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    New Member-----Returning ST-70 to stock specs.

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    breading

    Posts : 23
    Join date : 2010-01-08

    New Member-----Returning ST-70 to stock specs.

    Post by breading on Mon Jan 11, 2010 1:31 am

    Hello,

    What a great find the Dynaco Tube Audio Forum has been! I am totally stoked about all the great information I have discovered regarding my Dynaco components, which include a ST-70 amp, PAS 3 preamp, and FM-3 tuner, all of which I assembled back in the mid sixties.

    One of the immediate benefits I have gained from the forum was learning about the selenium rectifier/diode swap out on the ST-70 amp, which I just completed yesterday. Unfortunately it was during that job that I discovered a previously unknown fact regarding my amp. I erroneously thought I still had a “stock” amp! However, when I opened the unit to do the swap, I discovered that several components have been modified.

    Some time back I had “repair” work done on the ST-70. The technician was asked to check out a hum in the amp and to repair as necessary. The work tag indicates the replacement of several resistors and capacitors and the installation of new tubes. However, I had always considered the unit to simply have been brought back to original factory specs with the work performed. I was then, and remain today, pleased with the results of his work and have experienced many trouble free hours of enjoyment, but I would like to return my unit to stock conditions.

    I found the following modifications:

    1) The original installation called for the connection of 1000 ohm resistors (brown-black-red) between pins 5 and 6 of V2, V3, V6, and V7, respectively. In each case, they have now been replaced by 680 ohm resistors (blue-gray-brown-silver).

    2) There are now 60UF-500VDC capacitors running from the quad filter capacitor to the ground lugs on the V6 and V7 sockets. Quad Pin 1 connects to V7 and pin 2 to V6.

    3) On the PCB, the 4 0.1UF/400VDC and the 2 0.05UF/200VDC capacitors have been replaced by SIX 0.1 UF/400VDC capacitors.

    4) Replacement tubes for my original Mullard “Dynaco” EL34/6CA7 fx2s are Ruby EL34Gs and the 7199s are now Sovtek. I understand about this part----Mullards are rare and expensive.

    My questions for anyone who can help are as follows:

    1). Do these mods make sense? I do not understand enough to know why the tech made the changes I have noted.

    2). With the exception of the tubes, can this ST-70 be brought back to “Stock”? If so, can you give me some directions on how to go about it? Is it as simple as removing the modified parts and replacing them with original spec components?

    3). Since I want to continue to operate the unit until I decide what direction to go, given the current modified configuration, is the bias setting still 1.56VDC?



    Thanks

    Bruce Reading

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: New Member-----Returning ST-70 to stock specs.

    Post by GP49 on Mon Jan 11, 2010 1:45 am

    > 1) The original installation called for the connection of 1000 ohm
    > resistors (brown-black-red) between pins 5 and 6 of V2, V3, V6, and
    > V7, respectively. In each case, they have now been replaced by 680
    > ohm resistors (blue-gray-brown-silver).
    ===============================

    Insignificant change.

    =====================================
    > 2) There are now 60UF-500VDC capacitors running from the quad filter
    > capacitor to the ground lugs on the V6 and V7 sockets. Quad Pin 1
    > connects to V7 and pin 2 to V6.
    ==================================

    Added filter capacitance to the power supply. Probably an improvement, especially if the original quad capacitor no longer performs to its original capacitance (it's at least 40-some years old!); the additional capacitors could have been added to get rid of the hum you complained about.

    ===================================
    > 3) On the PCB, the 4 0.1UF/400VDC and the 2 0.05UF/200VDC capacitors
    > have been replaced by SIX 0.1 UF/400VDC capacitors.
    ===============================

    Possibly an upgrade depending on what capacitors were used, although I would have gone higher in value on the four 0.1uF capacitors if I were doing that level of work on a Stereo 70.

    ==========================================
    > 4) Replacement tubes for my original Mullard “Dynaco” EL34/6CA7 fx2s
    > are Ruby EL34Gs and the 7199s are now Sovtek. I understand about this
    > part----Mullards are rare and expensive.
    ===========================================

    If the original tubes had been in good condition, someone may have kept the Mullard tubes for themselves while substituting lower-priced, currently-available substitutes.

    ===============================================
    > My questions for anyone who can help are as follows:
    > 1). Do these mods make sense? I do not understand enough to know why
    > the tech made the changes I have noted.
    ========================================

    To me, they basically do (if the tube replacement was done because the original tubes were actually bad).

    =========================================
    > 2). With the exception of the tubes, can this ST-70 be brought back
    > to “Stock”? If so, can you give me some directions on how to go about
    > it? Is it as simple as removing the modified parts and replacing them
    > with original spec components?
    ===========================================

    You've pretty much detailed the changes made. Reverse them. But you may get the hum back again unless you install a new four-section electrolytic capacitor.

    ===================================================
    > 3). Since I want to continue to operate the unit until I decide what
    > direction to go, given the current modified configuration, is the
    > bias setting still 1.56VDC?
    ===========================================

    Nominally yes, unless someone has changed the value of the cathode resistor across which the 1.56vDC is measured. However, with present-day EL34 tubes, many run the Stereo 70 on less bias, perhaps in the 1.1-1.2 volt range. This isn't due to any of the modifications, only due to the difference between modern EL34 tubes and the old standard, more rugged Mullards that could better withstand the higher bias current.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: New Member-----Returning ST-70 to stock specs.

