Can i use 1R on el84 cathode same as 6V6 ?

[12AX7]

I got the idea to tryEL84's in my 6V6 jcm 800 style build with 18 watt trannys. It originally had el84's but i converted it. I decided to make some octal>9pin converters to try 84's again in the amp. I have the 6V6 octals wired up with 1R resistors to ground as a bias check point. Any reason i can't do the same with el84s?

[pompeiisneaks]

Nope, no reason at all, go for it.  The whole idea is that you have a measuring point and with very little resistance.  The output current though the ground is pretty low, unless something is wrong, and then you get the added benefit of a cheap 1% 1ohm resistor melting down acting as a fuse for you.  So you get added protection too.

[12AX7]

I think you may have misunderstood, but i already have 1R resistors on the cathodes to measure bias and have done that for years. I just wanted to be sure it was ok to do the same with 84's. I asked because looking at the pinouts of both tubes the 6V6 shows it's cathode pin as "cathode" while el84's say "cathode and suppressor grid".

[sluckey]

I asked because looking at the pinouts of both tubes the 6V6 shows it's cathode pin as "cathode" while el84's say "cathode and suppressor grid".

The answer is still yes.

6V6 is a beam power tetrode. It has no suppressor grid. The EL84 is a pentode. It does have a suppressor grid and it is internally connected to the cathode.

[12AX7]

Well, i finished them and tried it. maybe if i reconfigured the PA with different component values it would sound good, but as it was (with just changing bias) the 6V6's sound worlds better.  Curiosity often drives me to the wrong path. Now i remember why i converted to 6v6 to begin with. Probably do this again in 5 years when i've forgotten again.   

[sluckey]

EL84s are much easier to drive than 6V6s and they need different biasing. Turn the signal down going into the PI and see if it gets any better.

[12AX7]

EL84s are much easier to drive than 6V6s and they need different biasing. Turn the signal down going into the PI and see if it gets any better.

Yeah, i noticed that. Maybe i'll try something at some point. Maybe using a 1.5 or 2.2k on the PI cathode instead of the 470R would help.

[HotBluePlates]

EL84s are much easier to drive than 6V6s and they need different biasing. ...

... Maybe using a 1.5 or 2.2k on the PI cathode instead of the 470R would help.

No, you'd need to go the other way. Probably 300Ω or less (maybe quite a bit less).

EL84's have higher transconductance (Gm) than 6V6's.  The Euro method for specifying Gm was mA/V, showing how many milliamps of plate current change you get per volt of grid voltage change.

Since the EL84 has higher Gm, it takes less driving V's for the same mA output.  And that also means less bias voltage (grid-to-cathode voltage) for the same idle current.

Same Ma * Smaller Cathode R = Smaller Cathode V, which is exactly what the EL84 needs for proper bias.

[12AX7]

Thanks. I was looking at the PI as a gain stage where a larger cathode gives less gain, but i just didn't know there was difference with the PI. Ok, well then if i try it i'll go with 100R and se what happens. That is IF i decide to. I've used el84 amps in the past and that short trial even if not optimal just brought back memories of why i didn't care for them in the first place.

[HotBluePlates]

Thanks. I was looking at the PI as a gain stage where a larger cathode gives less gain, but i just didn't know there was difference with the PI. ...

The phase inverter is a voltage amplifier, with a voltage gain often around 25-40.

The output tubes are power amplifiers, with a voltage gain often less than 10.  The EL84 has a gain closer to 20, but the voltage gain is only relevant in how much driving signal it requires to deliver the desired output power.

What you really care about in an output stage is the current the output tube can swing, and the size of the plate load it through which it swings that current.

Idle bias isn't so much about voltage gain, and only indirectly about power output.  It's all about keeping the plate from overheating while the tube is delivering that power output.  It also influences how much driving voltage is required for the output stage to make its maximum output power.