Here's one for ICBM I think...

Cirrus

My AC30 has a switch that I'm using to roughly double the value of the EL84's cathode resistor from 56 to 118 ohms for "half power" (give or take..)

What else would I need to switch at the same time to take out one pair of EL84s?

@ICBM and anyone else who knows, aidez moi!

jpfamps

My guess is that the half power switch will also switch off  a pair of the power valves (hence 1/2 power).

The reason the cathode resistor in increased in value is not tolimit the power, but because you will need twice the value to maintain the correct bias witj 1/2 the valves.

Cirrus

Nope, the latest handwire series half power switch is really lazy as far as I can tell. Stock, the "30 watt" setting is a 50 ohm resistor, and the "15 watt" setting puts a 150 ohm resistor in series with it which gives a pretty harsh sound as it's just 4 el84s biased really cold. And subjectively I'd say the power output drops to less than half.

I've done a bit of tinkering myself and settled on 56/ 118 ohms as a more practically useful power reducing switch, but I suspect it'd work better if it did cut out two valves as well. That way when I want half power without changing the feel too much I can switch to two valves and flick the switch I installed in the back of my 2x12 cab that bypasses one of the speakers.

That'll keep the whole power section/ speaker interaction much more consistant at half power mode I think, ignoring of course the output transformer not having to work quite as hard.

ICBM

The easiest way to do it is to have two separate cathode networks, one for the inner pair of valves and one for the outer. Each one should have double the resistor value and half the cap value of the normal 4-valve setup. You normally turn off one pair of valves by lifting the ground connection of one of the cathode networks.

If you want to get the true AC30 4-valve sound in full-power mode, you also need to connect the cathode connections of the two pairs together as well, so all four are paralleled. You can still do both things with one DPDT switch.

Ideally, when you're running in half-power mode you also need to reset the impedance of the output, to 8 ohms for the standard 16-ohm speaker pair. If you don't you'll actually get slightly more power reduction, due to the mismatch, but it will be a bit harder on the valves.

ecc83

Or, you can switch the valves to triode operation (yes, I know the loading is all over the shop but having a much lower ra the valves don't seem to mind!)

You would need a double pole 2 way switch.

 

Dave.

Cirrus

Cool, thanks very much for that! A wisdom for ye!

jpfamps

Cirrus said:

Nope, the latest handwire series half power switch is really lazy as far as I can tell. Stock, the "30 watt" setting is a 50 ohm resistor, and the "15 watt" setting puts a 150 ohm resistor in series with it which gives a pretty harsh sound as it's just 4 el84s biased really cold. And subjectively I'd say the power output drops to less than half.

I din't know this.

Cheap might have been a better adjective!

Cirrus

jpfamps said:

I din't know this.

Cheap might have been a better adjective!

Haha yeah, I think that's fair. As for the "hot/cool" switch which gives you more gain on the top boost channel, it's also a pretty simpledevice. It just lifts the ground of the EQ circuit - nice big boost but no way to control it.

It's strange, because the circuit's so simple I guess someone at Vox looked at the competition and decided they needed to add a couple of bells and whistles so they worked out the simplest way of getting a couple of extra switches on the control panel.

ICBM

jpfamps said:

Cirrus said:

Nope, the latest handwire series half power switch is really lazy as far as I can tell. Stock, the "30 watt" setting is a 50 ohm resistor, and the "15 watt" setting puts a 150 ohm resistor in series with it which gives a pretty harsh sound as it's just 4 el84s biased really cold. And subjectively I'd say the power output drops to less than half.

I din't know this.

Cheap might have been a better adjective!

Nasty is probably closer .

I'm not a big fan of over-hot biasing, but going the other way is worse - biased that cold, I would guess the harsh sound is because it will produce a fair amount of crossover distortion. Not the right way to reduce power at all.

jpfamps

ICBM said:

jpfamps said:

Cirrus said:

Nope, the latest handwire series half power switch is really lazy as far as I can tell. Stock, the "30 watt" setting is a 50 ohm resistor, and the "15 watt" setting puts a 150 ohm resistor in series with it which gives a pretty harsh sound as it's just 4 el84s biased really cold. And subjectively I'd say the power output drops to less than half.

I din't know this.

Cheap might have been a better adjective!

Nasty is probably closer .

I'm not a big fan of over-hot biasing, but going the other way is worse - biased that cold, I would guess the harsh sound is because it will produce a fair amount of crossover distortion. Not the right way to reduce power at all.

It does seem a strange approach, and must sound fairly bad,

Does not inspire confidence in the designer I would say.

thermionic

Speaking as an ex-designer (not of valve amps) I bet the designer didn't want to do it, but that the decision was made by the marketing/sales people!

jpfamps

thermionic said:

Speaking as an ex-designer (not of valve amps) I bet the designer didn't want to do it, but that the decision was made by the marketing/sales people!

A fairly point, however it would have been very easy (cheap) to switch the cathode resistor to the higher value and simultaneously turn off a pair of output valves.

OK you would not be running the remaining valves at the correct impedance, but this would have been a better solution.