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    Power Transformer Options - ST-70

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    Aljaheejus

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    Power Transformer Options - ST-70

    Post by Aljaheejus on Sat Aug 27, 2011 12:13 am

    I have an assembly consisting of a chassis with two A470 output transformers, some old tube sockets, and not much else. Thinking of rebuilding with a VTA low-output board and wondering if this transformer (below) would be appropriate in the opinion of this knowledge community.



    tubes4hifi
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    Re: Power Transformer Options - ST-70

    Post by tubes4hifi on Sat Aug 27, 2011 3:41 pm

    that should work fine, I'm currently building one using an Antek AS-4T360, they are $57.

    Aljaheejus

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    Transformer Options and Testing questions

    Post by Aljaheejus on Sat Aug 27, 2011 10:16 pm

    Hey, thanks! Do you have a schematic you can share regarding that unit? And that is even less expensive than the one I found on e-bay, shown in the photo, which is being sold for $76 with mounting hardware. I had no idea there were other options for the power trannies, until I ran across the auction for this one on ebay. Are these not as popular because of the break from traditional design? Just curious what other engineering bargains might be out there for repopulating a chassis with two a-470s.

    Also, I have not found how to test, individually the A 470 OPTs. Is there a simple method? (Don't have oscilloscope, but I have another ST-70, a DMM and now a variac.)

    Blitzen

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    Re: Power Transformer Options - ST-70

    Post by Blitzen on Sun Aug 28, 2011 11:11 am

    That Tomiko looks like a direct replacement for a stock ST-70 power transformer. You can go cheaper with the Antek, But you'll need to deal with the issues of not having a 5v supply for the rectifier (you could use solid state), and the bias voltage supply; not as simple.

    tubes4hifi
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    Re: Power Transformer Options - ST-70

    Post by tubes4hifi on Sun Aug 28, 2011 3:35 pm

    Blitzen is correct, the Antek doesn't have a 5v 2 or 3 amp winding for the rectifier tube, so that would be an additional transformer, so the Tomika seems to be an all-in-one solution.
    Check this link here for testing A470 transformers:
    [url="http://www.tubes4hifi.com/Dyna-testing.pdf"] www.tubes4hifi.com/Dyna-testing.pdf [/url]

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Mon Aug 29, 2011 1:54 am

    Thanks so much for the observation and confirmation. cheers You all are very helpful.

    I was reading about that missing feed in some DIYaudio forum postings, and next on my list was to look more closely at the VTA board to see if it has anything to do with this 5v tap requirement, and then to ask if there was a schematic for use of the Antek unit. I am not knowledgeable of these issues, and want to keep to the fun part (building/listening) rather than reinventing the wheel that I am not qualified to design. If the VTA board does not eliminate the tube rectifier, and still needs a 5v feed for bias voltage, then I guess I will move toward the Tomiko, which, yes, is even marked as an "ST-70" power transformer.

    I like what I read (what people hear with) about the VTA board, its triode mode switch-ability. I also like the savings and potential upgrade the toroid transformer may offer.

    Seems the general consensus would also be to avoid the quad-cap, but the chassis I have has one already, so I may use it on first round configuration. I will post a photo of my bare-bones build project soon.

    tubes4hifi
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    Re: Power Transformer Options - ST-70

    Post by tubes4hifi on Mon Aug 29, 2011 2:03 pm

    neither the transformer or the VTA driver board "changes" the schematic of the Dyna ST70 amplifier, you are just substituting one part for another (well, the VTA does actually have a different schematic than original, but the idea is, you are just swapping parts).
    In either case you'll probably want to retain the same rectifier tube using 5v, so the Tomika toroid is the easier option since it has all the correct voltages on it. The VTA board takes care of the improved bias circuit (from the -60 or 70v tap on the transformer).
    The wiring diagram for the Antek is probably very similar to the Tomika, which is similar to the original Dynaco PA060 transformer.

    Bob Latino
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    Re: Power Transformer Options - ST-70

    Post by Bob Latino on Mon Aug 29, 2011 2:16 pm

    From Aljaheejus > "I like what I read (what people hear with) about the VTA board, its triode mode switch-ability. I also like the savings and potential upgrade the toroid transformer may offer."

    The triode switching system is outboard of the VTA driver board and not on the board itself. If you want your amp to have triode/pentode ultralinear switching ability you will need the triode/pentode ultralinear switch kit to add to your amp. This kit is available from either Roy or myself.

    Bob

    Aljaheejus

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    Wire Colors original PA060 - ratings

    Post by Aljaheejus on Sat Apr 23, 2016 7:33 pm

    I am finally installing this toroidal transformer, and I need to know the voltages for the original PA060 transformer wires, according to the colors of the wires in the original PA060.

    All the wiring diagrams I have are for the original PA060 in the Dynaco ST-70.  

    Can someone please provide me with the PA060 wire colors and voltages?  I could then match the wires from my new transformer to those colors and figure out where to connect them!

