VTA driver board - resistors over-heating?

by dyna70 Wed Apr 21, 2010 7:19 am

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Post n°1

VTA driver board - resistors over-heating?

by dyna70 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.

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

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Post n°2

Re: VTA driver board - resistors over-heating?

by Bob Latino 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

by dyna70 Wed Apr 21, 2010 2:02 pm

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Post n°3

Re: VTA driver board - resistors over-heating?

by dyna70 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

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

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Post n°4

Re: VTA driver board - resistors over-heating?

by Bob Latino 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

by dyna70 Thu Apr 22, 2010 11:43 am

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Post n°5

Re: VTA driver board - resistors over-heating?

by dyna70 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

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

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Post n°6

Re: VTA driver board - resistors over-heating?

by Bob Latino 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

by dyna70 Thu Apr 22, 2010 4:13 pm

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Post n°7

Re: VTA driver board - resistors over-heating?

by dyna70 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

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

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Post n°8

Re: VTA driver board - resistors over-heating?

by Bob Latino 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

by dyna70 Thu Apr 22, 2010 4:25 pm

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Post n°9

Re: VTA driver board - resistors over-heating?

by dyna70 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

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

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Post n°10

Re: VTA driver board - resistors over-heating?

by Bob Latino 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

by dyna70 Thu Apr 22, 2010 6:46 pm

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Post n°11

Re: VTA driver board - resistors over-heating?

by dyna70 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

by dyna70 Tue Apr 27, 2010 9:17 am

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Post n°12

Re: VTA driver board - resistors over-heating?

by dyna70 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

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

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Post n°13

Re: VTA driver board - resistors over-heating?

by Bob Latino 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

by dyna70 Tue Apr 27, 2010 11:10 am

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Post n°14

Re: VTA driver board - resistors over-heating?

by dyna70 Tue Apr 27, 2010 11:10 am

Cheers Bob!

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VTA driver board - resistors over-heating?