ST120 B+ low

I have a fully upgraded ST120 with auto bias board.

If reading correctly I should be able to measure the B+ from the top of the board and it should range 375-410v DC

When plugged directly into wall with a WS1 copper cap in, B+ only reads 360. I have a variable voltage regulator. Should I run ST120 thru it to raise it up to 400?

what is your wall voltage and where exactly are you measuring B+, where in enters the driver pcb?

erhard-audio wrote:what is your wall voltage and where exactly are you measuring B+, where in enters the driver pcb?

Wall voltage is ~122

Measure from the top of the amp/board in middle. Just below the triode/pentode switches

OK, well yes, it should read a bit higher. I assume you are using a tube rectifier, have you tried a different one?
Any change in the sound etc of the amp?
360VDC is not a disaster but it should be closer to the spec voltages, I like to see the high 300's, around 390 or so for the ST120

erhard-audio wrote:OK, well yes, it should read a bit higher. I assume you are using a tube rectifier, have you tried a different one?
Any change in the sound etc of the amp?
360VDC is not a disaster but it should be closer to the spec voltages, I like to see the high 300's, around 390 or so for the ST120

Actually using a WS1 solid state copper cap so another reason i thought it would be higher.

Seems odd I’d have to crank the voltage up to 125+ to get a higher B+ voltage huh?

indeed it does. It should read quite a bit higher with the WS1. Do you have a rectifier tube you can try out just to see if it makes any difference?

erhard-audio wrote:indeed it does. It should read quite a bit higher with the WS1. Do you have a rectifier tube you can try out just to see if it makes any difference?

Yeah I have 2. I’ll give them a try later.

Outside of the low B+ to me it sounds great but this amp is new to me so not sure what it “should sound like” at higher B+

So just tested with a tube rectifier and it was lower than the WS1. Around 350.

So I put the WS1 back in. If I ramp of the voltage to 126, the B+ measures around 390.

I tested the output of the voltage regulator with multimeter first and They are only off by about 1v

Ref … https://dynacotubeaudio.forumotion.com/t4141-latino-st120-with-vta-sp9-ph12

What's the deal with the extra 3k9/390R? resistor and diodes across the HT rectifier on your amp Kray? Sort of hidden SS rectifier setup. Not quite a "yellow Sheet" modification where the diodes are located.

Also assume that the large wire wound resistor on the quad cap feeds the front displays. Could this be pulling down the HT voltage you are measuring? All depends if its prior or post the dropper resistor on the quad cap. Not too clear from the photo.

If this is suppling the front meters and is dropping the full HT (550v) down to 12v through 25k then this is running at maximum dissipation if a 10w component. ((500 * 500) / 25000) = 10W. A lot of wasted power in heat.

VERY CAREFULLY Check the HT at the quad cap with the two linked pins or the gold banded end (towards the rear) of the extra resistor on the tag strip with the red wires. Only one hand at a time near the amp when you're doing this and the other behind your back!

I'd also be wary of upping the supply voltage too far as you'll risk overrunning the tube filaments not to mention sky rocketing the main HT.

On a side note I wonder if twisting the cathode and grid wires together from the AB board has any effect? Fortunately both lines are relatively low impedance. Looks like the original amplifier was fairly neatly built and then the "meter" guy got involved

For the driver board on my amp I too have a voltage slightly lower than spec and this is measured after the dropper resistor on the quad cap. Makes little difference. But... if it's working ok and you're enjoying the sound why not leave alone and stop worrying..

Steve

OK - I will ask the so-far-unasked silly question:

How accurate is your VOM? For B+ it is measuring DC.
For the wall plate, it is measuring AC. If it is a True RMS meter, it should be reasonably accurate on both. If it is not, then the DC readings may be 'off' as much as 10%, and the AC readings will be dependent on the underlying frequency.

Is your meter capable of measuring AC over DC (few will do this at all, fewer well)? If so, see if there is an AC component on your B+.

Please let us know.

before you do ANYTHING, check to make sure you have the correct dropping resistor on the (quad cap) which should be 2.2K.
You don't want to crank up the line voltage input and find out you have 600vdc on the tubes and 400vdc to the driver board.
B+ with an SS rectifier should be around 450-480vdc. A 2.2K 3w resistor to the driver board should drop around 55vdc (25ma).

tubes4hifi wrote:before you do ANYTHING, check to make sure you have the correct dropping resistor on the (quad cap) which should be 2.2K.
You don't want to crank up the line voltage input and find out you have 600vdc on the tubes and 400vdc to the driver board.  
B+ with an SS rectifier should be around 450-480vdc.   A 2.2K 3w resistor to the driver board should drop around 55vdc (25ma).

I bought this amp from someone who said they bought it fully built direct from Bob.

So I was emailing Bob, and he mentioned this: "The B+ DC voltage on the driver board is controlled by the 4.7K 3 watt resistor on the quad cap."
So maybe that explains the lower B+ value?

pichacker wrote:What's the deal with the extra 3k9/390R? resistor and diodes across the HT rectifier on your amp Kray? Sort of hidden SS rectifier setup. Not quite a "yellow Sheet" modification where the diodes are located.

Also assume that the large wire wound resistor on the quad cap feeds the front displays. Could this be pulling down the HT voltage you are measuring? All depends if its prior or post the dropper resistor on the quad cap. Not too clear from the photo.

Also I was in there yesterday to take a bunch of measurements all at 120v (which all fall into normal range, except the quad #4 which mirrors the B+).

RECTIFIER:
- pin 4 = 408
- pin 6 = 411
- pin 2 = 500
- pin 8 = 510

QUAD CAP:
- 1 Square = 500
- 2 Half Circle = 492
- 3 Circle = 492
- 4 Triangle = 350

So, now I am confused. My kit came with supposedly a 4.7K ohm 3-watt resistor for the quad cap. I didn't check the resistance value at the time of construction. I am using a WZ68 Copper Cap rectifier and a bucking transformer to reduce my wall voltage of about 122 V to about 117 V. I just measured the B+ for my VTA ST120 at the driver board and get only 355 V DC. Do I need to change out the quad cap resistor?

from memory, the ST-70 uses a 2.2K 3W resistor and the ST-120 uses a 4.7K 3W resistor, unless those values have been changed in the meantime.
But it sounds as though, at least for the ST-120, that resistor value needs to be dropped as previously advised.

yes, the ST70 uses a 2.2K and the ST120 uses a 4.7K, and the voltage to the board isn't critical, it will work fine from 350-400vdc.
But if you want slightly more voltage, then replace the 4.7K with a 3.9K OR put a 27K 1 or 2w in parallel with the existing 4.7K
But as is 360vdc is perfectly fine . . .

tubes4hifi wrote:yes, the ST70 uses a 2.2K and the ST120 uses a 4.7K, and the voltage to the board isn't critical, it will work fine from 350-400vdc.
But if you want slightly more voltage, then replace the 4.7K with a 3.9K OR put a 27K 1 or 2w in parallel with the existing 4.7K
But as is 360vdc is perfectly fine . . .

Thanks Roy. Bob mentioned the same.

What exactly would the benefit be for running more B+ voltage to the board in this scenario? Sounds like making any changes wouldn't have much of a difference on sound/output?

OK, I won't obsess about this, but next time I have the bottom off, I'll tack a 22K 2-watt resistor on the quad cap and recheck the B+. Thanks for the information.