    Bob Latino
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    Re: Power Transformer Options - ST-70

    Post by Bob Latino on Sat Apr 23, 2016 8:08 pm

    Aljaheejus wrote:

    Can someone please provide me with the PA060 wire colors and voltages?  I could then match the wires from my new transformer to those colors and figure out where to connect them!

    On an original Dynaco PA-060 power transformer with about 117 VAC applied to the two BLACK wires ..

    Across the two RED wires > 360 VAC
    Across the two WHITE wires > 5 VAC
    Across the two GREEN wires > 6.3 VAC
    Across the two BROWN wires > 6.3 VAC
    RED/BLACK to RED/YELLOW > 55 VAC

    NOTE - the GREEN/YELLOW and BROWN/YELLOW wires on the PA-060 are the center taps of the two 6.3 VAC lines and will measure about 1/2 the 6.3 VAC OR about 3.15 VAC each to the RED/YELLOW wire. The RED/YELLOW wire on a PA-060 is grounded directly to the ST-70's chassis.

    Bob

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Thu May 12, 2016 11:39 pm

    This was very helpful information.  I have connected the transformer wires to where I believe they belong.  Other parts of my project amp are:  TRIODE Cap Board and the DIYtube driver board.

    I'm having trouble, and getting some advice over on the DIYtube forum, but wondering if anyone here can advise regarding anything unusual to look out for with this combination of parts.   That thread is at:  http://www.diytube.com/phpBB2/viewtopic.php?f=6&t=6831&start=15

    The only original parts from the ST70 are the chassis, the choke and the A470 output transformers.   I tested the A470s, and I had music coming through the amp but with what I thought was a significant humming/noise.  I only tested with my computer headphone jack line, so some noise was probably coming from there.   With shorted inputs there was still a significant hum.


    Last edited by Aljaheejus on Fri May 13, 2016 12:17 am; edited 1 time in total (Reason for editing : added link to another discussion forum regarding this build)

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Wed May 18, 2016 11:29 pm

    So, it seems to be working as it should, except for a hum. Seems evenly distributed between both channels. Next I'm going to disconnect the ground wire from the chassis star ground terminal. My house has a two-wire electrical system, so not much point in connecting that wire.

    Other ideas to trace and eliminate a hum are welcome.

    GP49

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    Re: Power Transformer Options - ST-70

    Post by GP49 on Thu May 19, 2016 12:18 am

    Is it more a "hummmmmmmmmm" or a "buzzzzzzzzzzzzzzzz"?

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Thu May 19, 2016 12:49 am

    GP49 wrote:Is it more a "hummmmmmmmmm" or a "buzzzzzzzzzzzzzzzz"?

    Not sure, now. Thanks. I will check.

    tubes4hifi
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    Re: Power Transformer Options - ST-70

    Post by tubes4hifi on Thu May 19, 2016 12:54 pm

    you could have a noisy power transformer (hum) or mechanical buzz, or radiated EMF from the power transformer.
    Usually when using a toriod power transformer it should either have a full shield around it, or a metal case around it.

    GP49

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    Re: Power Transformer Options - ST-70

    Post by GP49 on Thu May 19, 2016 2:54 pm

    Aljaheejus wrote:If the VTA board does not eliminate the tube rectifier, and still needs a 5v feed for bias voltage...

    You need a 50-60 volt feed for bias voltage.

    The original Dyna Company...not Tyco, and not Panor which in reality I consider "Dynaco" in name only and not in terms of design principles and philosophy...published a circuit to derive the bias voltage from a transformer that lacked the bias tap, by means of a capacitor. It would pass AC but block DC and was intended for a mono amplifier. However I found it supplied insufficient negative voltage for the KT88-6550, and was adequate but with no reserve for the EL34. It works for 6L6-type output tubes. Probably the effective series resistance of the capacitor was too high for the tubes requiring more negative voltage on the grids or for a stereo amplifier but I did not investigate further than to incorporate the circuit into a Mark II with a replacement transformer, with 6L6GC tubes, which I then put into a background music application with an FM-1 tuner. It survived for 20 years until the business closed.

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Sun May 22, 2016 1:10 am

    tubes4hifi wrote:you could have a noisy power transformer (hum) or mechanical buzz, or radiated EMF from the power transformer.
    Usually when using a toriod power transformer it should either have a full shield around it, or a metal case around it.

    Thank you. I'm looking for a metal coffee can or something to fit around the PT. Should it be grounded to the chassis, and does it need to be thick?

    Tonight I connected it to a grounded electrical outlet in my studio. (My house has a 1950s two-wire system, with no ground.) It was a fairly faint hum until connected to my computer for a music signal. It was like playing a note from a reed organ. Then I figured out that the external source of noise was an IPS computer monitor that had connected to the computer. The amp picks up very strongly the EMF, I guess.

    Could it be the inexpensive 12Au7 driver tubes I'm using while testing picking up EMF?

