The Dynaco Tube Audio Forum

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


    I'm in - new ST-70!

    Share

    johnnybgood

    Posts : 5
    Join date : 2011-04-02

    I'm in - new ST-70!

    Post by johnnybgood on Sat Apr 02, 2011 9:27 pm

    Hello all – just wanted to introduce myself and say that after a few months going back and forth, I finally took the plunge and got one of Bob’s ST-70’s! I’ve only had it a few days now, but I’m already suitably impressed. At present I’m just running WAV and FLAC files from a laptop through a Musical Fidelity V-DAC directly into the amp (stepped attenuator option). I have the Tekton Design Lore speakers on order, but since I got the amp before the speakers, I hooked up to my small 3-way JBL speakers that serve as the fronts to my Home Theater System. The best testament I can give right now is that my wife (who was 90% against the purchase) has been commenting all day on how freakin’ good this new amp makes the music sound! Believe me, she is not into hi-fi, and for her to comment like that blew me away! I don’t have more than 15 hrs on it yet, so things will probably get even better in the coming weeks. And when the new speakers arrive… Very Happy

    Now for a quick question – Bob mentioned not to “short cycle” the amp (i.e., not to turn the amp back on unless it’s been off for at least a minute). We occasionally experience brief (5 seconds) power outages that would effectively short cycle the amp if its on. I’ve thought of 3 possible solutions, and was hoping anyone with experience will comment:
    1)Hook up a simple self latching relay and momentary switch circuit on the power cord, so that power to the amp would stay off until I engaged the momentary switch.
    2)Hook up a time delay relay on the power cord so that power wouldn’t be restored to the amp until a specific amount of time has passed.
    3)Use a UPS (battery backup) similar to the ones used to keep home computers on for a few minutes during lost power.
    Bob thinks there’s a device available that would “shut off and stay off if power is interrupted”, but couldn’t think of the name. I’ve had little success with net searches. I don’t want to compromise the sound in any way, but I do want to avoid unintentional short cycling. Does anyone else have this problem, and have you effectively dealt with it?

    Thanks,
    John

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: I'm in - new ST-70!

    Post by j beede on Sat Apr 02, 2011 10:12 pm

    I know Bob likes the silicon rectifiers in place of the 5AR4. Are you running a 5AR4/GZ34 or are you using a silicon bridge? The bridge should tolerate power outages. If you are using a borrowed vintage Mullard 5AR4 from a British neighbor... I would consider ordering a quarter dozen Sovtek 5AR4 from www.guitarcenter.com for $13 each and using those until you decide whether or not "it is safe". A crude one shot switch could be tossed together using a pair of 6V or 12V ac adapters and a 6V or 12V DPST relay--all from your convenient neighborhood Radio Shack. This would keep your amp off after an interrruption until you turn it back on.

    davidness

    Posts : 39
    Join date : 2010-09-15

    Re: I'm in - new ST-70!

    Post by davidness on Sat Apr 02, 2011 11:30 pm

    I use a rather large APC 2200XL rack mount UPS that I had laying around to solve this problem.

    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: I'm in - new ST-70!

    Post by Sal on Sun Apr 03, 2011 12:25 am

    The UPS might create "switching" noise to your ST-70. Usually a UPS is not a true sine wave output but a modified sine wave.

    Sal

    johnnybgood

    Posts : 5
    Join date : 2011-04-02

    Re: I'm in - new ST-70!

    Post by johnnybgood on Sun Apr 03, 2011 9:20 am

    Hi folks – thanks for the replies so far. I do have the 5AR4 tube, and will order a few spares today. Are there any drawbacks to using a silicon rectifier? The APC brand was one of the UPS’s I was looking at, and it seems to be well-reviewed. Some newer models (CyberPower) are being advertised as “Pure Sine Wave”, I wonder if that makes a difference?

    Thanks,
    John

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: I'm in - new ST-70!

    Post by j beede on Sun Apr 03, 2011 11:19 am

    With the 5AR4 in the circuit the filaments of your output stage will have done 10-30 seconds of cathode pre-heating before the B+ is applied. With the silicon rectifier B+ is applied after a couple seconds. Some people say that this reduces the working life of your output tubes. I would think the rectifier vendor has commented on this. I think this is the one that Bob has mentioned in the past. I have never used one of these devices.

    http://www.webervst.com/ccap.html

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: I'm in - new ST-70!

