Metropoulos Forum

MOVED TO BEGINNER HELP SECTION.

v1 and v2 both 1 at pin 3 is ok. Your current from v4 to v7 seems to be a little low.

vanhalen5150 wrote:My v1 and v2 read both 1 at pin 3. I wouldn't worry about it. Your current from v4 to v7 seems to be a little low.

My tranny is only rated at 460 B+ voltage. Wouldnt this make my voltages right on?

Yeah, it would, but I'm thinking your bias would be a bit higher. Other than that, everything looks ok.

Yeah, I just checked it again and the neg bias voltage is right about -50 when in the middle, 46-54 is the trim pot sweep. 0- 22mA is way off. The formula says I need 38mA

vanhalen5150 wrote:Yeah, it would, but I'm thinking your bias would be a bit higher. Other than that, everything looks ok.

Why use the 68k instead of the 56k resistor?

As I was building, I saw many with a 68K resistor including the Billy Batz layout in the forums. I am going to change it out just to see what happens, it could even be bad.

vanhalen5150 wrote:Why use the 68k instead of the 56k resistor?

I just swapped to a 56K and it got better but still sweeps from -50 to -42. I may try a 47k to get it the -37mA or -38mA.

Start with 27k or 33k and go up. I needed the 68k in my 12xxxx because it was the only way to get the bias hot enough while dropping the plate voltage down to around 380 with a variac.

I believe I will re-illustrate my numbers as I may have been a bit confused in my expalination. The trimmer pot should have a wider range (-50 to -30 approx). Mine ranges from -53 to -46 with a 68K resistor across the capacitor and pot. I changed this resistor to a 56K and now the hottest setting reads -42. Only sweeps about 8-10 values, not 20 like the instructions. When reading PIN 8 with the 68K across the cap and pot, I read 15-18mV. With the 56K installed across the pot and cap I read 22mv. The dropping resistor at the diode is still a 27K.

Does your PT have its own bias tap, or is it derifved from the B+? If it has it's own tap, the only resistor you should be changing from stock is the one feeding the diode(27k?). If the bias is derived from the B+, you will have to change other resistors.

Yes, my MM tranny has 2 bias tap wires (GRN and White) White goes to the 27K resistor and green goes to ground on the PT mounting bracket. My question would be "is this totally common to change the 27K up and down to achieve this? Sorry for being an idiot...lol!

vh junkie wrote:Does your PT have its own bias tap, or is it derifved from the B+? If it has it's own tap, the only resistor you should be changing from stock is the one feeding the diode(27k?). If the bias is derived from the B+, you will have to change other resistors.

Are you refering to changing the dropping resistor(27K) or the 68K across the bias pot and capacitor?

Strat78 wrote:Start with 27k or 33k and go up. I needed the 68k in my 12xxxx because it was the only way to get the bias hot enough while dropping the plate voltage down to around 380 with a variac.

wrea398 wrote:Yes, my MM tranny has 2 bias tap wires (GRN and White) White goes to the 27K resistor and green goes to ground on the PT mounting bracket. My question would be "is this totally common to change the 27K up and down to achieve this? Sorry for being an idiot...lol!

vh junkie wrote:Does your PT have its own bias tap, or is it derifved from the B+? If it has it's own tap, the only resistor you should be changing from stock is the one feeding the diode(27k?). If the bias is derived from the B+, you will have to change other resistors.

I guess that would depend on the variance of raw AC bias voltage from PT to PT... if the bias has it's own tap you only need to change the 27k resistor upward to make the bias hotter... the other parts can stay stock.

Thanks for your help. I will leave the 56K for now on the trimmer and install a differnt dropping resistor value. We will see what happens.