VHT deliverance D120

  • Thread starter Thread starter Michiel Kemper
  • Start date Start date
311splawndude said:
I didn't click on any links in this thread but this is what I came to say. You need your plate voltage first as that can vary. @Michiel Kemper if you've biased amps before as you said then grab your plate voltage and then back into your cathode milliAmp target either by % or by ear. Ear is better honestly, but don't play the amp with a probe attached (assuming your are using a probe), your volume should actually be on zero.

I wrote up a tutorial for EL34 based amps but the math works the same. Just replace the EL34 25w with whatever tubes you are using.



MilliVolts is a constant that we captured earlier. (475 was my plate voltage)
Cathode bias is a variable that we will be adjusting.
Plate dissipation % we decide on up front but only as a guide. (60%)
Tube wattage is a constant. EL34s are 25 watts each. (25w)

If you take your tube watts and divide it by your plate voltage you will get your cathode reading in milliAmps. You then take this number and multiply it by your desired plate dissipation %. This will give you your cathode bias 'ball park' starting point. We will adjust the variable resistor up or down from this number but more importantly - we will adjust it to the best tone.

Here is the formula with my readings using EL34 tubes:

------------
25 watts per EL34 tube / 475 plate voltage = .052 amps
so 52 mA x .60 plate dissipation = .031 or 31 mA

bias amp to 31 mA to start
-----------

So, as you can see - I want to shoot somewhere in the 30 - 34 mA range.


(I made a few adjustments to call our EL34). Honestly not sure what your are but easy to find out.)
Click to expand...
I was thinking the same thing .... if he's installed bias pots and done other amps ... then ???

all he has to do is change the 25 watts to 35 watts in the equation for the KT88 or 6550
 
311splawndude said:
I didn't click on any links in this thread but this is what I came to say. You need your plate voltage first as that can vary. @Michiel Kemper if you've biased amps before as you said then grab your plate voltage and then back into your cathode milliAmp target either by % or by ear. Ear is better honestly, but don't play the amp with a probe attached (assuming your are using a probe), your volume should actually be on zero.

I wrote up a tutorial for EL34 based amps but the math works the same. Just replace the EL34 25w with whatever tubes you are using.



MilliVolts is a constant that we captured earlier. (475 was my plate voltage)
Cathode bias is a variable that we will be adjusting.
Plate dissipation % we decide on up front but only as a guide. (60%)
Tube wattage is a constant. EL34s are 25 watts each. (25w)

If you take your tube watts and divide it by your plate voltage you will get your cathode reading in milliAmps. You then take this number and multiply it by your desired plate dissipation %. This will give you your cathode bias 'ball park' starting point. We will adjust the variable resistor up or down from this number but more importantly - we will adjust it to the best tone.

Here is the formula with my readings using EL34 tubes:

------------
25 watts per EL34 tube / 475 plate voltage = .052 amps
so 52 mA x .60 plate dissipation = .031 or 31 mA

bias amp to 31 mA to start
-----------

So, as you can see - I want to shoot somewhere in the 30 - 34 mA range.


(I made a few adjustments to call out EL34). Honestly not sure what yours are but easy to find out.)
Click to expand...
Thanks that helps a lot!
 
Rocker68 said:
If your biasing KT88 tubes in the D 120 and your plate voltage is 500 VDC ,
( let's just say , )
what ma would you dial in for
a 60% idle condition ? What maximum watts plate dissipation is used in your formula ?
Click to expand...
So, when I first learned to bias I was measuring PV, loaded/unloaded etc and using the formula...but, eventually I learned that typically, in most amps there's a safe 'range' to bias in and I experiment with moving the number within that range, and pick the best sounding spot. So, in most vintage Marshalls pre 1990 the range is usually within 30-40ma. I experiment in that range, and if I don't hear much difference at 30 vs 40ma, I'll leave it at 30ma to save a little tube life vs biasing them hotter.
Hope that makes sense. With the VHT stuff, that message on the PCB tells you exactly what they recommend biasing 88s at. I did contact them on a pair of KT 90s I wanted to use, and they responded to use the same range as 88s.
 
Racerxrated said:
So, when I first learned to bias I was measuring PV, loaded/unloaded etc and using the formula...but, eventually I learned that typically, in most amps there's a safe 'range' to bias in and I experiment with moving the number within that range, and pick the best sounding spot. So, in most vintage Marshalls pre 1990 the range is usually within 30-40ma. I experiment in that range, and if I don't hear much difference at 30 vs 40ma, I'll leave it at 30ma to save a little tube life vs biasing them hotter.
Hope that makes sense. With the VHT stuff, that message on the PCB tells you exactly what they recommend biasing 88s at. I did contact them on a pair of KT 90s I wanted to use, and they responded to use the same range as 88s.
Click to expand...

Yep

And once you lock in with your 'healthy' range you can a.) bias by ear [probe removed] and b.) order tubes in the range you want :yes:

What's funny is that when I helped @OldGtrGuy bias his VHT I either don't remember or didn't know to look at the PCB for info :lo:
 
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311splawndude said:
Yep

And once lock in with your 'healthy' range you can a.) bias by ear [probe removed] and b.) order tubes in the range you want :yes:

What's funny is that when I helped @OldGtrGuy bias his VHT I either don't remember or didn't know to look at the PCB for info :lo:
Click to expand...
Yeah I don't remember either and unfortunately any pics of the guts are long gone from that amp. Sounded great whatever the bias was set to.
 
OldGtrGuy said:
Yeah I don't remember either and unfortunately any pics of the guts are long gone from that amp. Sounded great whatever the bias was set to.
Click to expand...

I did find this though....

(and evidently, I have quite a few clips)

esAAlknl.jpg



Carry on TS - didn't mean to highjack :lol:
 
I just biased my Deliverance Series II last week with new KT88s. I ran into the same issue of not seeing any recommendations from Fryette. The newer PCBs also don't have any of the helpful info the older ones had for some reason. I just ended up getting plate voltage and using one of the online calculators to get a range, think I ended up at 52mA at 480v plate voltage.
 
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