Systemic Development > Bipolar System Disorders

what recent changes have you made to your system?

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steve:
Something I changed, actually testing right now is lowering the filament voltage and checking
tube life of the JJ E88cc tube. With today's tube prices, would like to obtain maximum tube life.

Lowering filament voltage too much will cause premature tube failure. I am thinking that the
ratio of actual idling plate current vs the maximum listed in the particular tube specs will
determine how low the filament can go. However, this could be wrong due to an assumption
I am making.

A 12ax7 has a maximum plate current of 9ma while typical idle is listed as 1.2ma.
or 13.3%

A 6dj8, E88cc has typical idle plate current of 15ma while maximum of 20ma, or
75%.

Both are running the plate current and other characteristics in the linear region.

I have seen an ARC SP-3 preamp running the 12ax7 filaments nearly dark with
low filament voltage as there is plenty of reserve cathode current capability according
to the specs in the manuals.

I am testing the JJ E88cc for optimum tube life at ~8.6ma, plate current each section
or 43% of the maximum listed plate current.

cheers

steve

Rewrote this post due to parts on hand for a small test chassis. The 11A preamp will take
much longer to evaluate while the small chassis will be on 24/7.

Nick B:

--- Quote from: steve on August 04, 2024, 09:26:32 PM ---Something I changed, actually testing right now is lowering the filament voltage and checking
tube life of the JJ E88cc tube. With today's tube prices, would like to obtain maximum tube life.

Lowering tube life vs filament voltage will most likely vary with descent tube type. For instance:

A 12ax7 has a maximum plate current of 9ma while typical is 1.2ma.

A 6dj8, E88cc has typical plate current of 15ma while maximum of 20ma.

Both are running the plate current and other characteristics in a nice region.

I have seen an ARC SP-3 preamp running the 12ax7 filaments nearly dark with
low filament voltage as there is plenty of reserve cathode current capability.

I am testing the JJ E88cc at low, and lower filament voltage to check for optimum
tube life. Too low and the cathode will be depleted and short life. 

cheers

steve

--- End quote ---

Fascinating test, Steve. Keep us posted
Nick

steve:
This is not a truly representative test as only 1 used JJ E88cc tube with two sections,
A, B are being tested. I will have to perform more tests with more JJ E88cc tubes for
better overall accuracy but should give us a good indication. Tube has been turned
off just a couple of times so far.

Test started on 8-11-24, measurements below are from 8-15-24.
Very early on, approximately 90 hours, the tube appears stable.

I did make a change from 2 watt to dual 12 watt plate resistors on 8-11-24
for increased heat related stability. Different RL resistance values A and B
are due to availability from the old parts box.

RL A are two Mills 12 watt resistors in parallel for 7.11K ohms
RL B are two Mills 12 watt resistors in parallel for 7.71K ohms

Voltages and currents are approximates.
           
Filament                  5.85 volts dc

B+ voltage              142
Plate A voltage        71.8 volts
Plate B voltage        63.8 volts

Plate A current         9.86ma
Plate B current         10.13 ma         

We'll see how the tube acts over time.

cheers

steve

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