wiring 6SN7 instead 12AU7

[Helmholtz]

Bottom line ... Unless you are going to wind your own OT, or buy one of everything that's available, buy one of those Hammond Universal OT's and try all the taps.

I think I provided everything necessary to find the optimum plate load.

Arbitrarily changing plate load bears a high risk of exceeding dissipation limits and might result in unsafe operation.

[boroman]

I think I provided everything necessary to find the optimum plate load.
Arbitrarily changing plate load bears a high risk of exceeding dissipation limits and might result in unsafe operation.

I'm trying to know "the simple rule" but it seems way off in some cases, in my calculations

Let's have 2 or 3 examples below. With 12AU7 triode, that "rule" Load Reistance means the best primary value for OT (e.g. in 12AU7, LR is 7.7kohms = 7.7k primary would be best choice) - is not true. (because I know it needs 25k primary). But this guy proves otherwise https://www.youtube.com/watch?v=Yzo3A-NywSs

So, the next calculator (http://valvewizard.co.uk/se.html)

Z = Va^2 / Pa

Where:
Va = Anode voltage.
Pa = Maximum anode dissipation.

Let's say we have 12AU7 = 325v on the plates & 2.4w max dissipation
(325*325)/4.8 = over 22k, so 25k primary would be great fit.
**** 4.8 = 2x 2.4 - someone suggested me that I need to calculate two triode sides (?)

With 6V6 pentode, should I use the same formula as above? With multiplying max dissipation x2?
315v 5.5 (x2) would be like 9k primary needed...?

So, how it all trabslates to EF80 that I read I need to have 10k primary for it, and it have 0,65mOhm load resistance? It all just makes no sense.
And to get it straight: I don't want to know how it's done and study millions of calculations.. Just need a formula for single-ended amps.

:-/

[Helmholtz]

I'm trying to know "the simple rule" but it seems way off in some cases, in my calculations........

As said the "simple rule" is for pentodes only (under the conditions I stated above) and it works well, say with an accuracy of 20%.

It does NOT work with triodes (use the online calculator instead).

[boroman]

As said the "simple rule" is for pentodes only (under the conditions I stated above) and it works well, say with an accuracy of 20%.

It does NOT work with triodes (use the online calculator instead).

Helmholtz Thanks. So, for triodes that (pvoltage*pvoltage)/2x dissipation formula would work, right? This guy from the http://valvewizard.co.uk/se.html takes EL34 pentode with that calculator. That's why I'm confused.

[Helmholtz]

So, for triodes that (pvoltage*pvoltage)/2x dissipation formula would work, right?

NO! It does NOT work with triodes (and you quoted Merlin's formula wrong above)

There is no simple formula for triodes, sorry..

BTW, Merlin Blencowe is an acknowledged tube expert. Much more reliable than most self-appointed internet "experts".

[boroman]

NO! It does NOT work with triodes (and you quoted Merlin's formula wrongly above)

There is no simple formula for triodes, sorry..

BTW, Merlin Blencowe is an acknowledged tube expert.

Thanks for the patience.

I think I don't get it anyway... Will need to meet someone that could explain to me directly

If the triode can't be calulated easy, so there are TWO methods for that pentode single ended stuff, one with Z = Va^2 / Pa formula (like above), second with just load impedance being perfect OT number for primary (no calculations, just reading from data sheets). Still, way divergent results from both. I guess, this is a mess for me now, really ;-)

[Helmholtz]

A 6V6 datasheet example: Vp = Va = 315V, Pa = 12W (design-center rating).
Datasheet Ra = 8500 Ohm.
Simple formula Ra = 8269 Ohm.

[Helmholtz]

For your convenience I did a load line construction using the online calculator.

For a single triode of a 12AU7 I find an optimum load impedance Za (at plate voltage = 300V = datasheet limit) of about 20k with an output power around 0.7W.
Means for the 2 triodes in parallel: Za = 10k, Pout = 1.4W.

You can always use a higher than optimum load impedance, but that will lower output power.

Also a higher plate voltage would require higher load impedance.

Optimum plate load for an EF80 is around 30k.
10k would exceed the plate dissipation limit of 2.5W.

[boroman]

Optimum plate load for an EF80 is around 30k.
10k would exceed the plate dissipation limit of 2.5W.

In this layout it calls for 10K... but also says it's Hammond 125B which is... 22,5k:8ohm...

taken from: https://robrobinette.com/Champ_Micro.htm (EF80 section)

[Helmholtz]

In this layout it calls for 10K... but also says it's Hammond 125B which is... 22,5k:8ohm...

A design that ignores the dissipation limit is not very professional.
It might work but is likeky to "eat" tubes.