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 products and all products

    VTA St-70 Low output volume in right channel.



    Posts : 1
    Join date : 2015-09-26

    VTA St-70 Low output volume in right channel.

    Post by AlexDr on Wed Jan 20, 2016 9:15 pm

    Recently I built an St-70 based off a vta driver board and have been having some issues with unequal volume between the right and left channels. I've been combing over this problem for a few weeks using the trouble shooting guides posted here and just reading various forums and have failed to eliminate the problem. Let me give some specs on my amp and then ill get into the details of how far Ive gotten into the trouble shooting.
    Drive board is a vta st-70 board using 6sn7gt tubes. I bought a new power transformer and output transformers for the amp and am using dual 5ar4 rectifiers with an under chassis quad cap mod.

    That was a few months ago and I've made some changes as far as adding heat shrink to the output caps so they cant come in contact with the drive board as the upgraded output caps are kind of a tight fit. Additionally I've changed the output transformers since that picture was taken hoping it would fix this problem.

    Currently all the trouble shooting voltages are nominal and both sides bias correctly. Additionally the cathode resistors on all sides checked out ok. b+ runs around 410 volts and im getting correct voltages from the power transformer at each of the output tubes. Additionally I've tried various sets of output tubes and driver tubes including swapping them between the left and right channels. One thing I have noticed is that I'm getting cross-talk into the left channel from the right channel. I've been looking for bad solder connections on the drive board or any shorts to the chassis or bridged traces on the drive board but have yet to find any. I even went as far as re heating most of the solder joints on the drive board and output sockets to eliminate a faulty solder connection. Maybe someone here has encountered this problem before and has some tips for troubleshooting that aren't in the quick trouble shooting guide. I love the amp so far I am really hoping I can fix the unequal volume issues. I've got a tectronix 465b oscilloscope and signal generator I recently acquired but have yet to set it up and really dive into the amp. Considering most of the parts are new and I've checked the majority of the connections to the transformers and tube sockets multiple times the problem must be on the driver board somewhere. This is my first tube amp build so any advice is greatly appreciated.


    Posts : 63
    Join date : 2015-07-11
    Location : UT, USA

    Re: VTA St-70 Low output volume in right channel.

    Post by vtshopdog on Thu Jan 21, 2016 1:47 am

    To me, crosstalk issue says look at your inputs where signal level is lower.

    The photo is fairly fuzzy in my viewer, apologies if I'm not making out your wiring correctly: Are the two looping yellow wires your L&R input leads?  Is there a reason one channel (left, I presume) lead is looped and routed under what looks like two red jumper wires?

      Current date/time is Thu Oct 27, 2016 8:29 pm