New Build - Express

[amplifiednation]

Why did you change the jumper wire? I would change as few things as possible, you are so close to getting the amp running

Those 1ohm resistors aren't going to change anything, I'm pretty sure they are an option just to help calculate bias.

I woudlnt test fire the amp with good tubes.

[T Wilcox]

He needs a decrease in neg bias voltage right?
he was reading -29 and is trying to achieve -33 which would be a decrease.
Also I wouldnt use NOS tubes to fire up, but I would also not want to use questionable tubes either.

Respectfully

Todd

[Picker]

Basically, I removed the jumper because that is what the build guide said to do. Not sure why I was getting no reading though.

Anyway, it will have to wait now till after work. I may try replacing the R29 tonight to see if that helps.

[ToneMerc]

Doesn't the -29 indicat that I am to low to be able to bias it properly?

-29 is close or what you should have with no power tubes installed to load down the bias supply. Once the power tubes are installed and draw current, you should be able to bias around at least -32 or so.

I would wire back how you had it. If you are really concerned, you could slightly up the value of the 47K resistor in the bias string.

See diagram for bias resistor /pin wiring

With a speaker load attached and the amp volume turned to "0", bring the amp out of standby. Using the DC mV setting on your VOM meter, measure voltage at pin 8 on each power tube.

[amplifiednation]

He needs a decrease in neg bias voltage right?
he was reading -29 and is trying to achieve -33 which would be a decrease.
Also I wouldnt use NOS tubes to fire up, but I would also not want to use questionable tubes either.

Respectfully

Todd

Yes I had it backwards

[ToneMerc]

Also I wouldnt use NOS tubes to fire up, but I would also not want to use questionable tubes either.

Respectfully

Todd

Same here, I have known good plain jane tubes that I call "junk tubes". These are always the first tubes that I install when bring an amp up.

[T Wilcox]

He needs a decrease in neg bias voltage right?
he was reading -29 and is trying to achieve -33 which would be a decrease.
Also I wouldnt use NOS tubes to fire up, but I would also not want to use questionable tubes either.

Respectfully

Todd

Yes I had it backwards

Well now that I think about it did you have it wrong or did I?
You said increase negative voltage. If you said increase voltage that would be wrong. So I recant


Todd

[amplifiednation]

He needs a decrease in neg bias voltage right?
he was reading -29 and is trying to achieve -33 which would be a decrease.
Also I wouldnt use NOS tubes to fire up, but I would also not want to use questionable tubes either.

Respectfully

Todd

Yes I had it backwards

Well now that I think about it did you have it wrong or did I?
You said increase negative voltage. If you said increase voltage that would be wrong. So I recant


Todd

I was thinking that putting more volts into the bias circuit would result in a supply/reserve of more negative voltage. I'm still learning...

[Firestorm]

I think you want to get the bias voltage more negative before you start putting tubes in V4 and V5. You don't want to redplate a new one ...

With the amp unplugged and caps discharged measure the resistance from the junction of your 220K grid load/grid return/bias feed resistors to ground. Make sure you can adjust the bias pot to get up to 25K resistance. If not, you may have misidentified the wiper and wired the pot so the max resistance is only 12.5K. Or the pot could be screwy.

-- Edit: I should have said make sure you can get max resistance up to 72K since R31 is in series to ground.

[ToneMerc]

I'm still learning...

Me too, every build no matter what I usually will learn something.

TM

[Picker]

I didn't change the configuration back to the way I had it yet, but I did turn on the Power and Standby and take a reading at pin 5. I get 29.3v

Should I go ahead and change the R31 to a different value?

[Zippy]

He needs a decrease in neg bias voltage right?
he was reading -29 and is trying to achieve -33 which would be a decrease.

I would call that an increase in the magnitude of the negative voltage.

[T Wilcox]

So was I positively wrong or negatively right?

[Firestorm]

I didn't change the configuration back to the way I had it yet, but I did turn on the Power and Standby and take a reading at pin 5. I get 29.3v

Should I go ahead and change the R31 to a different value?

Maybe. What's the maximum resistance you can read across the pot? Should be very nearly 25K.

[Picker]

It presumably was about 25k. I don't remember now. I would have to take it all the way out to check wouldn't I?