6SN7 load resistance

[boroman]

I have spent over an hour to find load resistance of 6SN7. No luck. Can't find it in the data sheets. In terms of other parametres I find it pretty similar to 12AU7 - which in 5F1M circuit by Rob Robinette where he uses 25k primary OT with approx 320V on the plates.

The reason I want to use 6SN7 vs 12AU7 for output is just simple - I have like 10pcs of those tubes and not a single 12AU7, I like to use parts that were going to trash. Also I will have a bit lower voltage there because I am using 200V PT so will have approx 250V on the plates. Also have few OTs here, so I'd need to choose the right one...

[g1]

See pg.1 of attached, under class A amplifier, 250V

[Helmholtz]

See pg.1 of attached, under class A amplifier, 250V

The value from the datasheet is the tube's internal plate resistance, not a recommended load.

[Helmholtz]

A tube doesn't "have" a fixed load resistance.
Optimum load depends on tube limits, circuit parameters, load type (resistive/inductive) and application requirements.

You can use this calculator to play with different values: https://www.vtadiy.com/loadline-calc...ne-calculator/

[g1]

The value from the datasheet is the tube's internal plate resistance, not a recommended load.

Ok, I wasn't 100% sure which he was looking for.
pg.2 of the datasheet I posted shows 100K for 300V operation. The sheet Tom attached shows 50K for phase inverter application at 250V.

But I think we are talking about using the tube as a power amp, so should not the 'plate resistance' number be used to match with OT ?

[Tom Phillips]

I re-did this reply because I found more relevant data to post and when I tried to edit my original post everything got scrambled beyond repair.

Attached is another version of the 6SN7 data which I was copying as g1 was posting.
This data came from a 1946 Sylvania Tube manual.
Note that the 6SN7 page says to refer to type 7N7, which is identical electrically, for detailed data. That's why the 7N7 pages are included
HTH,
Tom

[Helmholtz]

But I think we are talking about using the tube as a power amp, so should not the 'plate resistance' number be used to match with OT ?

Calling the internal plate resistance rp, tube literature recommends an external load (OT primary impedance) Za = 2...4 rp for a triode power amp with transformer load.
Not that rp varies with idle current.

I think a Za around 20k should be fine.

[g1]

I think a Za around 20k should be fine.

Which ends up close enough to the Robinette example from post #1.

[boroman]

Thank you all! So it's like 12AU7 -> what wanted to hear :-)

[boroman]

Maybe it's a weird question... Is it possible to work with 2 pararell (or series) output transformers? to get 20K? I have two approx 10K... I know it's better to find higher impedance speaker, but I don't have one.

[Enzo]

Easier to change the speaker impedance.

[boroman]

Easier to change the speaker impedance.

Yes... but loosing a lot of power

[Enzo]

Half power is 3 decibels.

[Helmholtz]

Yes... but loosing a lot of power

How that?

[Helmholtz]

Maybe it's a weird question... Is it possible to work with 2 pararell (or series) output transformers? to get 20K? I have two approx 10K... I know it's better to find higher impedance speaker, but I don't have one.

OTs don't have an own fixed primary impedance.
Rather they have an impedance transfer ratio.

A 10k/8Ohm OT will provide a 20k primary impedance with a 16 Ohm load.

And yes, it is possible to combine 2 OTs, but there might be drawbacks.
The Marshall JTM45/!00 used 2 JTM45 OTs with paralled primaries.

Please specify desired primary impedance, load impedance and impedance ratios of your OTs.