    Post by Bob Latino on Sun Apr 03, 2011 11:48 am

    The Weber WZ34 (for the ST-70) and Weber WZ68 (For either ST-70 or ST-120) solid state rectifiers work well in Dynaco tube amps. Both have a short delay built in before high voltage is applied to simulate a tube warm up. The delay in the Weber rectifiers (3 to 5 seconds) is not as long as a GZ34 tube rectifier but it allows the bias system to come up and the filaments to start lighting up before high voltage is applied. That said, both the Dynaco ST-35 and SCA-35 tube amps have solid state rectifiers with NO time delay and there are still plenty of these amps kicking around after 40+ years of use. I use both solid state and tube rectifiers and feel that both have a place with Dynaco tube amps.

    One thing to rememeber also is > Always REBIAS your amp after any rectifier change. This is especially true when going from a tube to solid state rectifier or vice versa. Each rectifier will provide a slightly different B+ high voltage which will in turn require a slightly different bias adjustment.

    Bob

    WntrMute2

    Posts : 96
    Join date : 2010-11-21

    Re: I'm in - new ST-70!

    Post by WntrMute2 on Sun Apr 03, 2011 12:20 pm

    One thing you might use is a GFCI of the "manual power-up" or "Non-latching" type. These are designed for outdoor use so that the hedge clippers or other tool won't start back up unexpectedly when power is restored after an outage. I have my ST-120 plugged into one I bought at Home Depot. It plugs directly into the wall and the cord plugs into that (90 degrees). Simple, about 20 bucks. Trips incredibly fast and you need to push a reset button to restore power. I also purchased one of these.
    http://ericson.thomasnet.com/item/-protect-ground-fault-interupters/ed-protector-series-add-a-cord-90-degree-plug-gfci/g902?&bc=0|1159
    But I haven't wired it up. The important feature is the manual power-up feature. Look for outdoor Ground fault circuit interrupters.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: I'm in - new ST-70!

    Post by Bob Latino on Sun Apr 03, 2011 12:31 pm

    Dave,

    Thanks for this link. If you are in an area that experiences momentary loss of power for a few seconds this is a good item to have. Your just add this to the male end of your power cord. The amp kicks off during a momentary power loss and STAYS OFF unless you manually reset the cord.

    Bob







    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: I'm in - new ST-70!

    Post by Sal on Sun Apr 03, 2011 2:25 pm

    I would also install 2 1N4007 diodes in series to the B+ transformer leads going to the 5AR4. That will help with new manufactured 5AR4 tubes today. Take a look at the attached photo to see what I mean. Drawing courtesy by Kevin at Dynakitparts.com

    Sal


    WntrMute2

    Posts : 96
    Join date : 2010-11-21

    Re: I'm in - new ST-70!

    Post by WntrMute2 on Sun Apr 03, 2011 4:14 pm

    Sal wrote:I would also install 2 1N4007 diodes in series to the B+ transformer leads going to the 5AR4. That will help with new manufactured 5AR4 tubes today. Take a look at the attached photo to see what I mean. Drawing courtesy by Kevin at Dynakitparts.com

    Sal


    What does this modification accomplish?

    johnnybgood

    Posts : 5
    Join date : 2011-04-02

    Re: I'm in - new ST-70!

    Post by johnnybgood on Sun Apr 03, 2011 4:18 pm

    Wow - this is pretty cool - so many great ideas, so many options! Thanks everyone. I should be able to stop at the HD one night after work this week and check out the GFCI with the manual power up, or order the 90 degree plug with manual power up from Ericson. Seems like an ideal and inexpensive solution - thanks again. Very Happy

    John

    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: I'm in - new ST-70!

    Post by Sal on Sun Apr 03, 2011 9:44 pm

    You get the slow start feature of the 5AR4 tube but the AC from the transformer is pre-rectified to the 5AR4 tube. Many have said the new manufactured 5AR4 tubes last longer.

    Sal

    johnnybgood

    Posts : 5
    Join date : 2011-04-02

    Re: I'm in - new ST-70!

    Post by johnnybgood on Sun Apr 10, 2011 5:06 pm

    Just to update here - I purchased the Ericson GFCI earlier this week and it works exactly as advertised. When power is lost, even momentarily, you must hit the reset button on the plug before the ST-70 powers-up again, effectively ending the possibility of an unintentional "short cycle". Thanks again everyone! At some point I may add the diode mod too.

