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    Anybody used 6V6GT output tubes in ST-70?

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    david24x7

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    Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Mon Jul 14, 2014 4:08 pm

    Hi all,
    I was rummaging through some old tubes at an antique store warehouse and came across these 6V6GT tubes (zenith and Raytheon) and was wondering if anybody had experience or advice using them with Dynaco ST-70.

    My Dynaco ST-70 was rebuilt using the VTA kit which has new board and caps but same power transformer and output transformers.

    There isn't much on the internet about this combo so would love to hear your experience.

    Thx,
    David

    Bob Latino
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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by Bob Latino on Mon Jul 14, 2014 7:34 pm

    Hi David,

    A 6V6 tube should be looking at 350 to 375 volts DC on the plates. In an ST-70 you have 430 to 450 volts on the plates. That is too much voltage on the plates for a 6V6. If you did use a 6V6 in an ST-70, the 6V6 would probably have a short life. A 6V6 tube in a normal push/pull configuration is about a 14 - 15 watt amp.

    If you come across any 6L6 tubes, you could use those in an ST-70 without any problems ..

    Bob

    david24x7

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Mon Jul 14, 2014 7:40 pm

    Thanks Bob,
    I'll look for some 6L6's. In the meantime, what's the worst that could happen with the 6V6's? Is it just shortening the life on those tubes or could I risk further damage to the amp?

    Thanks for your advice.

    David

    Bob Latino
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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by Bob Latino on Mon Jul 14, 2014 7:50 pm

    david24x7 wrote:Thanks Bob,
    I'll look for some 6L6's.  In the meantime, what's the worst that could happen with the 6V6's?  Is it just shortening the life on those tubes or could I risk further damage to the amp?

    Thanks for your advice.

    David

    No - You won't hurt the amp.. You will just run these tubes harder than they are designed for .. Another thing ... 6V6's may not bias properly in an ST-70. If you have the VTA driver board you may have to change out R39, the main bias resistor, to get them to bias properly. (maybe .300 VDC per each output tube?)

    Bob

    david24x7

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Mon Jul 14, 2014 8:04 pm

    Thanks again and I'll keep my eye out for some 6L6's.

    Gregg R.

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    6L6GC power tubes

    Post by Gregg R. on Mon Jul 14, 2014 10:40 pm

    You might want to try this set of Shuguang 6L6s I found on eBay. This dealer also has some Russian versions at a similar price. I bought some Shuguang EL34s from this guy recently, and I am very happy with the quality and sound:

    Shuguang 6L6

    david24x7

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Tue Jul 15, 2014 12:48 am

    Thanks Greg. You can't beat the price. I have some russian 6P3S-E which I think is a variant of 6L6. I think that they sound pretty good. Would love to try some other NOS tubes without breaking the bank.

    dougmon

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by dougmon on Tue Jul 15, 2014 8:58 am

    My 2 cents: NOS 6L6 tubes are usually pretty expensive, and can be a crap shoot as far as quality. I got some Tung-Sol reissue 6L6G tubes that sound great. I'd spend the bucks on some NOS 12AU7. Again, that's just my opinion.

    david24x7

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Tue Jul 15, 2014 12:34 pm

    Thanks Dougmon. I have NOS GE 12AU7s and just swapped in a used but working Telefunken 12AU7 into the center position last week. The telefunken made a big difference. Do you know if you get the biggest bang for the buck from upgrading all 3 or is one more important than another?

    sKiZo

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by sKiZo on Tue Jul 15, 2014 2:23 pm

    So how about modding the amp to work with the 6V6 series? Not sure if that would lock you in on that variety or you'd still be able to use "normal" tubes, albeit at a lower output ...

    Might be a fun experiment anyway. If it's just a resistor or two, could even add as switching circuit.


    dougmon

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by dougmon on Tue Jul 15, 2014 2:35 pm

    david24x7 wrote:Thanks Dougmon.  I have NOS GE 12AU7s and just swapped in a used but working Telefunken 12AU7 into the center position last week.  The telefunken made a big difference.  Do you know if you get the biggest bang for the buck from upgrading all 3 or is one more important than another?

    My understanding (and perhaps someone can correct me if I've misunderstood) is that the center tube makes the biggest difference. The 12AU7 tubes I have are all of the same type, so I don't have any real evidence of that.

    A little off-topic: I put some fairly premium 12AU7 tubes in my preamp (SP-10) and it made an audible (and desirable) difference.

    Also, if you're going to go with 6L6, I _really_ like the SED =C= tubes -- they're a little pricey, but not compared with a lot of NOS tubes. Also, the SED =C= EL34s are really nice.

    david24x7

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Tue Jul 15, 2014 2:49 pm

    Sweet. I had made the same assumption that the center (driver?) tube would have the biggest impact and it did make a noticeable improvement. Not sure what is in my pre-amp (PAS-3) but now I'm going to have to take a look.

    I've been considering trying some of the new Gold Lions. I'll have to save up a bit because they aren't cheap but I have read great things about them. Has anyone compared GL KT-66 vs GL-KT-88?

    I know that Bob has cautioned against using KT-88 in a stock Dynaco ST-70 but I'm curious whether my upgraded VTA board with 4 military-spec Russian PIO output coupling capacitors and original type UPGRADED quad cap (80+40+30+20uF@525v) is sufficiently upgraded or whether I need to upgrade my power transformer (which is still stock). Bob?

    david24x7

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Tue Jul 15, 2014 2:54 pm

    sKiZo wrote:So how about modding the amp to work with the 6V6 series? Not sure if that would lock you in on that variety or you'd still be able to use "normal" tubes, albeit at a lower output ...