    Post by Bob Latino on Mon Jan 11, 2010 7:42 am

    Hi Bruce and welcome to the Forum,

    It appears from you description that your "technician" did a "patch job" on the amp to keep it running. He added capacitance to the quad cap when he really should have (IMHO) pulled and replaced the quad cap. The most common reason that you get hum in an old ST-70 is that the quad cap is leaking DC and just won't hold a charge anymore. The hum is always there and doesn't change it's level with the volume control on your preamp. The B+ DC voltage is usually lower than it should be. Off pin 8 of the rectifier you SHOULD measure about 420 to 440 volts DC to chassis ground but in an amp with a leaky quad cap the reading will almost always be lower than 400 volts DC.

    So what do you do about it? IMHO it depends on what you are going to do with the amp. If you intend to just play it now and then and it doesn't hum much anymore then I wouldn't do anything to it. If, however, you intend to play it on a daily basis as your main amp you should do some work on it .. You have already replaced the old selenium rectifier with a diode - so that's done ... What I would do next is ..

    A. Replace the quad cap with a new quad cap. If you do that the two "add-on" capacitors you have there now won't really be necessary.
    B. Replace the entire driver board with a new driver board and parts set. Even though your tech replaced some of the capacitors on the board, those carbon compostition resistors on there have probably drifted a good distance from their original values. The newer metal film resistors have much higher tolerances and won't drift anywhere near as much as the older resistors you have on there now. When you replace the driver board you can replace it with a driver board having the stock circuit or one having an upgraded circuit. I would opt for the latter because it gets you away from the 7199 driver tubes which are not made anymore.

    If you want to take a step further then you could do a complete rebuild. This would involve taking everything off the chasssis and replacing tube sockets and all the wiring. If your chassis is rusted you could also start with a new chassis. You could repaint the three transformers and make it look like a new amp. You would get a lot of self satisfaction with a project like this. If you use a replica of the original driver board you could follow the original manual for your rebuild. There is a copy at the link below.

    Dynaco ST-70 original assembly manual

    If you use the VTA replacement driver board on your rebuild there is a manual at the link below you could follow ...

    ST-70 complete rebuild with VTA driver board assembly manual

    Bob

    breading

    Posts : 23
    Join date : 2010-01-08

    Re: New Member-----Returning ST-70 to stock specs.

    Post by breading on Tue Jan 12, 2010 6:05 pm

    Bob and GP49,

    Thanks for all the great responses to my newbie questions. GP49, I did get the original Mullard EL34 fX2s back from the tech and still have them. Unfortunately, I do not have a means of testing them, other than installing them back in the amp. I did that, and they seemed to perform as well as the cheap Rubies, but I am sure they are tired after all these years.

    I think for now, until I decide which way to go, I will leave the unit as is, with the exception of new tubes.

    Which ones would you recommend I opt for, given the stock nature of the amp? The re issue Mullard EL34s or the KT666 or?????

    Thanks again.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: New Member-----Returning ST-70 to stock specs.

    Post by Bob Latino on Tue Jan 12, 2010 6:18 pm

    Hi,

    I honestly have not heard the new Mullard EL34 reissue. Maybe someone here on the forum has heard thse tubes and could comment on them ? This new production Mullard EL34 tube is manufactured at the Xpo-Pul factory (also known as Reflektor) in Saratov, Russia. These are maybe $90 a matched quad.

    I do now use the Genalex Gold Lion KT66's in my own ST-70. They are the best sounding tube that I have used in that amp. They are more expensive at $150 - $160 per matched quad.

    Bob

    breading

    Posts : 23
    Join date : 2010-01-08

    Re: New Member-----Returning ST-70 to stock specs.

    Post by breading on Tue Jan 12, 2010 6:26 pm

    Thanks Bob, I was leaning in the direction of the KT66s. I know the bias setting for the Mullards is 1.56VDC, do I recall seeing that the KTs are biased at 0.40VDC?

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: New Member-----Returning ST-70 to stock specs.

    Post by Bob Latino on Tue Jan 12, 2010 6:45 pm

    breading wrote:Thanks Bob, I was leaning in the direction of the KT66s. I know the bias setting for the Mullards is 1.56VDC, do I recall seeing that the KTs are biased at 0.40VDC?

    Hi,

    If you have a STOCK ST-70 you can bias them at 1.56 VDC which gives a bias current of 100 milliamps per each output tube PAIR or 50 milliamps per each individual output tube.

    The .40 volts DC bias for an EL34 or KT66 that you mentioned above is for the VTA driver board only which uses a 10 ohm bias resistor per each output tube.

    With respect to what I said above in the first paragraph > The 1.56 volts DC bias voltage was the bias voltage recommended by Dynaco for those tough original Dynaco Mullard EL34 tubes. Conventional wisdom nowadays is to drop the bias somewhat on stock ST-70's to about 1.25 volts VDC. This will give about 80 milliamps per each tube pair with the stock Dynaco 15.6 ohm bias resistor OR about 40 milliamps per each individual EL34 or KT66 output tube. With this somewhat lower bias setting your tubes will sound the same but will last longer. The tubes made now just don't seem to be as "tough and durable" as those tubes made in the 1950's and 1960's.

    Bob

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