EL84 tube amp build, how to start?

[wg_ski]

Originally Posted by Kay Pirinha

This might end in death cathodes in tubes that are run withoud cathode current for a long time.

Only heat one cathode. It means you’re stuck with 6 volts, rather than the option of running off 12.

[Kay Pirinha]

Oh, good idea! You can also feed them off 12.6 V if you connect the cathodes of two halved tubes in series.
Best egards!

[wg_ski]

If both are either AX7 or AU7. Can’t mix if the heater currents aren’t the same.

[Kay Pirinha]

Of course. Or 12AT7 and possibly more.
Best regards!

[Francois G]

Originally Posted by Jones880

Ok, thanks. I'm gonna start ordering parts I guess.

Just out of curiosity:

It's said a few times that a 12AU7 would be better for the concertina. This is I presume Because of the larger Ia? This would probably also mean that the grid leak resistors of the EL84 are less of a load to the concertina.

I was trying to use the 2 triodes of the same tube in the same channel. But I could use the 2 half's of an 12AX7 as input stages for 2 channels and the same with the 12AU7 as concertina for the 2 channels. However would this not also mean there's a high risk of x-coupling between channels?....

Jones880,

If you want to explore this direction I recommend a review of the diytube design by Parks Audio that uses one 12ax7 section for voltage amplification and both 12au7 sections for phase splitting. I built it and it is a very fine EL84 amplifier. Your concern about cross-coupling between channels in one double triode is valid, but in practice in this application that proved to be a non-issue.

Unfortunately the PCBs are no longer available, but it would not be difficult to hardwire build it.

See: http://www.diytube.com/st35/ST35REVE.pdf

[Ketje]

The EL84 is a very easy to drive tube.No problem for the ECC83.
Philips EL84 datasheet gives max Rg1 (the load for the concertina) 1M with auto(cathode) bias ande 300k with fix bias.
Most amps use 470k with cathode bias but there are stories about runaway tubes.
So, to be on the save side, I proposed 330k no need to go lower.
330k//100k= 77k the real AC-load.10Vtop (max drive) is a current change of 130µA on 740µA bias, no problem.
Mona

[Jones880]

Parts are ordered!

exciting!

Will start building around X-mas (if parts arrive in time)
Let's hope all goes well

Many thanks again to all who helped me. I'll probably be back to share some pictures while building. This is an awesome community

[Schmitz77]

Exciting!