    I'm having so much fun with this amp, revisiting music I haven't listened to in years as well as buying new stuff! It is SWEET! I bet I've got more than 60 hrs on it now, and when the Lores arrive (on Wednesday), I'm sure I'll be re-exploring everything again with these new speakers... Very Happy

    John

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: I'm in - new ST-70!

    Post by j beede on Sun Apr 10, 2011 5:33 pm

    A dollar's worth of 1n400x diodes in series with your recitfier is cheap insurance. Do it now! Smile

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: I'm in - new ST-70!

    Post by mantha3 on Mon Apr 11, 2011 10:35 am

    I did the same mod... I did use UF4007 diodes in place of the 1N4007. The two are the same but the UF4007 is an Ultrafast diode. The switching is quicker (Ultrafast) and otherwise it is the same diode (same specs for voltage/amp). The UF4007 is probably 3 times the cost compared to the 1N4007 (3 bucks or so and not a buck). www.digikey.com has both of these.

    I read a post or two on this modification. some mentioned the potential to hear the switching on the 1N4007 and advised the UF4007. Most (if not all) thought the 1N4007 was not audible with the switching. I wanted to do the mod (ST-120 tube amps eat GZ34s)... I just got the UF4007 as the extra buck or two for these didn't seem a big deal.




    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: I'm in - new ST-70!

    Post by j beede on Mon Apr 11, 2011 3:25 pm

    In a 60 Hz environment I suspect any silicon diode will be viewed as switching fast Smile I have built HV supplies for electrostatic speakers and headphones using $0.08 1n4007 and never "heard" them through the "blanket" of filter caps. I may try stringing 10-20 of these diodes in series to see if they become audible in some way.

    FYI: I have been having good results with the ultra-cheap Sovtek 5AR4 in my Mark IIIs so far. Many, many power cycles completed through debug, updates and occasionally actually listening to music(!). My only complaint is the sloppiness in the internal assembly--each of the 5AR4 filaments were trimmed differently so they glow with different color and brightness. I am still enjoying comparing my 6550-based Mark IIIs with point-to-point wiring to my VTL (EL-34, PCBs, bridge rectifiers). The Mark IIIs seem to have more authority and the VTL has the edge in stereo effects and subtle detail. Listener fatigue is low for both.
    ...j

    frank

    Posts : 95
    Join date : 2011-02-14
    Location : Pacific Northwest

    Re: I'm in - new ST-70!

    Post by frank on Sun May 29, 2011 12:50 am

    WntrMute2 wrote:One thing you might use is a GFCI of the "manual power-up" or "Non-latching" type. These are designed for outdoor use so that the hedge clippers or other tool won't start back up unexpectedly when power is restored after an outage. I have my ST-120 plugged into one I bought at Home Depot. It plugs directly into the wall and the cord plugs into that (90 degrees). Simple, about 20 bucks. Trips incredibly fast and you need to push a reset button to restore power. I also purchased one of these.
    http://ericson.thomasnet.com/item/-protect-ground-fault-interupters/ed-protector-series-add-a-cord-90-degree-plug-gfci/g902?&bc=0|1159
    But I haven't wired it up. The important feature is the manual power-up feature. Look for outdoor Ground fault circuit interrupters.

    Possibly the most useful tip in the history of forums. Thanks WntrMute2.

    johnnybgood

    Posts : 5
    Join date : 2011-04-02

    Re: I'm in - new ST-70!

    Post by johnnybgood on Thu Jun 02, 2011 5:44 pm

    fswidecki wrote:
    Possibly the most useful tip in the history of forums. Thanks WntrMute2.

    Yep - "saved me" twice already from a sure short cycle. Thanks again!

    Westy56

    Posts : 58
    Join date : 2011-05-09
    Location : Albuquerque

    Re: I'm in - new ST-70!

    Post by Westy56 on Thu Jun 02, 2011 7:36 pm

    Sal wrote:You get the slow start feature of the 5AR4 tube but the AC from the transformer is pre-rectified to the 5AR4 tube. Many have said the new manufactured 5AR4 tubes last longer.

    Sal

    Great info Sal, very easy DIY.


    Steve

    Sponsored content

    Re: I'm in - new ST-70!

    Post by Sponsored content Today at 6:34 am


      Current date/time is Sun Sep 25, 2016 6:34 am