Switchable EL84 cathode resistors...what's the best method?

[dehughes]

Hello again,

I'd like to wire in a switch to increase the resistor value on my quad of EL84s running in my AC30-ish amp. What's the best/simplest way to accomplish this? At present, I have a 62 ohm 10w Dale RS-10 in there that is super nice...and I have some 68 ohm RS-10s that I could switch in series with the 62 (a la the Matchless DC30) to get 130 ohms and a much cooler power section. This, again, is what Matchless does with the DC30.

However, in reading through other methods I came across the idea of running two cathode resistors in parallel and just switching out one in order to bump up the power section. That is, instead of using my 62 and 68 ohm resistors and running them in series, switching one out in order to kick things up, one could just run, say, in my case, two 130 ohm resistors in parallel and just switch one out of the circuit to cool things off, as with both you'd essentially have 65 ohms of resistance on the cathodes.

This method seems to be the simplest and easiest way to do it....what is your opinion?

Thanks!

[UR12]

Hello again,

I'd like to wire in a switch to increase the resistor value on my quad of EL84s running in my AC30-ish amp. What's the best/simplest way to accomplish this? At present, I have a 62 ohm 10w Dale RS-10 in there that is super nice...and I have some 68 ohm RS-10s that I could switch in series with the 62 (a la the Matchless DC30) to get 130 ohms and a much cooler power section. This, again, is what Matchless does with the DC30.

However, in reading through other methods I came across the idea of running two cathode resistors in parallel and just switching out one in order to bump up the power section. That is, instead of using my 62 and 68 ohm resistors and running them in series, switching one out in order to kick things up, one could just run, say, in my case, two 130 ohm resistors in parallel and just switch one out of the circuit to cool things off, as with both you'd essentially have 65 ohms of resistance on the cathodes.

This method seems to be the simplest and easiest way to do it....what is your opinion?

Thanks!

I think you are missing one inportant point. I think the matchless and others are actually disconnecting two of the EL84s to drop the power to 1/2 and when they do this they have to change the bias from 4 tubes at 65 ohms to 2 tube at 130 ohms. If you leave all of your tubes still hooked up and just change the bias resistor from 65 to 130 your going to be doubleing the bias voltage and running all 4 tubes a lot colder. It would be interesting to hear what that sounded like.

[dehughes]

Hello again,

I'd like to wire in a switch to increase the resistor value on my quad of EL84s running in my AC30-ish amp. What's the best/simplest way to accomplish this? At present, I have a 62 ohm 10w Dale RS-10 in there that is super nice...and I have some 68 ohm RS-10s that I could switch in series with the 62 (a la the Matchless DC30) to get 130 ohms and a much cooler power section. This, again, is what Matchless does with the DC30.

However, in reading through other methods I came across the idea of running two cathode resistors in parallel and just switching out one in order to bump up the power section. That is, instead of using my 62 and 68 ohm resistors and running them in series, switching one out in order to kick things up, one could just run, say, in my case, two 130 ohm resistors in parallel and just switch one out of the circuit to cool things off, as with both you'd essentially have 65 ohms of resistance on the cathodes.

This method seems to be the simplest and easiest way to do it....what is your opinion?

Thanks!

I think you are missing one inportant point. I think the matchless and others are actually disconnecting two of the EL84s to drop the power to 1/2 and when they do this they have to change the bias from 4 tubes at 65 ohms to 2 tube at 130 ohms. If you leave all of your tubes still hooked up and just change the bias resistor from 65 to 130 your going to be doubleing the bias voltage and running all 4 tubes a lot colder. It would be interesting to hear what that sounded like.

Ah. Yes. That would be an important point, wouldn't it? I'll go re-check my schematics and pay more attention.

However, wouldn't you need to adjust the secondary impedence on the OT if you dropped out two of the power tubes??? That is, use a different ohm jack (8?)?

Also, what type of switch would you use to switch the cathode resistors a la the DC30?

[UR12]

Ah. Yes. That would be an important point, wouldn't it? I'll go re-check my schematics and pay more attention.

However, wouldn't you need to adjust the secondary impedence on the OT if you dropped out two of the power tubes??? That is, use a different ohm jack (8?)?

Also, what type of switch would you use to switch the cathode resistors a la the DC30?

I actually built my Liverpool with a 1/2 power switch and used 2 130 ohm resistors like you were refering to. I can tell you that the 1/2 power is kind of a misnomer when it comes to how loud the amp is. It does drop the power by 1/2 but it is still very loud even in the 1/2 power mode. Unfortunatly the human ear isn't linear and you have to use a power factor of 10 to really get at what is going on. For example, if you had a amp running at 1 watt and you wanted it to sound twice as loud you have to increase the power to 10 watts. If you wanted to double the volume again then you would have to go to 100 watts. So dropping from say 36 watts to 18 watts really isn't going to sound like the amp is 1/2 as loud.

If you run 2 130 ohm resistors , one to each pair of tubes. All you need is a spst switch to open ou the cathode circuit from ground so two tubes will be out of the circuit. If you want to use the series method (ala DC30) you would need a DPDT switch.

As for the impedance, when you drop 2 tubes out the impedance would need to go up from say 4k to 8k. This can be acomplished by just changing your impedance switch. If you had say a 8 ohm cab when you flip to 1/2 power you can move the impedance switch to 4 ohms to keep the turns ratio or impedance the same as it was before you flipped the switch.

Hope this helps !