    Once I turned that monitor and the laptop off, connected it to my phone for a test it was pretty quiet. Also, just a faint hum when using shorting plugs on the inputs. Will try next with a proper preamp.


    tubes4hifi
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    Re: Power Transformer Options - ST-70

    Post by tubes4hifi on Sun May 22, 2016 11:29 am

    as you can see on the photo you provided, the BROWN wire on the TOMIKA toroid is the 60v bias wire.
    I still think you need a full shield around the toroid. You can try a coffee can, it will need to be grounded to the chassis.
    For a nicer solution, Antek has toroid covers. http://www.antekinc.com/steel-cases/

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Thu Jun 09, 2016 12:06 am

    tubes4hifi wrote:....
    For a nicer solution, Antek has toroid covers.   http://www.antekinc.com/steel-cases/

    I obtained the Antek CA-100 for a perfect fit. I slid the PT into the case with some dishwashing detergent to minimize the friction, as it had gotten stuck on my first dry-fit attempt.

    Re-assembled after double checking some of the wiring from the PT to the Triode Cap Board. All, looked good and the new PT case grounds to the chassis, etc.

    Now she's blowing fuses. The 3A that was in place, then a 2 1/2 amp I had laying around. Before I get more fuses and try again I thought I'd post to see if anyone can advise on troubleshooting this.

    Could the squeeze into the case be shorting the PT wiring? Some bits of the former sheet metal PT base may have ended up magnetically attaching to some terminals. I'll be vacuuming and maybe passing a stronger magnet around before powering up again. Other ideas? Resistance checks I should make?

    tubes4hifi
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    Re: Power Transformer Options - ST-70

    Post by tubes4hifi on Thu Jun 09, 2016 9:24 am

    very possible the transformer is shorting to the case since it was such a tight fit.
    Use a meter and check every transformer lead to ground

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Thu Jun 09, 2016 9:11 pm

    tubes4hifi wrote:very possible the transformer is shorting to the case since it was such a tight fit.
    Use a meter and check every transformer lead to ground

    Thanks very much.  I have done so, and here's what I find (leaving everything soldered in place, no tubes installed and unplugged.)

    None go lower than the first listing, below.  I don't know how to interpret these.

    Tomiko wire colors:  
    BROWN (bias tap) - 6.8 DROP TO 5.7 ohms
    GRN, GRN and YEL, YEL and the BLK center taps between these pairs ALL - 49.8 ohms
    RED to pin 6 on GZ34 - 98 drop to 34 ohms
    RED to pin 4 on GZ34 - 30 drop to 18 ohms
    BLU to pin 2 on GZ34 - 29M drop rise 50K unstable
    BLU to pin 8 on GZ34 - 15M drop 9M unstable

    No resistance registers on two BLK leads from power cord to ground.

    I removed the GZ34 and powered up and the fuse blew again. BTW, should I get more 2A slow blow fuses? What rating is best for this rig?

    Bob Latino
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    Re: Power Transformer Options - ST-70

    Post by Bob Latino on Fri Jun 10, 2016 7:04 am

    Aljaheejus wrote:

    I removed the GZ34 and powered up and the fuse blew again.   BTW, should I get more 2A slow blow fuses?  What rating is best for this rig?

    You should be using a 3 amp slo-blo fuse for any Dynaco ST-70 type amp.

    Bob

    GP49

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    Re: Power Transformer Options - ST-70

    Post by GP49 on Fri Jun 10, 2016 1:02 pm

    Unsolder the center-tap ground and any other ground leads from the transformer, then measure resistance between each winding and the center-tap ground, and between each lead the aftermarket toroid shell, again.  Come back to us with the results.

    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Mon Jun 13, 2016 12:25 am

    GP49 wrote:Unsolder the center-tap ground and any other ground leads from the transformer, then measure resistance between each winding and the center-tap ground, and between each lead the aftermarket toroid shell, again.  Come back to us with the results.

    Thanks to everyone, assisting with this build! I've unsoldered and removed the PT in its case for the following resistance results:

    First I can say there was no shorting to the case from any of the leads.

    RED 82 dropped to 33.6 -____
    RED 41 dropped to 17.6___ |
    BRN 6.7 dropped to 5.6__ | |
    to | | |
    BLK - center ground_____|_|__|

    BLU to BLU .5 to each other ( no shorting to any other leads or case)

    GRN leads both read 0.4 ohms to their BLK center ground (no shorting to other leads or case)

    YEL leads both read 0.4 ohms to their BLK center ground (no shorting to other leads or case)

    BLK input leads 6.4 dropped to 2.3 ohms to each other (no shorting to any other leads or case)

    Should I take it out of its metal case/cover, or start looking elsewhere?
    Could one of my tubes caused the fuse to blow?
    If the bias got way out of whack (because I'm using mis-matched tubes and put them back in different sockets)?


    Aljaheejus

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    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Wed Jun 15, 2016 9:29 pm

    Still blowing fuses. I thought that my readings, in the post above, seemed likely to be okay. Since there was no shorting to the case/ground, I put her back together and powered up with a new 3A slow-blow fuse. I'm ramping up the voltage supply with a variac, just because I have one.

    I would really appreciate any other ideas as to what I should be looking for. Should I change out some of the tubes / could they be causing the fuse to blow? If I got the output EL34s mixed up, and the bias balance is out of whack, can that cause fuses to blow?

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