bit in the process.

    I will try to contact the seller, but the amp did not come with any schematics.  I've been searching, but Google hasn't returned any amps that look similar to mine.  I guess that somebody did a dual power supply modification.
    I don't know what that toggle switch does on the front, and whether it's related to the power supply.

    Montana--I took a look at the smaller tube in the center and it is a AMPERITE 6N030T MINI DELAY Relay.  I guess that this is to prevent cathode stripping?  I don't think this amp has a tube rectifier.

    even more of a good reason to do a chassis up restoration. It will be a fun and very good way to learn lots about tube amps, correct wiring etc., and the satisfaction at the end when you listen to it...priceless!

    I'm pretty sure you'll not find an amp like this one, it has been heavily modified, like you said, mainly the power supply, but it also has two extra pots, I am assuming for bias adjustment. I t would be interesting to trace out the circuitry and see what exactly has been done to it!

    corndog71

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    Re: newbie with some questions about ST70

    Post by corndog71 on Tue Apr 01, 2014 11:54 am

    Wow.  That amp has a lot of mods added.  Without a schematic it could be a challenge troubleshooting it should things go bad.  Does it work?  It's hard to see what that front switch does.  Can you get some better shots of the inside?  Follow the wires from the switch.  Where do they go?

    For a newb I would recommend a complete overhaul and start fresh with a proven circuit such as the one by VTA. (it's excellent)  Definitely toss the old power transformer (or try selling it) and getting the newer one (dynakitparts).   Maybe even replace the power tube sockets as the old ones may not be in great shape and could cause problems.  If the output transformers are still good then keep them and consider sanding off the corrosion and repainting the covers.  The old chassis may be novel to keep but it's already been changed with numerous holes not from the original design.  You may want to consider getting one of the newer stainless steel chassis and save yourself a lot of potentially futile work refinishing the old one.

    Do not run KT88 with the old power transformer!  I'm pretty sure it's just not rated for them.  

    Should be a fun project.  As always be very careful as there will be high voltages present when powered.

    mazeeff

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    Re: newbie with some questions about ST70

    Post by mazeeff on Tue Apr 01, 2014 1:10 pm

    Can you ID the tube that was added between the bias pots? It looks to be wired as part of the rectification circuit. The can cap that was added in place of the original tube rectifier, appears to be a dual cap, with the two caps in parallel. I would simply recommend drawing yourself a new schematic, to make sure that you know what you are dealing with. The added switch on the front, could simply be an effort of the previous owner to move to on/off switch to the front. I also recently purchased a ST-70 from eBay, and went through the same effort. From the pictures, it looks like a nice late model (1977) ST-70.


    Last edited by mazeeff on Tue Apr 01, 2014 1:51 pm; edited 1 time in total

    colin86325

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    Re: newbie with some questions about ST70

    Post by colin86325 on Tue Apr 01, 2014 1:27 pm

    I will try to trace the wires to the switch on the front tonight and will post my findings.  Maybe together we can figure this out!

    I haven't plugged it in yet as I wanted to look it over visually before doing anything.

    mazeff--The small tube you asked about is an AMPERITE 6N030T MINI DELAY Relay.  A relay in a bottle.

    I might try to draw a schematic if I can't get one from the seller, though my attempt will probably look like a newb drew it...because one did!

    mazeeff

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    Re: newbie with some questions about ST70

    Post by mazeeff on Tue Apr 01, 2014 1:33 pm

    colin86325 wrote:I will try to trace the wires to the switch on the front tonight and will post my findings.  Maybe together we can figure this out!

    I haven't plugged it in yet as I wanted to look it over visually before doing anything.

    mazeff--The small tube you asked about is an AMPERITE 6N030T MINI DELAY Relay.  A relay in a bottle.

    I might try to draw a schematic if I can't get one from the seller, though my attempt will probably look like a newb drew it...because one did!

    That is what I thought. It appears that the modifier converted to solid state rectification, and was concerned about start up delay (Cathode Stripping). This would solve it. It adds a 30 second delay. Drawing the schematic is not too bad. Start by downloading a stock ST-70 Schematic, and just mark it up. Your driver board looks stock to me. No need to draw a schematic for it.


