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 Tubes4hifi.com products and all Dynakitparts.com products


    VTA driver board - resistors over-heating?

    Share

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    VTA driver board - resistors over-heating?

    Post by dyna70 on Wed Apr 21, 2010 7:19 am

    Hi folks.

    I'm not 100% sure if the following (as the pictures I took of my VTA board the last time I used it (2+ years back) are not too clear), however I am sure that the following resistors have discoloured (gone darker) a little:

    R19 / R20 / R21 / R22

    Could this be from heat or is this an indication of some problem perhaps? Again I must stress not 100% sure if they HAVE discoloured since I last used the board, or perhaps these 4 resistors were from a different brand. But now it's set up again and sounding great I want to make sure it's all running smoothly.

    Cheers,

    - John

    EDIT: I believe all resistors on this revision of the VTA board (high-gain, purchased 2007) are 1/2 watt types.

    Bob Latino
    Admin

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

    Re: VTA driver board - resistors over-heating?

    Post by Bob Latino on Wed Apr 21, 2010 10:11 am

    dyna70 wrote:Hi folks.

    I'm not 100% sure if the following (as the pictures I took of my VTA board the last time I used it (2+ years back) are not too clear), however I am sure that the following resistors have discoloured (gone darker) a little:

    R19 / R20 / R21 / R22

    Could this be from heat or is this an indication of some problem perhaps? Again I must stress not 100% sure if they HAVE discoloured since I last used the board, or perhaps these 4 resistors were from a different brand. But now it's set up again and sounding great I want to make sure it's all running smoothly.

    - John

    EDIT: I believe all resistors on this revision of the VTA board (high-gain, purchased 2007) are 1/2 watt types.

    Cheers,

    John,

    R19, 20, 21 and 22 are 51K resistors and are part of the phase splitter/phase inverter circuit and look at about 300 volts each. Measure and make sure that they are 51K. What I would do is try a complete voltage check on the amp to see if there is some issue with the older original components on your ST-70 that could be affecting the VTA driver board in some way. See if you get roughly the voltages listed below.

    Bob

    NOTE - These ST-70 amp voltages are for a VTA board driven ST-70 - not a stock ST-70

    GZ34 – Pin 2 to ground – 420 - 450 volts DC
    Pin 8 to ground – 420 - 450 volts DC
    Pin 4 to ground – 355 - 380 volts AC
    Pin 6 to ground – 355 - 380 volts AC

    Any EL34 – Pin 1 or pin 8 to ground – approx .400 volts DC (depends on bias setting)
    Across pins 2 and 7 – 6.0 – 6.5 volts AC
    Pin 3 to ground – 400 - 435 volts DC
    Pin 4 to ground – 400 - 435 volts DC (2 or 3 volts higher than pin 3)
    Pin 5 to ground – minus 30 to minus 40 volts DC
    Pin 6 to ground – minus 30 to minus 40 volts DC
    Pin 8 to ground – approx .400 volts DC (depends on bias setting)

    Quad cap –-- Section # 1 (wire to pin #8 of rectifier) –------ 420 – 450 volts DC
    Section # 2 (faces the back of the amp) --- 420 – 445 volts DC
    Section # 3 (faces to the right) ------------- 420 – 445 volts DC
    Section # 4 (faces the front of the amp) ------ 380 – 420 volts DC
    Central 12AT7 – Pin 1 or pin 6 to ground > 140 – 165 volts DC
    Two outer 12AT7’s – Pin 1 or pin 6 to ground > 250 – 280 volts DC
    Any 12AT7 – across the two incoming heater wires > 6.0 - 6.5 volts AC
    “-50 VAC” on driver board > 48 – 55 volts AC
    “B+” on driver board > 380 – 420 volts DC

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    Re: VTA driver board - resistors over-heating?

    Post by dyna70 on Wed Apr 21, 2010 2:02 pm

    Thanks for this Bob - I'll run those checks.

    Should add that my Dynaco ST70 is basically brand new. I built it from scratch 2 years back using all new parts from Dynakitparts, with the only original/old items being the output transformers. The 240V mains transformer was purchased from Triode Electronics.

    The only changes between then and now are a new Authenticap quad-cap, and I have reinstalled the VTA board using better quality caps.

    The 51K resistors all measure within spec. I will check the voltages as you suggest. I know our local mains here hovers around 248V so that might be pushing it slightly.

    This will be the first time I've tested a valve amp 'on the job' so to speak. I assume it's OK working on its side. Do I test these voltages with shorting plugs inserted, or playing music?

    Many thanks,

    - John

    Bob Latino
    Admin

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

    Re: VTA driver board - resistors over-heating?

    Post by Bob Latino on Wed Apr 21, 2010 3:04 pm

    John,

    Have the amp on and at idle when you do your measurements. Don't play any music while you are measuring. With a line voltage at 248 VAC, most of the voltages that you measure will probably be a little higher than normal.

    Bob

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    Re: VTA driver board - resistors over-heating?

    Post by dyna70 on Thu Apr 22, 2010 11:43 am

    Thanks Bob.

    Just started testing, and when I have one probe on pin 1 of the central 12AT7, and then touch the other probe to the chassis I get a slight 'crackle' through the speaker - is this normal? Not having tested valve equipment before I guess I have the jitters with all those high voltages running around the circuit.

