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6HF5 testing

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Hello everyone,
Did some 6HF5 testing. Plate = 500v, G2 = 170v, G1 = -70v. P-P is 3.3KΩ.
So, initially had some problems getting Ib under control. I started with G2 at 215v and with -70v on G1, Ib for 2 tubes was at 160ma. So, with -70 on G1 and G2 at 170v, I get 42ma/tube.
As with previous testing, I am feeding the tubes with the driver from a Dynaco Mark III.
I have not been able to find any graphs for the tubes, If someone can point me in the right direction, that would be great.
Anyway, not a bad tube. I can't give any numbers as to max power or distortion, but they sound fairly decent.

Ray
 
Davorin, thank you for the curves.
So if I am reading this one correctly, 3.3KΩ is not a good P-P load for these tubes. I'm seeing 2.5KΩ as the better choice when the plate is 500V. Anyone else agree with me on this?

Thanks,
Ray
 

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So if I am reading this one correctly, 3.3KΩ is not a good P-P load for these tubes........Maybee 2,2-2,3 KOhm including SAG of PS.

That number sounded familiar so I used the SEARCH feature.

The 6HF5's met the red board in post #168. With 520 volts of B+, 150 volts on the screen, and a 3300 ohm load I got 125 WPC out of 6HF5's.

In post #218 I tested the same tubes with a 2500 ohm load. Power did not increase, distortion went up and redness appeared.

hi don, the 6HF5 spec shows a positive bias for G3, do we need to bias this grid positve also for audio?

Many sweep tubes allow for a small positive voltage on G3 to stop Barkhausen Oscillation. This is not needed in audio.
 
Rough estimate (no consideration of dist.) of power for a given loading in class B or AB (tube diss. limit and max current not considered) (and OT resistive loss not considered):
Vknee is plate voltage of knee for 0 V on g1 at the given screen voltage (top plate curve usually)
R is loading per OT side (so P-P Ohms is 4xR)

Watts = (B+ - Vknee)x(B+ -Vknee)/(2R)

You do have to check the tube dissipation then and current capability against the load line and also leave some safety margin on diss. Current required = Iknee = (B+ -Vknee)/R

so at 120V on g2 (from curves), Vknee is around 50 V, and with 500 V for B+, and 3300 P-P OT:
Watts = (500 - 50)(500-50)/(1650) => 122.7 Watts
Iknee required: 545 mA


and at 150 V on g2 Vknee is around 62.5 V [ 62.5V = (150V/120V)x50V) ], and with 520 V for B+, and 3300 P-P:
Watts = (520-62.5)(520-62.5)/1650 => 126.8 Watts (George got 125 W after OT losses)
Iknee required: 554 mA

Usually you want to increase the above Vknee a little (maybe 10 V) over the graph Vknee to avoid heavy screen grid heating if over driven, for safety.

A higher B+ and higher P-P Ohm OT will usually get you lower distortion. (again within the max tube limits and assuming such an OT is practical)
 
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looking at davorin's graphs, this tube can produce large plate currents at Vg2 of 120 volts, which brings me to thinking, maybe this tube is another candidate for screen grid drive....

@davorin,
do you have simmilar graphs for the 6KD6, 6LQ6, 6LF6, if you do, can you send me please?

thanks,
tony
 
Ok, so I made some changes to the bias supply. So:
Plate = 500.0v
G2 = 185.0v
G1 = -72.0v
Idle plate current = 75ma (2 tube)
Idle G2 current = 1.5ma (2 tube)
Transformer is still 3.3KΩ P-P

They sound pretty decent. I'm baffled by the fact that it takes -72v on G1 to get Ib at 75ma for 2 tubes. Testing 6LB6's under the same conditions, I think I needed -48v. Has anybody else ran into this?

Ray
 
The triode Mu of the 6HF5 is 3 versus 4 for the 6LB6, so one can expect roughly 4/3 more g1 volts per g2 volt with the 6HF5 than the 6LB6.

Do you really need 185 V on g2? This gives a 0V g1 plate curve knee at slightly over an amp [580 mA x(185/125)^1.5] . Lowering Vg2 will lower Vg1 bias proportionately.
 
Don,
Where do you think the screen should be for best operation with the 3.3KΩ transformer and the 500V on the plate? Would you still set the idle Ib to 60ma for 2 tubes and how long would I be in class A operation?
Did you get the response to your email?


Thanks,
Ray
 
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