    Last edited by mazeeff on Tue Apr 01, 2014 1:47 pm; edited 2 times in total

    MontanaWay

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    Re: newbie with some questions about ST70

    Post by MontanaWay on Tue Apr 01, 2014 1:36 pm

    colin86325 wrote:I will try to trace the wires to the switch on the front tonight and will post my findings.  Maybe together we can figure this out!

    I haven't plugged it in yet as I wanted to look it over visually before doing anything.

    mazeff--The small tube you asked about is an AMPERITE 6N030T MINI DELAY Relay.  A relay in a bottle.

    I might try to draw a schematic if I can't get one from the seller, though my attempt will probably look like a newb drew it...because one did!

    I suggest that you layout your drawing like the amp itself. Draw the sockets, caps, driver board outline etc as they are on the chassis, then start drawing the wires to each point etc., from there, it would be easier to draw an actual circuit diagram.

    GP49

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    Re: newbie with some questions about ST70

    Post by GP49 on Tue Apr 01, 2014 1:59 pm

    Actually that is a pretty nicely done modification, I'd guess from the 1970s.  The solid state rectification MAY have permitted use of 6550 tubes, as the load from the filament of the 5AR4 rectifier would have been gone (I can't tell what the tubes are...some big-envelope 6CA7, the US equivalent of the EL34, looked like that).  Selenium rectifier is replaced with silicon, and B+ smoothing/reservoir capacitance has been added. Possibly there are two branches of the power supply from the rectifiers onward, with each channel having separate choke and capacitors; one can't really tell without actually tracing wires.  A time delay relay, the little tubelike thing between the Sprague capacitors, was added to hold off applying B+ to the tubes until filaments were warmed up.

    I'd still strip it and rebuild.  The tube sockets might not be the marginal originals but even if they have been replaced, they'll be a mess once you unsolder the wires from them.  The original parts on the circuit board have probably drifted off value by now, and you'll want to upgrade the capacitors and increase their value to increase LF bandwidth. So even if you want to maintain the original driver/inverter circuit, a nice, clean new board would be the best place to start.  Of course you can install an upgraded board, too. The only reason I wouldn't would be if you could get the circuit diagram and perhaps the provenance of the unit...who did the modification and who's had it since. Even then, and even with knowledge that the modification was well-engineered and well-built, I'd still be wary of old parts.

    mazeeff

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    Re: newbie with some questions about ST70

    Post by mazeeff on Tue Apr 01, 2014 3:52 pm

    Is it my imagination, or are there any wires hooked up to the bias plugs on the front. The left side bias plug does not appear to have any wires hooked up to it.

    GP49

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    Re: newbie with some questions about ST70

    Post by GP49 on Tue Apr 01, 2014 4:39 pm

    Just a guess: this amp could have a different cathode current measuring setup, involving the mystery toggle switch.

    daveshel

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    Re: newbie with some questions about ST70

    Post by daveshel on Tue Apr 01, 2014 5:47 pm

    I would plug it in and give it a listen before doing anything else. You never know.

    MontanaWay

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    Re: newbie with some questions about ST70

    Post by MontanaWay on Tue Apr 01, 2014 5:50 pm

    daveshel wrote:I would plug it in and give it a listen before doing anything else. You never know.

    I'll keep an eye out for the mushroom cloud!!  affraid 

    daveshel

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    Re: newbie with some questions about ST70

    Post by daveshel on Tue Apr 01, 2014 5:52 pm

    MontanaWay wrote:
    daveshel wrote:I would plug it in and give it a listen before doing anything else. You never know.

    I'll keep an eye out for the mushroom cloud!!  affraid 

    The only thing really at risk is the transformers. If there's no sound, power it down then go to work on it.

    mazeeff

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    Re: newbie with some questions about ST70

    Post by mazeeff on Tue Apr 01, 2014 6:53 pm

    GP49 wrote:Just a guess: this amp could have a different cathode current measuring setup, involving the mystery toggle switch.

