Cathode biasing multiple tube types (6V6, EL34, etc)

[jordan86]

So I am working on a 20W cathode biased plexi style amp. I've ordered two transformers from Hammond, an 18w Marshall style (8.4k), and a 20W Fender Deluxe style (6.6K).  Model numbers 1750pa and 1760h, for those who want to know or care.

I have octal sockets for the power tubes and plan to run either 6V6's, or possibly EL34s. Would love the flexibility to switch back and forth when desired. With the OT being in the 18-20W range, should I fuss with trying to rebias EL34's hotter or just hardwire in a bias resistor to keep me in the 20w range and call it good? I also have a variable cathode bias arrangement I am thinking about, but wondering if trying to bias the EL34's hotter would just mean problems for my OT. Plate voltage is expected to be ~400v or just under.

[shooter]

20w range and call it good

once you've played awhile, swapped in/out speakers, used all your pedals....then would be a good tine to evaluate power

[HotBluePlates]

So I am working on a 20W cathode biased plexi style amp. ... an 18w Marshall style (8.4k), and a 20W Fender Deluxe style (6.6K).  ... plan to run either 6V6's, or possibly EL34s. Would love the flexibility to switch back and forth when desired. ...

Cathode bias EL34 can do 20w easy.  Cathode bias 6V6, not so much (maybe 15w at best).  But that doesn't really matter because unless the amp is on-stage you won't miss the 5 watts.

This thread has an approach for adjustable cathode bias that might be suitable (exact values are up to you).

... should I fuss with trying to rebias EL34's hotter or just hardwire in a bias resistor to keep me in the 20w range and call it good? ...

I'm unsure how it would shake out in practice, but the EL34 will need something slightly different than 6V6 to bias up, just due to the difference of Gm.  But I think the adjustable setup in the link should serve both tube types.

[jordan86]

My amp currently has this variable bias scheme with bias check test points. Would this be sufficient for a pair of EL34s running off a shared cathode resistor scenario?

[PRR]

If you put the 1r at the bottom you can use a grounded voltmeter.

You are unlikely to want to be able to turn-down to 101 ohms, make the fixed resistor bigger. More than half the expected total.

[jordan86]

If you put the 1r at the bottom you can use a grounded voltmeter.

You are unlikely to want to be able to turn-down to 101 ohms, make the fixed resistor bigger. More than half the expected total.

Thanks PRR. I believe I have an ohmite 180R/10W in the stash. So does the resistor of the greatest value also carry the biggest current load? Ie bigger fixed resistor cause less strain in the 5W pot?

[tubeswell]

If you put the 1r at the bottom you can use a grounded voltmeter.

You are unlikely to want to be able to turn-down to 101 ohms, make the fixed resistor bigger. More than half the expected total.

Thanks PRR. I believe I have an ohmite 180R/10W in the stash. So does the resistor of the greatest value also carry the biggest current load? Ie bigger fixed resistor cause less strain in the 5W pot?

For resistors in series, the current is the same through each resistor, no matter what the resistance of each resistor is. (Kirchhoff’s current law). However, different resistances will produce different voltage across each resistor. (Ohm’s Law). Higher resistance will produce higher voltage, but less power dissipation (current being the same). And lower resistance cross the pot with higher resistance across the fixed series resistor will reduce the voltage (and dissipation) across the pot.

[PRR]

> For resistors in series, ........ Higher resistance will produce higher voltage, but less power dissipation (current being the same).

Uh, higher voltage and same current, isn't that more power dissipation?

{And under a tube, higher total resistance is lower current, so heat rises slower than Ohms.}

[tubeswell]

> For resistors in series, ........ Higher resistance will produce higher voltage, but less power dissipation (current being the same).

Uh, higher voltage and same current, isn't that more power dissipation?

{And under a tube, higher total resistance is lower current, so heat rises slower than Ohms.}

Yes - sorry brain fart.