Special VH4 bias question

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elfredoo

elfredoo

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hi, i got a special question regarding biasing a vh4.

there are two ways to bias:
a) measuring the mA of a tube pair at the OT
b) measuring the mV (1 Ohm resistor -> 1mV^=1mA) of each tube with bias rite.

i did both and got these results:
a) OT1: 56mA, OT2: 58mA
b) 37mA, 35mA, 33mA, 31mA

i think that OT1 is for inner tube pair, OT2 for outer tube pair. if i add up the single tubes it would be: INNER 68mA, OUTER 68mA, but that's different from the OT readings above.

QUESTION a) Why do they differ?
QUESTION b) Which way is better to bias the tubes?


PS: I measured all pins' resistances: all 1ohm but one 2ohms since it has the 1 ohm resistor in between. when I measure the cables coming from the bias rite I get 1,5 ohms.
 
I've used the shunt method, utilizing a multimeter at the wires intersection on the underside of the chassis. I have a Bias-Rite, but haven't used it on my VH4 or Hagen (yet). I would imagine the next time I go to swap tubes and the like, I'll be using the Bias-Rite and adjusting with the trim-pot.

Perhaps Peter can chime in about your question of mA discrepancies.

Cheers,
Mo
 
Ventura":1upcpv05 said:
I've used the shunt method, utilizing a multimeter at the wires intersection on the underside of the chassis. I have a Bias-Rite, but haven't used it on my VH4 or Hagen (yet). I would imagine the next time I go to swap tubes and the like, I'll be using the Bias-Rite and adjusting with the trim-pot.

Perhaps Peter can chime in about your question of mA discrepancies.

Cheers,
Mo
Click to expand...


Sure glad my herbert has external bias points and is super duper easy to bias :lol: :LOL: :lol: :LOL:

No but seriously that is kinda odd... Depends on the circuit and where peter says it should be biased at. I haven't heard of anyone biasing using readings from the OT... Hope he has something to say about it and it helps you out!

-Alex
 
Just three reasons for the differences . . .

a) firstly you are reading the anode (direct, OT), secondly the cathode which incl. screen currents (some mAs diff., cath currents are always higher then plate/anode current, they have to)
b) the inner resistance of your multimeter (depends on quality) may be too high so you get diff results reading current (OT method)
c) the 1 Ohm Rs could have more or less Ohms, so you get the wrong reading here (Bias rite method)
 
i have a 100€ multimeter (beha unitest hexagon 110)

all bias rite socket pins have 1.0 ohm, one pin has 2.0 ohms.

so what is the better/more precise method to bias? how is it done at diezel hq?
 
Personally I'm using both - adding about 10% when adjusting the idle current by reading the cathode.
 
We adjust second time after burn in.

70mA is for a pair at exactly 230 volt mains.

I´m watching the main power while adjusting.
If the main voltage is 235 volt I add 1,5 mA
for every single volt, so I adjust to 77,5 mA

In most cases there is a weak main voltage
in the practise rooms or on stage. In this case
the the bias current would not be lower than
60 mA at 220 volt.

Anyway, you just have to know that the bias
circuit voltage is not proportional to the plate
voltage which depends on the main voltage.
 
ok, so diezel hq uses the OT-method with additional precise main voltage readings. thanks for the detailed answer! but like duesentrieb said, you have to use good equipment.
 
No ... we are using the cathode measurement.

If you are into the OT measurement follow
Uncle Duesis insructions.
 
ah ok...thought because of the pair you were talking about. sorry :aww:
 
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