    I think that you are correct. I see four (one for each output tube) 15.6 ohm resistors. This would seem to indicate individual bias adjustment. The magic switch is likely the key. The power switch on the back, appears to be intact.

    j beede

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    Re: newbie with some questions about ST70

    Post by j beede on Tue Apr 01, 2014 9:51 pm

    If you have a 100W incandescent lamp handy, put it in series with your amp when you power it up the first time. This will give you a softer turn on and voltage drop for a first run. The bulb should light up brightly and then dim down to ~50% after 3-4 seconds as a crude check.

    mazeeff

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    Re: newbie with some questions about ST70

    Post by mazeeff on Wed Apr 02, 2014 6:28 am

    I found the original ad on eBay. The prior owner seems to have used this for years, and recently powered it up just prior to sale. Based on the info in the ad, i would be inclined to power it up, once the bias control is well understood. This looks like a great amp, that someone built in the late 70's when solid state amps were the norm. The original owner must have wanted that "tube" sound, and knew enough to add some nice mods.

    http://www.ebay.com/itm/Dynaco-Dynakit-Stereo-70-ST-70-Vacuum-Tube-Power-Amplifier-Good-Condition-/231182672594?pt=Vintage_Electronics_R2&hash=item35d38fa2d2&nma=true&si=jywwF0FQvanHcwVmrX2%252Bg8QN1HM%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

    colin86325

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    Re: newbie with some questions about ST70

    Post by colin86325 on Wed Apr 02, 2014 10:37 am

    Wow, you guys are good--you even found the correct Ebay ad!  Smile
    Your hunch about the switch and its effect on bias was correct.  I just had a look at it and found this:

    When the switch is "UP" there is contact with pin 8 on the Right Bias port, and pin 8 of the EL34 on the front left of the amp (on this EL34 pin 8 is bridged to pin 1, which in turn goes to the Left Bias port).

    When the switch is "Down" there is contact with pin 7 on the Right Bias port. There is also contact with pin 7 of the Left Bias port, and with pin 1 of the back left EL34. This pin 1 is bridged to pin 8, which has a wire that I didn't trace.

    Does this make sense? Would I flip the switch DOWN to set bias on the back two EL34s and flip the switch UP to set it for the front pairs?  Recall that I have 4 bias pots, not just the standard 2.
    I will try to get a wiring diagram over the weekend.

    Thanks again for the continuing help.

    tubes4hifi
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    Re: newbie with some questions about ST70

    Post by tubes4hifi on Wed Apr 02, 2014 11:20 am

    seems you've got it right, down for the back tubes, up for the front tubes.
    BTW, unless those tubes are totally worn out, those Sylvania 6CA7s are going to sound great!

    mazeeff

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    Re: newbie with some questions about ST70

    Post by mazeeff on Wed Apr 02, 2014 11:33 am

    colin86325 wrote:

    When the switch is "UP" there is contact with pin 8 on the Right Bias port, and pin 8 of the EL34 on the front left of the amp (on this EL34 pin 8 is bridged to pin 1, which in turn goes to the Left Bias port).


    Are you saying that pin 1&8 of the front left el34 goes to pin 8 of both the left & right bias ports? If so, this explains the need for the switch! Also, can you confirm that each of the EL34's has a 15.6 ohm resistor? When it comes time to bias, you will need to set the bias to 1/2 of the normal 1.56v. .78v for 50ma, or .62v for 40ma. I have a similar setup, but no front/back switch.

    tubes4hifi
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    Re: newbie with some questions about ST70

    Post by tubes4hifi on Wed Apr 02, 2014 3:02 pm

    .78v for 50ma, or .62v for 40ma.
    yes, if you have a 15.6 ohm on each tube, then those are the correct bias voltages!

    colin86325

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    Re: newbie with some questions about ST70

    Post by colin86325 on Wed Apr 02, 2014 6:51 pm

    mazeeff wrote:
    Are you saying that pin 1&8 of the front left el34 goes to pin 8 of both the left & right bias ports? If so, this explains the need for the switch! Also, can you confirm that each of the EL34's has a 15.6 ohm resistor? When it comes time to bias, you will need to set the bias to 1/2 of the normal 1.56v. .78v for 50ma, or .62v for 40ma. I have a similar setup, but no front/back switch.

    On the front left EL34, pins 1&8 go to pin 8 on the left bias port only. (It does not extend to the right side bias port.)

    Each EL34 has actually two 15.6k resistors in series (so 31.2k resistance).  Will this change my ideal bias voltage?

    Thanks for the help! Once I get this business figured out I think I will be ordering the VTA driver board.

    tubes4hifi
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    Re: newbie with some questions about ST70

    Post by tubes4hifi on Wed Apr 02, 2014 6:56 pm

    are they in series, or in parallel?
    So there is a total of 8 of those resistors (2 on each output tube?)

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