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    ST-70 just finished - very little bass

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    fmorelli

    Posts : 16
    Join date : 2013-08-27

    ST-70 just finished - very little bass

    Post by fmorelli on Thu Oct 10, 2013 10:30 pm

    I just finished my ST-70 kit. It is beautiful and whisper quiet. It is driving a set of Klipsch KG4's which have wonderful full range sound. The ST-70 is equipped with Genalex KT88's which are new, along with a JJ Electronics rectifier and Genalex 12au7's. It has the TDR, stepped attenuator and Russian PIO upgrade.

    For some reason I'm getting very little bass, almost like a high pass filter in play. It's being fed by an iPod which was previously driving another tube amp to the KG4's and had full range sound. The ST-70 has several hours of use thus far. I'd be amazed if this was a break-in issue as the roll off is hardly subtle.

    Any suggestions or thoughts? As I said, the rest of it sounds great!

    Filippo


    Last edited by fmorelli on Fri Oct 11, 2013 2:04 pm; edited 1 time in total (Reason for editing : 12au7 not 12ax7)

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 just finished - very little bass

    Post by Bob Latino on Fri Oct 11, 2013 8:50 am

    Try driving the amp with a CD player instead of an Ipod. Also - your KG4 is a 6 ohm speaker. If you are using the 8 ohm tap right now, try the 4 ohm tap. The 4 ohm tap may be a better match to your speaker.

    Also - you did not get your tubes for this amp from me. Try replacing the center driver tube and/or other tubes. Used tubes with many hours on them will lose output at the top and bottom of the frequency scale ..

    If properly built, the VTA ST-70 amp has plenty of bass.

    Bob

    Blitzen

    Posts : 47
    Join date : 2009-07-10

    Re: ST-70 just finished - very little bass

    Post by Blitzen on Fri Oct 11, 2013 9:00 am

    I'm assuming you didn't mean 12AX7's? That's not the right tube for that amp...

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 just finished - very little bass

    Post by Bob Latino on Fri Oct 11, 2013 9:11 am

    Blitzen - I guess that I read right over the "12AX7" in the first post ... Yes - 12AX7's are the WRONG driver tube for that amp. You need 12AU7/12BH7 type tubes. A 12AX7 has too much gain to be used as a driver tube in that amp.

    Bob

    fmorelli

    Posts : 16
    Join date : 2013-08-27

    Re: ST-70 just finished - very little bass

    Post by fmorelli on Fri Oct 11, 2013 2:04 pm

    Well it would be helpful if I actually provided the correct information Smile.  I have 12au7's not 12ax7. So we are okay there. These are all brand new, matched tubes, so nothing vintage or worn out.

    I will try driving it with a CD first. If that's not the issue, I'll set it up for 4 ohm. Report back soon. Thanks for everyone's input!

    Filippo

    fmorelli

    Posts : 16
    Join date : 2013-08-27

    Re: ST-70 just finished - very little bass

    Post by fmorelli on Fri Oct 11, 2013 6:37 pm

    I've decided I'd like to go ahead about put a DPDT On/On switch in the back of the chassis for 4/8 ohm switching. Any issue with doing this? Obviously one wants to switch this only when the power is off ...

    Filippo

    Bob Latino
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    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 just finished - very little bass

    Post by Bob Latino on Fri Oct 11, 2013 7:15 pm

    Hi Filippo,

    Yes - no problem in using a DPDT switch for changing the two output transformer secondaries from 4 to 8 ohms.

    Bob

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-70 just finished - very little bass

    Post by peterh on Sat Oct 12, 2013 4:24 am

    fmorelli wrote:I've decided I'd like to go ahead about put a DPDT On/On switch in the back of the chassis for 4/8 ohm switching. Any issue with doing this? Obviously one wants to switch this only when the power is off ...

    Filippo
    Sorry bob, but i have the opposite opinion.
    Do not install a switch to select output circuit. It is a unneeded source of failure, many switches
    that stays in the same position and is subject to heating cycles may experience dust and
    oxidation product on the unused surfaces. Thus some day it might be an open circuit and
    subsequently risk your amp.

