Bias range Marshall 9005 poweramp

[JimSoprano]

Hi All,

I've got a Marshall, I want to bias, but the range is way to insufficient for my new JJ EL34II tubes. I can only get it to approx. 20mA

What is the best wat to extend the range?

Please find schematic and picture below.

[Stevem]

You have too much negative bias voltage that's keeping you from idling the outputs hotter.

We need to sink more of that unnecessary voltage to the chassis, to do such the value of R23 needs to be lowered.

First type placing another 56K 1/2 watt resistor across R23 and set the bias pot to the middle of its sweep range.
This will lower that total resistance from 56K to like 27K or so.
Next turn the amp on and see what you have for adjustment range now.
If now you have not enough voltage and the pots range will at allow you to now run the tubs cool enough then you will need to either change out R23 for a resistor of maybe 33K or play around with putting other resistors of high value then 56K across R23.

The rule of Thumb for resistors placed in parallel is that whatever the smaller value of the two resistors are your working with, the resistance you end up with will always be less then that smallest value resistor.

I need to ask about your old outputs your replacing.
What where they idling at?

I ask because if your new tubes are not idling at least as hot as your old ones then your new tubes are NO better then your old ones if the old ones where weak!

[pdf64]

Rather than the now typical 1ohm value, those are 10ohm cathode current sensing resistors. Are you sure you not looking at 200mA, rather than 20mA?

[JimSoprano]

You have too much negative bias voltage that's keeping you from idling the outputs hotter.

We need to sink more of that unnecessary voltage to the chassis, to do such the value of R23 needs to be lowered.

First type placing another 56K 1/2 watt resistor across R23 and set the bias pot to the middle of its sweep range.
This will lower that total resistance from 56K to like 27K or so.
Next turn the amp on and see what you have for adjustment range now.
If now you have not enough voltage and the pots range will at allow you to now run the tubs cool enough then you will need to either change out R23 for a resistor of maybe 33K or play around with putting other resistors of high value then 56K across R23.

The rule of Thumb for resistors placed in parallel is that whatever the smaller value of the two resistors are your working with, the resistance you end up with will always be less then that smallest value resistor.

I need to ask about your old outputs your replacing.
What where they idling at?

I ask because if your new tubes are not idling at least as hot as your old ones then your new tubes are NO better then your old ones if the old ones where weak!

The old tubes we're at approx. 38mA. But were very old and it had the origional marshall 12ax7's.

The amp is lacking power and punch imo. Maybe it's just it's design. Maybe rebuild one side to BE50 deluxe power amp. with half 12ax7 input stage. Currious to the VHT internals...

[JimSoprano]

Rather than the now typical 1ohm value, those are 10ohm cathode current sensing resistors. Are you sure you not looking at 200mA, rather than 20mA?

Yes I red 200mV, so 20mA

[JimSoprano]

You have too much negative bias voltage that's keeping you from idling the outputs hotter.

We need to sink more of that unnecessary voltage to the chassis, to do such the value of R23 needs to be lowered.

First type placing another 56K 1/2 watt resistor across R23 and set the bias pot to the middle of its sweep range.
This will lower that total resistance from 56K to like 27K or so.
Next turn the amp on and see what you have for adjustment range now.
If now you have not enough voltage and the pots range will at allow you to now run the tubs cool enough then you will need to either change out R23 for a resistor of maybe 33K or play around with putting other resistors of high value then 56K across R23.

The rule of Thumb for resistors placed in parallel is that whatever the smaller value of the two resistors are your working with, the resistance you end up with will always be less then that smallest value resistor.

I need to ask about your old outputs your replacing.
What where they idling at?

I ask because if your new tubes are not idling at least as hot as your old ones then your new tubes are NO better then your old ones if the old ones where weak!

On the new tubes are rated at 36mA according to the matches, it's writen on the tube cartons. No idea what this rating means?

I've now set bias to -40V, measured at negative side of C8.

[Stevem]

The ma rating on those new tubes was as tested per industry standards for the spec on those tubes which is not how your amp run's them .

That being said your -40 setting sound much more like what's needed because that schematic number of -45 had those tubes running way too cold.
This makes for no punch and terrible sound distortion if you drive the amp up into that range.

[JimSoprano]

The ma rating on those new tubes was as tested per industry standards for the spec on those tubes which is not how your amp run's them .

Hi Steve,

The sound wat pretty good from the new tubes. It sounded louder then the old tubes as well.

What do you think, the bias of the amp should be modded as the measurement on the cathode should be leading over the bias voltage at C8?

Or the new tubes are not good?

Sorry I've not much experience with these bias issues.

[Stevem]

I think now your ok.
That amp at idle likely has about 420 volts on pin 3 of the outputs.

There is a range of between 50% and 70% of the tubes wattage rating that is good/ safe to have them idle in.

With 420 volts the numbers look like this.

50% = 29 ma

60% = 35 ma

70% = 41 ma

Any bias setting above 70 % with your amps type of output stage will start to chew thru output tubes kinda fast depending on how loud you play it.