    Might be a fun experiment anyway. If it's just a resistor or two, could even add as switching circuit.


    There is an old thread on the internet for modding the ST-70 to use 6L6GT using different rectifier and rebiasing. Some say it works while many more were skeptical.
    http://rec.antiques.radio-phono.narkive.com/dacuYzUw/answer-to-an-faq-using-6v6-s-in-a-dynaco-st-70

    Bob Latino
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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by Bob Latino on Tue Jul 15, 2014 3:33 pm

    david24x7 wrote:Sweet.  I had made the same assumption that the center (driver?) tube would have the biggest impact and it did make a noticeable improvement.  Not sure what is in my pre-amp (PAS-3) but now I'm going to have to take a look.

    I've been considering trying some of the new Gold Lions.  I'll have to save up a bit because they aren't cheap but I have read great things about them.  Has anyone compared GL KT-66 vs GL-KT-88?

    I know that Bob has cautioned against using KT-88 in a stock Dynaco ST-70 but I'm curious whether my upgraded VTA board with 4 military-spec Russian PIO output coupling capacitors and original type UPGRADED quad cap (80+40+30+20uF@525v) is sufficiently upgraded or whether I need to upgrade my power transformer (which is still stock).  Bob?

    The biggest "upgrade bang for the buck" you can make on any VTA amp is the CENTER driver tube. The center driver tube is the initial voltage amplifier and does have more influence on the sound of any VTA amp than the two side (phase splitter/inverter) driver tubes. On the VTA M-125 amps it is the FRONT driver tube that is the voltage amplifier tube. You can also try other tubes in the side driver positions on the VTA ST-70 and VTA ST-120 if you want. Any sound difference from swapping these two side tubes out will be subtle.

    I get questions a lot about what is the BEST driver tube ? I politely refuse to answer such questions because your associated gear (speakers, preamp, CD player and your room) must be taken into consideration. Example - sometimes a speaker that may be a little "bright" in your room can be offset by using a center driver tube that maybe does not have the top end extension of the average driver tube.

    Bob

    corndog71

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by corndog71 on Tue Jul 15, 2014 5:49 pm

    david24x7 wrote:Sweet.  I had made the same assumption that the center (driver?) tube would have the biggest impact and it did make a noticeable improvement.  Not sure what is in my pre-amp (PAS-3) but now I'm going to have to take a look.

    I've been considering trying some of the new Gold Lions.  I'll have to save up a bit because they aren't cheap but I have read great things about them.  Has anyone compared GL KT-66 vs GL-KT-88?

    I know that Bob has cautioned against using KT-88 in a stock Dynaco ST-70 but I'm curious whether my upgraded VTA board with 4 military-spec Russian PIO output coupling capacitors and original type UPGRADED quad cap (80+40+30+20uF@525v) is sufficiently upgraded or whether I need to upgrade my power transformer (which is still stock).  Bob?

    The center tube of the VTA ST70/120 is the voltage amplifier for BOTH channels. It doesn't have to have super low noise but it would probably be good if both triodes were matched for gain.

    A friend of mine got a set of Gold Lion KT77s for his Mark IV amps and he loves them! He was enjoying Valve Art EL34s but said the Genalex tubes were clearly superior. Upscale Audio is a good source. I would also recommend RAM tubes as they are selected from large batches for low noise, low grid leakage, as well as other variables. They're a bit more expensive but nobody else selects tubes the way RAM does. My RAM 12BH7 outperformed a new Genalex 12AU7 in my ST120.

    Is there a particular reason you don't seem to want EL34s?

    david24x7

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by david24x7 on Tue Jul 15, 2014 6:11 pm

    corndog71 wrote:Is there a particular reason you don't seem to want EL34s?

    No particular reason. Just trying to see if I can do better. I don't really have a point of reference as I only have one tube amp and don't really know anybody else with one I can listen to for reference. Plus, I really like the shape of the KT-88s. Is "cooler looking" a good reason?

    dougmon

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by dougmon on Tue Jul 15, 2014 6:52 pm

    david24x7 wrote:
    corndog71 wrote:Is there a particular reason you don't seem to want EL34s?

    No particular reason.  Just trying to see if I can do better.  I don't really have a point of reference as I only have one tube amp and don't really know anybody else with one I can listen to for reference.  Plus, I really like the shape of the KT-88s.  Is "cooler looking" a good reason?

    If "cooler looking" is your reason, then it's a good enough reason. And if they sound good, that's a bonus.

    I got both good sound and cool looks with the Tung-Sol 6L6G (the new production). Very clear and present in my setup.

    MarcVBelgium

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    6V6GT tubes in ST 70

    Post by MarcVBelgium on Tue Jul 15, 2014 7:38 pm

    There is a beautifull site in Germany with a tremendous choice of selected new and NOS tubes, a site that I have not seen mentioned here before.
    www.tubeampdoctor.com (TAD )

    please note that their prices are in €, including 21% sales tax (what did Bob say about Taxachusetts?)

    They have a beautifull cataloque, shipment the same day and they ara known to deliver quality.

    6L6 is available there.

    corndog71

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    Re: Anybody used 6V6GT output tubes in ST-70?

    Post by corndog71 on Tue Jul 15, 2014 11:18 pm

    Oh yeah, I also recommend the upgraded transformer. It runs slightly cooler and has higher current taps.

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