    If you will have the opportunity to change output circuit then install proper connectors
    ( like the ones dynaco used , or maybe like the ones audio research uses)

    Those "banana connectors" that seems to be popular at the moment suffers from no locking, any activity behind the amp may accidentally pull them out , thus risking your amp.

    Real men uses screw connections :-)

    Zimmer64

    Posts : 113
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Re: ST-70 just finished - very little bass

    Post by Zimmer64 on Sat Oct 12, 2013 5:41 am

    Hi there,

    I would suggest to use tripple posts. I bought the ones from tubes4hifi and never looked back.

    Best

    Michael

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 just finished - very little bass

    Post by Bob Latino on Sat Oct 12, 2013 6:41 am

    I have in the past use a switch to switch between two output impedances on a tube amplifier.. Worked fine with no issues. As Zimmer64 has mentioned, another alternative is to use the optional 3 post binding post that Roy has on his web page.

    Bob

    fmorelli

    Posts : 16
    Join date : 2013-08-27

    Re: ST-70 just finished - very little bass

    Post by fmorelli on Sun Oct 13, 2013 8:46 am

    Thanks you all. I prefer the triple binding posts and ordered those up. I'll report back once I've installed them. In the meantime, I continue to burn in the amp. We'll see if we can figure out what's causing the mystery bass roll off.

    Filippo

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: ST-70 just finished - very little bass

    Post by tubes4hifi on Sun Oct 13, 2013 3:28 pm

    your order going out when the post office is open again on Tuesday.
    Just make sure none of the connectors are touching the chassis.
    You can use the extra nuts provided to space the terminals away from the chassis.
    The PCB part should be on the outside of the chassis (same as Dyna original)



    fmorelli

    Posts : 16
    Join date : 2013-08-27

    Re: ST-70 just finished - very little bass

    Post by fmorelli on Tue Oct 15, 2013 10:40 pm

    I did go ahead and order the triple posts, which makes sense to set up with this amp, especially given my speakers.

    That said I discovered the issue, and pass it along should someone else search for this problem. Turns out the bias on the two front power tubes was off from original adjustment (I'm not sure what happened, but replaced multimeter batteries and went to manual setting rather than auto ranging). Anyway with bias set, it sounds exactly as it should.

    Thanks for sending the triple posts - I'll look forward to installing them.

    Filippo

    rockinrob86

    Posts : 18
    Join date : 2012-03-26

    Re: ST-70 just finished - very little bass

    Post by rockinrob86 on Thu Oct 17, 2013 8:13 pm

    I felt like I should add two things to this for future searchers...

    Bass Issues - I thought I was having these as well, and all of them ended up being something I had done. The two main ones were related to me using multiple 12au7s that I assumed one of which had to be good, along with the same issue with power tubes. I had a set of Mullard XF3 EL84s, the best EL84s made, which are known for having a very long life. They worked and biased up okay, but sounded thin. I had also bought a lot of 12au7 tubes listed as untested on ebay, which I think were actually rejects. Don't be cheap. Buy at least some decent tubes so you have a frame of reference.

    My other issue was in my Klipsch speakers. I damaged a tweeter when I was working on the finish, so I replaced them with some supposedly factory replacements from a third party vendor.

    They were either more efficient, or just screechy and annoying, which made me think the st-70 was light on bass. My other amp rolled off the treble so much that the tweeter wasn't noticeable.


    A correctly biased VTA ST70 is a fantastic sounding amp with plenty of bass (I listen to everything from Coltrane to the Beatles to Rage Against the Machine to Black Sabbath to George Jones). It is capable of shaking my house at 105db through my Klipsch Chorus II's without breaking a sweat.




    When you put in new tubes and bias them, the bias will drift as the tube gets older. This is why you should buy tubes that are matched AFTER a burn in period.

    When I set bias, I set it as soon as I turn the amp on so I dont blow anything up, then I play it for awhile (you can hear if there is an issue) and check it again after about 20 minutes, and then again after an hour.

    I check my st70 about once a month and adjust the bias to around 450 mv on my DMM, which I think translates to 45ma, but I am not sure of that...


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