The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


3 posters

    VTA MK3 Oscillation. NFB?

    avatar
    MHProd


    Posts : 7
    Join date : 2022-08-31

    VTA MK3 Oscillation. NFB? Empty VTA MK3 Oscillation. NFB?

    Post by MHProd Wed Jan 11, 2023 8:42 pm

    I built 4x VTA MK3s (quad amping) from scratch a few months ago. I posted a thread when I had a phase issue I couldn’t figure out… turns out out of the 4 transformers, 2 had the secondaries taps reversed, the colour coding was wrong… that’s why it took me forever to figure it out.

    Anyway, since then the amps have been working ok until I did some tube rolling. If I use a 12BH7 or E80CC, I started to get an oscillation. While monitoring the bias, I notice that if I turn the volume up on my preamp past a certain point, the bias goes up by about 10% and the amp oscillates. If I turn down the bias, the “pitch” of the oscillation lowers and then disappears.

    So it’s like an input sensitivity. The 12BH7 and E80CC have a bit more gain but even with a 12AU7, if the volume is loud enough, the amps will oscillate.

    I realized I neglected something about the NFB and I wonder if this could be the issue: the schematic takes the NFB (6.8K/680pf) to the 16ohms tap. The OTs I use has a single 6ohms secondary tap (and the speakers are 4ohms).

    I soldered a 8k2 in parallel with the 6k8/680pf network to bring it down to 3K9 ish but now as soon as the the tubes light up, the oscillation appears right away with no signal at the input...
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1791
    Join date : 2008-11-30

    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by tubes4hifi Thu Jan 12, 2023 2:58 pm

    you really need an oscope and a sig gen to get the NFB correct,
    but try 10K and 220pf instead of the orginal
    the resistor controls the gain, the capacitor controls the high frequencys and square wave response
    avatar
    MHProd


    Posts : 7
    Join date : 2022-08-31

    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by MHProd Mon Jan 16, 2023 4:55 pm

    Thanks Roy! I only changed the resistor to 10K and it works fine with 12AU7s and 12BH7s. Unfortunately, still seem to oscillate a bit with E80CC although I'm not sure... it sounds more like a hum?

    Otherwise, would lowering the capacitor to 220pF brighten the sound a bit? Unfortunately, I wouldn't know how to use a scope... Thanks!
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1791
    Join date : 2008-11-30

    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by tubes4hifi Mon Jan 16, 2023 5:30 pm

    I don't have a MK3 amp here to try anything, so it's all trial and error without an amp, scope, and sig gen
    avatar
    MHProd


    Posts : 7
    Join date : 2022-08-31

    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by MHProd Sat Jan 21, 2023 6:36 pm

    Regarding the E80CC, I think I'm getting the issue because it's biased too hot. If I'm reading the schematic correctly, the cathode resistor is 780r (680+100), right? If I change R6 to something like 680, the 1K36 should be good as far as bias. I'm just not sure I'm reading the schematic properly...Or should I change R10 to 1K2 instead of R6?
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1791
    Join date : 2008-11-30

    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by tubes4hifi Sat Jan 21, 2023 9:34 pm

    not sure why you're having trouble with an E80CC tube unless it has some defect, as it has the same operational characteristics as a 12AU7 except
    that it uses twice as much filament current. R6 should remain 680 ohm and R20 should remain 20K unless you are measuring voltages at plate and cathode that are more than
    10% different than nominal in the VTA circuit (140v plate, 0.47v cathode)
    R10 affects the percentage of NFB and shouldn't be a factor.
    There are tons of great quality 12AU7 tubes out their, so why mess with something that works?
    avatar
    MHProd


    Posts : 7
    Join date : 2022-08-31

    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by MHProd Sat Jan 21, 2023 11:00 pm

    I just have a few of them and I really like how they sound. In previous builds where I used them (not MK3s), I remembered having to use a higher cathode value, like 1k2, 1k5 otherwise it would distort… it has a higher mu than 12au7 (17) vs 25 with the 80. I’ll check the voltages.

    I’ve just been spending time tube rolling and coming up with pretty interesting results, using 4 amps really expands what can be done between balancing highs and lows. I’ll post the results of that in another thread once I’ve figured out the 80 thing.
    avatar
    sayntjack


    Posts : 10
    Join date : 2020-11-26

    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by sayntjack Tue Jan 24, 2023 12:21 pm

    Hi MHProd,

    You say, "The OTs I use has a single 6ohms secondary tap (and the speakers are 4ohms)." So, you are not using the original Dynaco transformer design. This could have something to do with your problem.  The Dynaco transformers are very high quality and less prone to ringing.  With the original Dynaco transformers in my upgraded driver board MkIIIs the 10kHz square wave response was very respectable.  The same amp fitted with the Edcor "equivalent" transformers rang / oscillated severely on a 10kHz square wave. Not only did I have to play with the global negative feedback I had to add some local damping upstream to solve the issue.  Note also that not all oscillation can be heard.  You may think that you have solved the problem but then start experiencing tweeter failures at higher volumes.  So, as mentioned in this thread, an oscilloscope (and audio frequency generator) are essential in troubleshooting this kind of problem. Otherwise you are pretty much shooting in the dark.  

    Jack

    Sponsored content


    VTA MK3 Oscillation. NFB? Empty Re: VTA MK3 Oscillation. NFB?

    Post by Sponsored content


      Current date/time is Thu Feb 02, 2023 1:10 pm