    Cheers,

    - John

    Bob Latino
    Admin

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

    Re: VTA driver board - resistors over-heating?

    Post by Bob Latino on Thu Apr 22, 2010 1:23 pm

    dyna70 wrote:Thanks Bob.

    Just started testing, and when I have one probe on pin 1 of the central 12AT7, and then touch the other probe to the chassis I get a slight 'crackle' through the speaker - is this normal? Not having tested valve equipment before I guess I have the jitters with all those high voltages running around the circuit.

    Cheers,

    - John

    John,

    Yes - you might get a little crackle when you touch the probe to a voltage point. Just be careful with the red probe that you don't accidentally cross two tube pins or the place you are testing with anything else.

    Bob

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    Re: VTA driver board - resistors over-heating?

    Post by dyna70 on Thu Apr 22, 2010 4:13 pm

    Thanks Bob - well have only had time to test the voltages on the 12AT7s, but two of them already look too high:

    Middle 12AT7 - Pin 6 to ground: 162 volts
    Left 12AT7 - Pin 6 to ground: 287 volts
    Right 12AT7 - Pin 6 to ground: 296 volts

    The middle seems fine, but the outer two are above the limits you mentioned above. Are these dangerously high? Wondering if I might be wise to construct a 'buck' transformer to shave off 12V or so from our 248/250V local voltage here.

    Cheers,

    - John

    Bob Latino
    Admin

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

    Re: VTA driver board - resistors over-heating?

    Post by Bob Latino on Thu Apr 22, 2010 4:18 pm

    dyna70 wrote:Thanks Bob - well have only had time to test the voltages on the 12AT7s, but two of them already look too high:

    Middle 12AT7 - Pin 6 to ground: 162 volts
    Left 12AT7 - Pin 6 to ground: 287 volts
    Right 12AT7 - Pin 6 to ground: 296 volts

    The middle seems fine, but the outer two are above the limits you mentioned above. Are these dangerously high?

    Cheers,

    - John

    John,

    Because your line voltage is a little high then sometimes the target voltages will be a little high. Those voltages are OK.

    Bob

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    Re: VTA driver board - resistors over-heating?

    Post by dyna70 on Thu Apr 22, 2010 4:25 pm

    Well that's a relief Smile I'll measure the under-chassis ones tomorrow.

    Other than the possible discolouration of those 4 resistors it's certainly playing well at present and bias on all 4 tubes remains stable.

    - J

    Bob Latino
    Admin

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

    Re: VTA driver board - resistors over-heating?

    Post by Bob Latino on Thu Apr 22, 2010 5:09 pm

    John,

    Those 51K resistors could be discolored and overheating a little because your voltage is a little high. They are 1/2 watt resistors. You could pull them and put some 51K 1 watt resistors in their place ?

    Bob

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    Re: VTA driver board - resistors over-heating?

    Post by dyna70 on Thu Apr 22, 2010 6:46 pm

    Yeah I was thinking that earlier today and was looking at the Kiwame 2W jobs - they seems to offer the most wattage in terms of relatively small size. Not sure if the leads will fit in the VTA boards hole though :s

    Still I prefer to deal with the problem at the source, so will wire up a 'buck' transformer to shave off 12V from our mains, and put this into an aluminium enclosure. Should solve the high voltage problem effectively. I think I'll still replace those four resistors though to be on the safe side. At some point I intend to buy another VTA circuit board, but this time will source the components myself, and will build the lower gain version. That's a way off in the future though.

    I haven't (yet) been able to address the AC balancing on my VTA board either - both are set to central position. Would these have any effect on the higher voltages I measured on the (Mullard NOS) 12AT7s, other than our high local mains? Bit concerned why the right 12AT7 was nearly 10V higher than the left one.

    For the AC balance I am going to transfer a 100Hz WAV file to my iPod, and set the volume to about a quarter and output that directly into the ST70. Hoping that will do the trick.

    - John

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    Re: VTA driver board - resistors over-heating?

    Post by dyna70 on Tue Apr 27, 2010 9:17 am

    Sorry to be a dunce, but how does one measure the -50V AC, and also the B+ voltage? Is it between those respective points on the circuit board, with the other probe on the chassis? It doesn't say in the voltage checking document you posted a few posts above.

    Have replaced those four 51K with Kiwame 2W resistors - have mounted them with a 1mm gap between resistor and PCB to aid cooling. Also have a 2 x 12V transformer on the way so I can construct a 'buck' transformer.

    Cheers,

    - John

    Bob Latino
    Admin

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

    Re: VTA driver board - resistors over-heating?

    Post by Bob Latino on Tue Apr 27, 2010 9:55 am

    Hi John,

    Measure 50 VAC at the point from where the RED/BLACK wire from the power transformer attaches to the driver board TO chassis ground.

    Measure B+ DC high voltage from any one of the 4 sections of the quad cap to chassis ground. The section connected with a wire to pin 8 of the rectifier tube will have the highest voltage. The others will be somewhat lower.

    Bob

    dyna70

    Posts : 25
    Join date : 2010-03-30
    Location : Hampshire, England

    Re: VTA driver board - resistors over-heating?

    Post by dyna70 on Tue Apr 27, 2010 11:10 am

    Cheers Bob!

    Sponsored content

    Re: VTA driver board - resistors over-heating?

    Post by Sponsored content Today at 4:48 am


      Current date/time is Sat Dec 10, 2016 4:48 am